Deserts (Ecosystem)

DESERTS Revised Edition

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DESERTS Revised Edition

Michael Allaby Illustrations by Richard Garratt

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DESERTS, Revised Edition Copyright © 2008, 2000 by Michael Allaby All rights reserved. No part of this book may be reproduced or utilized in any form or by any means, electronic or mechanical, including photocopying, recording, or by any information storage or retrieval systems, without permission in writing from the publisher. For information contact: Facts On File, Inc. An imprint of Infobase Publishing 132 West 31st Street New York NY 10001 ISBN-10: 0-8160-5929-2 ISBN-13: 978-0-8160-5929-4 Library of Congress Cataloging-in-Publication Data Allaby, Michael. Deserts / Michael Allaby ; illustrations by Richard Garratt. — Rev. ed. p. cm. — (Ecosystem) Includes bibliographical references. ISBN 0-8160-5929-2 1. Deserts. I. Title. QH88.A45 2008 577.54—dc22 2007000477 Facts On File books are available at special discounts when purchased in bulk quantities for businesses, associations, institutions, or sales promotions. Please call our Special Sales Department in New York at (212) 967-8800 or (800) 322-8755. You can find Facts On File on the World Wide Web at http://www.factsonfile.com Text design by Erika K. Arroyo Illustrations by Richard Garratt Photo research by Elizabeth H. Oakes Printed in the United States of America BP Hermitage 10 9 8 7 6 5 4 3 2 1 This book is printed on acid-free paper.

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Contents

Preface

ix

Acknowledgments

xi

Introduction

1 ✦ Geography of Deserts Location of the World’s Deserts What Makes a Desert? Climatic Optima and Minima Characteristics of the Sahara and Arabian Desert Studying Ancient Climates Characteristics of the Gobi, Takla Makan, and Thar Deserts Characteristics of the Kalahari and Namib Deserts David Livingstone (1813–1873) Boundary Currents Land and Sea Breezes Characteristics of the Atacama Desert Alexander von Humboldt (1769–1859) Patagonia Characteristics of the Australian Desert Continental Climates Characteristics of the North and Central American Deserts Cold Deserts Ice Sheets and Glaciers Isotopes The Larsen Ice Shelf Greenland or Kalaallit Nunaat Alfred Lothar Wegener (1880–1930) Adiabatic Cooling and Warming When Northern America, Europe, and Asia Were Cold Deserts Pliocene, Pleistocene, and Holocene Glacials and Interglacials

2 ✦ Geology of Deserts Plate Tectonics and Orogenies The Tsunami of December 2004

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1 1 6 8 11 12 16 18 20 22 22 24 25 27 28 31 32 34 38 39 41 42 43 45 45 47

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The Formation, Development, and Aging of Soils Desert Soils How Soils Are Classified Ergs and Sand Dunes Desert Pavement, Erosion, and Varnish Deserts and Water Polar Molecules Aquifers, Oases, and Wells Rate of Groundwater Flow Poisoned Wells

3 ✦ Atmosphere and Desert Hadley Cells George Hadley (1685–1768) Why Sinking Air Is Dry General Circulation of the Atmosphere Conservation of Angular Momentum Intertropical Convergence Zone, Monsoons, and Jet Streams Air Masses and Fronts Thermal Wind Vilhelm Bjerknes (1862–1951) Oceans and Climates Thermohaline Circulation Desert Weather Dust Storms, Sandstorms, Dust Devils, and Whirlwinds

4 ✦ Biology of Deserts Photosynthesis Osmosis Melvin Calvin (1911–1997) Transpiration and Why Plants Need Water Roots, Stems, and Leaves That Conserve Water Desert Plants Typical Desert Plants Cacti Plants of Continental and Polar Deserts Desert Animals How Heat Can Kill and How Animals Keep Cool How Tolerating a Slightly Higher Temperature Pays Dividends How Freezing Kills and How Animals Avoid It Why Small Animals Tolerate Heat and Large Animals Tolerate Cold Estivation and Hibernation Do Bears Hibernate? Finding Water and Conserving Water The Ship of the Desert Convergent Evolution and Parallel Evolution

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55 57 57 59 62 66 68 69 69 72

73 73 74 76 77 79 79 82 83 84 87 88 90 93

96 96 97 98 101 103 105 108 116 119 122 123 125 128 130 132 135 136 138 142

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Desert Invertebrates Locusts Reptiles of Old and New World Deserts Mammals of Old and New World Deserts Birds of Old and New World Deserts Desert Ecology Animal Life of the Arctic Saving the Saiga The “Glutton” Animal Life of the Antarctic Life in Polar Seas

5 ✦ History and the Desert When Deserts Grew Crops Desert Civilizations Anatolia, Mesopotamia, and the Birth of Western Civilization Irrigation, Rivers, and Dams Aquifer Depletion, Waterlogging, and Salination Rising Water Table Due to Irrigation Extent of Soil Degradation on Irrigated Dry Land by Continent Egypt Chinampas Nomadic Peoples of the Sahara Peoples of the Arabian Desert Bedouin Population by Country Nomadic Peoples of the Gobi Desert Caravans and the Silk Road Peoples of the American Desert Peoples of the Arctic and Antarctic Deserts Desert Buildings Modern Bricks Explorers of the Polar Deserts Fritjof Nansen (1861–1930) Lincoln Ellsworth (1880–1951) James Clark Ross (1800–1862) Explorers in Africa, Arabia, and Asia

6 ✦ Economics of the Desert Oil and the Economies of Modern Desert Countries Per Capita GDP in Desert Countries, 1999 (US $) Proven Oil Reserves of the Top 20 Countries, 2006 Manufacturing as a Percentage of GDP and Employment Vital Statistics for Desert Countries Solar Energy Minerals, Metals, and Textiles Tourism Tourism in Desert Countries (Millions of US$)

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145 147 150 153 156 158 161 165 166 167 170

173 173 176 178 182 184 186 188 189 192 194 196 197 198 200 202 205 209 210 212 212 214 216 217

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7 ✦ Health of the Deserts Climate Change and the Future for Deserts Global Dimming and Changing Clouds Svante Arrhenius (1859–1927) Sunspot Cycles Edward Walter Maunder (1851–1928) Milankovitch Cycles Rainmaking Vincent Schaefer (1906–1993) and Bernard Vonnegut (1914–1997) Halting the Spread of Deserts and New Crops for Dry Climates Improved Irrigation

8 ✦ Management of the Desert Desert Advances and Retreats Pastoralism Overgrazing and Desertification Providing Water Tragedy of the Commons China’s History of Flooding Diverting Rivers Oasis Farming and Artificial Oases Conflicts over Water Resources Water Availability per Person per Year Desalination and Mining Icebergs How Distillation Purifies Water Annual Production of Desalted Water, 1992–2000 Food from the Arctic and Antarctic The International Whaling Commission

240 242 244 246 248 249 253 255 255 261

264 264 266 268 269 270 271 275 280 282 283 284 285 286 289 290

Conclusion

292

Appendixes

293

Areas of the Major World Deserts, by Continent Climatic Averages and Extremes for Major Deserts SI Units and Conversions

293 294 295

Glossary

297

Bibliography and Further Reading

308

Books and Articles Web Sites

Index

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308 309

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Preface

Increasingly, scientists, environmentalists, engineers, and land-use planners are coming to understand the living planet in a more interdisciplinary way. The boundaries between traditional disciplines have become blurred as ideas, methods, and findings from one discipline inform and influence those in another. This cross-fertilization is vital if professionals are going to evaluate and tackle the environmental challenges the world faces at the beginning of the 21st century. There is also a need for the new generation of adults, currently students in high schools and colleges, to appreciate the interconnections between human actions and environmental responses if they are going to make informed decisions later, whether as concerned citizens or as interested professionals. Providing this balanced interdisciplinary overview—for students and for general readers as well as professionals requiring an introduction to Earth’s major environments—is the main aim of the Ecosystem set of volumes. The Earth is a patchwork of environments. The equatorial regions have warm seas with rich assemblages of corals and marine life, while the land is covered by tall forests, humid and fecund, and containing perhaps half of all Earth’s living species. Beyond are the dry tropical woodlands and grassland, and then the deserts, where plants and animals face the rigors of heat and drought. The grasslands and forests of the temperate zone grow because of the increasing moisture in these higher latitudes, but grade into coniferous forests and eventually scrub tundra as the colder conditions of the polar regions become increasingly severe. The complexity of diverse landscapes and seascapes can, nevertheless, be simplified by considering them as the great global ecosystems that make up our patchwork planet. Each global ecosystem, or biome, is an assemblage of plants, animals, and microbes adapted to the prevailing climate and the associated physical, chemical, and biological conditions. The six volumes in the set—Deserts, Revised Edition; Tundra; Oceans, Revised Edition; Tropical Forests; Temperate Forests, Revised Edition; and Wetlands, Revised Edition—

between them span the breadth of land-based and aquatic ecosystems on Earth. Each volume considers a specific global ecosystem from many viewpoints: geographical, geological, climatic, biological, historical, and economic. Such broad coverage is vital if people are to move closer to understanding how the various ecosystems came to be, how they are changing, and, if they are being modified in ways that seem detrimental to humankind and the wider world, what might be done about it. Many factors are responsible for the creation of Earth’s living mosaic. Climate varies greatly between Tropics and poles, depending on the input of solar energy and the movements of atmospheric air masses and ocean currents. The general trend of climate from equator to poles has resulted in a zoned pattern of vegetation types, together with their associated animals. Climate is also strongly affected by the interaction between oceans and landmasses, resulting in ecosystem patterns from east to west across continents. During the course of geological time even the distribution of the continents has altered, so the patterns of life currently found on Earth are the outcome of dynamic processes and constant change. The Ecosystem set examines the great ecosystems of the world as they have developed during this long history of climatic change, continental wandering, and the recent meteoric growth of human populations. Each of the great global ecosystems has its own story to tell: its characteristic geographical distribution; its pattern of energy flow and nutrient cycling; its distinctive soils or bottom sediments, vegetation cover, and animal inhabitants; and its own history of interaction with humanity. The books in the Ecosystem set are structured so that the different global ecosystems can be analyzed and compared, and the relevant information relating to any specific topic can be quickly located and extracted. The study of global ecosystems involves an examination of the conditions that support the planet’s diversity. But environmental conditions are currently changing rapidly. Human beings have eroded many of the great global

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ecosystems as they have reclaimed land for agriculture and urban settlement and built roads that cut ecosystems into ever smaller units. The fragmentation of Earth’s ecosystems is proving to be a serious problem, especially during times of rapid climate change, itself the outcome of intensive industrial activities on the surface of the planet. The next generation of ecologists will have to deal with the control of global climate and also the conservation and protection of the residue of Earth’s biodiversity. The starting point in

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approaching these problems is to understand how the great ecosystems of the world function, and how the species of animals and plants within them interact to form stable and productive assemblages. If these great natural systems are to survive, then humanity needs to develop greater respect and concern for them, and this can best be achieved by understanding better the remarkable properties of our patchwork planet. Such is the aim of the Ecosystem set.

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Acknowledgments

All the maps and diagrams in this book were drawn by my friend and colleague Richard Garratt. Richard and I have been collaborating for more years than I am sure either of us would care to count, and, as always, I am deeply grateful to him. I drew up a list of photographs to illustrate the text, but the task of finding them fell to Elizabeth Oakes, with

results that are obvious. Without her enthusiasm and skill this would have been a much duller and less informative book. Finally, I thank my friend and editor of many years Frank K. Darmstadt, whose encouragement and guidance helped me greatly.

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Introduction Seen from space, the Earth is blue and green, with patches of white cloud, and it shines brilliantly in the light of the Sun. It is breathtakingly beautiful, and astronauts have told how they have stared at it transfixed, no matter how many times they have seen it before. Our planet is blue because of the oceans that cover more than three-quarters of its surface and because the air is blue, which is why a cloudless sky is blue. It is green because of the plants that grow on the surface—the vast forests of the Tropics and the northern taiga, the grasslands, and the farmed lands that provide our food. People often look no further, content to admire the beauty but remaining unaware of all that remains hidden. Orbiting satellites take photographs that reveal more. Cameras on satellites are sensitive to light beyond the visible spectrum, especially in the infrared, and they can photograph what our eyes cannot see. Their eyes can penetrate cloud and distinguish different types of vegetation and different types of land surface. They transmit radio messages to receiving stations on the surface, to be processed by computers to form pictures rich in information. The pictures are still beautiful, but with a beauty that tells those who can interpret them what the surface of the Earth is really like.

DESERT COVERS NEARLY HALF THE WORLD

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Satellite pictures reveal that although forests, grasslands, and farms cover more than half the land surface, it is barely more than half. Over about 48 percent of the land, plants are widely scattered, with bare ground between them, and in some regions there are no plants at all. These areas are classified as dry (10 percent), semiarid (18 percent), arid (12 percent), or hyperarid (8 percent). The arid and hyperarid areas are deserts, and the remaining areas are dry enough to resemble deserts at least some of the time. These

inhospitable areas are vast, and about 13 percent of all the people in the world live in them. Deserts are also different one from another. Some are hot, some are cold, and some are covered with great oceans of sand that is piled into high dunes. Some are frozen and thickly covered with ice. All deserts are dry, even those buried beneath thousands of feet of ice, but this does not mean they are necessarily far from the sea. There are coastal deserts, bordered by the ocean. Other deserts lie in the heart of Asia, thousands of miles from the sea, or in the rain shadow of mountain ranges, such as the Rockies.

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DESERT GEOGRAPHY

This book is about deserts. It begins, as it must, by describing where on Earth deserts are found and the ways in which one type of desert differs from another. The Geography of Deserts, the first section of the book, has to be qualified, because deserts appear and disappear over long periods. They are produced by climatic conditions, and climates change. During ice ages, for example, deserts expand. Having established the locations and general types of deserts, the section continues with a more detailed account of the most important deserts. These include the hot deserts, such as the Sahara, the cold continental deserts, such as the Gobi, and the coastal deserts, such as the Namib and Atacama as well as the North American deserts. The section also describes the polar deserts, with their ice sheets and glaciers.

DESERT SURFACES AND LAND FORMS

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Deserts have a variety of surfaces. Some are sandy, others covered with gravel or bare rock. The next section of the

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book, Geology of Deserts, explains how these surface types and shapes are produced and describes what they are like. It explains, for example, why sand dunes have particular shapes, each with its own name. Dryness is the one feature that all deserts have in common. The section continues by describing how water moves in a desert, and it explains how oases are formed. The movement of water results from the rain that falls occasionally in most deserts or that drains into a desert from elsewhere. Rain is a meteorological phenomenon, and the third section of the book, Atmosphere and Deserts is about climate. It describes how the circulation of the atmosphere produces desert climates in certain places and also explains some of the spectacular types of desert weather, including whirlwinds, dust devils, and sandstorms.

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DESERT PLANTS AND ANIMALS

It is true that deserts are barren, but this does not mean they are devoid of life. There are many plants and animals that have evolved to tolerate the harsh conditions of even the hottest, coldest, windiest, and driest deserts. The fourth section of the book is concerned with desert life. It describes how plants and animals economize their use of water and how some species can remain alive but dormant for years and then come to life when a rare opportunity occurs for them to complete their life cycles. That is what happens when the desert blooms and within hours is ablaze with the color of flowers that vanish as rapidly as they appeared. Some animals are especially famous—or notorious. The section describes the camels and locusts as well as the wolf, wolverine, and polar bear of the far north. It also describes the animals of the polar seas and shores, the walrus, seals, fishes, and penguins.

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DESERT PEOPLES

With its description of desert plants and animals, the first half of the book draws to a close. The second half deals with people. It tells of how human societies have thrived in and beside deserts for thousands of years and of how it was, in Egypt and the land between the Euphrates and Tigris Rivers that Western civilization was forged. It describes how desert dwellers managed their water supply, and it also tells of the dangers that accompany the irrigation of fields in a hot, dry climate. In Mexico, however, those dangers were avoided in one of the most successful and remarkable of all methods of irrigation and cultivation, the chinampas, or floating gardens. If this is how life in the desert was made possible, what is it actually like? The fifth section of the book, History and

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the Desert continues with a series of accounts of a number of desert peoples. Some are famous, such as the Berber and Tuareg peoples of North Africa, the Mongol peoples of Central Asia, the Inuit of the far north, and the Hopi and other Pueblo peoples of North America. Others described here are less well known. The section also describes the Silk Road that formed a communications link between Europe and China. It also tells of the explorers who were the first outsiders to penetrate these difficult and unmapped regions.

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DESERTS TODAY

The final three sections of the book discuss the deserts as they are today, dealing with the economics of desert nations and industries, the possibilities of climatic change, and the management of desert lands. It includes an account of the development and impact of the oil industry in Africa and the Middle East and of the opportunities for tourism. In the discussions of climate change the book explains the climatic effect of changes in the orbit and rotation of the Earth and in energy output from the Sun. It also explains the ways in which local weather, though not climates, can sometimes be manipulated. No book about deserts can ignore the widespread fear that at least some of the world’s deserts are spreading. Here you will read about the way deserts do indeed expand and also how they contract again. The danger arises much more from poor land management that leads to overgrazing and the resulting degradation of land than from changes in the climate. Those dangers are described together with their causes. Egyptian culture has always depended on the Nile, and the book describes the river itself, the way its water is used, and the effect of damming the river at Aswān. Moving water from one place to another can have catastrophic consequences. The book continues with the story of irrigation in the arid lands of the former Soviet Union and its effects on the Aral Sea. Finally, the book shows how deserts can be improved successfully. It tells of ancient artificial oases made by building underground water channels and of the way seawater can be made drinkable. It warns of the risk of conflict over access to water but points out that so far water has not been used as a reason for going to war. Deserts, Revised Edition, contains a large amount of scientific and technical detail, but it is presented clearly and should not be difficult to follow. So far as possible, technical terms are avoided, and where they cannot be avoided they are defined in simple language. These terms are also defined in a comprehensive glossary. Scientists use units of measurement that are defined in the Système International d’Unités, known as the SI system. These units are listed in an appendix, together with their

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Introduction equivalents in units that may be more familiar. In the text the customary units are given first, with the SI units in parentheses. Many SI units are also metric units, and in these cases the conversion in the text is between the most obvious comparisons—inches and centimeters, miles and kilometers, pounds and kilograms, for example. There is one exception to this rule. By convention amounts of rainfall are always expressed in millimeters, not centimeters. This is to avoid confusion, so precipitation in different places and at different times can be compared without the risk of making mistakes.

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A REVISED EDITION

Several years have passed since the first edition of Deserts appeared. Scientific research never ceases, and much has been learned about deserts during those years, especially about the climatic conditions that produce them and the climate cycles that make them expand and contract. I was very glad, therefore, when my friends at Facts On File invited me to revise the book for a new edition. I have updated all the facts and figures contained in the book and have rearranged or rewritten much of the text. This makes the revised edition substantially different from the first edition. I hope the changes I have made explain important ideas and concepts more clearly. Some explanations are necessary for an understanding of deserts that would interrupt the flow if they formed part of the main text. Coastal deserts are strongly affected by land and sea breezes, for example, and deserts in the interior of continents have an extreme continental type of climate.

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Many of the characteristics of water arise from the fact that its molecule is polar—but what is a polar molecule? It would be possible to rely on brief glossary definitions to explain such ideas, but it would be unsatisfactory and would mean having to turn to the back of the book to find them. Instead, I have placed these explanations in sidebars. Sidebars allow me to explain an idea fully without getting in the way of the main text. Sidebars can also be used in other ways. Many people have heard on television or read in the news about the collapse of the Larsen ice shelf in Antarctica. I have included a sidebar describing the Larsen ice shelf and the sequence of events that led to its breakup. The December 2004 Asian tsunami attracted worldwide attention. In describing seafloor spreading and plate tectonics I thought it worthwhile to provide a reminder of what happened in that tragic event, and I placed my summary in a sidebar. Finally, I have used sidebars to provide brief biographical details about individuals who have made important contributions to our knowledge of deserts. In all there are more than 30 sidebars distributed through the book. At present the world is growing slowly warmer, and climate change may mean that some present-day deserts disappear and new deserts appear elsewhere. Consequently, interest in deserts has never been keener. I hope that with all its many revisions Deserts will supply useful information to help readers engage in the ongoing debate about the implications of global warming. Michael Allaby Tighnabruaich, Scotland www.michaelallaby.com

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1 Geography of Deserts There are deserts in every continent. Europe has no desert, but geographically Europe constitutes the westernmost part of Eurasia and is not a continent in its own right. All deserts are dry—it is lack of moisture that defines a desert—and aridity is the only characteristic that all deserts share. There are sandy deserts, rocky deserts, and deserts of ice. There are deserts where no plant grows and deserts that support some vegetation. There are even deserts that are frequently blanketed in fog yet remain dry. Deserts vary according to where they occur and the climatic conditions that give rise to them. This chapter begins with brief descriptions of the principal types of desert and where they are found, followed by an explanation of the climatic conditions that produce them. Climates change, and the lands that are barren today were not always so inhospitable. There have been times when climates were warmer and wetter than those of today and also times when they were cooler and drier. The chapter then describes the climate and physical features of the world’s deserts, how it is possible for a desert to lie beside an ocean, and why the interior of a continent is dry. As it does so the chapter outlines the lives and adventures of some famous explorers.

LOCATION OF THE WORLD’S DESERTS

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As far as the horizon in every direction there is only sand, rocks, and gravel. The landscape has no features— landmarks to guide the traveler. One rock looks just like another, the sand dunes are identical, nothing stands out from its surroundings. It is no wonder people can find themselves walking in circles, repeatedly passing the same rocks and the same dunes and failing to recognize them. Only the Sun indicates direction—due south at noon, east at dawn, west at sunset—and the Sun beats down mercilessly from a pale, cloudless sky.

This is the desert. At least, it is the desert we see in movies. It is hot, dry, and lifeless except for the lost travelers of the story and occasional bands of nomads moving from one oasis to the next with their sheep and camels or strings of camels transporting goods to some distant city. Many deserts are like this, but there are also deserts that are cold—the coldest place on Earth lies in what is possibly the driest of all deserts—and deserts without a single sand dune. Nor are most deserts altogether lifeless, although conditions in some deserts are so harsh that nothing can live there. Most deserts have some plants and animals but look barren because many of the plants appear only occasionally and most of the animals remain out of sight. Hot, dusty deserts lie in the subtropics, the subtropical deserts.

Subtropical Deserts A glance at the map of the world in the figure shows that most deserts lie close to the Tropics. These are the regions to either side of the equator bounded by latitude 23.5°. Latitude 23.5°N is the tropic of Cancer and 23.5°S is the tropic of Capricorn (the names refer to those lines of latitude). Within these latitudes the Sun appears directly overhead at noon on at least one day in the year. To the north or south of them the Sun is never directly overhead. Every day the Earth turns on its axis through one complete revolution. Each year the Earth completes one journey, or orbit, around the Sun. The Earth is not upright, however, in respect to the Sun. Its axis is tilted. Consequently, as Earth travels around its orbit, first one hemisphere and then the other is tilted toward the Sun. As the diagram shows, in June it is the Northern Hemisphere that faces the Sun, and in December it is the Southern Hemisphere. In March and September both hemispheres are exposed to sunlight equally, because the Sun is directly above the equator. The days when the effect is most extreme are known as the solstices. They fall on June 21–22 and December 22–23 and are also called midsummer day and midwinter day, which is which depending on the hemisphere, because midsummer

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60°N

Gobi Mojave

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Sonoran 30°N tropic of Cancer

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Arabian

equator OCEAN INDIAN

tropic of Capricorn 30°S

Atacama

Kalahari OCEAN

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(above) Location of the world’s deserts. The map shows all of the major deserts and semideserts.

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(below) How Earth’s tilted axis causes the seasons. As the Earth orbits the Sun, its tilted axis means that the Northern and Southern Hemispheres face the Sun alternately.

March

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Geography of Deserts

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23°30' tropic of Cancer

solstice

equinox 23°30'

equator

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23°30' tropic of Capricorn

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day in the Northern Hemisphere is midwinter day in the Southern Hemisphere. The days when both hemispheres are equally exposed are called the equinoxes and fall on March 20–21 and September 22–23. The Earth’s axis is tilted at an angle of 23.5°, which can also be written as 23°30' (23 degrees and 30 minutes). At each solstice a person at the equator would see the noonday Sun at an angle of 23.5° from the vertical, and at each equinox the noonday Sun would be directly overhead. The geometry shows that at the solstices the noonday Sun is directly overhead at latitude 23.5°N or S. That is why the tropics of Cancer and Capricorn are located where they are. The part of the Earth lying between them is known as the

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Angle of tilt and the Tropics. The angle by which the Earth’s axis is tilted defines the latitudes of the tropics of Cancer and Capricorn. At the equinoxes the Sun appears directly overhead at noon at the equator. At the solstices it appears overhead at noon at one or the other of the Tropics.

Tropics, and the regions of each hemisphere lying just outside the Tropics, centered on about latitude 30°, are known as the subtropics. Sunshine is more intense in the Tropics than it is anywhere else on Earth. This is because the Sun rises higher

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in the sky over the Tropics than it does anywhere else, and this makes a big difference. The Sun radiates light and heat equally in all directions. Imagine, though, the amount of solar radiation contained in a narrow beam and picture how a beam that size affects people living in two different parts of the world, one group in the far north, say in Greenland, and the other near the equator, say in southern Mexico. For the people in the south, where the noonday Sun is almost overhead, the sunlight shines nearly vertically downward, so the beam illuminates a quite small area because of the angle at which it strikes the surface. To the Greenlanders, the Sun appears much lower in the sky, so its light reaches them not vertically, as it does for the Mexicans, but at a low angle. The beam is the same width as the one shining on Mexico, but it illuminates a much bigger area. Both beams deliver the same amount of light and heat, but in Mexico the energy is concentrated into the small area the beam illuminates; in Greenland it illuminates a much bigger area. So the Mexicans receive their solar energy in a much more concentrated form than do the Greenlanders. Remember, of course, that there is not just one beam of radiation—the sunlight falls everywhere. That is why the Sun shines more brightly over Mexico than it does over Greenland and why Mexico is a much warmer place than Greenland. Latitude explains why the Tropics and subtropics have a hot climate. Deserts located in the Tropics and subtropics are hot, but latitude alone does not explain why the climates are dry.

Subtropical climates are dry because of the way air circulates. This is described in Hadley Cells, Equatorial Rain, and Hot Deserts on page 75. The world’s largest desert is the Sahara, covering most of Africa north of about latitude 15°N, with an area of about 3.5 million square miles (9.1 million km2), although it seems much larger because the desert continues through part of Ethiopia as the Denakil Desert and into Sudan. Farther east, across the Red Sea, lies the Arabian Desert, covering the whole of the Arabian Peninsula, an area of about 1.6 million square miles (3 million km2). North of the Arabian Desert lies the Syrian Desert, occupying much of the Middle East. The Thar, or Great Indian, Desert straddles the border between northwestern India and eastern Pakistan. It is 500 miles (805 km) long and 300 miles (485 km) wide. South of the equator the Kalahari Desert covers 275,000 square miles (712,250 km2) in southwestern Botswana, northern South Africa, and southeastern Namibia. The Australian Desert, covering much of the western side of the interior, is not a single desert but several, formed in depressions

Why low-latitude deserts are warmer than those in high latitudes. The Sun always appears higher in the sky in low latitudes than it does in high latitudes. The higher the Sun, the smaller the area of the Earth’s surface that it illuminates, but the more intense that illumination is.

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The huge sand dunes of the Namib Desert (Tom Claytor)

between large, uplifted highland areas. The Great Sandy Desert is in the north of Western Australia, with the Gibson Desert to its south and the Great Victoria Desert to the south of that, extending eastward into South Australia. To its south, along the coast bordering the Great Australian Bight, the Nullarbor Plain is also dry and empty. Its name, null arbor, means “no trees.” In the center of the country, where the Northern Territory, Queensland, and South Australia meet, is the Simpson Desert. A region of northern Colombia, on the Gaujira Peninsula, is desert, and there are arid conditions in northeastern Brazil, inland from the forested coastal strip in the states of Pernambuco, Paraíba, Rio Grande do Norte, and Ceará. All of these are subtropical deserts.

Continental Interiors There are also deserts located outside the subtropics, in the center of continents and a very long way from the sea. Air laden with moisture gathered as it drifted over the ocean must travel thousands of miles over land before it reaches the center of Eurasia, the world’s biggest continent. In the course of that journey it loses almost all its moisture and can bring very little rain to the parched continental interior. The Gobi, lying mainly in Mongolia, is the most famous desert at the center of a continent. Latitude 40°N passes approximately through its center, and in some places the desert is 1,000 miles (1,610 km) from east to west and 600 miles (970 km) from north to south. It is bounded by mountain ranges—the Greater Khingan Mountains to the east, Altun Shan and Nan Shan to the south, Tien Shan to

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the west, and the Yablonovyy Mountains to the north. To the east, inside China between the Huang He and the Great Wall of China, the Gobi continues as the Ordos Desert. The Takla Makan, or Taklimakan, Desert is also bordered by mountains. It lies farther west, in the Xinjiang Uygur Autonomous Region of China, between the Tien Shan in the north and Kunlun Mountains in the south. It covers about 115,000 square miles (297,850 km2), and its surface consists mainly of drifting sand dunes. Patagonia, in Argentina, is also a desert lying at the heart of a continent.

West Coasts and Rain Shadows Some of the driest deserts lie along the western coasts of continents, beside an ocean that should moisten the air sufficiently to give them a pleasantly rainy climate. Unfortunately, these deserts lie close to the edges of large areas of permanently high air pressure centered over the adjacent ocean, where air is sinking, and air that is sinking is always dry (see the sidebar “Why Sinking Air Is Dry” on page 76). Their climates are also influenced by an ocean current flowing close to the coast and parallel to it, carrying cold water from the Southern Ocean. These currents cause fog to form in air crossing over them (see the sidebar “Boundary Currents” on page 22). The Atacama is the principal South American desert— and the driest. It runs through northern Chile as a north– south strip from about 5°S to 30°S, covering an area of about 140,000 square miles (363,000 km2). A little way offshore, the Peru Current chills any air moving from west to east.

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The Namib Desert, one of the world’s driest, is a similar west coast desert in Africa. On its eastern side it merges with the Kalahari Desert. The Namib runs along the coast, mainly in Namibia, for a distance of 1,200 miles (1,930 km) and extends up to 100 miles (160 km) inland. The Benguela Current flows a short distance from the coast. Both the Atacama and the Namib Deserts are located in the subtropics, where the winds are usually from the southeast. Air must travel across the continent to reach them. Despite being so close to the ocean, they are dry partly because the air over them has arrived from the opposite direction. Air approaching the Atacama must also cross the Andes Mountains, and it loses almost all its moisture as it rises and cools on the west side of the mountains. Land that is dry because air must cross a mountain range to reach it and lose its moisture as it does so is said to lie in a rain shadow. The North American deserts, on the western side of the United States, also lie in a rain shadow, in this case of the ranges of the Rocky Mountains. The Mojave Desert lies to the southeast of the Sierra Nevada, California, approximately between latitudes 34°N and 37°N, with the most extreme conditions in Death Valley. Desert conditions also occur west of the Great Salt Lake, Utah, centered on latitude 40°N, and desert continues in the Southwest with the Sonoran Desert, in Baja California, Mexico, and inland.

The Arctic and Antarctic Desert nights can be cold. Winters are bitter in the Gobi, but the coldest deserts of all are found in the most unexpected places—in Greenland and, driest of all, in Antarctica. Away from the coasts, Greenland and Antarctica are as dry as any sandy desert. This sounds paradoxical. After all, central Greenland lies beneath approximately 10,000 feet (3,050 m) of ice, and the average thickness of the Antarctic ice sheet is about 6,900 feet (2,100 m). Ice is frozen water that fell as snow, so how can these places be dry deserts? It is not heavy snowfall that produced the ice sheets, however, but the fact that the climate is too cold for snow to melt. Snowfall is light, but the snow has been accumulating for thousands or, in the case of Antarctica, millions of years. That is why it lies thickly. The polar climate is as dry as that of the central Sahara.

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exceptional. It rains sometimes in most deserts, and when it does the rain is often torrential. What matters is not whether it rains, but what happens to the rain as it is falling and once it has reached the ground. Air inside a cloud is saturated with water, and water vapor is constantly condensing to form liquid droplets. Droplets merge to form bigger droplets, and when they are too big and heavy to be supported by the vertical air currents inside the cloud, they fall as rain. Snowflakes, formed when water vapor changes directly into ice crystals that link together, also fall when they are too heavy to remain aloft. As soon as they leave the cloud, raindrops and snowflakes enter drier air—air that is not saturated—and they begin to evaporate. This happens everywhere, not only over deserts. Some of the rain and snow that fall from clouds never reaches the ground. Water that does reach the ground continues to evaporate—puddles left after a summer shower soon dry in the warm sunshine that follows the shower. Climate scientists, known as climatologists, measure the rate at which water evaporates by leaving a pan of water, like the one in the drawing, out in the open and measuring the amount of water that evaporates from it in a day or week. If the amount of water that could evaporate in the course of a year, assuming a limitless supply of water, is greater than the average amount that falls as rain or snow during the year, then the ground surface will be dry most of the time. The area will then be desert. An average annual rainfall of less than 10 inches (250 mm) will produce desert conditions in most parts of the world. In liquid water, the water molecules are linked together, forming small groups. These are constantly breaking and reforming and can slide past one another. Give them

Evaporation pan. The pan consists of a rectangular container of standard size. The volume of water that evaporates over a given period is measured.

diameter 48"

stillwell

height 10"

WHAT MAKES A DESERT?

Some deserts are hot, some cold, and some frigid, but all deserts are dry. It is its aridity—dryness—that defines a desert. This does not mean that a desert is a place where it never rains. True, there are deserts, or parts of deserts, where no rain has fallen for several decades, but these are

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Geography of Deserts enough energy, however, and the molecules start breaking free. These energetic molecules escape into the air, and as more and more of them do so, the amount of liquid water diminishes. This is evaporation. At the same time some of the water molecules already in the air and moving in all directions collide with the water surface. This traps them, because they dissipate their energy in the impact of the collision and are then part of the liquid water mass. These events take place just outside the boundary between the liquid and the air—just outside a water droplet or above the surface of a pool of water. Here, energetic water molecules are both leaving and joining the liquid. If the number leaving is greater than the number joining, the liquid water will evaporate. If the number joining the liquid is equal to the number leaving, the liquid will not evaporate. The rate at which the liquid evaporates depends on the number of water molecules in this boundary layer of air. The more water molecules the air contains, the more slowly the liquid will evaporate, and it will evaporate faster the fewer airborne molecules there are.

Partial Pressure Whether water evaporates depends on the amount of water vapor already present in the air. Air exerts a pressure on the surface due to the weight of all the air in a column above an area on the surface, all the way to the top of the atmosphere. That weight is the sum of the weights of all its constituent gases, and so each of these gases contributes proportionally to the surface pressure. The amount each gas contributes is called the partial pressure for that gas. If the pressure is, say, 1,000 millibars (mb), then the partial pressure of nitrogen will be about 781 mb and that of oxygen about 209 mb, because air consists of 78.1 percent nitrogen and 20.9 percent oxygen. Water vapor is also a gas, and it, too, exerts a partial pressure. The partial pressure of water vapor is usually called the vapor pressure. It is highly variable because the amount of moisture present in the air changes frequently. If the air is moist, the vapor pressure is high, and the vapor pressure is low in dry air. The vapor pressure rises where liquid water is evaporating and entering the air as water vapor.

Why a Rise in Temperature Makes Air Drier As water continues to evaporate, the vapor pressure continues to rise because the amount of water vapor in the air is increasing. At the same time, however, the rising vapor pressure forces more water molecules to return to the liquid. Eventually the two reach a balance when molecules are returning to the liquid at the same rate as molecules are

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leaving the liquid. The dry air can then hold no more water vapor; it is saturated. If more moisture enters saturated air, a similar amount of moisture will condense into liquid, and the amount in the air will remain the same. The vapor pressure is then called the saturation vapor pressure. If the temperature rises, water will evaporate faster, and the vapor pressure will reach a higher value before saturation is reached. In other words, saturation vapor pressure depends on the temperature, and so the higher the temperature the more water vapor air can hold. The effect is quite dramatic. At freezing temperature (32°F; 0°C) 1,000 parts of dry air can hold only 3.5 parts of water vapor by volume, and the saturation vapor pressure is 6.1 millibars (mb). At 68°F (20°C) air can hold 14 parts of water vapor per 1,000, with a saturation vapor pressure of 23.4 mb, and at 104°F (40°C) it can hold 47 parts per 1,000, with a saturation vapor pressure of 73.8 mb. If the temperature were to reach 212°F (100°C), the saturation vapor pressure would be 1,013 mb. That is the same as the average sea-level air pressure, and the water would vaporize—in fact, it would boil. As the temperature rises, it takes more and more water vapor to saturate the air. Air must be saturated before water vapor will condense to form clouds, and the clouds must be large before they hold enough water for raindrops or snowflakes to form, so the warmer the air the drier it is likely to be. If saturated air is heated, its saturation vapor pressure rises, and any clouds will disappear. This is what happens when the warmth of the morning sunshine burns off a layer of overnight fog. A rise in temperature makes the air drier.

Saturation and Humidity The amount of water vapor present in the air—the “wetness” of the air—is called the humidity, and there are several ways to measure it and report its value. The mass of water vapor present in a unit volume of air (as ounces of water per cubic yard of air, for example, or grams per cubic meter) is called the absolute humidity. This measure is seldom used because it takes no account of changes in temperature and pressure that can alter the volume of the air without affecting the amount of water vapor it holds. The mass of water vapor present in a unit mass of air including the water vapor (as ounces of water vapor per pound of air, for example) is called the specific humidity. This is very similar to the mixing ratio, which is the ratio of the mass of water vapor to a unit mass of dry air (ounces of water vapor in one pound of air, for example). Relative humidity is the measure most often used, however. This is what weather forecasters mean when they talk of humidity, and the figure they quote is always a percentage. Relative humidity is calculated as the amount of water vapor in the air divided by the amount of water vapor needed to saturate the air at that temperature, multiplied by 100. Or, to

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put it another way, it is the actual vapor pressure expressed as a percentage of the saturation vapor pressure.

Evaporation and Transpiration Think of what this means for hot deserts. As a raindrop falls from its cloud, it enters unsaturated air. At once it starts to vaporize. If the cloud base is high enough, the droplet will probably have evaporated completely before it reaches the ground. This can be seen happening, even in temperate latitudes, as a gray, almost transparent sheet extending for some distance below the base of a cloud. It is called virga, and it is rain that evaporates before reaching the ground. If the rain does reach the ground, it will soak a little way into the surface, finding its way through the tiny spaces between sand or other mineral grains. These spaces are filled with air, and, like the air above, it is warm and dry. The water will evaporate into it. Plants take water from the ground and release it as vapor into the air. The process is called transpiration (see “Transpiration and Why Plants Need Water” on pages 101–103), and in practice it is almost impossible to measure it separately from evaporation, so the two are usually combined as evapotranspiration. This is what is measured. The maximum amount of water that will evaporate and be transpired if the supply is unlimited is called the potential evapotranspiration (PE). If the annual PE is greater than the annual rainfall, desert will develop. An occasional downpour will not bring an end to desert conditions, although plants and animals will be quick to take advantage of it. A place in the Thar Desert of India once received 33.5 inches (850 mm) of rain in two days, but it was a very long time before it rained again. Such water that falls soon disappears, and the ground remains dry. The desert will remain desert until such time as the annual rainfall exceeds the annual PE. An annual rainfall of less than about 10 inches (250 mm) will produce a desert anywhere in the world.

Polar Deserts A hot climate and low rainfall will produce a desert, but how does an extremely cold climate achieve this? The answer lies in the relationship between the temperature of air and the saturation vapor pressure. Air moving toward the polar regions (see “General Circulation of the Atmosphere” on pages 77–79) travels at a high altitude, where the air temperature is very low. Because it is so cold, its water vapor condenses and falls as precipitation early in the course of its journey. By the time the air reaches the polar regions, where it descends to surface level, it is very dry indeed. At a temperature of -40°F (-40°C), which is about the average temperature at 15,000 feet (4,575 m) over northern Greenland on a summer afternoon, air can

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hold no more than one part of water vapor to every 10,000 parts of air. Occasionally snow falls, just as rain falls in the tropical deserts, but this does not alter the fact that the climate is extremely arid. The air is so dry that despite the low temperature some snow evaporates directly into it without melting first (called sublimation). Near the South Pole, in latitudes 70–90°S, an average of about 2.8 inches (71 mm) of snow (measured as the equivalent amount of rain) falls each year, and about 1.2 inches (30.5 mm) of snow sublimes, leaving about 1.6 inches (40.6 mm) to accumulate. For comparison, the city of Tamanrasset, Algeria, in the middle of the Sahara, receives an average of 1.8 inches (46.7 mm) of rain a year. The relationship between air temperature and saturation also has an effect in long-term climate changes. During ice ages (see “When Northern America, Europe, and Asia Were Cold Deserts” on pages 45–49), temperatures were lower than they are today. This meant the air could hold less water vapor, and, therefore, climates were drier than they are now. Indeed, one way scientists identify past ice ages is by the dust trapped in layers of polar ice laid down long ago. Dry air is dusty, so if the world climate grows cooler, it also becomes drier, and if it grows warmer, the amount of rainfall and snowfall increase.

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CLIMATIC OPTIMA AND MINIMA

Who knows what a keen-eyed observer might find, walking along and kicking up the dust, stones, and sand to see what may be hiding beneath? In the right places in some parts of the Sahara, a scrape with the foot might turn up one or two fish bones. These are not the remains of someone’s lunch, but of fish that died when a sea dried up so all that remained was a lake, known today as Lake Chad (or Tchad). Lake Chad lies just to the south of the Sahara, in the region known as the Sahel, where four countries meet— Niger, Chad, Cameroon, and Nigeria. As the map shows, the lake is still large. It covers an area of 10,000 square miles (25,900 km2). Its appearance on the map is deceptive, however, because during the dry season the area is sometimes as little as 4,000 square miles (10,360 km2), and the lake is very shallow. Nowhere is it more than 20 feet (6 m) deep, and in the shallower northwest there are places where the depth is only three feet (1 m). At one time Lake Chad was much bigger. The fish, whose remains can still be found, lived when the lake was an inland sea, in places more than 150 feet (45 m) deep, and it is possible to identify its former shores. The sea existed about 5,000 years ago. Around its shores were elephants, giraffes, and rhinoceroses, although 5,000 years ago these animals were already becoming rare. Hannibal (d. 406 b.c.e.), the

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Lake Chad

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diles. There are even pictures of people traveling in some kind of canoe. Farther east, there were settlements in the heart of what is now the Arabian Desert. Obviously, at that time the Sahara had a much wetter climate than the climate it suffers today. In the driest part of the Sahara, where nowadays it rarely rains at all, the average annual rainfall about 8,000 years ago is believed to have been eight to 16 inches (200–400 mm). The Lake Chad area received much more than that, and rivers flowed all year round from the Tibesti Mountains. Lake Turkana (also known as Lake Rudolf), with an area of 2,741 square miles (7,100 km2) in the Rift Valley mainly in Kenya with a part across the border in Ethiopia, held much more water than it does now and discharged some of it into the Nile.

Maiduguri

Ancient Floods

lake during dry season lake after rains © Infobase Publishing

CAMEROON

Lake Chad

Carthaginian general whose armies used elephants in their assault on Rome, obtained his elephants from Algeria, where they were still living naturally but isolated from the main elephant population to the south of the desert. In the Tibesti Mountains, on the border of northern Chad and southeastern Libya, caves contain wall paintings that are between 7,000 and 8,000 years old. The people who painted them were hunters, and the pictures depict the game they pursued. There are pictures of elephants, rhinoceroses, hippopotamuses, antelope, deer, giraffes, buffalo, and croco-

Lake Chad as it was seen by the crew of Apollo 7 in 1968 (NASA)

The Sahara was not unique. Around the world at that time lands in the same latitude as the Sahara had wetter climates than they have today. In parts of Asia the rains brought by the summer monsoon were so heavy they are probably what gave rise to the flood legends that form part of the tradition of many cultures. Certainly there is archaeological evidence of flooding between about 6,000 and 4,400 years ago at Ur, Nineveh, and other ancient cities. Around 4,000 years ago, melons, dates, wheat, and barley were being grown in the region that is now the Thar Desert, in northwestern India, where the annual rainfall was 16 to 32 inches (400–800 mm). Many parts of Australia had much wetter climates than they do now, before the Australian climate became drier around 4,500 years ago. Wet weather is also warm weather. When the weather is warm, more water evaporates from the oceans, there is more cloud, and rainfall increases. During cold periods the opposite happens, and climates become drier. A prolonged period of weather that is markedly warmer and wetter than present-day weather is called a climatic optimum, and its opposite, a period of cold, dry weather, is called a climatic minimum. It was a climatic optimum that allowed hunters to travel the Sahara in search of game, sometimes traveling by boat.

Tree Rings This was not the only climatic optimum, although it may have been the warmest and longest since the end of the last ice age (see “When Northern America, Europe, and Asia Were Cold Deserts” on pages 45–49). Climate records for more recent times are obtained from the annual growth rings in trees, especially in the bristlecone pines growing in the mountains of California. There are two species: the Great Basin bristlecone pine (Pinus longaeva) and the

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Mountain bristlecone pine (Pinus aristata) is one of the longlived tree species that are used to calibrate the chronology of past events. (Maurice J. Kaufman)

mountain bristlecone pine (P. aristata). Some of these trees are more than 4,600 years old, and when growth rings in living trees are used in conjunction with those from dead trees nearby, they record growing conditions year by year since more than 8,200 years ago. Dating past events from tree rings is called dendrochronology. Woody plants, such as trees, grow taller each year, and their branches grow longer. This is called primary growth. Trunks and branches also grow thicker each year unless the weather is so bad that the plant fails to grow at all. This is called secondary growth, and it is secondary growth that allows scientists to measure the age of a tree and to tell what the weather was like in each year of the tree’s life. In secondary growth the tree lays down a layer of new cells around the cells formed in previous years, so the new growth takes place just beneath the bark. The cells produced in spring are large and have thin walls. Those produced in late summer

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are smaller, with thicker walls. Each year this produces a growth pattern that is clearly visible in a cross-section of the trunk when the tree is felled as a set of pale rings separated by thinner, dark rings. It can also be revealed without harming the living tree by drilling a core of wood from the surface to the center of the trunk. There is one pair of rings for every year of the plant’s age, but the rings reveal more than that, because plants do not always grow at the same rate. If the weather in a particular year is especially favorable, the plant puts on more growth and therefore a wider pair of annual growth rings than it does in a year when the weather is bad. In very good years a plant may even lay down two pairs of rings, and in very bad years it may put down none. This means that tree ring records must be checked against records of other kinds, but it also means they provide a faithful record of the weather. Using tree ring data to study past climates is called dendroclimatology. Using tree ring data, scientists have reconstructed the climate of the Columbia River Basin, in the northwestern United States, over the past 250 years. The rings reveal that between 1750 and 1950 there were six droughts each lasting for several years, including one that began in the 1840s

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Geography of Deserts and continued for 12 years. A severe drought in the 1930s was part of the drought that produced the dust bowl, and droughts starting around 1775, 1805, 1890, and 1925 each lasted between three and five years.

The Warm Middle Ages Tree ring records from around the world show a climatic optimum (warm episode) that peaked around 3,100 years ago, leading to a minimum (cold episode) about 2,500 years ago. This was followed by another optimum lasting much longer that is sometimes called the Roman Warm Period. Starting from the minimum 2,500 years ago, it reached its peak about 2,000 years ago, and although there were relatively warm and cool episodes within it, the European climate did not start cooling again until about 300 c.e., when Europe entered the Dark Ages. In North America the cooling began up to 150 years earlier. It was during the 2,300 years of that climatic optimum that the Roman Empire reached its greatest extent, and the Romans introduced vine growing in Britain and Germany. The Roman occupying authorities turned Britain into a major exporter of metals and, most of all, of cereals. North Africa was also farmed intensively at that time, and the outlines of fields can still be seen from the air in what is now desert. Eventually the climate cooled, but only to grow warmer again. The Middle Ages were another period of warm, wet weather known as the Medieval Warm Period. It began in Europe around 900 and a little later in North America, and it ended in the 13th century. While it lasted, Britain was an important wine-producing country, and the weather was so good that French wine growers tried to have the English vineyards closed down because of the competition. Within these long periods of relative warmth there were shorter episodes, lasting a few years or decades, when the average weather was a little warmer or cooler. The average summer temperature over the entire period was probably 1.3–1.8°F (0.7–1.0°C) warmer than the 20th-century average in England and 1.8–2.5°F (1.0– 1.4°C) warmer than that average for central Europe.

The Little Ice Age By about 1300, though, the climate was certainly deteriorating. Wine harvests started failing in northern France. Several winters in the 1430s were very severe in Europe, and from all over northern Europe there are reports of wolves approaching close to villages and farms. In parts of Scandinavia the bad weather triggered a major movement of people from the countryside into the cities, and farmers were deserting their holdings in England and Germany as well. Average temperatures were falling toward a climatic minimum known as the Little Ice Age. In Love’s Labour’s Lost (Act V, Sc. ii), probably written in 1590 or 1591, Shakespeare

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described a winter that would have been familiar to his English audiences: When icicles hang by the wall, And Dick, the shepherd, blows his nail, And Tom bears logs into the hall, And milk comes frozen home in pail

Between 1500 and 1550 the River Thames at London froze over at least three times—and that was a relatively mild period. There was worse to come. The average summer temperature between 1690 and 1699 was about 2.7°F (1.5°C) cooler than the average between 1920 and 1960. It was a time when glaciers descended farther into their valleys and sea ice extended farther south over the Arctic Ocean. During the 1580s there were some winters when ice completely blocked the Denmark Strait between Iceland and Greenland. The Little Ice Age affected the whole of the Northern Hemisphere, and there is evidence that it also affected the Southern Hemisphere. In Maine the weather was so severe in the winter of 1607–08 that many people died, both Europeans and Native Americans. There were bitter frosts in Virginia, and there was ice along the edges of Lake Superior in June 1608. It is not easy to say precisely when a climatic period begins and ends (see the sidebar “Studying Ancient Climates” on page 12). The Little Ice Age may have started around 1200 and was certainly established by the first quarter of the 15th century. It ended around 1850, which is when glaciers in the European Alps began to retreat, and recovery from the Little Ice Age probably explains the sharp rise in the average global temperature between about 1880 and 1940.

CHARACTERISTICS OF THE SAHARA AND ARABIAN DESERT

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There is an Arabic word, sahrá, that means “wilderness,” and Arabic speakers apply it to the barren emptiness of the world’s largest desert, also known as the Great Desert. That wilderness is the Sahara, and it occupies more than 3.5 million square miles (9.1 million km2) in North Africa, not far short of the area of the United States (3.7 million square miles or 9.5 million km2). The desert covers all or parts of 10 countries: Morocco, Mauritania, Mali, Algeria, Niger, Tunisia, Libya, Chad, Egypt, and Sudan. The Sahara is bounded in the west by the Atlantic Ocean, in the north by the Atlas Mountains and Mediterranean Sea, and in the east by the Red Sea. In the south it is bounded partly by the River Niger. Elsewhere the boundary is climatic, as the desert gives way to the semiarid lands known as the Sahel. Conventionally, the desert is divided into the sections shown on the map: the Atlantic, Northern, Central,

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Studying Ancient Climates Paleoclimatology is the scientific study of the climates of the distant past, and the scientists who practice it are paleoclimatologists. They try to discover what climates were like long before there were any people to leave written records, so they must seek clues, searching for them in the soil, in the fossils of once-living plants and animals, and in polar ice. Male flowering plants produce pollen that is transported, mainly by the wind or by insects, to female flowers. The flowers produce vast quantities of pollen, and some of it falls to the ground. Each tiny pollen grain is enclosed in a tough protective coat that can survive for thousands of years under the right conditions. Each type of plant produces its own pollen, and scientists can identify the pollen—and therefore the plant—by the shape of its coat and markings on it. Knowing the kinds of plants that grew in a particular place at a certain time in the past tells paleoclimatologists a great deal about the climate. For example, the discovery of the pollen of mountain avens (Dryas octopetala) in soils across northern Europe told scientists that 11,000 to 10,000 years ago the European climate was bitterly cold, with Scandinavia and western Scotland buried beneath an ice sheet. Mountain avens is a plant of cold climates, found today on mountainsides and in the Arctic. Beetles also provide clues. Most species can survive only where the temperature and moisture suit them.

Mountain avens (Dryas octopetala) is an alpine plant that grows at low elevations during periods of very cold climate. (Alastair Rae)

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Their shiny wing cases are tough and can remain unaltered in the soil long after their owners have disappeared, and they can be identified because each beetle species is different. Wing cases—the scientific name is elytra—also reveal the climate at the time the beetles were living. Some fossils are still older, and they, too, can reveal secrets. When construction workers were excavating the foundations of buildings in central London, they found the fossilized remains of elephants and hippopotamuses. Clearly, at one time central London had a climate as warm and wet as the present-day Tropics. There are two varieties, or isotopes, of oxygen, known as 16O (oxygen-16) and 18O (oxygen-18). The two are chemically identical, but 18O is slightly heavier than 16 O, and water (H2O) containing 16O (H216O) evaporates more readily than water containing 18O (H218O). Water that evaporates falls again as rain or snow, much of it over the oceans, where tiny animals use oxygen they obtain from water to make the calcium carbonate (CaCO3) of their shells. When the animals die their shells sink to the sea floor to form part of the sediment. If the CaCO3 in the sediment contains a higher proportion of 18O than is present in the air, it shows that during the lifetime of the animals 16O was being removed by evaporation and not returned, indicating that it must have been accumulating in ice sheets. This shows that the climate was growing colder and the ice sheets were expanding. Polar ice sheets also have a story to tell those who know how to read it. The sheets are very thick, and they were built from snow that fell on them year by year, so the youngest ice is at the top and ice at the bottom is very ancient. Ice 200,000 years old has been recovered from the Greenland ice sheet. The amount of dust present in the ice tells whether the world climate was cold, dry, and dusty or warm and wet so that the dust was quickly washed from the air by rain. The ice also contains the two kinds of oxygen, and it records changing temperatures very accurately. An increase in the proportion of 18O means the climate was growing warmer, and a decrease means it was growing cooler. Paleoclimatologists are like a police forensic team that examines the crime scene to reconstruct what happened, but in their case the “crime” took place many thousands of years ago. Despite the difficulty, they have become very skilful at recognizing clues and understanding what they mean.

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Geography of Deserts

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Southern, and Eastern Sahara. The Eastern Sahara, which is very large, is subdivided into the Libyan Desert in the west and the Nubian Desert to the east of the River Nile in Sudan. The region to the east of the Nile in Egypt is counted as part of the Arabian Desert.

Structure of the Sahara Much of the Sahara is a plateau 1,300–1,600 feet (395–490 m) above sea level. It comprises sedimentary rocks, such as sandstone and limestone—rock made from sediment that accumulated on the sea floor and was later compressed and heated. Some of the Saharan rocks are more than 2,500 million years old. They have been folded and then eroded, after which further sediments were deposited on top of them. In some places volcanic rocks, made from solidified lava, have intruded into the sedimentary structure. Several of the highest mountains, such as Mount Tousidé (10,712 feet; 3,265 m) in the Tibesti Mountains, are extinct volcanoes. Dry river valleys (called wadis) cut through the sedimentary rocks, but there are also deep gorges. Movements in the Earth’s crust produced these by thrusting great masses of rock upward while others sank lower. The incessant wind then wore down the exposed rocks. Despite being a plateau, the land is far from level. Large areas are below 600 feet (183 m), and some are below sea level. To the west of Cairo, for example, the elevation of the Al Qattarah Depression, a land of soft sand, salt lakes, and marshes, averages 440 feet (134 m), but parts of it are more than 400 feet (122 m) below sea level. There are salt

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lakes, called chotts, south of Biskra, Algeria, one of which, the Chott Melghir, also lies below sea level. Such low-lying areas contrast with mountains. The Atlas range lies along a southwest–northeast line. In the Central Sahara the highest peak of the Ahaggar Mountains is Mount Tahat, at 9,842 feet (3,000 m). It is sometimes capped with snow, giving it a dramatic appearance in such a dry landscape. The Ahaggar Mountains extend into a range of hills to the southwest, then rise again to form the Tibesti Mountains in Chad, extending into southern Libya. The highest peak in the Tibesti range is Emi (Mount) Koussi, at 11,204 feet (3,415 m). There is low-lying land in northeast Chad, but then the ground rises again into the mountains of western Sudan, where the highest peak rises to 10,132 feet (3,088 m), and continues again as the mountains of Ethiopia. Together, these mountains form a ridge crossing the Sahara from west to east.

Seas of Sand There are three types of land surface in the Sahara: erg, reg, and hammada. Of these it is the ergs that conjure the image of the “typical” desert. Ergs (the singular is areg, the Arabic word for ocean) are “sand seas,” vast, rolling expanses of dunes like gigantic ocean waves. In the Great Eastern Erg the dunes are 6.5 to 16 feet (2–5 m) high, but some of those in the Great Western Erg are 1,312 to 1,970 feet (400–600 m) high, far bigger than the waves even the worst sea storm can generate. The dunes are made from sand that has blown into basins and depressions and been trapped there and sand carried there by rivers of former times. In some places the dunes are still shifting; in others they have stabilized. There are dunes in many parts of the desert, but the ergs, the sand seas, are in the Northern Sahara, extending

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A sea of sand, or erg, in the Sahara

across the Libyan Desert and into Egypt. The Great Western and Great Eastern Ergs are in Algeria, between Béni-Abbès in the west and Ghudàmis (or Ghadames) just across the border in Libya. These two towns are both at latitude 30°N. The other principal ergs are Erg Iguidi, to the southwest of the Great Western Erg, and Erg Chech, to the east of Erg Iguidi. A chech is the length of colored cloth men wear as a turban that covers the nose and mouth to keep out the sand.

Shifting Dunes Wind makes sand dunes shift. Day after day and year after year, the incessant wind blows sand grains up one side of a dune to the crest, from where they fall forward. Gradually the entire dune advances downwind. This is a gentle process, but the desert wind can also be violent. A wind of 12 MPH (19 km/h) is enough to lift average-sized sand grains, and the winds frequently exceed this speed. The winds are produced either by strong surface heating during the day or by low-pressure systems. During the hottest part of the day the desert surface can reach temperatures as high as 180°F (85°C). Air, heated by contact with the surface, expands and rises. This produces an area of very low pressure near ground level. Denser air rushes in to compensate, and its movement causes strong, gusty winds. These are called thermal winds because they are caused by temperature differences (see the sidebar “Thermal Wind” on page 83). Low-pressure weather systems are much bigger. They affect much larger areas and can generate stronger winds than those due to daytime heating. A wind that carries sand can cause a sandstorm, and sandstorms are common. Their winds need not be blowing with the force of a gale. A 35-MPH (56-km/h) wind is sufficient. That is about the speed of the wind that closed the Suez Canal and Cairo Airport in March 1998. It was a wind called the khamsin. The airborne sand reduced visibility to about 600 feet (180 m). It affected Lebanon and Jordan as well as Egypt. The khamsin is a hot, dry wind that blows from the southeast very regularly at 50-day intervals in late winter and early spring—khamsin means 50. As well as dust

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and sand, the khamsin brings air temperatures between 100°F and 120°F (38 and 49°C). It is caused by the passage of low-pressure systems. A similar wind, though not always so hot, occurs widely in North Africa and in countries bordering the Mediterranean whenever a depression (see “Frontal Depressions” on pages 83–85) moves eastward through the Mediterranean. It is called the sirocco (or scirocco), and winds of this type are known as ghibli in Libya and leveche in Spain. Although the sirocco brings dry weather to North Africa, after it has crossed the Mediterranean it brings wet weather to southern Europe.

Reg and Serir Reg is the name given to ground covered by boulders and gravel. The reg is fairly level, with gradients as low as 1:5,000 in some places. These areas are bleak, windswept, and monotonous. This type of surface is formed by wind action. Small particles are blown from around and between the larger, heavier stones. The process is called deflation, and it keeps the reg swept clean. Indeed, it keeps the surface very clean, because the finest particles are often carried very long distances. Saharan dust has been identified in the United States. Where the stones are a little larger and mixed with some sand, the surface is called serir. There are areas of serir in Libya and Egypt. The Serir Tibasti is a large expanse of serir in southern Libya, extending across the border into Chad. It also occurs on fairly level ground. Serir formed originally from material deposited by rivers flowing along entwined, braided channels or by water flowing in sheets across the surface. Subsequently, the climate changed, and dry winds gradually altered the surface into the condition found today.

Hammada Hammada (or hamada) is the rocky desert. It has no small particles at the surface, neither of sand nor anything else.

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Geography of Deserts There is just rock, either as large boulders or exposed bedrock. Stony hammada is the jagged, broken surface of crystalline rocks—igneous rocks formed by volcanic activity. Pebbly hammada forms over sedimentary rocks. As the name suggests, it consists of pebbles mixed with broken fragments of bedrock. This type of desert surface is widespread. There is a very large area of it in northern Libya, called the Hammada al Hamra. Where it comprises nothing more than exposed bedrock, hammada resembles a vast and rather uneven parking lot. It is probably the most inhospitable type of desert surface. Very little can live on it.

Saharan Climate The Sahara is a hot desert. At Tamanrasset, in southern Algeria, the mean temperature ranges from 54°F (12°C) in January to 84°F (29°C) in July, but averages can be misleading, because they include nighttime as well as daytime temperatures. Summer daytime temperatures at Tamanrasset exceed 90°F (32°C) for eight months of the year, and in summer they often exceed 100°F (38°C). The hottest place of all is Al-‘Aziziyah (also spelled Azizia), in Tripolitania, northern Libya, at about 32.5°N and 367 feet (112 m) above sea level, where a temperature of 136.4°F (58°C) was recorded on September 13, 1922. This is the highest temperature ever measured at the Earth’s surface, breaking the previous record of 134°F (56.7°C) at Greenland Ranch, in Death Valley, California, on July 10, 1913. Other

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places may have been hotter, but if so at times when no one was around or no one had a thermometer. Such a high temperature was very unusual, however, even for Al-‘Aziziyah, where the temperature through the year ranges from an average minimum of 56.5°F (13.6°C) to an average maximum of 83.1°F (28.4°C). July is the hottest month, but even then the average temperature reaches no more than 99.7°F (37.6°C). It is hot, but not really any hotter than other parts of the Sahara, and the official 1922 record has still not been broken. At night, however, the temperature at Tamanrasset drops sharply, by as much as 50°F (28°C). Nights can be cold, and temperatures often fall below freezing in winter. Rainfall is sparse, of course. The annual total at Tamanrasset amounts to less than two inches (51 mm).

The Arabian Desert In the east the Sahara extends over virtually all the Arabian Peninsula. Geologically, Arabia lies above ancient, hard rocks (called the shield) in the west and sedimentary rocks in the east. These slope down into the basin in which much of the Middle Eastern desert lies. A range of mountains dominates the western side of Arabia. The mountains run parallel to the eastern coast of the Red Sea, the highest peaks rising to more than 9,000 feet (2,745 m). In some places two mountain ranges run parallel to each other, separated by a plateau. The northern section of the coastal belt, from Aqaba in the north to a point about 200 miles (320 km) south of Mecca, is called Hejaz, which

The barren dunes of the Arabian Desert (Tom Claytor)

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means “the barrier.” The southern section is called Asir, meaning “difficult.” To the east of the mountains the central part of the Arabian shield is known as Najd (“highland”). Wadis—river channels that are usually dry—running across the Najd toward the east carry water during the rainy season, from January to May. The biggest sand desert in the world lies to the south of the Najd. It is called Ar Rub’ al Khalli, the “empty quarter”—although the Bedouin who live there know it as Ar Ramlah, “the sand”—and it covers about 230,000 square miles (595,700 km2). Despite its name, there are watering places, so crossing it is not too dangerous for those familiar with the desert. The second-biggest desert is more difficult to cross and has fewer watering places. Covering about 26,000 square miles (67,300 km2), it is called An Nafud and lies to the north of the Najd. These two deserts are linked by Ad Dahna, a stream of sand 800 miles (1,290 km) long that is flowing slowly southward, like a river. Several roads and a railroad cross it, and during the rainy season it provides some grazing for livestock. In many places its sands are a reddish color, and they form parallel ridges up to about 150 feet (45 m) high.

Arabian Climate Arabia has a desert climate, but in places the relative humidity (see “Saturation and Humidity” on pages 7–8) can be high, making conditions clammy, especially at night. Rainfall is low, however, averaging no more than about four inches (100 mm) a year. In Riyadh the average is 3.2 inches (81.3 mm). The climate is hot. December and January are the coolest months, with an average temperature of 70°F (21°C) during the day and 46°F (8°C) at night, although the averages conceal a wide range of extremes. A daytime temperature of 87°F (31°C) has been recorded in December and a nighttime minimum of 19°F (-7°C) in January. From May to September the average daytime temperature is between 100°F (38°C) and 107°F (42°C), but 113°F (45°C) has been recorded. At night the temperature falls to between 72°F (22°C) and 78°F (26°C).

CHARACTERISTICS OF THE GOBI, TAKLA MAKAN, AND THAR DESERTS

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The Sahara and Arabian Desert lie in the Tropics and subtropics. They are hot deserts produced by descending air on the poleward side of Hadley cells (see “General Circulation of the Atmosphere” on page 77–79). This air movement

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produces a belt of fairly permanent high pressure. Farther north the deserts of Central Asia are also caused by persistent high pressure, but they are well clear of the Tropics and much cooler.

The Gobi There are no towns in the Gobi Desert, but Ulaanbaatar (formerly known as Ulan Bator), the capital of Mongolia, lies not far from its northern edge. In July, the hottest month of the year, the average daytime temperature in Ulaanbaatar, calculated from records gathered over 12 years, is 71°F (22°C), falling to 51°F (11°C) at night. The highest July temperature recorded over this period was 92°F (33°C), but there was also a nighttime temperature of 34°F (1°C). January is the coldest month, when temperatures average -2°F (-19°C) during the day and -26°F (-32°C) at night, with a record high of 21°F (-6°C) and low of -47°F (-44°C). The city is 4,347 feet (1,325 m) above sea level, but much of the Gobi is also at about that elevation. Central Asian deserts are cold. The wind blows most of the time. Where there is sand to be blown, it is blown, but blown sand does not move outside the desert area. Precipitation falls mainly in the summer. Ulaanbaatar has an average annual precipitation of about eight inches (203 mm). This is typical for the northern and southeastern margins. In the center the desert receives one to two inches (25–50 mm) a year. Only the southeastern part of the desert is completely without water. There is some confusion about the definition of Gobi. The word suggests a series of shallow basin structures that have been scoured by the wind. Mongolians often apply the word gobi to the bottoms of such basins. These are usually level and sometimes marshy or covered with grass. Adding to the confusion, the Chinese name for the desert is Sha-mo, which means “sand desert,” although sand dunes are confined to quite small areas. Overall, the Gobi is not a sandy desert. With these qualifications, the location of the Gobi is shown in the map. The desert extends for about 1,000 miles (1,600 km) from west to east and about 600 miles (970 km) from north to south. It lies mainly inside Mongolia but extends into China. Mountains border the Gobi on all four sides. The Da Hinggan Ling (Greater Kinghan Mountains) lie on the eastern side, the Altun Shan and Qilian Shan (Nan Shan) to the south, the Tien Shan range to the west, and the Altai, Hangyan Nuruu, and—across the border in Russia—the Yablonovyy Mountains to the north. The desert occupies a plateau at a height of 3,000 feet (914 m) in the east rising to 5,000 feet (1,524 m) in the west. The desert surface consists mainly of bare rock or gravel forming rolling plains with isolated hills and low ranges of

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Geography of Deserts

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hills with tops that have been flattened by erosion. About three-quarters of the area supports sparse vegetation consisting of grass, shrubs, and thorn bushes. This provides pasture for the herds of livestock belonging to the nomadic peoples who inhabit the desert.

The Takla Makan To the west of the Gobi, in the Sinkiang Uighur (Xinjiang Uygur) Autonomous Region of China, there is another desert, the Takla Makan (or Taklimakan). It occupies the center of the Tarim Basin, a low-lying area adjoining the River Tarim, on the northern edge of the desert. Extending for about 600 miles (970 km) from east to west and 250 miles (400 km) from north to south, the Takla Makan is a much harsher place than the Gobi. In winter the average temperature falls to about 15°F (-9°C), and in summer it rises to about 86°F (30°C) but can reach

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100°F (38°C). The climate is extremely dry because air must cross high mountains to reach the desert, and in doing so it loses any moisture it carries. The western side of the desert receives an average 1.5 inches (38 mm) of rain each year, but the drier eastern side receives only 0.4 inch (10 mm). This is a sandy desert, especially in the south and southwest, where there are great expanses of shifting dunes interspersed with small, eroded hills and bare ground swept clear by the incessant wind. Sandstorms are frequent and often last for days on end. There is some vegetation near the eastern and western edges, where there are permanent rivers, but nothing lives in the interior.

The Great Indian, or Thar, Desert Farther south, in the Indian subcontinent, the Thar, or Great Indian, Desert covers about half the Indian state of Rajasthan and part of eastern Pakistan, with a total area of 77,000 square miles (200,000 km2). New Delhi is about 250 miles (400 km) to the northeast of its center. As the map shows, the desert is bordered to the south by the Rann

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Thar (Great Indian) Desert

of Kutch—an area of sparse grassland, marshes, and salt flats in the delta of the Indus River—and to the west by the plain across which the Indus flows. In the southeast the desert ends at the Aravalli Range of hills, rising to 820 feet (250 m) in the northeast and 1,620 feet (495 m) in the southwest. In the northeast the desert gives way to less arid conditions in Punjab State. The distance from its southwestern to northeastern margins is about 500 miles (805 km) and from the southeast to northwest is about 300 miles (485 km). This is a tropical desert—the tropic of Cancer passes through the Rann. In July the afternoon temperature can reach 127°F (52.8°C), but in January frosts can occur at night, although the mean January nighttime temperature is 55°F (12.7°C). The daytime temperature in January averages 70°F (21°C). Dust storms are common in April and May and again in October. There is little rain. The amount ranges from about 20 inches (500 mm) a year in the east to about four inches (100 mm) in the west, near the Indus, but the rainfall is very erratic. Rain falls mainly during the summer monsoon season, from June to September. Air pressure over the Thar Desert is usually low due to the monsoon regime that governs the climate of the subcontinent. During the winter monsoon the daytime temperature is high enough to heat the ground, causing air

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to rise and producing low pressure near the surface. The air flowing in to fill the low is drawn not from the ocean, but from the region of high pressure to the northwest that extends over much of central Asia. Its air is very dry, and as it flows outward it produces offshore winds, carrying air away from the land and preventing moist air from entering across the Indian Ocean. During the summer monsoon the intertropical convergence zone (see “Intertropical Convergence Zone, Monsoons, and Jet Streams” on pages 79–82) lies to the north of the desert at ground level and to the south of the desert at a height of about 10,000 feet (3,000 m). This allows some moist air to enter from the sea, but air continues to flow from the continental landmass at high altitude and at ground level in the east. It is the monsoons, therefore, that produces a dry climate in a low-pressure region. At its boundary with the Aravalli Range, the Thar Desert is at an elevation of about 1,500 feet (457 m). From there it slopes to about 200 feet (61 m) near the Rann and the Indus Plain. It is a sandy desert. The name thar means “sandy waste.” There are rock outcrops, but the surface consists mainly of rolling sand hills. In places the sand is mixed with silt and fine soil. Crops can be grown where irrigation is provided, although salt-laden dust is also carried into the area from the delta and the Rann.

CHARACTERISTICS OF THE KALAHARI AND NAMIB DESERTS

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South of the equator there are tropical deserts matching those of the north. They are smaller, but this is only because there is less land in the Southern Hemisphere than in the Northern. The two deserts of southern Africa are the Kalahari and the Namib. The Kalahari Desert covers an area of about 275,000 square miles (712,250 km2) in southwestern Botswana, northern South Africa, and southeastern Namibia, mostly between latitudes 20°S and 28°S. The map shows its general location. Its southern boundary is the Orange River in Cape Province. In the north it ends at the valley of the Okovango River in Botswana. It merges into the Namib Desert in the southwest, but down the remainder of its western side two ranges of hills, the Nama and Namara, separate the two deserts. On its eastern side the Kalahari gradually gives way to cultivated land.

Ngamiland and the Makarikari Pan The northern edge of the Kalahari, near the Okovango River, is low-lying. After heavy rain the river overflows into this basin, producing extensive mud flats and swampy areas.

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Geography of Deserts

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Although the flow of water is irregular, the river terminates in an inland delta with an area of about 4,000 square miles (10,360 km2) in a region called Ngamiland. A little way to the south of the river, a lake called Lake Ngami was discovered by David Livingstone in 1849 (see the sidebar on page 20). It is 40 miles (64 km) long and six to 10 miles (10–16 km) wide but very shallow, and it often dries up completely. Water from several of the delta channels flows into it, but in 1887 the Taokhe channel, which used to be the main source of its supply, became blocked by papyrus beds. Water reaches the head of the delta in March and arrives at the northeastern end of the lake by July or August, but the amount varies greatly from year to year. There is no natural outlet from the lake, and two valleys would be inundated if it were to fill. To the west

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of Lake Ngami are salt flats in the Makarikari Pan, formed by the inflow and evaporation of water from the Okovango. The river water carries dissolved mineral salts that remain as evaporite deposits when the water evaporates. The Makarikari Pan is in the lowest part of the basin. Elsewhere the elevation is fairly uniform. The Kalahari occupies part of the wide southern African tableland that covers much of the interior of Africa south of the equator. The average elevation is about 3,000 feet (900 m) above sea level in the west, rising to about 4,000 feet (1,220 m) in the east and sloping into the low-lying Ngamiland region in the north. The desert lies on rocks of Archaean age, formed more than 2,500 million years ago and covered by more recent material. In most places the desert surface consists of red, sandy soil. There are sand dunes in the drier parts, especially in the east.

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David Livingstone (1813–1873) Possibly the greatest explorer of tropical Africa of the 19th century, David Livingstone was born on March 19, 1813, in the small mill town of Blantyre, Scotland. Like other village children, at the age of 10 he began work in a cotton mill, starting at 6 A.M. and finishing about 8 P.M. After work all the children were required to attend night school. David was a keen student, and by the age of 22 he enrolled for courses at Anderson College, in Glasgow, where he studied Greek, theology, and medicine. Livingstone read an appeal for missionaries in China, and by the time he graduated from college the London Missionary Society had already accepted him as a candidate for missionary work, but in Africa, not China. He qualified in medicine in 1840 and was ordained a missionary in the same year. The society sent him to Kurumun in southern Africa, where he arrived in 1841. Kurumun was a 10-week journey from the coast by ox wagon, horse, and on foot, and Livingstone found he had a talent for coping with the rigors of traveling through remote country. He was also quick to understand African people and developed a deep affection for them. After only a few weeks at Kurumun he set off again in search of a new place to establish a mission, penetrating about 200 miles (320 km) into the Kalahari—farther than any European

Climate of the Kalahari Despite being a desert, most of the Kalahari supports at least some vegetation, and except in the most arid parts it is more accurate to describe it as poor scrub rather than desert. In the north the annual rainfall averages 25 inches (635 mm) and in the south 10 inches (254 mm). The climate is barely dry enough to produce a desert, but the rate of evaporation is high enough to remove much of the surface water before it can be absorbed by the ground. In the east the rainfall is only about five inches (127 mm) a year. Average daytime temperatures range from about 90°F (32°C) in summer to 70°F (21°C) in winter, but frosts are fairly common on winter nights. Summer temperatures can exceed 104°F (40°C). Francistown, Botswana, at an elevation of 3,294 feet (1,004 m) to the east of the Makarikari Pan near the Zimbabwean border, has a typical Kalahari climate. The average daytime temperature varies little throughout the year. June is the coolest month, with a temperature

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had ventured previously. He spent the next seven years setting up missions, learning local languages, and deepening his understanding of Africa. While working at one of his missions he was attacked by a lion and had to return to Kurumun to recuperate. During his stay at Kurumun he married Mary Moffat, the daughter of the missionary there. They had six children, one of whom died in infancy. In 1849 he crossed the Kalahari in the company of William Oswell, a wealthy hunter, reaching Lake Ngami and the swamps of the Okavango River. He returned to the same area the following year, accompanied by his wife and children, and this time traveled farther north, into Makololo territory, where he met and became very friendly with the Makololo chief, Sebituane. It was there that Livingstone saw for the first time the practical effects of the slave trade. He determined that the best way to fight slavery was to open up trade routes into the interior of Africa, thereby stimulating economic development. He sent his family back to England so he might continue alone in his efforts to undermine slavery. In 1853, traveling with 27 men led by Sebituane’s nephew Sekeletu, he found a route to the Atlantic coast, arriving at Luanda, Angola, in May 1854. The journey took six months, and in the course of it Livingstone suf-

average of 74°F (23°C), and October is the warmest, when the temperature averages 90°F (32°C). Nighttime temperatures average 41°F (5°C) in midwinter and 65°F (18°C) in midsummer. Francistown receives an average of 18 inches (457 mm) of rain a year, but most of this falls in summer, between November and March, and little or no rain falls from May to the end of September.

The Namib To the west of the Kalahari, along the coastal strip between the hills that line the edge of the tableland, the Namib Desert is much drier. It receives an average of about two inches (51 mm) of rain a year. Walvis Bay, on the Atlantic coast of Namibia and 24 feet (7 m) above sea level, has a total of 0.8 inches (20 mm) of rain a year, and there are seven months during which no rain at all can be expected. The climate is not especially hot, however. At Walvis Bay the average daytime temperature is between 66°F (19°C) and

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fered from 27 bouts of malaria. When he arrived people in Luanda urged him to sail for Britain, but Livingstone knew that the Africans with him could not return home without his help, so he led them back by an even longer route. He then set himself the task of finding a route to the eastern coast, departing with 100 men in November 1855 to follow the Zambezi River. After 50 miles (80 km) he saw and named Victoria Falls. This time when he reached the coast he did depart for England, where he was greeted as a hero. He returned to Africa in 1858 to explore the eastern and central parts of the continent, but this was an unhappy time. Livingstone proved unable to lead a large expedition. Members of the party quarreled, the aim of navigating the Zambezi proved unattainable, and in 1862 Livingstone’s wife Mary died from fever. He was recalled to England in 1864. In April 1866 Livingstone sailed for Africa for a third time with the aim of discovering the source of the Nile. He landed on the eastern coast and passed around Lake Nyasa, but most of his party left him. He fell sick and had no choice but to seek help from Arab slave traders. They treated him kindly rather than let him return to tell of their misdeeds, and he spent four years with them. He reached Ujiji on the shore of Lake Tanganyika,

from where he explored the upper reaches of the Congo River, still searching for the source of the Nile, but broke from the Arabs when he saw further examples of their cruelty to the Africans. Nothing had been heard from Livingstone for a long time, and there were rumors that he had died. In 1869 John Gordon Bennett Jr., the editor of the New York Herald, sent Henry Morton Stanley (1841–1904) to find Livingstone. Stanley reached Ujiji in November 1871 and found Livingstone, greeting him with the words “Dr. Livingstone, I presume.” Livingstone and Stanley continued exploring the region around Lake Tanganyika, determining that it could not be part of the drainage system that fed the Nile. Stanley then left but was unable to persuade Livingstone to return to the coast with him. Livingstone continued alone down the eastern side of Lake Tanganyika but became lost in swampy land during the rainy season. He managed to cross the swamps, but attacks of dysentery, which had troubled him for a long time, were now almost continuous. He reached a small village called Chitambo, where he died some time in May 1873. The men accompanying him embalmed his body and carried it with all his papers to Zanzibar, from where it was sent to England. Livingstone was buried in Westminster Abbey.

75°F (24°C) throughout the year. At night the temperature falls to 46–60°F (8–16°C).

Land Breezes, Sea Breezes, and Fog

The Benguela Current Walvis Bay lies just miles north of the tropic of Capricorn, where it might be expected to have a much warmer and more markedly seasonal climate. Its cool climate is due to the Benguela Current. Benguela is a town on the coast of Angola, and the Benguela Current is an eastern boundary current (see the sidebar on page 22). The Benguela Current carries cold water northward, parallel to the coast, and it moves slowly, at about 5.6 MPH (9 km/h). The current originates near the edge of the Southern Ocean as a stream of water that is blown in an easterly direction by the West Wind Drift (also called the Antarctic Circumpolar Wind) and that turns to the left just south of the African continent. For part of its journey the current flows beneath the warmer, less-dense water, but there are many upwellings, where cold water reaches the surface.

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In this part of Africa the prevailing winds are the very dependable southeasterly trade winds (see “Hadley Cells” on pages 73–77), but land and sea breezes are more evident along the coast (see the sidebar). At about 10 a.m. on most mornings the sea breeze begins as the land warms, air over the land rises, and cool air blows in from the west to fill the low pressure. Despite having crossed the ocean, however, this air brings no rain because it was chilled as it crossed the cold water of the Benguela Current and its water vapor condensed. Cool air does produce cool conditions over land, however. During the course of the day the wind direction changes slowly from west to south-southwest. The sea breeze is often strong, especially to the south of the Orange River. There its speed exceeds 30 MPH (48 km/h) for most of the afternoon, driving sand and dust inland. Soon after sunset the wind drops. The land cools very quickly, and the calm

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Boundary Currents The winds on either side of the equator are very dependable. In the days of sailing ships, when winds were more important than they are now, sailors gave them the name that is still used today. They called them the trade winds— trade is an old word for “track,” referring to the fact that the winds always blow in the same direction. The trade winds blow from the northeast in the Northern Hemisphere and from the southeast in the Southern Hemisphere. Winds that blow across the ocean push water before them, driving currents, which are streams of flowing water, like rivers that flow through the sea rather than across the land. On either side of the equator, therefore, there is an Equatorial Current flowing from east to west, pushed along by the wind. Each current is deflected as it approaches a continent, turning away from the equator. Once the moving water is more than about 5° from the equator, it begins to experience the Coriolis effect, pushing it to the right in the Northern Hemisphere and to the left in the Southern Hemisphere. The current is then flowing away from the equator, parallel to the coast of a continent. When it is more than about 30° from the equator, it enters a region where the winds in both hemispheres blow mainly from the west, so the water is pushed from west to east. This pressure strengthens the Coriolis effect, and the current crosses the ocean once more, this time in middle latitudes. Again, as it approaches a continent

the current is deflected, and the Coriolis effect causes it to turn to the right in the Northern Hemisphere and to the left in the Southern Hemisphere. The current is then flowing toward the equator, parallel to the coast of a continent. As it reenters the Tropics, it encounters the trade winds pushing it in a westerly direction, and it rejoins the Equatorial Current. As the map shows, the currents in all the world’s oceans move in a circle. These circular patterns are called gyres. Each stage in a gyre has its own name, even though all the currents belong to the same gyre. In the North Atlantic, for example, the gyre comprises the North Equatorial Current, Antilles Current, Florida Current, Gulf Stream, and Canary (or Canaries) Current. Currents flowing parallel to continental coasts are called boundary currents. Western boundary currents, flowing on the western sides of oceans and therefore along the eastern coasts of continents, carry water away from the equator. These currents are deep, narrow, fastflowing, and warm. Eastern boundary currents flowing toward the equator, parallel to the western sides of continents, are wide, shallow, slow-flowing, and cool.

(opposite page) Ocean gyres and boundary currents. The map shows the general circulation of ocean water and names the principal ocean currents.

Land and Sea Breezes Land absorbs heat much faster than does water, but it also loses it much faster. In coastal regions the land and sea are often at different temperatures. Sometimes the land is the warmer, and sometimes it is the sea. The difference in temperature generates breezes that blow from land to sea or from sea to land. During the day in summer and in those parts of the world with a warm climate, the strong sunshine rapidly warms the land, and the warm ground heats the air in contact with it. The warm air rises by convection, producing an area of low air pressure near ground level. Air over the water is much cooler and denser, so the air pressure is higher over the sea than it is over land, caus-

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ing air to flow from the sea to the land. This is the cool sea breeze. It usually begins in the early afternoon as the ground temperature approaches its maximum. Toward the end of the afternoon, as the Sun sinks lower in the sky, the ground temperature ceases to rise and as dusk approaches begins to fall. The land cools quickly, lowering the temperature of the air immediately above it and causing the cool, dense air to subside. Air pressure over land increases. The sea loses its warmth more slowly than the land, and soon after nightfall the air pressure is higher over the cool land than it is over the warm sea. Air flows from the land to the sea. This is the warm land breeze.

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N. At lan tic C.

Geography of Deserts

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Land and sea breezes. During the day warm air rises over the land, and a sea breeze of cooler air flows from the sea to replace it. At night the sea is warmer, so the air flows in the opposite direction, with a land breeze blowing over the sea.

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desert extent of desert

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If the Namib Desert is dry, its South American counterpart may be even drier. There are parts of the Atacama Desert (Desierto de Atacama) where the average annual precipitation amounts to about 0.4 inch (10 mm), and it arrives as fog, not rain. Rain can be expected no more than two to four times a century. Over a period of 21 years the town of Iquique (20.3°S) received an average 0.06 inch (1.5 mm) of rain a year. This was not spread evenly over the 21 years. There was one fiveyear period during which no rain at all fell for the first four years, and in July of the fifth year—July is in the “rainy” season!—a single shower delivered 0.6 inch (15 mm). It has been known for a shower to deliver 2.5 inches (64 mm) of rain, however. Although rain is so rare, Iquique is on the coast, and the relative humidity is usually about 75 percent. The air is so moist that bare iron rusts rapidly. Arica, in the north, received an average of less than 0.03 inch (0.75 mm) of rain

BOLIVIA

Arica

ARGENTINA La Serena Coquimbo Region

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CHARACTERISTICS OF THE ATACAMA DESERT

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A Long, Narrow Desert The northern margin of the Namib Desert is located some distance to the south of Luanda, Angola. Luanda, at 8.82°S, receives an average of 13 inches (330 mm) of rain a year. The Angolan seaport of Namibe, at 15.2°S, receives two inches (50 mm). Farther south the rainfall is even lower. The desert extends southward for about 1,200 miles (1,930 km), across the Orange River and into Cape Province, South Africa. Nowhere is the Namib Desert more than about 100 miles (160 km) wide. Over the northern part of the Namib the surface is covered mainly with gravel. The ground is level, but there are isolated, steepsided hills called inselbergs. In the south there is more sand, with large dunes shaped into parallel lines by the sea breezes and berg winds.

PERU

ATAC AMA

air soon changes into a gentle land breeze blowing from the north. This continues throughout the night. Easterly winds, called berg winds, are also fairly common, bringing warm, very dry air. They occur mainly on winter mornings. In the afternoon the sea breeze overpowers them. Warm, moist air from the tropical Atlantic is cooled as it crosses the Benguela Current. Its water vapor condenses over the sea, but as low cloud and fog rather than rain. Fog— known in Angola as cacimbo—is the most characteristic feature of the Namibian climate. At night it is cool enough for the cloud and fog to drift over the coast, and there is often a fine drizzle. The sky soon clears in the morning. In Walvis Bay there is fog an average of 55 days a year.

Atacama Desert

a year over a 19-year period. Antofagasta and Copiapó each receive 0.6 inch (14 mm) a year. In the case of Antofagasta that is the average rainfall recorded over a 22-year period. Rainfall increases toward the southern end of the desert. La Serena, at 30°S, receives 5.6 inches (142 mm), and Valparaíso, at 33°S, receives 19.7 inches (500 mm). Local people say that an earthquake is much more likely than a week of rain (not so extraordinary as it may sound, because the western coast of South America is an earthquake zone). Perhaps the driest desert in the world, the Atacama covers about 140,000 square miles (363,000 km2) in northern Chile, mainly in the Antofagasta and Atacama Regions. The tropic of Capricorn passes through its center, so it lies farther north than the Namib. It is a narrow desert, running parallel to the coast for about 600 miles (965 km). The map shows the location of the desert.

Moderate Temperatures and the Peru Current Despite its tropical location, the desert climate is not hot. The average temperature is about 65°F (18.3°C), and, the latitude being tropical, it varies little throughout the year. January and February are the warmest months at Antofagasta, with daytime temperatures averaging 76°F (24°C). In August, the coolest month, the average daytime temperature is 62°F (16.6°C). Nor is there much difference between daytime and nighttime temperatures, which range from about 51°F (11°C) to 63°F (17°C).

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Geography of Deserts A cool, very dry climate in a coastal region suggests the presence of a cool ocean current, and South America has its equivalent of the Benguela Current (see the sidebar “Boundary Currents” on page 22). It is called the Peru Current, or sometimes the Humboldt Current, after the German traveler and scientist Alexander von Humboldt (see the sidebar). Humboldt measured the sea temperature in the current in the course of his exploration of South America between 1800 and 1804.

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The Peru Current flows northward close to the coast, turning to flow westward as it meets the “bulge” of South America. The current originates in the Southern Ocean, as does the Benguela Current, and although it flows beneath warmer and therefore less dense surface water, there are frequent upwellings that bring cool water to the surface. These occur because the prevailing tropical winds, the southeast trade winds, drive the Equatorial Current, which carries warm water away from the South American coast and toward

Alexander von Humboldt (1769–1859) Alexander von Humboldt was a Prussian geologist and geographer. His full name and title were Friedrich Heinrich Alexander, Freiherr (Baron) von Humboldt. Humboldt was born in Berlin, then the capital of Prussia, on September 4, 1769, the son of an army officer who served as an official at the court of the king, Frederick II (Frederick the Great). Following the death of his father in 1779 Humboldt was educated privately before enrolling at the University of Göttingen in 1789 to study science. While there he met and became friendly with Georg Forster, who had accompanied James Cook (1728–79) on the second of his voyages of exploration. Humboldt spent only one year at Göttingen before he and Forster set off on a journey through the Netherlands and England, where they met many leading scientists. On his return to Prussia Humboldt realized he would need a formal qualification if he were to make any useful contribution to science. In 1791 he became a student at the Freiburg Bergakademie (School of Mining). He spent two years there before graduating in geology. While studying mining he became fascinated by the plants that grow in and around mines. Humboldt was appointed assessor and later director of mines in the Prussian principality of Bayreuth. He founded a school of mining, improved conditions for the miners, and also conducted his own research into the magnetic declination of the rocks in the area. He spent the years from 1792 to 1797 on a diplomatic mission that took him to the salt mining regions of several central European countries. In the course of these travels he met more of the most senior scientists of the day. Humboldt’s mother died in 1796, and Alexander inherited a share of the family fortune. This meant he no longer needed to earn a living and could indulge his passion for travel. He went first to Paris and from there to Marseilles, accompanied by the French botanist Aimé Bonpland (1773–1858). They planned to travel to Egypt,

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where they hoped to join Napoleon, but instead they went to Madrid, where the prime minister, Mariano de Urquijo, became their patron. With his support they changed their plans, heading instead for the Spanish colonies in South America. They sailed from Spain in 1799, landing in New Andalusia (modern Venezuela), and early in 1800 they began a four-month expedition through tropical Latin America. Having completed that journey, they sailed to Cuba, stayed there for several months, then returned to South America in March 1801, arriving at Cartagena, Colombia, to undertake a second expedition across the Andes. As they climbed Humboldt noted the changes in vegetation at different elevations and recorded the decrease in air temperature with height. He also made many geophysical observations of the alignment of volcanoes and Earth’s magnetic field. When they reached the Pacific coast Humboldt measured the temperature of the water offshore and discovered the existence of the cold current that is sometimes named after him. Humboldt and Bonpland left South America in February 1803, spent a year in Mexico, visited the United States, and sailed for Europe on June 30, 1804. In the course of their explorations the two men had traveled about 6,000 miles (9,600 km). Humboldt spent the following years in Berlin and Paris arranging the vast amount of material he had collected during his travels. His major work, Voyage de Humboldt et Bonpland, appeared in 30 volumes between 1805 and 1834. Most of his fortune had now been spent, and in order to secure an income Humboldt agreed to serve as a Prussian diplomat in Paris. His many discoveries and his liberal opinions had made Humboldt a celebrity. He was said to be the second-most-famous man in Europe, after Napoleon. He died in Berlin on May 6, 1859, at the age of 89 and was given a state funeral.

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Asia, where it forms a deep pool in the region of Indonesia. Off the South American coast the layer of warm water is kept thin enough by this constant removal of surface water for the cooler water to be drawn up from the ocean depths. During an El Niño–Southern Oscillation (ENSO) episode the pattern changes, and heavy rain falls in Peru, where the climate is usually dry. This does not help the Atacama. It receives no benefit from the El Niño rains. During the day the prevailing wind is from the southwest. At night land breezes blow from the northeast or east. Gales are rare north of 30°S—south of this latitude their frequency increases toward the roaring forties. Warm air is chilled as it crosses the Peru Current, and low cloud and fog are common, just as they are along the Namibian coast.

A High Plateau There the similarity ends, because most of the Atacama Desert lies a little farther inland and is separated from the coast by the Cordillera de la Costa range of mountains. The coastal mountains rise to about 5,000 feet (1,500 m), with individual peaks reaching 6,560 feet (2,000 m). There is no coastal plain, and in places the cliffs are more than 1,600 feet (500 m) high. Inland from the mountains the desert lies in a depression forming the Tamarugal Plain at an elevation of more than 3,000 feet (900 m). The altitude of the Atacama contributes to the moderate temperatures. High mountains that form part of the Andes chain border the desert on its eastern side. These include the Cordillera Domeyko range, containing many volcanic peaks, some more than 16,000 feet (4,900 m) high. The Atacama Plateau, up to 13,000 feet (4,000 m) above sea level, lies in the northeast, on the border between Chile and Argentina. Like the other subtropical deserts, the Atacama lies beneath a permanent area of high pressure produced by the subsiding air of the Hadley cells (see “Hadley Cells, Equatorial Rain, and Hot Deserts” on page 75). This air is dry, and the high pressure means it flows outward, preventing moister air from entering. The subtropical anticyclone (high-pressure region) is the principal cause of the extreme dryness of the Atacama, but although rain is so rare cloud is fairly common. Sometimes the cloud is low enough to be called fog. Cloud and fog arrive with air reaching the coast from the ocean to the west. Air approaching from the sea is chilled by contact with the cold water of the Peru Current (see “Moderate Temperatures and the Peru Current” on pages 24–25). This contact produces a layer of cool surface air lying beneath a layer of warmer air, a condition known as a temperature inversion. The inversion is very stable and forms a cap that prevents air from rising through it. Moist air can rise high enough for condensation to form clouds, but not to form clouds that are big enough

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to produce rain. Air approaching from the southeast crosses the South American continent, then loses any moisture it still carries during its ascent over the Andes. This is the combination of geographical circumstances that make the Atacama so extremely arid. The aridity continues on the eastern side of the Domeyko Mountains, where the desert rises to a height varying from 7,000 feet (2,100 m) to 13,500 feet (4,100 m). The high, bleak Andean plateau is known generally as the altiplano, and in this part of Peru and Chile as the puna. The desert region is called the Puna de Atacama. Within the Puna there is another region, the Salar de Atacama, where there are basins rich in mineral salts. Away from the influence of the ocean current, temperatures are higher despite the elevation, and the diurnal variation is greater. In summer the temperature can rise to 90°F (32°C). On winter nights, however, the temperature often falls to below freezing as the ground loses its heat by radiation, and radiation fog forms.

Bolsons and Minerals The desert is not level. The mountains were formed by a process that caused much faulting—fracturing of rocks so that one section moves in relation to the other. Blocks were thrust upward, leaving deep basins between the faulted blocks (see “Plate Tectonics and Orogenies” on pages 50– 55). Such a basin is called a bolson, and in the Atacama their surfaces are 2,000–3,000 feet (610–915 m) below the level of the equivalent rocks in the raised blocks that form the surrounding mountains. The structure produces a characteristic surface. In the center there is a level plain called a playa or salina. Near its edges it begins to slope gently upward in a region called the pediment, covered with broken rock. Nothing lives in the Puna de Atacama. There is just brown earth and dust. In the lower desert, to the west of the Domeyko Mountains, some tough grasses and a small amount of other vegetation manage to survive. There is some very sparse vegetation on the coastal strip. In general, though, the Atacama is one of the bleakest, most forbidding places on Earth.

Fighting for the Desert Despite being uninhabitable, Chile, Peru, and Bolivia spent much of the 19th century competing for control of the part of the Atacama Desert lying between latitudes 23°S and 26°S. Much of the area belonged originally to Bolivia and Peru. The War of the Pacific began in 1879 and ended with the signing of the Treaty of Ancón on October 20, 1883, leaving Chile in control of the area. Bolivia lost its coastline and became an entirely landlocked country.

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Geography of Deserts The war was not fought simply for control of a worthless desert. The Salar de Atacama, in the bolsons, contains some of the world’s richest deposits of sodium nitrate and copper, and Chilean companies owned the mines. At first the attraction was the nitrate deposits, located inland from Iquique and to the northeast of Antofagasta. Nitrate is the raw material for fertilizer and explosives manufacture. Taxes on the export of Chilean nitrate sometimes amounted to half the government’s revenue. Railroads were built to bring the nitrate to the coast, where ports were constructed to export it. The Chilean nitrate industry declined from about 1920, following the introduction of the Haber process for manufacturing ammonia industrially. Discovered by the German chemist Fritz Haber (1868–1934), the process makes atmospheric nitrogen (N) react with hydrogen (H) to form ammonia (NH3): N2 + 3H2 ↔ 2NH3. (The ↔ sign means the reaction can proceed in either direction.) The reaction takes place in the presence of a catalyst (a substance that promotes the reaction without being altered itself) in a vessel where the pressure is about 3,625 pounds per square inch (25 MPa) and the temperature is about 840°F (450°C). When the nitrate industry could no longer compete with the Haber process, the mining companies switched their effort to the copper reserves. These are still mined, mainly at Chuquicamata, where mining commenced in 1915. Chuquicamata, at 9,350 feet (2,850 m) above sea level, is the biggest open-cast mine in the world and produces one-fourth of Chile’s copper.

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PATAGONIA

Far to the south is Patagonia, another South American desert that lies between the mountains and a cold ocean current. When the first Spanish explorers entered this region, they met the Tehuelche Indians. The Spaniards called the Tehuelche Patagones, and that is how Patagonia acquired its name. Patagonia occupies the whole of Argentina lying to the east of the Andes and mainly south of the Colorado River, although part of Patagonia extends to the north of the river. In the south it continues to the Strait of Magellan, and Tierra del Fuego, the region shared between Argentina and Chile to the south of the Strait of Magellan, is often included. Patagonia lies between latitudes 31°S and 51°S, or 55°S if Tierra del Fuego is included; the map shows its location. Its total area is about 260,000 square miles (673,000 km2), making it the biggest desert in either North or South America. Patagonia is also unique. There is no other desert in the world lying on the eastern side of a continent in a latitude higher than 40°. It is far less arid than the Atacama Desert, but nowhere is the average annual rainfall as high as 10 inches (254 mm), the precipitation threshold below which

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Santa Maria B R A Z I L Pôrto Alegre Cordoba

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a desert will form regardless of the temperature. Sarmiento, a town near the center of Patagonia, at 45.6°S, has a climate that is fairly typical of the region. The wettest months are in winter, from April to August, but barely more than five inches (127 mm) of rain falls in an average year. The average daytime temperature ranges from 45°F (7°C) in July to 78°F (26°C) in January.

A Series of Terraces Patagonia occupies a high plateau about 5,000 feet (1,500 m) above sea level. In the north, close to the River Negro, the land rises in a series of terraces. These lie about 300 feet (90 m) above sea level at the top of the coastal cliffs in the east, rise to about 1,300 feet (396 m) in the center, and finally reach about 3,000 feet (900 m) where the tableland meets the Andes in the west. Farther south the terrain is broken by deeply incised valleys that are aligned from west to east. Some of the valleys are dry, rivers flow through others for part of the year, but few carry rivers that flow all the time. Hills rise from the plain, made from hard rocks resistant to the weathering that has eroded the material around them. On the western side, where the plateau meets the Andean foothills, there are lakes sealed on one side by glacial moraines—large heaps of rock and gravel deposited by glaciers—extending westward into deep mountain canyons.

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The Falkland Current Patagonia is far enough south to lie in the belt of westerly prevailing winds. These bring air across the Andes, where it loses its moisture. Patagonia lies in the rain shadow of the high mountain range. Winds blowing from the South Atlantic Ocean to the east must cross a cold ocean current. Along the northern edge of the Southern Ocean, the West Wind Drift (also called the Antarctic Circumpolar Current) is a surface current that encircles the globe. As it approaches each of the continents, a part of the West Wind Drift is deflected, turning northward to become an eastern boundary current (see the sidebar “Boundary Currents” on page 22). In the South Pacific the deflection forms the Peru Current. In the South Atlantic another deflection forms the Benguela Current, but there is also a smaller part that turns northward around the southern tip of South America. This is the Falkland Current, an eastern boundary current that chills any air approaching Patagonia from the east. The Falkland Current flows northward between the eastern coast and the Falkland Islands (also called Las Malvinas) and continues northward to about 30°S, where it meets and merges with the Brazil Current, a western boundary current flowing southward. Since the prevailing winds are from the west, the Falkland Current has only a limited influence on the Patagonian climate, but its effect is to enhance the dry conditions. Comodoro Rivadavia, on the eastern coast at latitude 45°S, receives an average of only 8.8 inches (224 mm) of rain a year. The climate is somewhat wetter in the south of Patagonia than it is farther north, however, and heavy snow falls in winter near the narrowing tip of the continent. The Patagonian Desert is bleak, but not all of it is barren. In the north there are tough grasses and shrubs, the grasses providing pasture for sheep. There is little vegetation farther south, where the climate is colder as well as drier.

CHARACTERISTICS OF THE AUSTRALIAN DESERT

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The tropic of Capricorn passes across the center of Australia. This means that the country lies close enough to the trade wind latitudes (see “Explaining the Trade Winds” on pages 73–74) for the prevailing winds to be from the southeast. These bring maritime conditions, with abundant rain, to the coast of New South Wales and Queensland. Brisbane, Queensland, for example, enjoys an average annual rainfall of about 45 inches (1,136 mm), while Sydney, New South Wales, receives about 46 inches (1,183 mm). In both cities the rainfall is distributed fairly evenly throughout the year.

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Inland from the densely populated coastal strip is a range of mountains, the Great Dividing Range, that runs the entire length of the country parallel to the eastern coast (and continues into Tasmania). Consequently, lands to the west of the mountains lie in a rain shadow. A second mountain range extends along an approximately southwest– northeast line from the Yorke Peninsula, South Australia, until it merges with the Great Dividing Range at about the tropic of Capricorn. The southern part of this range is called the Main Barrier Range, and the northern section is the Grey Range. The map shows the principal ranges.

Rainfall There are thus two lines of high ground over which air must rise as it moves inland and where the southeasterly trade winds lose moisture. The 20-inch (500-mm) isohyet (line joining places of equal rainfall) runs along the Great Dividing Range. To its west there is the 15-inch (380-mm) isohyet, and to the west of the second range there is the 10-inch (250-mm) isohyet. An average rainfall of 10 inches (250 mm) a year is the limit below which desert usually forms, regardless of the average temperature. As the map shows, most of the interior of Australia is either semiarid or desert. Of all the continents, Australia is the second-driest, after Antarctica. About 70 percent of the total area receives less than 20 inches (500 mm) of rain a year, making it arid or semiarid, and approximately 35 percent of the country receives so little rain that it is virtually desert. True desert covers 18 percent of the total land area, amounting to 529,000 square miles (1.371 million km2). To Australians the figure that matters is not so much the amount of rainfall as how useful the rainfall is to farmers. If the amount of rain that falls during a month is greater than one-third of the amount that would evaporate from an open water surface during the same period, the rain is said to be effective, meaning that precipitation exceeds potential evaporation. Agriculture is possible if there is an effective amount of rain for at least five months of the year. With less than five months of effective rain the only crops that can be grown are those that mature rapidly in valley bottoms and other depressions, where the water table is within reach of their roots. The five-month line runs across the north of the country on the southern side of Arnhem Land to the Gulf of Carpentaria. From the western side of the Cape York Peninsula in the north it runs southeast about to the tropic, then south-southwest to approximately the latitude of Sydney, and then west more or less parallel with the coast as far as Perth. Most of the land enclosed by this line is semiarid, and much of it is desert. Alice Springs, just south of the tropic and almost exactly at the center of Australia, has an average rainfall of just under 10 inches (250 mm) a year.

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Alice Springs is in the foothills of the Macdonnell Ranges, the highest of which is Mount Ziel, at 4,954 feet (1,510 m), and the town itself is at 1,901 feet (579 m). The hottest month is January, with an average daytime temperature of 97°F (36°C), but the temperature usually exceeds 90°F (32°C) from November through March. In midwinter the temperature falls to about 67°F (19°C). These are averages, however, and the temperature can fall below freezing in winter, and in summer it has been known to reach 111°F (44°C). Kalgoorlie, Western Australia, is at a similar elevation (1,247 feet, 370 m) farther south, to the west of the Great Victoria Desert and Nullarbor Plain. It receives an average of 9.7 inches (246 mm) of rain, about the same as Alice Springs, and its temperatures are also fairly similar. In

Australian Climate Australia is an island, but an island the size of a continent, and away from the coasts it has a continental climate (see the sidebar on page 31). A large proportion of the total area of Australia experiences a continental climate because of the shape of the continent. It is very compact, especially along its west–east axis, with no deep embayments to bring the ocean closer to the center of the landmass and extend its climatic influence inland. This combines with the rain shadow effect to increase the aridity, partly by increasing the extremes of temperature.

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January the average daytime temperature is 93°F (34°C), but 115°F (46°C) is not unknown. In July the daytime temperature averages 62°F (17°C), but has been known to reach 81°F (27°C). High temperatures increase the rate of evaporation.

Plateaus, Tablelands, and Lowlands Geologically, Australia is a very ancient continent in the sense that its rocks were not affected by movements of the Earth that elsewhere raised great mountain ranges such

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as the Himalayas, Atlas, Alps, and American Cordillera (see “Plate Tectonics and Orogenies” on pages 50–55). Much of Australia lies on rocks of Precambrian age (more than 542 million years old), and, except in the east, typical Australian landscapes are those of plains and plateaus. Where rugged features do occur, they are due to ravines or steep-sided valleys cut through the plateaus by rivers that flowed in the remote past. Even the ranges of hills are rounded, but there are also isolated, flat-topped hills, the larger ones called mesas and the smaller ones buttes. Only about 7 percent of the total land area lies more than 2,000 feet (610 m) above sea level, and the average elevation is between 1,000 feet and 1,500 feet (305 and 460 m).

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Geography of Deserts

Continental Climates After it has remained over a continent or ocean for several weeks, the temperature and humidity at any particular altitude of an air mass are approximately the same everywhere. The physical characteristics of the air are constant throughout it. Air masses may be continental or maritime and arctic, polar, or tropical depending on the latitude in which they form. Maritime air masses are moist because of evaporation from the water surface, and contact with the surface moderates extremes of temperature. Continental air masses are much drier. Compared with the ocean, there is a much smaller total area of exposed water surface to feed moisture into the air. As the air rises to cross high ground it will lose much of the moisture it has acquired. Continents also experience a much wider temperature range—between day and night and summer and winter—than do islands and coastal regions. This is because dry land absorbs warmth much faster than ocean water and also loses it much faster. The land warms more rapidly than the ocean in spring and summer and cools more rapidly in winter. Consequently, summers are warmer and winters cooler than those influenced by the ocean. Air masses move. As a maritime air mass drifts over a continent its characteristics slowly change, and it becomes a continental air mass. Continental air masses slowly change into maritime air masses as they move away from the land and cross the ocean. The climate of a region, and hence the dayto-day weather, depend on the type of air masses that move across it.

The main platform of Precambrian rock, known as the shield, is broken into a number of distinct blocks separated by lowland areas. It is in the lowland regions that the deserts have formed.

Australia’s 10 Deserts Australia has not one desert but 10! The largest is the Great Victoria Desert, with an area of 134,617 square miles (348,750 km2). It stretches across nearly half the width of Western Australia and to halfway across South Australia. The Great Sandy Desert covers 103,158 square miles (267,250 km2). It lies partly in the Canning Basin, Western Australia, less than 600 feet (183 m) above sea level. Nowhere

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does the Great Sandy Desert rise more than 1,200 feet (370 m) above sea level. The Tanami Desert, with an area of 71,217 square miles (184,500 km2), lies to the east of the Great Sandy Desert, partly in Western Australia and partly in the Northern Territory. On the southwestern side of the Great Sandy Desert, the Little Sandy Desert covers about 43,040 square miles (111,500 km2) around Lake Dora, Western Australia. To the south of the Great Sandy Desert, lying on slightly higher ground, is the Gibson Desert. With an area of about 60,216 square miles (156,000 km2) entirely inside Western Australia, the Gibson Desert lies right on the tropic of Capricorn around the aptly named Lake Disappointment. Tableland rises to the south of the Gibson Desert. The Macdonnell Ranges lie to the east of the Gibson Desert, with Alice Springs in their southeastern foothills. On low ground to the southeast of the ranges is the Simpson Desert, less than 500 feet (150 m) above sea level, occupying 68,130 square miles (176,500 km2) in the southeastern corner of the Northern Territory and part of northern South Australia and western Queensland. It lies to the north of Lake Eyre, one of the largest of a number of salt lakes and the lowest point on the continent, about 60 feet (18 m) below sea level. Despite being called a lake, most of the time Lake Eyre is a sheet of salt 90 miles (145 km) long and 40 miles (64 km) wide that is firm enough to drive a truck across. It is named after the explorer Edward John Eyre (1815–1901). Farther to the east the Strzelecki Desert is a field of sand dunes covering 31,000 square miles (80,250 km2) in the area where South Australia, Queensland, and New South Wales meet. The desert extends in a northwesterly direction from just inside the New South Wales border to Cooper Creek. Strzelecki Creek runs across its center, and several salt lakes, including Lake Frome, lie within its borders. Sir Paul Edmund Strzelecki (1797–1873) was a Polish-born explorer and geologist. The Sturt Stony Desert and Tirari Desert are adjacent to the Strzelecki Desert, to the east and northeast of Lake Eyre. The Sturt Stony Desert, named after the explorer Charles Sturt (1795–1869), covers an area of 11,483 square miles (29,750 km2) on the borders of South Australia, Queensland, and New South Wales. The Tirari Desert, with an area of 5,900 square miles (15,250 km2), is entirely inside South Australia. Straddling the border between South Australia and the southeastern corner of the Northern Territory, the Pedirka is the smallest of Australia’s deserts, covering 482 square miles (1,250 km2). The Nullarbor Plain lies immediately to the south of the Great Victoria Desert. It is not called a desert, but the plain is a very arid and virtually uninhabited limestone plateau. Nullarbor is Latin and means “no trees.” Forrest, Western

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Australia, located at 30.83°S, 128.10°E, and 511 feet (156 m) above sea level, has an average annual rainfall of 7.3 inches (184.7 mm).

Gibber, Sand Dunes, and Desert Scrub The desert surfaces vary. In places the ground is bare rock, and elsewhere it is gravel or pebbles shaped by the wind. These small stones are called gibber in Australia, and a large expanse of them is a gibber plain. There are sand dunes in the many of the deserts, especially in the Simpson, Great Victoria, and Strzelecki Deserts. The deserts are not lifeless, except locally. In some areas the desert is bordered by a savanna type of vegetation comprising grasses and Acacia scrub called mulga, in others by dwarf Eucalyptus scrub called mallee. There are also areas of Acacia scrub called brigalow. Over most of the Great Sandy, Gibson, and Great Victoria Deserts the soil is sandy, and the principal vegetation consists of various species of porcupine grasses (Triodia species) and cane grass (Spinifex paradoxus). Farther south and southeast the soil is salty and supports saltbushes and other salt-tolerant plants.

CHARACTERISTICS OF THE NORTH AND CENTRAL AMERICAN DESERTS

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It is one of the hottest places on Earth. In July the average daytime temperature is 116°F (47°C), and it has been known to rise to 134°F (57°C) in the shade. At night the temperature falls to an average of 87°F (31°C). Winter temperatures are much lower, but 85°F (29°C) has been known in January, the coldest month. It is dry and often windy. Sandstorms and whirling dust devils are common. The average rainfall amounts to no more than two inches (50 mm) a year. Death Valley, California, was named in 1849. That was the year a party of 30 emigrants used it as a short cut to the California gold fields, and 12 of them perished. By the end of the 19th century, however, the valley had become a winter resort. The valley is hotter and drier than anywhere else in the United States. It is also lower. About 4.75 miles (7.6 km) west of Badwater is the lowest point on the North American continent, 282 feet (86 m) below sea level. The region is a national park with an area of 5,315 square miles (13,765 km2), and about 550 square miles (1,425 km2) of that area lie below sea level. In the lowest parts of the valley the surface consists of salt flats. It is not devoid of water. There are springs, marshes,

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and pools, but the water is salt—in many places much saltier than seawater. Despite this the pools support several species of pupfish, one of which, the Devil’s Hole pupfish (Cyprinodon diabolis), is found nowhere else in the world. On higher ground is a mixture of salt and sand grains. Cacti and other succulent plants grow in the valley, and when there is sufficient rainfall, annual herbs appear in the early spring. Lizards, snakes, rabbits, rodents, and foxes live in the valley throughout the year. Death Valley lies between two mountain ranges. Part of the Amargosa Range forms its eastern boundary, and it is bounded by the Panamint Range in the west. To the southwest, beyond the mountains, is the much larger Mojave Desert, covering an area of 15,000 square miles (38,850 km2).

Mojave Desert The Mojave Desert lies to the south and east of the Sierra Nevada, extending south to the San Bernardino Mountains and the Colorado River. Both it and Death Valley are in the rain shadow of the mountain ranges to the west. Maritime air crossing the mountains from the ocean is forced to rise—the process of orographic lifting—and this causes it to expand, cool to below its dew point temperature, and lose most of its moisture. By the time it reaches the lee side of the mountains the air is very dry. The average rainfall in the Mojave Desert is less than five inches (127 mm) a year. Occasionally, the precipitation falls as snow, and frosts are fairly common on winter nights, when the temperature falls to 15–30°F (-9.4 to - 1.1°C). Winter days are mild, however, with temperatures of about 55°F (12.8°C). In summer daytime temperatures often exceed 100°F (37.8°C). Surrounded by mountains, the scenery of the Mojave is dramatic. The mountains are rugged and rise steeply from level basins, where the surface is of gravel and sand. In the center of the desert are salt flats from which a variety of chemical compounds are extracted. Cattle graze on part of the Mojave, and in the mountains there are open juniper and piñon woodlands. Elsewhere the vegetation consists of low shrubs (see “Typical Desert Plants” on page 108–116).

Colorado Desert The San Bernardino Mountains mark the southern boundary of the Mojave Desert. Beyond the mountains, starting at the San Gorgonio Pass, the Colorado Desert extends into Baja California, Mexico. As well as the San Bernardino Mountains, the Chocolate, Chuckawalla, and Cottonwood ranges separate the Colorado and Mojave Deserts. The Colorado Desert is about 200 miles (320 km) long and 50 miles (80 km) wide, and much of it lies below sea level.

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The Mojave Desert is rocky and dusty, but its scenery is magnificent. (Ronald and Charlotte Williams)

The Salton Sea is a brackish lake that formed when an irrigation scheme failed (see “The Salton Sea” on page 279). Its bed is 235 feet (72 m) below sea level. The Imperial and Coachella Valleys are also low-lying. The map shows where these and the other North American deserts lie in relation to each other.

desert extent of desert

Rainfall rarely exceeds four inches (102 mm) a year, and temperatures can change rapidly, from 32°F (0°C) to 115°F (46.1°C). Between May and September daytime tempera-

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tures average 90°F (32°C), and they can reach 125°F (52°C). There are shifting sand dunes in the northwest and among the sand hills to the east. Surprisingly, the soils of the Imperial Valley are very fertile, and irrigation is supplied from a canal leading from the Colorado River. The valley is renowned for its cotton, vegetables, and fruit.

The Painted Desert To the northeast, near the upper end of the Grand Canyon, the Little Colorado River flows into the Colorado River. The Painted Desert, in Arizona, stretches for about 150 miles (241 km) along the northeastern side of the Little Colorado. It is between 15 and 50 miles (24 and 80 km) wide and covers an area of about 7,500 square miles (19,425 km2). Part of the eastern region of the Painted Desert lies within the Petrified Forest National Park. Lieutenant Joseph C. Ives is said to have given the Painted Desert its name. An explorer employed by the government, Lt. Ives visited the desert in 1858 and was struck by the brilliant colors of its rocks. These include shales, sandstones, and marls and have bright bands of red, yellow, blue, white, and lavender running through them. The Navajo and Hopi peoples, who live in the desert, use its colored sands for their ceremonial sand paintings. The Vermilion Cliffs, which are the sides of large mesas, bound the desert on its northern side. This desert lies on high ground, with elevations ranging from 4,500 feet (1,373 m) to 6,500 feet (1,983 m). Isolated buttes rise from a generally rolling surface. It is a colorful place but barren. The annual rainfall varies from five inches to nine inches (127–229 mm), and temperatures are extreme. They can fall to -25°F (-32°C) and rise to 105°F (41°C).

The colors of the Painted Desert are striking.

(Dale Franks)

Sonoran (Yuma) Desert Yuma is a town in southern Arizona, near the Mexican border, and it gives its name to the largest desert in North America, the Yuma Desert. This is not the desert’s only name. It is also called the Carson Plains and the Sonoran Desert, Sonora being the name of a state in Mexico. The Sonoran or Yuma Desert lies mainly in southwestern Arizona, southeastern California, and northwestern Sonora. On its western side the desert borders the Gulf of California. On its other sides it borders other deserts or semiarid regions— the Mojave Desert to the northwest, the dry Arizona highlands to the northeast, and the Sierra Madre Occidental in Chihuahua, Mexico, to the east. In all, the desert covers an area of 119,692 square miles (310,000 km2). Most of the desert is low-lying, with an average elevation of about 1,000 feet (305 m), but mountains rise steeply from the floors of the broad basins, where the surface is of sand and gravel. There are salt flats in the lowest basins, especially in the Coachella–Imperial Valley in the northwest, which lies below sea level. The Chocolate and Chuckwalla Mountains of California, the Kofa and Harquahala Mountains of Arizona, and Mount Pinacate in Mexico all lie within the Sonoran (Yuma) Desert. Few parts of the desert receive more than 10 inches (254 mm) of rain a year, and the annual rainfall is often only about five inches (127 mm). Frosts can occur in winter, but more usually the weather is warm in winter and hot in summer. Winter temperatures rise to 60–70°F (15.5–21°C) by day and fall to 40–50°F (4–10°C) at night. In summer the daytime temperature often exceeds 110°F (43°C). The desert supports a variety of vegetation. Tall shrubs and trees grow along dry riverbeds, creosote bushes and sage grow in the basins, and cacti, including the giant saguaro, are found on higher ground. The desert also provides habitats for many reptiles, including at least six species of rattlesnakes and the venomous lizard the Gila monster (Heloderma suspectum). Much of the area is protected. There are several wildlife refuges, and the Joshua Tree and Saguaro National Parks lie within the desert. Its mild winters have long made Palm Springs, Tucson, and Phoenix popular resorts.

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COLD DESERTS

Not every desert is a vast expanse of sand, gravel, or bare rock, nor is every desert hot, even at noon in the height of summer. One of the driest of all deserts has a surface that is so brilliantly white travelers are advised to wear dark glasses, and it is so cold that even in the middle of summer the temperature never rises above freezing. It averages between -30°F and -4°F (-34°C and -20°C). Naturally, win-

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Geography of Deserts ters are even colder, with temperatures between -94°F and -40°F (-70°C and -40°C). These are average temperatures, and they can fall lower. On July 21, 1983, Russian scientists at the Vostok Station measured the temperature at -128.6°F (-89.2°C). That is the lowest temperature ever recorded at the Earth’s surface.

Antarctica Vostok Station is, of course, in Antarctica, at 78.75°S, and the interior of Antarctica is a dry desert. Paradoxically, it is a dry desert most of which lies beneath an ice sheet. The thickness of the sheet varies from place to place, but it averages about 6,900 feet (2,100 m), and it contains about 90 percent of all the ice in the world.

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A desert is not a place where rain or snow never fall, but a place where the rate of potential evapotranspiration (see “Evaporation and Transpiration” on page 8) exceeds the rate of precipitation. If water is able to evaporate from the surface faster than rain falls from the sky, the land will be a desert. The higher the air temperature, the more rapidly water will evaporate, so a desert can form more easily in a hot climate than in a cold one, but regardless of the temperature it will form anywhere if the average annual precipitation is low enough. The threshold is about 10 inches (254 mm) a year.

Rainfall in Antarctica. As the map shows, the interior of Antarctica has an extremely dry climate.

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It is almost certain that a place receiving less than 10 inches of precipitation a year will be desert, no matter how cold its climate. As the map shows, most of Antarctica receives less than eight inches (200 mm) of precipitation a year, and a substantial area at the center of the continent receives less than two inches (50 mm). Near the South Pole the average precipitation is probably no more than about one inch (25 mm) a year. Visitors usually travel no farther than the coastal strip or the edge of the long Antarctic Peninsula that extends toward the tip of South America. They experience a different type of weather, with warmer temperatures and much more precipitation. In summer, which is the only season when the continent is easily accessible, the temperature near the coast can rise to 32°F (0°C). This is not warm enough to melt the ice and snow, but it is not too uncomfortable provided there is only a light wind. There is abundant snow. Precipitation near the coast averages about 15 inches (380 mm) a year. It will seem like much more, because snow is about 10 times bulkier than liquid water, and precipitation is always measured as its equivalent in rain. This allows direct comparisons to be made between snowfall and rainfall, and melting the snow before measuring it eliminates inaccuracies that arise due to some kinds of snow being much bulkier than others.

Gales and Blizzards The winds are often ferocious. Vast oceans surround the continent. Air gathers moisture during its long journey across the oceans, and it is warmer than air over the snow-covered land, which reflects most of the solar radiation falling on it. Huge storms are produced along the front where the warm, moist, maritime air meets cold, dry, continental air. These storms generate strong gales and heavy precipitation. They are confined to the coastal area, however, and tend to travel fairly quickly around the edge of the continent. Inland, the weather is different. There, precipitation is light, but the winds are at least as strong. Gales have been known to blow at 200 MPH (320 km/h). Strong winds produce blizzards, but these are not blizzards of falling snow. The fine, powdery snow that the winds carry is lifted from the ground and transported. Blizzards in the Antarctic interior are the equivalent of the sandstorms and dust storms of tropical deserts. Some of the snow is lost by ablation. This is a fairly general term describing losses by melting and evaporation and also by sublimation. Although the air temperature seldom rises above freezing, even at the coast, the surface layer of snow can absorb enough solar warmth for a little of it to melt, and liquid water evaporates quickly in the very dry air. Inland, where the air carries very little water vapor, some snow vaporizes directly, passing from

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the solid (ice) to gaseous phase without passing through the liquid phase. This is called sublimation; the reverse process, in which water vapor turns directly into ice, is known as deposition. Losses by sublimation are very small, however, and most of such little snow as does fall inland becomes permanently trapped. This is how a land receiving less precipitation than the Sahara can lie buried beneath such a thick layer of ice.

Why Antarctica Is Cold and Dry The fact that snow accumulates does not explain why Antarctica is so cold and so dry. There are several reasons for this. The first and most obvious explanation is astronomical. Antarctica receives only diffuse sunlight (see the bottom illustration on page 2), and even in summer the Sun never rises high above the horizon. In latitudes higher than the Arctic and Antarctic Circles, at 66.5°N and 66.5°S, there is at least one day in each winter when the Sun does not rise above the horizon and at least one day every summer when it does not sink below the horizon. Polar regions are where the seasons become most extreme. Summer is a time of almost constant daylight in the “lands of the midnight Sun” and winter of almost perpetual twilight or darkness. Climatically, this means that any warmth the ground absorbs during the long days of summer is lost during the long nights of winter, and during winter there is little or no direct heating of the surface by the Sun. When the Sun does shine, most of its light and heat are reflected. It is the glare of sunlight reflected from the snow that makes the wearing of dark glasses advisable, and radiation that is reflected is radiation that has not been absorbed. The reflectivity of a surface is called its albedo, and it can be measured. Freshly fallen snow has an albedo of 75–95 percent. That is the proportion of solar radiation it reflects. Dry sand also has a fairly high albedo, but of only 35–45 percent. For comparison, a field of grass has an albedo of 10–20 percent. Its high surface albedo means the snow-covered ground is able to absorb only a small fraction of the weak sunshine that falls on it. Antarctica is the fifth-largest continent. Its total area, of about 4.8 million square miles (12.4 million km2), is more than half that of North America and considerably more than that of Australia. Its center is far from the ocean, and air has ample opportunity to lose any moisture it may be carrying long before it arrives there. It is also the highest of all the continents. The average elevation of the rock surface beneath the ice is about 8,000 feet (2,440 m) above sea level, and there are several mountain ranges. The highest point on the continent is the Vinson Massif, in the Ellsworth Mountains of West Antarctica, which

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Point Hope

Fort Yukon

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CA N A DA

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Beaufort Sea Banks Island

ARCTIC OCEAN

Laptev Sea

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RUSSIA Nordvik

North Pole

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Baffin Bay © Infobase Publishing

GREENLAND Godthåb

Greenland Sea

The Arctic

rises to 16,860 feet (5,139 m). The continent is divided into distinct eastern and western parts by the Transantarctic Mountains. The height of its surface makes the climate even colder, because air temperature decreases with height in this dry air at about 5.4°F per 1,000 ft (9.8°C per km)—a rate of decrease known as the dry adiabatic lapse rate. If the sea-level temperature is, say, 32°F (0°C) and the air is unsaturated, the temperature at 8,000 feet (2,440 m) will be -11.2°F (-24°C). The actual surface is higher because of the ice. A person standing on the ice will be about 14,900 feet (4,545 m) above sea level, and the temperature will be about -48.5°F (-44.7°C). Air as cold as this is very dense, and over both North and South Poles cold, dense air subsides, producing areas of permanently high surface atmospheric pressure. Air flows outward from areas of high pressure. This increases the aridity, because moist air is unable to penetrate the region at low level. In Antarctica it is this outward movement of air that produces the constant winds. High pressure near the center of the continent pushes surface air outward, but as it moves the air also sinks toward lower ground. This accelerates the

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gravitational flow, often producing winds of hurricane force (speeds greater than 75 MPH, 121 km/h).

Arctic Weather Cold, dense air also sinks over the North Pole, producing an Arctic region of permanently high atmospheric pressure similar to that in the Antarctic. Air flows outward from it, but the climate it produces is less extreme than that of Antarctica. Svalbard, sometimes called Spitzbergen, which is the name of the biggest island, is a group of Norwegian islands at latitude 78.06°N. The islands are in almost exactly the same latitude as the Vostok Antarctic Station, but the climate is very different. In July the average temperature is 45°F (7°C), and over a 10-year period the lowest temperature recorded on a July night was 30°F (-1°C). January and February are the coldest months, with average temperatures rising to 19°F (-7°C) by day and falling to 7°F (-14°C) by night. Qaanaaq (formerly called Thule), in northern Greenland, is a little to the south of Svalbard, at 76.55°N, but its winters are colder. There the average daytime temperature in February is -4°F (-20°C), although July has an average daytime temperature of 46°F (8°C). There are colder places. The coldest place in the Northern Hemisphere—known as the cold pole—is Verkhoyansk,

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Siberia, where -90.4°F (-68°C) has been recorded. The lowest temperature ever recorded in North America was -85°F (-65°C) at Snag, Yukon.

Why Antarctica Is Colder Than the Arctic Both Verkhoyansk and Snag lie well to the north of the Arctic Circle, and, cold though they are, their temperatures illustrate the fact that the Antarctic is much colder than the Arctic. Antarctica is also drier, although most of the region enclosed by the Arctic Circle also has a desert climate. Qaanaaq receives an average annual precipitation of 2.5 inches (64 mm). Svalbard is rather wetter, with 13.3 inches (337 mm) a year. The difference between the climates of the two poles arises partly from the astronomical fact that Antarctica receives seven percent less solar radiation than does the Arctic. This is because in the middle of its winter (June) the South Pole is 3 million miles (4.8 million km) farther from the Sun than the North Pole is in the middle of its winter (December). Elevation also has an important effect. Antarctica is a large continent with a high elevation, and the Arctic region is not: The North Pole lies beneath the Arctic Ocean. The Arctic Circle passes across northern Europe, Asia, and North America and includes about two-thirds of Greenland, but sea covers most of the area. This means that except in Greenland (see “Greenland or Kalaallit Nunaat” on pages 42–45), the surface elevation is low. It is the sea that exerts the strongest influence on the polar climate, however. The Arctic is warmer than the Antarctic mainly because of the sea. Ocean currents carry warm water into the Arctic Basin. The sea is frozen for most of the year, but there are gaps in the ice, called leads, that appear and disappear. Winds move the ice, piling it up in some places and leaving it thin in others. Heat escapes from the ocean wherever there are open water surfaces, but ice insulates the areas it covers. Seawater freezes at about 28.56°F (-1.91°C), varying slightly according to the salinity, and the temperature at the surface of the sea never falls below 29°F (-1.6°C). Below this temperature the water approaches its greatest density, and sinks below warmer water that flows in at the surface to replace it. When the air temperature over the water falls below the temperature of the sea surface, heat passes from the water to the air. This warmer air then moves across the ice. Consequently, air temperatures over the entire Arctic Basin are markedly higher than they would be were there land rather than sea beneath the ice. The coldest air temperature ever recorded over the ice in the Arctic was -58°F (-50°C), and over most of the basin the average temperature ranges

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between about 4°F (-20°C) and -40°F (-40°C). On July 21, 1983, the temperature at Vostok fell to -128.6°F (-89.2°C).

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ICE SHEETS AND GLACIERS

Most of the continent of Antarctica—about 98 percent of its total surface area—lies beneath a layer of ice averaging about 6,900 feet (2,100 m) in thickness and containing about 90 percent of all the world’s ice and about 98 percent of all the freshwater on Earth. Such a large volume of ice can exist in a region with a desert climate if the annual rate of precipitation is higher than the rate at which water is lost and if the ice has been accumulating for a long time. Scientists at the Vostok Station have drilled out cores of ice in order to study details of past climates. In 1985 the first drilling reached a depth of 7,225 feet (2,202 m), and in 1990 a second hole drilled by a joint Russian–French–U.S. team reached 8,353 feet (2,546 m). A third hole was opened in 1990, and in 1998 it reached 11,887 feet (3,623 m). Scientists have also drilled ice cores near a research station called Byrd that has since closed. The cores were taken in Marie Byrd Land, at about latitude 80°S, and the scientists calculated that ice at a depth of 1,000 feet (300 m) had formed from snow that fell 1,600 years ago, a formation rate of about 0.6 inch (1.8 cm) a year. This suggests the ice has been accumulating at an average rate of about half an inch (1 cm) a year. The ice at the bottom of the deepest Vostok hole is about 420,000 years old. The rate of ice accumulation is not constant over very long periods, however. When the world climate is cool, and especially during ice ages, it is also dry, because the lower the air temperature the less moisture the air can hold (see “Why a Rise in Temperature Makes Air Drier” on page 7). At such times precipitation decreases, and polar ice accumulates more slowly. During warm periods the reverse happens, and climates become wetter. Much can be learned about climates from the chemical composition of tiny bubbles of air that were captured between ice crystals as the ice formed and from the ratio of oxygen isotopes (the ratio of 16O to 18O) present in the air and in the ice (see the sidebar on page 39). Ice cores provide a record of temperature. It is a record that can be dated, because each year’s accumulation of snow forms a distinct layer, and the layers can be counted.

Snow into Ice Place some water in a freezer to make ice cubes, and as its temperature falls the water changes into ice. Its molecules move toward one another and form crystals. A water molecule consists of two atoms of hydrogen (H + H = H2) and one atom of oxygen (O), making the familiar H2O. These atoms are arranged in a V shape, with the oxygen at the bottom of

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Isotopes At the center of every atom is the nucleus. The nucleus of a hydrogen atom consists of a single proton. Nuclei of the atoms of all other elements contain both protons and neutrons. A proton is a particle that carries a positive electromagnetic charge; a neutron is a particle, very slightly heavier than a proton, that carries no charge. An atomic nucleus therefore carries a positive charge. This is balanced by the negative charge carried by electrons. These are particles that surround the nucleus. The number of protons in a nucleus and the number of electrons surrounding it determine the chemical characteristics of an atom, which are based on attraction between opposite charges. An element is a chemical substance that cannot be broken down into simpler substances. Every atom of an element contains the same number of protons. Consequently, the chemical characteristics of an element are constant. A variation in the number of neutrons in the nucleus cannot affect the chemical behavior of the atom because neutrons neither attract nor repel other particles. Most elements have more than one type of atom; the atoms have an identical number of protons but different numbers of neutrons. These different types of atoms are called isotopes of the element. Although all the isotopes of an element are identical chemically, they have different masses because of the different numbers of neutrons they contain. Oxygen (chemical symbol O) has several isotopes. The most abundant, accounting for 99.76 percent of all naturally occurring oxygen, has eight protons and eight

the V and the two hydrogen atoms separated by an angle of 104.5° at the ends of the two arms, as shown in the illustration on page 40. The bond holding the molecule together is ionic. That is to say, it is based on the attraction between the positive electric charge on the protons in the atomic nuclei and the negative charge on the electrons surrounding the nuclei. A hydrogen atom has only one proton and one electron, and in a water molecule both the hydrogen electrons are pulled to the oxygen side of their atoms. This leaves the molecule with a slight positive charge on the hydrogen side, away from the electrons, and a slight negative on the oxygen side, due to the excess of electrons (shown as plus and minus signs in the illustration on page 40). A molecule of this type, which has a charge at either end but is neutral overall, is called polar. Water molecules are attracted to one another. The positive charge on the hydrogen end of one molecule can

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neutrons in its nucleus. It is known as oxygen-16, often written as 16O. The addition of one neutron produces 17 O, accounting for 0.04 percent of all oxygen, and adding one more neutron produces 18O, accounting for 0.2 percent of all oxygen. There are also three radioactive isotopes, 14O, 15O, and 19O, which are very short-lived. Information about past climates can be inferred from the ratio of 16O to 18O present in ice cores and in the shells of marine animals called foraminifers, or forams. When hydrogen is oxidized to form water (2H2 + O2 → 2H2O), this ratio remains constant, but water molecules containing 16O (H216O) are lighter than those containing 18 O (H218O). Being lighter, they travel faster and are better able to escape into the air from liquid water. When atmospheric water vapor condenses, therefore, the liquid contains a slightly higher proportion—about 0.7 percent—of 16O than is present in the water from which it evaporated, and that water is left slightly depleted of 16 O and enriched in 18O. The ratios of 16O to 18O in air and water reflect the amount of evaporation, so they provide an indication of temperature. Polar ice and the air bubbles in it record the temperature at the time the original snow fell. If the entrained air and ice have more than 499 atoms of 16O to every atom of 18O, the weather was warm when the ice formed. Water was evaporating and falling as precipitation. Fossil seashells are made from calcium carbonate (CaCO3) taken from seawater. Analyzing the oxygen isotope ratios in the shells reveals the temperature of the water in which the animals lived.

form a bond with the negative charge on the oxygen end of another. This is called a hydrogen bond. It is weaker than an ionic bond, but it causes the molecules in liquid water to link together in short strings. Heating a substance imparts energy to its molecules. If water is heated sufficiently, the energy its molecules absorb will allow them to break free from the hydrogen bonds linking them. First the strings of molecules become shorter as individual molecules escape, and then free molecules are able to escape from the surface of the liquid. In other words the water starts to vaporize. Cool the water and the strings grow longer as more hydrogen bonds form. Having less energy, the molecules move more slowly. The volume of the liquid decreases as they are gradually drawn together. Finally, the groups form links, and the water crystallizes. Uniquely, it then expands, because the crystals that water forms have an open structure.

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hydrogen +

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© Infobase Publishing

Water molecule. The water molecule is made of one atom of oxygen and two of hydrogen. The hydrogen atoms are both on the same side of the oxygen atom, separated by an angle of 104.5°.

That is how ice forms in a freezer. The ice in a polar ice sheet forms rather differently, because it starts not as liquid water but as snow. Warm air rises, and as it rises it cools. As its temperature falls below a particular value called the dew point water vapor present in the air starts to condense into droplets, and a cloud will form. Vertical air currents within the cloud then carry the tiny cloud droplets to a height at which the temperature is below freezing. They then freeze, and water vapor condensing at that height will do so as ice, not liquid water. The change between vapor to solid without passing through a liquid phase is called deposition. Ice crystals collide, adhere to one another, and form snowflakes. The flakes fall, and if they fall into a region where the temperature is above freezing, they melt into drops of water. If the snowflakes or water drops are too heavy to be carried aloft by air currents, they fall from the bottom of the cloud. Snowflakes may still melt as they fall toward the ground if the air below the cloud is warmer than about 39°F (4°C). Most of the rain that falls in middle latitudes is melted snow, even in summer. Only a small amount of snow falls in polar regions, but very little of the snow melts. December 27, 1978, was the warmest summer day ever recorded at the South Pole. On that day the temperature rose to 7.5°F (-14°C). Each fall of snow lies on top of the preceding fall, and even if the surface does thaw briefly it quickly freezes again. Large, soft snowflakes form only where the air temperature is higher than about 23°F (-5°C). In colder air the snowflakes are much smaller. In middle latitudes this powdery kind of snow falls only during very severe winter cold, but in the Arctic and Antarctic it is the commonest type of snow. Ice crystals can have many shapes. An international system for classifying them recognizes 10 basic shapes. They do not stack well, so a randomly scattered mass of ice crystals of

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various shapes forms a loose arrangement. Powdery snow is easily lifted from the ground. A strong wind can cause a blizzard when there is not a single cloud in the sky. Snow is heavy, however, and the weight of overlying snow at the base of a thick accumulation of the layer packs the crystals tightly together. Cars and trucks driving over a snow-covered road have a similar effect. They pack the snow down hard, making it almost solid. Beneath the much greater weight of a thick snow layer the snow actually does become solid. It is packed into ice. This is not the way ice forms in a freezer or on the surface of a pond in winter, and the resulting ice looks different. It is white rather than transparent. This is because crushing the snowflakes together traps large numbers of tiny air bubbles. These are the bubbles scientists release from ice cores and then capture for analysis.

Ablation, Ice Sheets, and Glaciers Some of the snow is lost. Where the air is extremely dry, as it is in the interior of Antarctica, ice can sublime directly. The amount involved is small, however. Fine, powdery snow can also be lifted by the wind, and Antarctica is a very windy continent. It will fall again, of course, but a small proportion of it will be carried over the coast and dropped into the sea. Losses by wind and sublimation are called ablation. Most of the snow remains close to where it fell, and slowly it is compacted into the ice that forms the ice sheet. An ice sheet is an expanse of ice covering an area of at least about 20,000 square miles (52,000 km2). At the base of the ice sheet compaction has a further effect. The ice is squeezed outward to the sides away from the center, and very, very slowly it begins to flow. The pressure from the center is sufficient to force the moving ice over small hills, but generally the ice moves down slopes. Where there is a natural valley or soft rock that is easily eroded, the moving ice will be confined by the valley walls. It is then called a valley glacier and conforms to the popular impression of a glacier. In fact, though, there is no clear distinction between an ice sheet and a glacier. Both consist of ice, and in both cases the ice moves.

Ice Shelves To the west of the Transantarctic Mountains, West Antarctica, the smaller section of Antarctica, is low-lying, and its coast has large bays and offshore islands. The Antarctic Peninsula projects from West Antarctica, and it also has many bays. Where the flowing ice sheet reaches a bay on a low-lying coast, it does not halt. The flow continues into the sea. The ice is secured to the land on either side of the bay, and inshore it is in contact with the sea floor. That part of the ice sheet that rests on the solid rock is said to be grounded. Farther from the shore, in deeper water, it is not supported from below. There is water beneath the

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Geography of Deserts ice. The boundary between floating ice and ice that rests on a solid surface is called the grounding line. This extension of the ice sheet is an ice shelf, and the Antarctic ice shelves are big, especially the Ronne and Filchner shelves in the Weddell Sea and the Ross shelf in the Ross Sea.

Icebergs At the outer margin of the shelves the ice is floating on water that moves with the tides and currents. The ice is constantly moving seaward, and movement of the floating ice weakens it. From time to time large blocks of ice break free. These are icebergs. Antarctic icebergs enter the Southern Ocean from the Weddell and Ross Seas. They are flat-topped, very clean-

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looking, often have a blue tinge, and can have surface areas of 1,200 square miles (3,100 km2) or more—the size of Rhode Island. An iceberg that broke away (the technical term is calved) from the Ross shelf in October 1987 had an area of 1,834 square miles (4,750 km2), almost the size of Delaware, and was at least 825 feet (250 m) thick. Arctic icebergs are different in appearance. They calve from valley glaciers rather than the edges of ice shelves. Consequently, they are smaller and much more irregular in shape than Antarctic icebergs. They are also darker in color because they contain rock particles scoured out by the glacier. People should not be too alarmed, therefore, at reports of large blocks breaking away from the Antarctic shelves. In recent years several large sections have broken away from the Larsen Ice Shelf (see the sidebar). These events were

The Larsen Ice Shelf The Antarctic Peninsula extends northwestward from the mainland of Antarctica and forms the southwestern boundary of the Weddell Sea. Its covering of ice and the sea ice that surrounds it make the peninsula appear wider than it is; in fact, it is a narrow strip of land with many offshore islands. On the eastern coast near the tip of the peninsula, between 65° and 66°S, 60° to 62°W, lies the Larsen Ice Shelf, extending over the sea and covering several of the islands. The Larsen is one of the smaller of the Antarctic ice shelves as well as being the most northerly, the whole of it lying outside the Antarctic Circle (66.5°S). Scientists divide the Larsen Ice Shelf into four sections labeled A through D. Larsen A is the most northerly section, with Larsen B to its south. Summer temperatures at the northern end of the peninsula have risen by about 4.5°F (2.5°C) since 1940, and summers have lengthened from about 50 days to 80. Over the same period the extent of the sea ice off the peninsula has decreased by about 20 percent. The higher temperatures allowed pools of water up to 0.5 mile (1 km) wide to lie on the surface of the ice. Liquid water drained into fractures in the ice called crevasses, weakening the ice at the bottom. Slowly over the years, Larsen A was growing weaker. Until 1975 the entire shelf was stable and possibly expanding in some places, but in that year Larsen A was seen to be shrinking and thinning. In late January 1995 (midsummer in Antarctica) a ferocious storm struck the tip of the peninsula, and a section of Larsen A broke away. The detached iceberg had an area of about 770 square miles (2,000 km2), and as it moved it knocked a 656-square-mile (1,700-km2) block of ice from

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Larsen B. The break-up of Larsen A left Larsen B exposed. Several more icebergs broke from it, and in November 1998 Larsen B lost a section 395 square miles (1,024 km2) in area. By 2000 Larsen B was 1,550 square miles (4,016 km2) smaller than it had been in 1975. The summer of 2002 was the warmest ever recorded on the peninsula. A further section of Larsen B disintegrated between January 31 and March 5, reducing the area of the ice shelf by a further 1,254 square miles (3,250 km2). Icebergs continue to break away from the Larsen Ice Shelf. On January 31, 2005, a section about 735 square miles (1,900 km2) in area broke away from the southern end of the shelf. Clearly, the Larsen Ice Shelf is disintegrating. Its loss will remove the large volume of ice at the seaward end of the glaciers that flow from the peninsula into the Weddell Sea, perhaps causing them to accelerate. If that happens, the part of the West Antarctic Ice Sheet covering the peninsula may grow thinner and eventually disappear. No one knows why summer temperatures have been rising over the Antarctic Peninsula. The rise is much too rapid to be linked directly with general global warming, and it began when global average temperatures were steady or falling. The rise is probably due to changes in the prevailing winds and ocean currents that carry sea pack ice across the Weddell Sea, piling it against the peninsular coast. Although the Larsen Ice Shelf has diminished greatly, this does not affect the main part of the West Antarctic Ice Sheet, and the amount of ice that has been lost is only a very small proportion of the total covering Antarctica.

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reported as an indication of global warming that heralded the imminent collapse of all the ice shelves and possibly of the entire West Antarctic Ice Sheet. In fact, such calvings are perfectly normal events, and the loss of the Larsen Ice Shelf may not be due to global warming.

GREENLAND, OR KALAALLIT NUNAAT

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Greenland is mountainous, but, unlike Antarctica, its mountains run along the coasts rather than dividing the island into two parts. Mountains running down the east coast rise to about 7,000 feet (2,100 m), and down the eastern side of the country there are mountains 5,000–6,000 feet (1,525–1,830 m) high. The highest mountain is Mount Gunnbjørn, the peak of which is at 12,139 feet (3,700 m). It is on the eastern coast, south of the Arctic Circle. Between the coastal mountain ranges, the interior of the country is a high plateau covered by an ice sheet averaging 5,000 feet (1,525 m) in thickness and on the highest ground, west of Ittoqqortoormiit (Scoresbysund) on the western coast, raising the surface to about 10,000 feet (3,050 m) above sea level. At its deepest point the ice sheet is more than 8,000 feet (2,440 m) thick. The central plateau is a remote, empty place, and several attempts to cross it failed. The first to succeed was in 1888, when a team of six people led by the Norwegian explorer Fridtjof Nansen (1861–1930) crossed from east to west at about latitude 64.42°N. They set out from the east coast on August 16 and reached the west coast on September 26. Alfred Wegener, the German meteorologist who proposed the theory of continental drift, also crossed the plateau, and in 1930 he lost his life on the ice cap (see the sidebar on page 43). Scientists continue to visit the interior, but Greenlanders live near the coast, which is where most earn their living.

Greenland is a land of rock and ice. (Peter Bjørstad)

The Ice Sheet The ice sheet covers an area of 670,272 square miles (1,736,095 km2), more than the areas of Oregon and Alaska combined and equal to 85 percent of the area of Greenland. The ice sheet measures 1,550 miles (2,500 km) from north to south and about 620 miles (1,000 km) from east to west, and it contains about 10 percent of all the freshwater in the world. In addition, local ice caps and glaciers cover a smaller area of approximately 18,763 square miles (48,599 km2)—more than the combined areas of Vermont and New Hampshire. This leaves 15 percent of the country ice free. This area amounts to 135,100 square miles (350,000 km2), an area considerably larger than New Mexico and eight times that of Denmark. The ice-free land is located around the coasts.

Place Names and Geographic Links Greenland, known in Danish as Grønland and in Greenlandic, the Greenland Eskimo language, as Kalaallit Nunaat, is the largest island in the world. The population in 2003 was estimated to be about 56,700, of which approximately 45,000 (79 percent) are Greenland Eskimo and 11,700 (21 percent) are European, mainly Danish. Greenland is a self-governing dependency of Denmark and an integral part of the kingdom of Denmark. Since it was granted self rule the Greenlandic names of its towns and villages have replaced the Danish names. Older maps still show the Danish names, however, so at the first mention of a place with two names the modern name is given here with the Danish name in parenthesis—for example, Qaanaaq (Thule). Subsequent references to the place give only the modern name, in this case Qaanaaq. Geographically Greenland forms part of the North American continent. The rocks beneath the ice sheet are ancient—some of them are as old as the planet itself— and they are related to rocks in adjacent Canada. In the north only Smith Sound, 25 miles (40 km) wide, separates Greenland from Ellesmere Island, Canada. The earliest traces of life are also found in rocks from Greenland. The map shows how close Greenland is to Canada. Cape Farewell, the southernmost tip of Greenland, is at latitude 59.77°N, and the Arctic Circle passes a little way to the south of the town of Sisimiut (Holsteinsborg). Most of the country lies inside the Arctic Circle. The northernmost point, Cape Morris Jesup, is at 83.65°N.

Temperatures The average winter temperature over the ice cap is about 27°F (-33°C), and it can fall much lower. In 1930–31 German scientists of the Alfred Wegener expedition (see

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Alfred Lothar Wegener (1880–1930) Alfred Wegener is remembered today mainly as the scientist who first suggested that the continents and oceans have not always occupied their present positions. He proposed that the continents were once joined, forming a supercontinent he called Pangaea, and that since Pangaea broke up the continents have continued to move across the surface of the Earth constantly but extremely slowly, reconfiguring the map of the world. This was his passion, but Wegener was first and foremost a meteorologist who devoted considerable time, much effort, and eventually his life to studying the climate of Greenland. Alfred Lothar Wegener was born in Berlin on November 1, 1880, the son of a minister of religion who was also director of an orphanage. Wegener was educated at the Universities of Heidelberg, Innsbruck, and Berlin. In 1905 the University of Berlin awarded him a Ph.D. in planetary astronomy. Wegener immediately changed his field of interest to meteorology and took a job at the Royal Prussian Aeronautical Observatory, not far from Berlin. (Berlin, now the capital of Germany, was then the capital of the kingdom of Prussia.) He used balloons and kites to carry instruments into the air high above the surface, and he and his brother Kurt also flew hot air balloons. In 1906 the two men remained airborne for 52 hours, setting a new world endurance record for balloonists. Later in 1906 Wegener joined a Danish two-year expedition to Greenland as official meteorologist. Wegener used kites and tethered balloons to study conditions in the polar air, returning to Germany in 1909. He was then appointed lecturer in meteorology and astronomy at the University of Marburg. In 1912 Wegener married Else Köppen, the daughter of a famous climate scientist, Wladimir Köppen (1846–1940). Later that same year Wegener returned to Greenland as a member of a four-man team that crossed the ice cap. Theirs was the first expedition to spend the winter on the ice.

the sidebar above) recorded an average February temperature of -52.9°F (-47.2°C) and an average summer temperature in July of 12.8°F (-10.7°C). Several times during the winter they spent there the temperature fell to -85°F (-65°C), and in summer it never rose above 26.6°F (-3°C).

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War broke out in 1914, and Wegener was drafted into the army. He was wounded almost at once, however, and spent a long time recuperating in hospital, passing the time by developing his theory of continental drift. When he was well enough he joined the military meteorological service. After the war Wegener returned to Marburg, and in 1924 he took up a post created for him as professor of meteorology and geophysics at the University of Graz, in Austria. In 1930 he embarked on a third expedition to Greenland as leader of a 21-member team of scientists and technicians. They planned to study the weather over the ice cap from three bases, all at 71°N, one on each coast and one called Eismitte 250 miles (450 km) inland, but bad weather delayed them. A party set out on July 15 to establish the Eismitte base, but bad weather prevented necessary supplies from reaching them. These included their radio transmitter and the hut in which they were to live. On September 21 Wegener and 14 companions headed for Eismitte with 15 sleds loaded with supplies. The weather was so bad that all but Wegener, Fritz Lowe, and Rasmus Villumsen were forced to turn back. The three finally reached Eismitte on October 30. They stayed long enough to celebrate Wegener’s 50th birthday on November 1, then Wegener and Villumsen departed for the base camp; Lowe was too exhausted and too badly frostbitten to accompany them. Wegener and Villumsen never reached the base camp. At first the people at the base camp assumed they must have decided to overwinter at Eismitte, but by April they had still not appeared, and a party went in search of them. They found Wegener’s body on May 12, 1931. He appeared to have suffered a heart attack. Villumsen had carefully buried him. The would-be rescuers marked the grave with ice blocks, and later a huge iron cross was erected there. Despite a long search, Villumsen was never found. The Alfred Wegener Institute for Polar and Marine Research at Bremerhaven, Germany, commemorates the life, work, and bravery of Alfred Wegener.

At the same time as the German expedition, a British expedition was recording similar weather conditions 300 miles (483 km) to the south. Several more teams of meteorologists visited the ice sheet during the 1930s and 1940s, and in the 1950s French scientists confirmed the earlier temperature records.

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ARCTIC OCEAN

Ellesmere Island

Baffin Bay

Svalbard

GREENLAND

Greenland Sea

Nuuk

Labrador Sea

Iceland Reykjavik

Norwegian Sea

Oslo

AT L A N T I C O C E A N © Infobase Publishing

Glasgow Dublin blin

Edinburgh Amsterdam

Greenland (Kalaallit Nunaat)

These temperatures demonstrate the effect of elevation on climate. At sea level it is much warmer. In South Greenland, described as “lush” by Greenlanders, about 100 families earn their living raising sheep and growing vegetable crops. At 51.72°N, Nuuk (Godthåb), the capital of Greenland, has an average winter (November through March) daytime temperature of about 23°F (-5°C). Much farther north, Qaanaaq, at 68.82°N, enjoys an average daytime temperature during those five months of about 1.5°F (-17°C). In summer (May through August) the daytime temperature averages 40°F (4.4°C) at Qaanaaq and 48°F (9°C) at Nuuk. Plants will grow only at average temperatures higher than about 43°F (6°C), so summers at Nuuk, but not at Qaanaaq, are warm enough to grow vegetables and grass. Elevation is only part of the explanation, however. Nuuk is on the western coast, where the climate is relatively mild due to the West Greenland Current, a branch of the Gulf Stream that flows northward parallel to the western coast (see the sidebar “Boundary Currents” on page 22). Greenland’s western coast also has a milder climate than that of Baffin Island on the opposite side of the Davis Strait separating Greenland from Canada. The cold Labrador Current flowing southward on the Canadian side of the strait affects the Baffin Island climate. The western coast of Greenland also has a milder climate than the eastern coast, which is affected by the East Greenland Current flowing southward from the

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Arctic Ocean. Consequently, most of the towns and villages are located along the western coast. The prevalence of warm föhn winds (originally this was the name of a local wind in the Swiss Alps) also contributes to the mild coastal climate. It seems paradoxical, but it is the extreme cold of the central plateau that causes warm winds at the coast. Over the ice cap, light and heat are reflected strongly from the surface of the snow, which has an albedo of 75–95 percent (see “Why Antarctica Is Cold and Dry” on pages 36–37). This maintains the low temperature over the ice sheet, since only 5–25 percent of the incoming solar energy is absorbed by the surface. Air immediately above the surface is chilled, making it contract the way an inflated balloon will contract if it is left in a refrigerator. Contraction makes the air denser because the same number of air molecules occupies a smaller volume. Cold, dense air flows out from the ice sheet and down the glacial valleys. As it descends from the high interior the air is compressed and grows warmer adiabatically (see the sidebar on page 45), producing warm, dry winds at the coast.

Precipitation Rainfall is moderate in the southwest. Nuuk receives an average of 23.5 inches (597 mm) a year, distributed fairly evenly throughout the year, although August and September are somewhat wetter than other months. Qaanaaq is much drier. It receives an average of only 2.5 inches (63.5 mm) of rain a year, of which 1.6 inches (41 mm) falls between June and September. Much of the precipitation falls as snow, of course, but precipitation is always measured as rainfall or its equivalent; one inch (25 mm) of rain is roughly equivalent to 10 inches (254 mm) of snow, but the volume of a given weight of snow can vary considerably. Snow that does not melt in summer and is eventually turned into glacier ice is called firn or névé. Qaanaaq probably has a climate quite similar to that in the uninhabited center of the country, on the ice sheet. There almost all the precipitation falls as snow, and on average the amount is equal to about 0.3 inch (8 mm) of rain a year, or approximately three inches (8 cm) of snow, giving the interior of Greenland a climate that is dry even by desert standards. The snow accumulates, ice forms, and in the 1990s the ice sheet overall was found to be growing thicker by about 0.8 inch (2 cm) a year.

Glaciers and Icebergs The ice cap has a domed shape, and pressure exerted by the weight of ice at the center squeezes the ice at the base of the dome outward. At the edges of the ice sheet ice is forced through the valleys in the coastal mountains. Bounded by valley walls, the ice forms valley glaciers flowing toward

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Geography of Deserts

Adiabatic Cooling and Warming Air is compressed by the weight of air above it. Imagine a balloon partly inflated with air and made from some substance that totally insulates the air inside. No matter what the temperature outside the balloon, the temperature of the air inside remains the same. Imagine the balloon is released into the atmosphere. The air inside is squeezed between the weight of air above it, all the way to the top of the atmosphere, and the denser air below it. Suppose the air inside the balloon is less dense than the air above it. Denser air will push beneath it, and the balloon will rise. As it rises the distance to the top of the atmosphere becomes smaller, so there is less air above to weigh down on the air in the balloon. At the same time, as it moves through air that is less dense, the balloon experiences less pressure from below. This causes the air in the balloon to expand. When air (or any gas) expands, its molecules move farther apart. The amount of air remains the same, but it occupies a bigger volume. As they move apart, the molecules must “push” other molecules out of their way. This uses energy, so as the air expands, its molecules lose energy. Because they have less energy they move more slowly. When a moving molecule strikes something, some of its energy is transferred to whatever it strikes, and part of that transferred energy is converted into heat. This raises the temperature of the struck object by an amount related to the number of molecules striking it and their speed. In expanding air the molecules are moving farther apart, so a smaller number of them strike an object each second. They are also traveling more slowly, so they strike with less force. This means the temperature of the air decreases. As it expands, air cools. If the air in the balloon is denser than the air below, it will sink. The pressure on it will increase, its volume will decrease, and its molecules will acquire more energy. Its temperature will rise. This warming and cooling has nothing to do with the temperature of the air surrounding the balloon. It is called adiabatic warming and cooling, from the Greek word adiabatos, meaning “impassable.”

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the sea. Near the coast the glaciers enter fjords. These are deep, U-shaped valleys open to the sea, where the glaciers partly melt. Fjords were formed by glacial erosion during past ice ages and were flooded when the sea level rose. The Humboldt Glacier is the biggest. It is more than 62 miles (100 km) wide and enters the sea in the north, presenting a wall of ice more than 300 feet (118 m) high. Some of the glaciers on the western coast move rapidly. The fastest have been known to advance 100 feet (40 m) in 24 hours, but this rate was not sustained. A movement of 97 feet (38 m) in 24 hours has been sustained, however. Eastern glaciers move more slowly. As it enters the sea the forward edge of a glacier begins to float and become unstable. Sections of ice calve from it as icebergs. These differ from Antarctic icebergs because the ice originates in valley glaciers rather than ice shelves. Arctic icebergs, most of which come from Greenland, are smaller and more irregularly shaped than Antarctic ones. When they first calve they are often more than 200 feet (60 m) high and extend up to 800 feet (244 m) below the sea surface, but they are rarely more than about 0.5 mile (0.8 km) long. They carry rock fragments and soil scoured by their parent glaciers from the bases and sides of their valleys, and this makes them darker in color than Antarctic icebergs. It was with an iceberg from the Illulissat (Jakobshavn) Glacier, on the western coast of Greenland, that the ocean liner Titanic collided on the night of April 14–15, 1912.

WHEN NORTHERN AMERICA, EUROPE, AND ASIA WERE COLD DESERTS

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In 1836 and 1837 Louis Agassiz spent his summer vacations exploring glaciers in his native Switzerland. At that time, Agassiz—his full name was Jean Louis Rodolphe Agassiz (1807–73)—was professor of natural history at the University of Neuchâtel, in Switzerland, and was already famous as a leading authority on fossil fishes. Glaciers had come to fascinate him because they presented a puzzle. Boulders lying on the ground at various places on the plains of eastern France as well as in Switzerland were made of rock quite different from that beneath the soil on which they lay. Such misplaced rocks are called erratics. The puzzle was how they had arrived in the localities where they were found. The boulders resembled rocks found in other parts of Switzerland, and a number of scientists had suggested that glaciers might have transported them. The idea seemed rather unlikely, and Agassiz was deeply suspicious. For that to happen, glaciers would have to move, pushing the rocks along in order to shift rocks from one

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place to another, but there were no glaciers within miles of the mysterious erratics. At some time in the past, therefore, the glaciers would also have to have extended very much farther than they did in the early 19th century. There was no proof that glaciers moved at all, it was difficult to believe that they were formerly much more extensive, and in any case the origin of the erratics could be explained in other ways. A popular view was that they had been carried in icebergs drifting in a sea that once covered much of Europe and that they were deposited when the sea retreated and the icebergs melted. Glacial deposits were called drift deposits, referring to the idea of drifting icebergs, and the word is still used even today. Nevertheless, in 1836 Agassiz and his colleagues built a hut they called the “Hôtel des Neuchâtelois” on the Aar Glacier to use as a base for their studies. They quickly observed that the sides of the glacier were lined with broken rocks, and there were also broken rocks at the lower end of the glacier. The most reasonable explanation for their presence was that the glacier had torn them from the sides of its valley, smashed them into smaller fragments, and then pushed them ahead of itself. This suggested that glaciers might move after all. Two years later, in 1839, Agassiz found another hut that had been built on the glacier in 1827. There was a record of its original position, but when he found it the hut was almost one mile (1.6 km) from that location. To check that it really was the glacier that had carried the hut, he and his friends drove a straight line of stakes firmly into the surface of the ice across the glacier from one side to the other. When he returned in 1841 to check them, he found they were no longer in their original positions, nor were they in a straight line. The stakes had moved and now formed a U shape. They had all moved, but those in the center had moved farther than those at the sides, presumably because friction between the ice and the valley walls slowed the movement of the ice at the edges.

The Great Ice Age Agassiz was forced to conclude that glaciers do indeed move. He then linked this movement to the distribution of boulders and other erratics—deposits of gravel and smaller rocks that were unrelated to the rocks on which they lay. If glaciers moved and if they scoured rocks from the valley walls on either side and from the ground over which they moved, dragging and pushing these rocks along with them and grinding them into ever smaller fragments, then at one time the glaciers must have extended over much of western Europe. He described his studies of Swiss glaciers and the conclusions he had reached in a book, Études sur les glaciers (Studies on glaciers), which he published in 1840. His principal conclusions were that in the geologically recent

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past Switzerland lay beneath a single ice sheet and that ice sheets similar to those still found in Greenland once covered all those regions where erratic boulders and other deposits are found. There had been, in fact, an ice age. In 1846 Louis Agassiz was invited to the United States to deliver a series of lectures at the Lowell Institute, in Boston, followed by another series in Charleston and single lectures in several other cities. The welcome he received and his fascination with the natural history of North America persuaded him to remain. In the course of his travels he found signs that the ice age had affected North America just as it had Europe. He called it the Great Ice Age. Agassiz was appointed professor of zoology at Harvard University in 1848 and became an American citizen. Louis Agassiz was elected to the Hall of Fame for Great Americans in 1915.

Not One Ice Age, but Many The ice age that Agassiz identified began about 1.81 million years ago and marks the beginning of the Pleistogene period (also called the Quaternary subera) of geologic time. Most of the Pleistogene constitutes the Pleistocene epoch, which ended about 10,000 years ago. We are now living in the Holocene (or Recent) epoch of the Pleistogene. This was not the first ice age—the technical name is a glacial. The earliest that is known occurred about 1,250 million years ago. There were three between 900 and 600 million years ago, and there have been many since then. One of the longest lasted from about 320 million years ago until 250 million years ago. The recent Pleistocene glacial discovered by Agassiz turned out to be not one ice age, as he supposed, but many. By early in this century geologists had identified four separate glacials in countries adjacent to the European Alps and named them after alpine rivers, in order, starting with the earliest: Günz, Mindel, Riss, and Würm. Four in northern Europe were called (oldest first) Elsterian, Saalian, Warthian, and Weichselian. The Weichselian is called the Devensian in Britain. North America experienced five: the Nebraskan, Kansan, Illinoian, Iowan, and Wisconsinian (which is equivalent to the Devensian and Weichselian). There were also glaciations in New Zealand and South America. The onset of the Baventian (British) or Eburonian (northwest European) glacial marks the commencement of the Pleistocene, but this was not the first of the sequence. There were possibly two earlier ones during the preceding Pliocene epoch. These episodes of extreme cold were interspersed with warmer periods, called interglacials. The interglacials lasted for an average of between 10,000 and 20,000 years, the glacials about 100,000 years. Then smaller oscillations were identified, the cold ones called stades (or stadials) and the warm ones interstades (or interstadials). The warmer

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Pliocene, Pleistocene, and Holocene Glacials and Interglacials APPROXIMATE DATE (’000 NORTH AMERICA YEARS BP)

GREAT BRITAIN

NORTHWEST EUROPE

Holocene

Holocene

Holocene

75–10

Wisconsinian

Devensian

Weichselian

120–75

Sangamonian

Ipswichian

Eeemian

170–120

Illinoian

Wolstonian

Saalian

230–170

Yarmouthian

Hoxnian

Holsteinian

480–230

Kansan

Anglian

Elsterian

600–480

Aftonian

Cromerian

Cromerian complex

800–600

Nebraskan

Beestonian

Bavel complex

Holocene 10–present Pleistocene

740–800

Pastonian

900–800

Pre-Pastonian

Menapian

1,000–900

Bramertonian

Waalian

1,800–1,000

Baventian

Eburonian

1,800

Antian

Tiglian

1,900

Thurnian

2,000

Ludhamian

2,300

Pre-Ludhamian

Pliocene

Pretiglian

BP means “before present” (present is taken to be 1950). Names in italic refer to interglacials. Other names refer to glacials (ice ages). Dates become increasingly uncertain for the older glacials and interglacials, and prior to about 2 million years ago evidence for these episodes has not been found in North America; in the case of the Thurnian glacial and Ludhamian interglacial the only evidence is from a borehole at Ludham, in eastern England.

or cooler conditions these brought were less extreme than those of glacials and interglacials, and they did not last so long. The sidebar lists the Pliocene and Pleistocene glacials and interglacials with their approximate dates and their North American, British, and Northwest European names. Scientists now know there have been many episodes of cold and warm conditions, and that climates are constantly changing. These fluctuations have not ceased, and the era of ice ages has not come to an end. Naming the present time the Holocene, meaning “whole of the new,” is based on the supposition that the glacial epoch ended with the retreat of the glaciers about 10,000 years ago. Today, however, scientists who study the history of climate believe we are living in an interglacial that began about 10,000 years ago, and it is very likely that one day the ice sheets will start expanding once more. If this is correct, and the Pleistocene is defined

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as a time of alternating glacial and interglacial climates, perhaps we are still living in the Pleistocene.

Last Glacial Maximum Around 18,000 years ago the ice sheets covered a larger area than at any other time during the Wisconsinian (Devensian, Weichselian) glacial. This period is often called the last glacial maximum (LGM). As the map on page 48 shows, at that time North America lay beneath ice that extended from southern Alaska to south of the Great Lakes. The ice formed several distinct sheets, spreading in the directions indicated by the arrows. The largest ice sheets were the Laurentide, covering the whole of the eastern side of the continent, and the Cordillera in the west. Greenland lay beneath its own ice sheet, as it does still. Sea ice covered the surface of the

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ARCTIC

OCEAN

sea ice

sea ice

AT L A N T I C OCEAN cool temperate PACIFIC OCEAN

warm temperate

ice tundra subarctic temperate subtropical

© Infobase Publishing

Pleistocene ice sheet in North America. The map shows the maximum extent of the ice sheet and the types of climates to the south of it.

ocean everywhere north of the latitude of Nova Scotia and southern Newfoundland. Europe also lay beneath ice. The Fennoscandian ice sheet covered the whole of Scandinavia, all but the south-

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ernmost parts of Britain, and mainland Europe as far south as the Alps. In Russia it extend as far east as the Laptev Sea, approximately in latitude 110°E. There were smaller ice sheets in eastern Siberia and central Asia, but in general those areas remained free of ice because their climates were too dry. The ground there was frozen hard, but there was insufficient precipitation for snow to be compacted into ice. Over the ice sheets covering so much of the Northern Hemisphere the climate was very similar to the present climate of central Greenland (see “Greenland or Kalaallit

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Geography of Deserts Nunaat” on pages 42–45). Temperatures rarely rose above freezing, and precipitation was very low. Most days brought clear blue skies, bitter cold, and fierce winds. To the south of the ice was tundra. In North America this covered most of Alaska and a broad belt across the continent in about latitude 40°N. Denver, Kansas City, and Cincinnati probably enjoyed weather much like that experienced today in the Northwest Territories of Canada. At Norman Wells, Northwest Territories, at latitude 65.25°N, July is the warmest month, and temperatures average 72°F (22°C) by day and 50°F (10°C) by night. Average temperatures remain above freezing from May through September. The average daytime temperature is -11°F (-24°C) in January, the coldest month, and at night the temperature falls to an average of -26°F (32°C). The highest temperature known is 89°F (32°C), which can be reached between May and July. The lowest is -66°F (54°C) in February. July and August are the wettest months, but the climate is dry. The average annual precipitation is about 13 inches (330 mm). Churchill, Manitoba, on the coast of Hudson Bay at latitude 58.78°N, has a very similar

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climate. There the annual precipitation averages 16 inches (406 mm), and daytime temperatures range from an average -11°F (-24°C) in January to 64°F (18°C) in July. These are not desert climates, but they are dry. Today, Kansas City receives an average of 37 inches (940 mm) of rain a year. At one time, therefore, most of North America and Europe north of about latitude 50°N was a cold, dry desert. To the south of the ice sheets, the climate was semiarid. The last glacial maximum occurred about 18,000 years ago. That is a very remote, distant time, long before the dawn of the earliest civilization. Compared with the 4,600-millionyear age of the Earth, however, the interval between then and now is very short indeed, and modern humans hunted the game animals migrating across the tundra. The oldest fossil of a modern human found in Europe lived in what is now the Czech Republic about 31,000 years ago. Climates change constantly due mainly to fluctuations in the amount of energy received from the Sun (see “Constantly Changing Climate” on page 241). One day it is likely that the ice and the arid climate associated with it will return.

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2 Geology of Deserts It is their arid climates that define deserts, and the world’s climates, including desert climates, form distinct belts around the planet. That is why deserts are where they are, but there is overwhelming evidence that places far from today’s desert regions were once deserts as dry and hostile as any that exist today. This apparent paradox makes sense if the present continents have not always occupied their present positions. Occasionally moving continents collide. When this happens the continents may crumple, pushing up mountain ranges made from rocks that once lay on the seabed. This chapter begins by explaining the theory of plate tectonics, which describes the way continents move and mountain ranges rise. As soon as rocks are raised above the surface, wind, rain, and ice begin to wear them away. Eventually, they are reduced to level plains, and the rocks of which they were made are ground into the mineral grains that form the soil. The chapter describes the formation of soils, the particular characteristics of desert soils, and the physical features of desert surfaces. It explains how sand dunes form and what their different shapes mean. Deserts are dry places, but sometimes rain falls. In most deserts there are places where water lies at or close to the surface, forming oases. The chapter describes what happens to the rain that falls on the desert and explains why there are oases.

PLATE TECTONICS AND OROGENIES

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In the south of England the county of Devon is renowned for its gently rolling pastures and coastal vacation resorts. Along some parts of the coast are distinctive cliffs formed, like all sea cliffs, by erosion as the sea has cut into what were once low hills. The cliffs are distinctive because of their color: They are brick red because they are made from sandstone.

Sandstone consists of sand grains that have been packed and cemented together. Sand grains are crystals, mainly of silica (SiO2), produced by the slow, relentless wearing away of hard rocks such as granite. Once freed, the grains are blown by the wind and carried by water, and they often find their way into deserts, where they accumulate to form dunes. If the silica is mixed with fragments of rock containing iron compounds, exposure to the air may cause the oxidation of the iron—it rusts and turns brick red as it does so, just the color of the Devon rocks. Red sandstone, then, is rock made from desert sand grains, some of which are particles of an iron oxide called hematite (Fe2O3). Devon, a land of lush grass and dairy cows, is as far from being a desert as it is possible to imagine, but the presence of so much sandstone seems to contradict the present climate. What the sandstone shows is that a very long time ago what is now the south of Devon formed a large river delta, rather like the Nile Delta today, where a major river met the sea after flowing across a very dry desert. Part of that ancient desert delta now extends from southern Devon across the coast and for some distance beneath the English Channel. The low hills that have been cut away to form sea cliffs were once desert sand dunes. The rocks of Devon have given their name to an entire period of Earth history, the Devonian, which began about 416 million years ago and ended about 359.2 million years ago. This is not the only experience Devon has had of desert conditions, however. It was a dry, sandy desert again later, during part of the Permian period, which lasted from 299 to about 251 million years ago, and the shapes of dunes can still be discerned in the sandstone. Obviously, Devon experienced a very different climate in the Devonian and Permian from the one it enjoys today, but how can this have been possible? If the climate along that ancient Devonian delta was at all like the climate of presentday Egypt, then what can the climate of North Africa have been like? Perhaps in those days it was even hotter than it is now, but if deserts in those times extended all the way to northern Europe, most of the world must have been a des-

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Geology of Deserts ert. In fact, deserts were once more extensive than they are now, but not because the Sun burned more fiercely. There is an alternative explanation, and the Devon sandstone is not the only puzzle.

When Fish Swam in the Himalayas The Himalayas are the world’s highest mountains. The peak of Mount Everest, the tallest, towers 29,035 feet (8,850 m)—5.5 miles (8.8 km)—above sea level. K2, also called Mount Godwin Austen, is 28,251 feet (8,611 m) tall, and the peak of Kānchenjunga, the third-tallest, is at 28,169 feet (8,586 m). As well as towering so high above the sea, the Himalayan peaks are also a very long way from the nearest coast. Yet, despite having no contact with the sea whatsoever, Himalayan rocks contain fossils of ammonites, lamp shells (known scientifically as brachiopods), and other animals that swim in the oceans or live on the seabed—or that once did so, for these are mainly fossils of organisms that are now extinct. In the course of their formation most of the Himalayan rocks melted and then cooled again, destroying utterly any fossils they might have contained, but it is reasonable to suppose that those fossils once existed. If so, the ones that have survived to the present day represent only a small fraction of what must have been a teeming population. It seems strange to think of shellfish, and presumably many other kinds of fish, living in the mountains near the center of a vast continent. There must be an explanation.

Fitting the Continents Together It is a curious fact that the west coast of Africa looks as though it ought to fit snugly against the east coast of South America, and the search for an explanation of the Devon desert and Himalayan fish began with attempts to account for this apparent fit. Scientists produced many theories throughout the 19th century, all of them based on the idea that the continents have not always been in the positions they occupy today. The continents had moved, in fact, and the various theorists proposed reasons for their movements and routes they may have followed. By the early years of the 20th century many scientists accepted this idea, but they did so with great reluctance, because it was very difficult to imagine how bodies the size of continents could move about. Then, in 1912, a German meteorologist, Alfred Wegener (1880–1930), began giving lectures in which he expounded his own ideas. He described these in detail in a book, Die Entstehung der Kontinente und Ozeane, which was first published in 1915 and then expanded in later editions. It was not until the third German language edition appeared in 1922 that the book was translated into English. The trans-

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lation was published in 1924 as The Origin of Continents and Oceans. (See the sidebar “Alfred Lothar Wegener” on page 43.) Wegener summarized all the evidence for what he called continental displacement, now known as continental drift, that had been accumulating over several decades and added observations of his own. He showed that not only did the coastlines on either side of the Atlantic fit together, but the actual rock formations also matched. The fit was not merely between rocks, it was between rocks of similar formations. Fossils in those rocks were of the same organisms, species that had evolved into distinct American and African forms after the split had occurred. He found evidence to show that parts of South America, Africa, India, and Australia had experienced glaciations (ice ages) all at the same time, around 270 million years ago. All these strands of evidence made sense, Wegener argued, if all the continents had once been joined together as a single supercontinent around the South Pole that had then drifted northward. As the supercontinent crossed the equator it would have experienced equatorial climates, evidence of which is widespread. Coal, for example, is formed from the remains of plants that once grew in tropical coastal swamps, yet coal is now found far from the Tropics in such places as North America, Europe, and China. Wegener called this huge continent Pangaea, from Greek words meaning “all Earth.” Eventually, Pangaea broke apart, finally producing the present continents. These moved and are still moving independently of one another. Wegener thought the mountain ranges running down the western side of North and South America were due to crumpling of the Earth’s crust as the continents were pushed westward and that the Himalayas were also formed by crumpling caused by the collision between India and Asia. The continents, he suggested, are made from rocks that are less dense than the material beneath them, so they float like rafts.

Related Species Separated by Oceans Part of Wegener’s evidence was based on the way certain groups of plants and animals are distributed. Australia is famous for its marsupial mammals, for example, such as kangaroos, koalas, and many more found in New Guinea as well as Australia. But opossums are also marsupials, and they occur in North, Central, and South America, separated from Australia and New Guinea by the vast expanse of the Pacific Ocean. Southern beeches (Nothofagus species) are now widely cultivated, but they occur naturally in New Guinea, southeastern Australia, Tasmania, New Zealand, and also along the southwestern coast of South America. The monkey puzzle tree, or Chile pine (Araucaria araucana), is a popular ornamental tree. As its name suggests, it is native to South

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America, where it forms forests. Its close relative bunyabunya (A. bidwilii) grows naturally in New Guinea, northeastern Australia, and on islands in the South Pacific Ocean. There are two species of tulip trees. Liriodendron tulipifera is native to eastern North America, and L. chinense is native to central China. These are individual species or genera, but there are entire families of plants with this type of disjunct distribution. For example, the papaya, or pawpaw family (Caricaceae), consists of four genera and about 30 species of trees. They are most abundant in South America, with some in Central America, but one genus, Cylicomorpha, grows in tropical Africa. Occasionally, plant seeds may cross a narrow stretch of sea, blown on the wind or carried by birds, but it would be impossible for seeds to travel all the way from America to Africa or China. Wegener argued that the only plausible explanation for disjunct distribution is that continents that are now widely separated were once joined. Populations of plants and animals were divided when the continents separated, then continued to thrive as the two populations moved farther and farther apart.

Seafloor Spreading Few geologists supported these ideas, although Wegener was widely respected. In 1924 he was appointed professor of geophysics at the University of Graz, Austria. Much of his research was conducted in Greenland, about which he was an expert. He died there during his fourth expedition, in the winter of 1930. Geologists hesitated because neither Wegener nor anyone else had suggested a convincing mechanism that could move continents. Several geologists attempted to explain how this could happen—after all, there was a large mass of evidence to support the idea that, somehow, continents had moved. One of those explanations came close to what is now believed to be the case, but it was largely ignored. Arthur Holmes (1890–1965), a British geophysicist, became convinced that heat generated by the decay of radioactive elements deep below the Earth’s surface produces masses of hot rock that rise slowly upward, spread out and cool, then sink once more. By the middle 1920s Holmes was proposing that this motion produces convection currents powerful enough to move the cold, solid rock above them. During the late 1940s and 1950s new evidence began arriving, when oceanographers were able to study the floor of the oceans for the first time. The scientists found that a ridge runs across the center of each of the ocean basins. Harry Hammond Hess (1906–69), an American geophysicist at Princeton University, proposed that these midocean ridges were the key to what he called seafloor spreading. Hot material rises through the Earth’s crust along the midocean ridges, pushing apart the rocks on either side and thereby

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widening the ocean basin. Where the ocean floor adjoins continents, there are regions where the rocks of the ocean floor sink beneath the rocks of the continent in a process called subduction. The theory of seafloor spreading supported Wegener’s idea of continental drift, but still the evidence was inconclusive.

When Compasses Point the Wrong Way Magnetic compasses point to the North Pole. This is not the geographic North Pole, but the magnetic North Pole some distance from it. Compasses work because the Earth behaves like a bar magnet, with north and south magnetic poles. From time to time, however, the Earth’s magnetic field reverses its polarity. What had been the North Pole becomes the South Pole. This has not happened since magnetic compasses were invented, but the next time it occurs all the compasses will turn around and point in the opposite direction. Polarity has reversed three times during the last 60 million years, but in earlier periods there have been intervals of 50 million years between reversals. Many rocks contain minerals that are affected by the Earth’s magnetic field. When the rocks are molten, so their molecules can move freely, those molecules affected by the magnetic field align themselves with it, like tiny compasses all pointing to the north. As the rock cools its minerals form crystals, but with the molecules in those crystals still aligned magnetically. Once the crystal has formed, however, and the rock has cooled and solidified, the molecules can no longer move. They are locked in position and continue to point to the north—or to where north was at the time they turned to point to it. Geophysicists are able to measure the magnetic orientation of mineral crystals, and they have been able to compile a list and time scale of the reversals of the magnetic field. When they applied this knowledge to the rocks on either side of the ocean ridges, they made an interesting discovery: The rocks forming the oceanic crust were arranged in bands of alternating magnetic polarity lying parallel to the ridges. As molten rock erupts, it adopts the prevailing polarity, then solidifies. As rock continues to erupt, the earlier rock is pushed away, and when the magnetic polarity reverses, rock that is then molten aligns with it. The bands on the ocean floor record changing polarity, and because the changes can be dated, so can the bands. Magnetic polarity reversals provide clear evidence not only that seafloor spreading has taken place but of the rate at which it has occurred.

Plate Tectonics Finally, in 1967, the British geophysicist Dan McKenzie (born 1942) at the University of Cambridge unified the

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where convection currents carry hot material into the base of the crust. The hot rock, called magma, tends to break through in volcanic eruptions and to form volcanic islands. Hot spots remain stationary as plates move over them, so chains of islands sometimes form, their alignment recording the direction in which the plate has moved. Where plates border each other, their margins are of four types. At constructive margins volcanoes bring magma to the surface, where it solidifies, pushing apart the plates on either side. That is how seafloors spread from midocean ridges. At destructive margins, two plates collide, and one is made from denser material than the other. The denser rock (ocean floor) sinks beneath the lighter (continental) rock, and dense rock that has already descended below the crust drags the rest of the plate behind it. The process is called subduction, and as the oceanic plate sinks, the sedimentary rock and loose sediment on the surface of the dense underlying rock is scraped off, forming mountains along the coast

theories of continental drift and seafloor spreading within a new theory of plate tectonics. Tectonic means “deforming.” McKenzie proposed that the Earth’s crust is composed of a number of solid blocks, called plates, of greatly differing sizes. Some of the plates are relatively thin and made from dense rock. These form the floors of the oceans. Other plates are much thicker and made from rocks that are less dense. These form the continents, rising above the seas. The plates are able to move in relation to each other, but particular plates can also remain stationary for long periods, and in some places there are sutures where an ocean basin has closed and two continental plates have joined permanently. There is a suture, for example, linking the valleys of the Indus and Tsangpo rivers in Pakistan and Tibet that formed when the Indian plate collided with the Eurasian plate about 40 million years ago. The map below shows the major tectonic plates and the directions they are moving at present. North America and Europe are moving apart by roughly 0.8 inch (2 cm) a year, for example. It also shows the ridges where new crust is forming and the trenches where crust is being removed by subduction. Hot spots, also marked on the map, are places

divergent boundary convergent boundary uncertain boundary

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Major tectonic plates and mantle hot spots

transform fault direction of plate motion hot spot

Reykjanes Ridge EURASIAN PLATE NORTH

Aleutian Kuril Trench Trench Japan Trench

Java Trench

PHILIPPINE PLATE Mariana Trench

INDIAN PLATE

ANATOLIAN PLATE

AMERICAN PLATE Mid

CARIBBEAN PLATE

Atlantic COCOS PLATE

Ridge

PACIFIC PLATE

ARABIAN PLATE AFRICAN PLATE

Carlsberg Ridge SOMALI Mid PLATE Indian

SOUTH NAZCA PLATE Tonga Trench

AMERICAN PLATE

East Pacific Rise

Ridge

Peru

Chile Trench

South West Indian Ridge

South East Indian Ridge ANTARCTIC PLATE

0 miles 0 Km

3,000 4,000

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of the continent. When two continental plates collide, both are of equal density. Neither can sink below the other, so the rocks of both are crumpled upward to form mountains. The Himalayas are the most recent mountains to be produced in this way, their rise having begun about 40 million years ago when India, moving northward, collided with Asia. (India is still moving northward.) At conservative margins the adjacent plates are moving past one another. This motion produces a series of cracks in the crust, called transform faults, at right angles to the line of the plate margin. Margins of all these types, where plates are actively moving, produce volcanoes and earthquakes, and when oceanic plates move jerkily their sudden movement can generate tsunamis. That is what caused the devastating tsunami of December 2004 (see the sidebar below).

The Caledonian Orogeny The movement of tectonic plates and the consequences of collisions between them produced the sand from which the sandstones of Devon were made. During the Devonian period the land that is now Europe, including Devon, lay close to the equator, and it remained there for millions of years. It had a hot climate. It was also a dry climate because it was located where the descending side of Hadley cells (see “Hadley Cells, Equatorial Rain, and Hot Deserts” on page 75) brings warm, dry air that produces deserts. The Devonian period was preceded by the Silurian period, and at the end of the Silurian, about 416 million years ago, Europe, traveling westward, collided with North America. The colliding rocks were of similar density and were

The Tsunami of December 2004 Ocean waves are driven by the wind. They can be large and dangerous, but even in the fiercest storm they affect only the uppermost layer of water. Wavelength is the distance between one wave crest and the next, and a wind-driven wave moves the water to a depth equal to approximately half the wavelength. Below that depth the water hardly stirs. A tsunami is different because a movement on the ocean floor generates it rather than wind blowing across the surface. Consequently, all the water moves, not merely the surface layer. A typical tsunami has a wavelength of about 160 miles (200 km) and a period—the time that elapses between two successive crests passing a fixed point—of 15–20 minutes. Tsunamis cross the open ocean at about 450 MPH (725 km/h) or faster, but they are seldom more than about 20 inches (50 cm) high. Sailors on ships far out at sea often fail to notice them. When a tsunami enters shallower coastal water, it slows, and as it does so its wavelength decreases. Farther out at sea the following waves are still moving at their original speed, so they catch up with the slower waves ahead of them, with the result that the wave height increases and a huge volume of water approaches the coast very fast. Tsunami is the Japanese word for “harbor wave.” In the eastern Indian Ocean the Indian tectonic plate pushes against the much smaller Burma microplate. The two plates move against one another at an average rate of about 2.5 inches (6 cm) a year, but the movement is not steady. Long periods with no movement at all are followed by sudden violent jerks. The tip of the Indo-

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nesian island of Sumatra and the Indian Andaman and Nicobar Islands are carried on the Burma microplate. At 00:59 hours Universal Time on December 26, 2004, the edge of the Indian plate slid beneath the Burma microplate, releasing tension on the Burma microplate that caused this plate to spring upward by about 33 feet (10 m) along a rupture nearly 1,000 miles (1,600 km) long. The ocean floor moved approximately 50 feet (15 m) in the direction of Indonesia. (Maps of the region are being redrawn.) The first movement occurred to the west of the tip of Sumatra and was followed within seconds by two further movements to the north. The initial earthquake was followed by a series of aftershocks, many of them very powerful. The event released a huge amount of energy as an earthquake measured at magnitude 9.3 on the Richter scale, and the earthquake triggered a tsunami 30–40 inches (80–100 cm) high that moved outward at about 500–560 MPH (800–900 km/h). The tsunami reached the tip of Sumatra 30 minutes later and arrived at the eastern coast of Sri Lanka two hours after the earthquake. After three and a half hours the wave reached the Maldive Islands. Eventually, the tsunami reached Madagascar and Africa, causing serious damage in Somalia, 5,600 miles (9,000 km) from the earthquake epicenter, and 14 and a half hours after the earthquake it produced waves 8.5 feet (2.6 m) higher than usual on the coast of Mexico. When it reached coasts around the Indian Ocean, the tsunami produced waves more than 33 feet (10 m) high that swept inland, causing terrible devastation. More than 283,000 persons lost their lives.

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Geology of Deserts crumpled into a mountain range extending from New York to Norway. Further crumpling then produced another range stretching from southwestern England to central Europe. An episode of mountain building is called an orogeny, or orogenesis (from the Greek oros, “mountain,” and gen-, “be produced”). This late-Silurian episode is known as the Caledonian orogeny. At once the newly raised mountains began eroding. Sand washed into rivers and was carried toward the sea across desert lands where iron in the sand was oxidized. This is how the cliffs and fields of South Devon acquired their attractive color. It shows that what are deserts today are not necessarily doomed to remain so for ever, but nor are lush, fertile lands immune from changes that could reduce them to arid wilderness.

THE FORMATION, DEVELOPMENT, AND AGING OF SOILS

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From the moment rocks are exposed to the air they start to erode, to wear away. Picture a range of mountains about 10,000 feet (3,050 m) high. If these are worn away at a rate of one-tenth of an inch (about 0.3 cm) a year during a human lifetime of about 75 years, their average height will have been reduced by about 7.5 inches (19 cm). No one would notice such a small amount of erosion, but compared with the life span of the Earth, which was formed about 4,600 million years ago, 75 years is but an instant. At this rate of erosion it would take a little more than 1 million years for the mountains to be worn away altogether. The landscape they once dominated would be a level plain. Yet even 1 million years is a very short time in the history of the Earth. Compare its 4,600 million years to a human life span of 75 years, and 1 million years is equivalent to slightly less than six days. That erosion, caused by sun, wind, and water, is called weathering. Contributing to it are also chemical reactions between minerals in rocks and substances, mainly acids, present in water. All rain and snow is naturally slightly acid because carbon dioxide, sulfur dioxide, and oxides of nitrogen dissolve into it, producing acid solutions. Some of the compounds present in rocks are soluble in water, so they dissolve to form solutions that engage in reactions with other mineral constituents. These processes are known as chemical weathering, and they form part of the overall weathering. Sunshine weakens rocks, especially in warm climates. During the day the rocks are heated by the sun, sometimes so strongly it can be painful to walk across them with bare feet. Rocks are not very good conductors of heat, however, so it is only the surface layer that grows hot. Inside, the rock

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remains cool. As it warms, the outer layer of rock expands, but the cool, inner part of the rock remains the same size. At night the rock cools again and contracts. Repeated day after day, expansion and contraction produce tiny cracks in the rock and cause flakes to separate and fall away. In colder climates water freezes in winter. This also shatters rock. Liquid water penetrates all the small fissures in a rock and moves slowly through them until it reaches hollows inside the rock where ice has formed. The water freezes onto the ice, forming lenses of ice. Chemical reactions between the ice and minerals in the rock alter the structure of the rock. Flakes detach and are washed out of the rock when the ice melts. At the same time, removing flakes enlarges the hollows, and they grow larger with each successive winter until entire sections of rock break away. The detached fragments are washed and blown by water and wind. They are rolled along the ground, tumble down hillsides, and are thrown against each other and against solid rock surfaces, knocking off yet more fragments, until they have been shattered, ground, and battered to grains, some so small as to be barely visible to the naked eye.

Living on Bare Rock Among the chemical compounds that are released from rocks by the processes of weathering, some can nourish bacteria. Bacterial spores carried by the wind fall onto the rock, and bacterial colonies establish themselves in sheltered crevices, where they are protected from direct exposure to heat, wind, and rain. The bacteria absorb substances from the rock and water, their own cells forming a layer, thin but rich in nutrients, on which other organisms can grow. Lichens arrive. A lichen is a kind of double organism. It consists of a fungus that lives in intimate association with an alga—a single-celled photosynthetic organism (see “Photosynthesis” on pages 96–101), or cyanobacterium, a type of photosynthesizing bacterium. The fungal partner (called the mycobiont) absorbs mineral nutrients from the rock, and the alga or cyanobacterium (the phytobiont) carries out photosynthesis, the process in which sunlight provides the energy to drive reactions that produce sugars from water and carbon dioxide. Lichens grow across rock surfaces, and as their cells die and are renewed opportunities begin to appear for other organisms. Perhaps a moss will arrive. This is a more complex plant, and beneath the green mat it presents to the world the moss forms a layer consisting of mineral grains, decaying plant material, and organic acids. At first the layer is thin, but as it fills small depressions in the surface it thickens until finally a wind-borne seed lands and is able to germinate. A blade of grass appears, or a tiny herb, and when that dies its own tissues decompose and join the layer of organic matter. One blade of grass is joined by another and then another.

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More and different herbs arrive until what had once been bare rock bears patches of vegetation that spread slowly to cover wider areas of rock surface and eventually to join as a single, continuous covering. Beneath the plants there is now a thin layer of soil. Soil consists of mineral grains mixed with organic material in varying stages of decomposition. It is made by the combined actions of physical and chemical weathering and biological activity.

How Soil Forms At this stage there is only a thin surface layer of soil. Soil formation—the technical term is pedogenesis—has only just begun, and the soil is in its infancy. Over the years and centuries that follow, generation after generation of plants will contribute to its further development. Animals will arrive to feed on plant material and each other. Some of them will live on the surface or above it, among the plants. Many will live below the surface, in the soil itself, where they will also help with the formation of the soil. Tunnels made as they move through the soil will allow air and water to circulate. By feeding on dead plant and animal material they will break large pieces into small fragments, and their own wastes and dead bodies will provide food for fungi and bacteria. These will complete the processes of decomposition, converting large, complex organic molecules into simple compounds with molecules small enough to be absorbed by plant roots. As life proliferates in and upon the soil, the soil grows deeper. No longer a mere film clinging to bare rock, it develops distinct layers. A trench cut vertically through the soil all the way to the underlying rock reveals the layers, distinguishable by their color and general appearance. A vertical cut of this kind is called a soil profile, and the layers are called soil horizons. Soils vary according to the kind of rocks from which they develop, the kind of plants that grow in them, and their age. A young soil that has only recently started to form will be thin and will have no horizons. A mature soil may have a number of horizons. Scientists identify soil horizons by letters and numbers that represent subdivisions of the two or more main horizons found in most soils. The illustration shows how this is done. The O horizons consist of twigs, leaves, and other plant and animal material that is lying at or just below the surface. Material lying on the surface is often called litter, so the O1 horizon is sometimes called the L layer, or litter layer. Where the O1 designation is used, the O2 horizon is called the F layer if the organic material can still be recognized and the H layer if it has decomposed into a mass of unidentifiable matter. At the bottom of the O2 horizon the organic material is partly decomposed: Beneath the leaves lying on the ground in a forest are partly rotted leaves. Organic matter continues to decompose in the A horizons, and as it does so the soluble chemical compounds that are released drain downward. The loss of com-

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O1

leaves, twigs, organic debris

O2

partly decomposed organic debris

A1

dark layer, into which organic compounds have drained

A2

pale layer from which organic compounds have drained

A3

transitional layer

B1

transitional layer

B2

dark layer in which organic compounds accumulate

B3

transitional layer

C

partly weathered parent material

R

bedrock © Infobase Publishing

A soil profile. This is an idealized profile; not every soil has all the horizons shown here.

pounds by vertical draining is called eluviation, so together the A horizons constitute the eluvial zone. The eluvial zone is also what farmers and gardeners call the topsoil.

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Geology of Deserts Beneath the eluvial zone lies the illuvial zone, the region into which the eluviated compounds drain and where they accumulate. This is also called the subsoil, and it constitutes the B horizons, where the soil consists mainly of mineral particles. Below this region is the partly weathered mineral material from which the overlying soil has developed— the soil’s parent material. The bedrock itself is sometimes included as well and called the R horizon. Additional information about horizons can be added to these designations, as subscripted lower-case abbreviations. Ap, for example, describes an A horizon that has been disturbed by plowing, and B3ca is a B3 horizon that is enriched with calcium carbonate.

How Soils Age This full complement of horizons develops slowly as a soil matures and is present in what soil scientists call a virile soil. As the soil ages further, material in the C horizon becomes increasingly weathered. Soluble compounds released from it by weathering move upward and are absorbed by plant roots, but more and more are lost in water that drains out of the soil. Eventually, flowing water transports the plant nutrients to the sea. In the senile stage to which a virile soil progresses, only the less soluble mineral nutrients remain, and in its final stage anything that can dissolve has dissolved. The soil is then very deep but very infertile, even though it may continue to support luxurious plant growth. This is possible where the nutrients are held mainly in the living plants themselves, their roots absorbing nutrients as fast as the decomposition of organic material releases them. Ancient, exhausted soils of this kind are widespread in the humid tropics, where nevertheless they support the rain forests. Soils in different places have developed from different parent material, have supported different types of vegetation, and are often at different stages in their development. There are so many different kinds of soil that soil scientists have struggled long and hard to classify them. Today there are standard classification schemes that allow scientists to describe particular soils unambiguously (see the sidebar).

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Soils result from the weathering of rock, in other words from the action of sunshine, wind, and water. How long it takes for a mature soil to develop, with a full complement of horizons, varies greatly from place to place. The soils over much of the northern United States and northern Europe have developed since the end of the last glaciation (see “When Northern America, Europe, and Asia Were

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How Soils Are Classified Farmers have always known that soils vary. There are good soils and poor soils, heavy soils containing a large proportion of clay, sandy soils that dry out rapidly, and light soils rich in loam that retain moisture and nutrients. In the latter part of the 19th century Russian scientists were the first to attempt to classify soils. They thought that the differences between soils were due to the nature of the parent material—the underlying rock—and the climate. They divided soils into three broad classes and gave soils Russian names that are still widely used, such as chernozem, solonchak, and podzol. American soil scientists were also working on the problem, and by the 1940s their work was more advanced than that of their Russian colleagues. By 1975 scientists at the U.S. Department of Agriculture had devised a classification they called soil taxonomy. It divides soils into 12 main groups, called orders. The orders are divided into 64 suborders, and the suborders are divided into groups, subgroups, families, and soil series, with six phases in each series. The classification is based on the physical and chemical properties of the various levels, or horizons, that make up a vertical cross-section, or profile, through a soil. These were called diagnostic horizons. National classifications are often very effective in describing the soils within their boundaries, but there was a need for an international classification. In 1961 representatives from the Food and Agriculture Organization (FAO) of the United Nations, the United Nations Educational, Scientific, and Cultural Organization (UNESCO), and the International Society of Soil Science (ISS) met to discuss preparing one. The project was completed in 1974. Like the soil taxonomy, it was based on diagnostic horizons. It divided soils into 26 major groups, subdivided into 106 soil units. The classification was updated in 1988 and has been amended several times since. It now comprises 30 reference soil groups and 170 possible subunits.

Cold Deserts” on pages 45–49). Moving glaciers scour away the soil, so when the ice retreats bare rock and gravel are exposed. These mature soils are, therefore, no more than about 10,000 years old, because that is the length of time that has elapsed since the glacial retreat.

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Obviously, this is a maximum time that takes no account of when the soils first covered the surface and supported deep-rooting plants. There are mature soils in Alaska that are known to be about 1,000 years old, and in some circumstances soils can develop much more quickly than this. The reclamation of the huge piles of waste produced by mining provides a good example. Heaps of waste from the mining and processing of china clay, or kaolin, consist mainly of mineral grains, material that is very deficient in the soluble nutrient compounds needed by plants. Using a machine resembling a water cannon, the heaps have been reclaimed by spraying onto them a mixture of water, fibrous organic material to bind the mineral particles together, and plant seeds, followed by an application of fertilizer. Within a few years the heaps were covered with vegetation, and soil was starting to develop, although no horizons had yet formed. With a little help from added fertilizer, soil was produced on the waste heaps by plants and other living organisms. They were able to colonize and root into material that was already finely fragmented. Fine fragmentation is ordinarily the result of weathering, so the waste heaps consisted of the equivalent of weathered rock. Soils that began forming as the last ice age drew to its close were able to do so because rock was exposed to the forces of weathering.

Water and Warmth Weathering and biological colonization must take place if soils are to develop, but they can do so only if certain criteria are met. Water must be available. Indeed, it should be abundant. The reactions that cause chemical weathering take place in aqueous (water) solutions. It is water that transports the compounds from which plants derive nourishment, and it is in solution that those compounds enter plant roots. Plants also need water to transport nutrients within their own tissues and for their own metabolic purposes (see “Transpiration and Why Plants Need Water” on pages 101–103). Living organisms also require warmth. Many are able to survive periods of extreme cold, but they do so by slowing their life processes to the barest minimum. In Antarctica there are species of moss that continue to photosynthesize at temperatures as low as 14°F (-10°C) and lichens that do so down to 1.4°F (-17°C). Most plants, however, cannot photosynthesize at temperatures below about 21°F (-6°C), and the chemical reactions proceed very slowly at that temperature. Between about 32°F (0°C) and 95°F (35°C) the rate of photosynthesis approximately doubles for every 18°F (10°C) rise in temperature. What is true for photosynthesis also obtains for most of the biochemical reactions on which living organisms depend, although there are many variations and this is very much a generalization. It is true, though, that falling

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temperature slows life processes, and, up to a limit, rising temperature accelerates them.

Polar Soils Thick ice sheets cover most of the ground in polar regions, and there is no soil at all. Beyond the edge of the permanent ice of Antarctica there are ice-free areas where bare rock and gravel support a few lichens, liverworts, and other small plants that survive in places sheltered from the wind. They remain dormant through most of the year and add new growth during the few weeks in summer when the temperature rises above freezing. These ice-free areas are known as dry valleys, or oases, and their total area amounts to about 2,200 square miles (5,700 km2). The exposed rocks are certainly exposed to physical weathering, but soil development has barely begun. The low temperature severely restricts biological activity, and precipitation is very low. Despite being called oases, they are extremely cold, dry, inhospitable places. The low temperatures and extreme dryness inhibit both plant growth and chemical weathering. Except for the outermost tip of the Antarctic Peninsula, all of Antarctica lies within the Antarctic Circle (66.5°S), and the continent is surrounded completely by the Southern Ocean. From the tip of the peninsula it is approximately 600 miles (965 km) to Tierra del Fuego, the southernmost point of South America, which is the nearest continent. Conditions around the edge of the Arctic Circle (66.5°N) are quite different. The South Pole is located near the center of a vast continent; the North Pole lies near the center of the Arctic Ocean, a sea, albeit a sea much of which is permanently frozen. Instead of the land being surrounded by ocean as it is in the south, in the north it is the ocean that is surrounded by land. The ocean helps prevent arctic temperatures from falling as low as those in Antarctica (see “Why Antarctica Is Colder Than the Arctic” on page 38), but there can be no soil around the North Pole because there is no land. Away from the region of permanent ice, in northern Canada, Alaska, Scandinavia, and Siberia, there is a belt of tundra vegetation bordered by the sea or permanent ice to the north and by the coniferous boreal forest, or taiga, to the south.

Tundra Soil The word tundra comes originally from tunturi, a Lappish or Finnish word that describes a hill with no trees. In fact, there are a few trees on the tundra, including black spruce (Picea mariana) in North America and dahurian larch (Larix dahurica) in northeastern Siberia. They are able to grow because their roots are quite shallow, extending to the sides rather than vertically. Most of the woody tundra plants are no bigger than shrubs, however. In places

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Geology of Deserts dwarf willow (Salix species) forms forests that are only knee high. Dwarf birch (Betula nana) and dwarf juniper (Juniperus sibirica) are common, growing to a height of about 12 inches (30 cm). There are also a few flowering herbs and sedges, but the most abundant plants are lichens and mosses. It, too, is a dry desert. The tundra covers a vast area— amounting to 15 percent of the land area of Russia, for example—and the climate is not the same everywhere. In western Eurasia, where the North Atlantic Drift, a branch of the Gulf Stream (see “Ocean Gyres” on pages 87–88), washes the coast, the climate is wetter and milder than it is farther east. Precipitation everywhere is between eight inches and 12 inches (200–300 mm) a year, and the ground is not covered by snow. Heavy falls are uncommon, and the perpetual wind quickly blows away any snow that does fall. Winds can reach 90 MPH (145 km/h). Winter temperatures often fall to -58°F (-50°C) in central and eastern Siberia, and the average temperature in summer is only about 40°F (4°C).

Permafrost Surprisingly, perhaps, considering the low annual precipitation, much of the level, low-lying tundra is swampy. Water is trapped at the surface because the water held below ground is permanently frozen. This frozen material beneath the surface may be sand, gravel, clay, or rock as well as soil. It is known as permafrost. Permafrost forms in ground that remains frozen through at least two winters and the intervening summer. Where it exists, it forms an impermeable layer through which surface water cannot drain, and in places the permafrost layer is up to 3,000 feet (900 m) thick. Permafrost covers nearly 3 million square miles (7.8 km2). Above the permafrost, in the active layer, the ground thaws during the brief summer. Plant roots can absorb water only as liquid. Ice is useless to them, but the trapped surface water becomes available during the thaw. So in the tundra it is the low temperature that imposes the main restriction on biological activity, not the availability of water. In some places the active layer is up to 16 inches (40 cm) deep, but in others it is no more than four inches (10 cm) thick. The summer thaw allows plants to grow, but the low soil temperature greatly slows the rate at which organic material decomposes. Consequently, tundra soil usually contains a large amount of organic matter in the shallow surface layer. No soil horizons form in it, and it is classified as a soil at the very beginning of its development. Pedologists (soil scientists) using the U.S. Department of Agriculture soil taxonomy scheme classify tundra soils in the order Inceptisols, a name derived from the Latin inceptum, which means “beginning.” In the international classification used by the Food and Agriculture Organization (FAO) of the United Nations, they are Cryosols, from the Greek kruos

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meaning “frost”; sol is an abbreviation of solum, which is Latin for “ground.”

Soils of Hot Deserts Soils of hot deserts are also placed in an order with a descriptive name. They are classified as Aridisols—arid soils—in the soil taxonomy and Arenosols or Regosols in the FAO scheme. Aridisols are the most widespread of all soil types, occupying more than 19 percent of the land surface of the world, nearly 10 million square miles (26 million km2). They are very different from Inceptisols (Cryosols), because it is lack of water that restricts plant growth in hot deserts, not extremes of temperature. There is vegetation, of course, but it is sparse, and the soil contains little organic matter. Deserts are windy places where soil, dry as dust, is blown from place to place, burying such plant debris as there is. There are no soil horizons. Unlike the Inceptisols of the tundra, Aridisols are not beyond redemption. In many places they contain plant nutrients, so if water is supplied it is possible to cultivate them (see “Halting the Spread of Deserts and New Crops for Dry Climates” on pages 255–260). With sustained cultivation they will eventually mature into soils that are more like those of the savanna grasslands, classified as Mollisols in the soil taxonomy. It is fairly easy to remedy a lack of water, but there is no practical way to raise the average temperature, although this will happen if the present global warming is sustained (see “Climate Change and the Future for Deserts” on pages 240–246). This makes it impossible to cultivate the Inceptisols at present, but if they were to thaw due to global warming over large areas they would turn into equally uncultivable swamp and marsh. Such wetlands are of great value to wildlife and probably society would decide to protect them. If it was decided to cultivate them—probably to grow coniferous forests—they would first have to be drained to remove the surplus water, and over so large an area this would be prodigiously expensive.

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ERGS AND SAND DUNES

Think of a desert, and the image most likely to spring to mind is of high sand dunes stretching endlessly in all directions. It is a landscape devoid of life, empty, desolate, and still, on which the Sun beats down mercilessly from a clear blue sky. Deserts are not like this everywhere. Many are mainly mountainous, and others are rocky. In parts of the southwestern United States there are large areas in which movements in the Earth’s crust have raised some huge blocks of rock and lowered others, forming what is called a basin-and-range topography. There are similar landscapes in the Gobi and Kalahari Deserts. Elsewhere, the surface is rocky, consisting of exposed horizontal bedrock, in places partly covered by boulders or smaller rocks. This surface is widespread in the Sahara and is known by its Arabic name of hammada.

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There is sand, of course, and where it is plentiful it does form dunes. It is in the Sahara and Arabian Desert that sand covers the greatest area (the tallest dunes, up to 1,640 feet (500 m) high, are in the Badain Jaran Desert in northern China). For much of the Carboniferous period (359.2–299 million years ago) what is now the Sahara formed the bed of a sea, and coal formed around the edges of that sea. Some of the sand of the seabed was compressed into sandstone during the Carboniferous. Parts of the ancient seabed were covered by the sea again later, about 70 million years ago. When the sea retreated for the second time, erosion wore away at the sedimentary seabed rocks. Sand and dust are the products of that erosion. Sand is abundant in the Sahara.

Draas and Ergs—Seas of Sand The biggest dunes form ridges or chains more than 1,000 feet (300 m) high and from about half a mile to three miles (0.8–5 km) apart. Landscape features produced by the wind are known as bedforms. Each of these very large desert bedforms is called a draa. A draa is so big that there are smaller dunes on its surface, and there are ripples—tiny dunes— across the surfaces of the dunes. Draas move by up to about two inches (5 cm) a year. Dunes also move, and where the sand covers a large area, as it does in the Sahara and in the Rub’ al-Khali, the Empty Quarter of Arabia, it resembles a sea, with the dunes as waves. A “sand sea” is called an erg, which is the Arabic word for “dunes” (the singular is erag). One of the biggest ergs, the Grand Erg Oriental (Great Eastern Erg) in Algeria, extends for about 74,000 square miles (192,000 km2) and is

separated by about 60 miles (100 km) of gravel desert from the smaller Grand Erg Occidental (Great Western Erg). The biggest of all ergs is the Rub’ al-Khali, which covers approximately 216,160 square miles (560,000 km2). It is the wind that builds sand dunes, just as it is the wind that pushes water at the sea surface into waves. Although the image of a desert as sandy is partially correct, it is quite wrong to suppose it is a place of stillness. Deserts are windy places, and the wind seldom abates.

How Dunes Form A light wind is enough to lift sand grains, provided they are dry and not sticking to one another. Whenever the wind speed exceeds 12 MPH (19 km/h)—a moderate breeze—grains start moving. Along city streets a wind of that strength blows small pieces of paper and loose leaves about. The wind is strong enough to lift sand grains, but it cannot keep them airborne for more than a few seconds. They quickly fall again. Some collect in a particular place, making a small mound, and then wind eddies over the sand surface lift and drop grains so they move in little jumps up one side of the mound, as shown in the illustration below. What happens next is determined by gravity. The height of a mound of dry sand is limited by the angle of the sides.

How sand dunes form. The wind blows sand grains up the slope, forming a sand pile. Grains fall from the top of the pile down the much steeper slope that is sheltered from the wind.

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It is impossible for dry sand to form a mound with sides steeper than about 35°. At that angle additional grains simply roll down the slope to the bottom. It is called, appropriately, the angle of repose for dry sand. Sand grains are driven by the wind up one side of the mound. The other side is sheltered from the wind, so on the downwind side gravity is the only force acting on the grains. As grains reach the top of the mound they roll down the other side. Sand accumulates at the bottom of that side until the pile is level with the top of the mound. The side away from the wind has a slope of about 35°. That is as steep as it is possible for the slope to be. Consequently, a sand dune usually has a long, gentle slope on the upwind side—the side against which the wind blows— and a slope of about 35° on the other, sheltered downwind side, and the entire dune moves in the direction of the wind. The mechanism is the same regardless of the height of the dune. Dunes grow bigger by accumulating sand on the shallow-slope side, and the angle of slope on the steeper side is always close to the angle of repose. Of course, this supposes the wind blows mainly from a particular direction. That is a reasonable supposition. Desert winds usually blow from one direction more than from others, although there are exceptions and even a wind that is generally from one quarter may well vary to either side of that direction.

Types of Sand Dune Wind blows the desert sand into dunes, and it also shapes the dunes, different winds acting on varying amounts of

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Types of sand dunes

sand to produce particular types of dune. The illustration shows some of the commonest dune shapes. If there is a virtually limitless supply of sand and the wind blows almost always from a particular direction, transverse dunes will form. These are long, with the gradual slope on the side facing the wind and the line of the dunes at right angles to the wind direction. If the wind blows steadily from the south, for example, the transverse dunes will be aligned east to west. Transverse dunes can be up to 60 miles (96 km) long and up to 300 feet (90 m) high, and they move downwind at up to about 80 feet (25 m) a year. Transverse dunes are not always straight. They can also develop wavy crests, so the face of the dune faces alternately into and away from the wind direction. Wavy dunes of this type are called aklé dunes. The prevailing wind is the most frequent direction from which the wind blows at a particular place, but that does not mean the wind never blows from any other direction. For instance, a prevailing southerly wind may blow sometimes from the south-southwest, sometimes from the south, and sometimes from the south-southeast. Provided there is an abundant supply of sand, this wind pattern will produce longitudinal dunes. The sand is blown first from one side then from the other, so longitudinal dunes are aligned in the average direction of the wind—in this case from south to north—and the slope is at the angle of

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A longitudinal sand dune in Utah (Edenpics.com)

repose on both sides. Longitudinal dunes have long, sharp, sinuous crests. In some places, especially in the western Sahara, the biggest of them are known as seif dunes. Seif dunes can stretch for hundreds of miles, and their curved shapes resemble sword blades; seif is from sayf, the Arabic word for “sword.” Where the supply of sand is limited, winds blowing from either side of a predominant direction will blow the sand into a crescent shape, with the horns pointing downwind. If the crescent is narrow, so the horns are close together, this is called a parabolic dune—the shape is that of a parabola. If the horns are fairly wide apart, so the crescent is open, it is a barchan dune. Barchans are up to about 100 feet (30 m) high, and the tips of their horns are up to 1,200 feet (370 m) apart. Where there is enough sand, adjacent barchans may join to form an aklé dune, and sometimes one limb of a barchan can be blown away altogether, leaving the remaining limb as an isolated seif dune. Winds may not blow mainly from one direction. In some places there is no prevailing wind. The wind blows from the west as often as from the east, south, and north. Such variable winds blow the sand into star-shaped, or stellar, dunes. Dunes of this shape can also form where other dunes intersect. A dune of this intersecting type is called a rhourd. Intersecting draas also form rhourds. It seems there is a name, in English, Arabic, or French, for every shape possible for a large heap of sand to adopt. This has generated a list of names that may appear bewildering. Order can be brought to it, however, because these

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dune shapes resolve themselves into just two or three principal types, of which the others are variants. Dunes may be long, more or less straight, and arranged parallel to each other. These are the transverse, longitudinal, and seif dunes, and they constitute one principal type, or two types if transverse and longitudinal dunes are considered separately. Alternatively, dunes may be crescent shaped. These are of the second (or third) type and include barchans and parabolic dunes. Stellar dunes and rhourds are derived from these basic shapes.

DESERT PAVEMENT, EROSION, AND VARNISH

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Despite the ergs—their vast seas of sand—sand dunes cover no more than about 30 percent of the Arabian Desert and 11 percent of the Sahara. There are dunes in the North American desert, but covering only about 2 percent of the total area. Dunes used to be more widespread. During ice ages (see “Not One Ice Age but Many” on pages 46–47), when climates are drier, deserts extend farther and dunes are more widespread. Traces of these fossil dunes show that at the time of the last glacial maximum, around 18,000 years ago, there were dunes in West Africa more than 350 miles (563 km) south of the edge of the present Sahara. Sandy deserts once occupied what are now savanna grasslands in Venezuela and northern Brazil.

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Geology of Deserts Although sand may be abundant, however, the supply is not limitless, and wind that moves it also removes it. Some sand moves by rolling along the surface. This is called creep. Deflation is the lifting of grains by the wind and their removal downwind in a series of short hops. Smaller particles, of silt and clay, are lifted more easily and carried higher and farther. They can cause dust storms (see “Dust Storms and Sandstorms” on pages 94–95). Continued for long enough, these eolian (wind) processes remove all the small particles from one place and pile them up in another. Fluvial processes, those involving transport by water, also remove small mineral particles. Rain is infrequent in deserts, but when it falls it often does so torrentially. Water rushes across the surface, washing clay, silt, and sand particles into ephemeral streams that carry them away, eventually depositing them where the stream slows down as it crosses level ground. Material transported by water is called alluvium, and when the stream dries the transported particles remain as an alluvial fan. The wind may then continue to move these deposits. It is the presence of features that look very much like alluvial fans that leads planetary geologists to conclude that water once flowed across the surface of Mars. The effect of wind and water is to remove the smaller material, leaving a surface of bare rock or one covered by stones that are too big and heavy to be transported. These remains constitute what is known as a lag deposit, and there are several types. A stony desert surface is sometimes called a reg, from Greek rhēgos, meaning “blanket,” but reg is used more strictly to describe a level or slightly sloping gravel surface made from rounded pebbles. Where gravel is mixed with sand, the surface is called serir.

Desert Pavement A rocky surface, including a surface consisting of the exposed bedrock, is called desert pavement, or hammada. Exposed bedrock is fairly rare, but desert pavement made from a covering of closely packed stones is one of the most widespread desert surfaces. It occurs widely in the North American deserts. Desert pavement sometimes forms as a result of deflation. Wind removes the sand and gravel particles, leaving behind the larger stones. These settle onto the material beneath them until the surface is entirely covered by large stones. Once this process is complete, the surface stones rest on finer material, protecting it from further erosion. If the pavement is the result of eolian processes, the material beneath it should be well mixed. As the diagram of desert pavement shows, however, the surface stones often lie above a layer of sand, silt, or a mixture of the two and below that a layer of gravel. There are no stones mixed with the sand or silt, and this layer is usually from about one inch

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Desert pavement. The diagram shows a profile, with silt beneath the pavement and gravel beneath the silt.

(2.5 cm) to one foot (30 cm) deep. This sorting of particles according to size cannot have been due to the action of wind. A different process was at work. At one time the stones were mixed with the fine particles, and over a long period they have risen to the surface. It seems strange to think that big, heavy stones can float upward through a mass of much finer particles, but this is what happens. It is easily demonstrated by placing a few dry pebbles at the bottom of a jar, half filling the jar with very dry sand, then shaking the jar for some time. The pebbles will rise to the top because the small particles fall around the bigger ones and fill any spaces that appear beneath them, then pack together so that the pebbles cannot sink downward. Little by little this pushes the pebbles to the top. No one shakes deserts, of course, but occasionally the ground may be soaked with water. In some deserts water below the surface may freeze. Some types of clay expand when they are soaked, and water expands when it freezes. Expansion pushes upward, producing a bulge at the surface. When the clay dries or the ice melts, the bulge subsides, and as it does so the small grains fall or are washed beneath the larger stones, just as happens in the shaken jar. Given enough time—and the surface of a desert has thousands of years in which to form—the stones accumulate at the surface to form a desert pavement.

Ventifacts Sand blasting is an industrial process that is used to clean stone and metal surfaces by driving a high-velocity stream of sand grains at them from a nozzle. Billions of sand grains striking an object at high velocity are extremely

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abrasive. Desert rocks are subjected to a natural form of sand blasting, and, not surprisingly, they erode rapidly but also unevenly. The softest rocks are removed first, leaving the more resistant rocks behind, like the stone and metal that remain after sand blasting has removed the dirt. It is not only large rock formations that experience this treatment; so do smaller rocks lying on the surface. The effect is to polish the exposed faces. Stones that are polished in this way are called ventifacts, from the Latin ventus, meaning “wind,” and factum, the past participle of the verb facere, “to make.” A stone with a single polished facet is called an einkanter (German for “one edge”), and one with three facets is a dreikanter. The dreikanter form is believed to develop as the wind polishes one facet but at the same time removes sand from beneath one side of the stone until it topples. The wind then repeats the process on a second and then a third facet.

penetrated the crust and then cooled and solidified. Erosion often levels the surface of a landscape formed in this way, producing an extensive plain. Further erosion then reduces the plain, removing the softer sedimentary rock until isolated fragments of the harder rock are left standing. They look like steep-sided hills with large, flat tops. Such a fragment of plain is called a mesa, the Spanish word for “plateau.” As erosion continues a mesa grows gradually smaller. Eventually all that remains of it is a flat-topped tower of rock. This is called a butte, an old word meaning hill or mound. Where a more substantial mass of a hard igneous rock such as granite has been intruded into softer sedimentary rock, the softer rock may be eroded away, leaving the granite core. This forms a large hill standing alone on the plain and is called by the German name inselberg, which means “island mountain.”

Mesas, Buttes, and Inselbergs

Salts can also cause weathering in areas where these are plentiful in the rocks or near the ground surface. When it rains salts dissolve, and salt solutions fill tiny cracks and crevices on the surface of rocks. The high temperature and almost constant wind cause water to evaporate very rapidly, and the salts are left behind. As the water evaporates, the salt solution becomes increasingly concentrated until the remaining water is fully saturated. When the water is unable to hold all the salt molecules, these begin to form crystals, as will happen in a shallow dish of very salty water that is left exposed to the air in a warm room. The salts expand as they recrystallize, splitting rocks. Crystalline salts also expand and contract in response to changes in temperature to a much greater extent than the rocks holding them. This type of weathering, called salt weathering, can produce single rocks standing on top of other rocks to which they are joined by a narrow column of rock. These are known as pedestal rocks, and the narrow column begins as an alcove eroded from around the rock along a shelf where water can accumulate. Salts dissolve in the water, then crystallize on the rock surface when the water evaporates. This also separates sand grains from the rock. Grains and crystals are then blown away by the wind. Between them, salt weathering and wind erosion also produce cuplike hollows, water pockets in which water collects as it does in the formation of alcoves, resulting in honeycombs of holes, or pits, in vertical or nearly vertical rock faces. This can leave the rock looking like Swiss cheese. Large holes are called tafoni and small ones alveoles. The facing drawing illustrates some of these features. On a much larger scale, some of the rivers that flow after desert rainstorms deliver water to shallow lakes that fill depressions in low-lying land. Water then evaporates rapidly from them, and as it does so the remaining water becomes increasingly saline. The lake may dry completely, leaving a surface salt deposit. Deposits resulting from the evaporation

Igneous rocks were once molten magma—igneous is from Latin ignis, meaning “fire.” As the magma cooled it solidified into rocks that are very hard. Sedimentary rocks, such as sandstone and limestone, form from particles that settled onto the sea floor, and they are much softer. Sediments form horizontal layers that are heated and compressed into solid rock, but during this process or later, movements in the underlying crustal rocks may bend, tilt, crumple, or even turn the layers completely upside down. There are many places, however, where sedimentary rocks have remained as approximately horizontal strata through which harder, igneous rocks have been intruded at some time in the past as magma from the Earth’s mantle

A line of mesas (NASA)

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Erosional features. The drawing shows how the actions of wind and water erode desert rocks into strange shapes.

varnishes are highly durable. Some are many thousands of years old.

Weathering Rind of water, called evaporite deposits, are sometimes extensive. They form salt deserts in which very little can survive. Salt lakes that periodically evaporate entirely can also be large. When it is full, the surface area of Lake Eyre, South Australia, is about 3,000 square miles (7,800 km2).

Desert Varnish As well as salt weathering, desert rocks are affected by other chemical processes. In some places rocks have a black or orange coating, called desert varnish. This patina is made from iron and manganese oxides and the clays that contain them. Oxides rich in manganese are black and those poor in manganese are orange. The patina is less than 0.04 inch (0.1 cm) thick. The oxides and clays that turn into desert varnish are carried by the wind and adhere to the surfaces they coat. Scientists believe microorganisms play an important part in the conversion of the coating into varnish. Once formed, the

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Rocks may also be covered in a thicker layer of red, orange, or yellow material. This resembles varnish but is called weathering rind, and it forms differently. The layer, which can be one inch (2.5 cm) thick or more, is produced by the weathering of the rock surface. As the surface layer of rock disappears, minerals beneath the surface are exposed to the air and occasionally to rain. In minerals that contain iron—and many do—the iron oxidizes. It is the oxides that give the rind its color.

Desert Rose Despite their covering of stones, gravel, or sand and their apparent similarity, desert soils vary from place to place. Some, called Gypsisols in the FAO soil classification, are rich in calcium sulfate (CaSO4). This may occur in a horizon within 40 inches (100 cm) of the surface or be concentrated in particular places at deeper levels, where more than 15 percent is in the form of hydrated calcium sulfate

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When Rain Does Not Reach the Ground Most of the water has returned to the air. It has evaporated, some of it during its fall from the base of a cloud, so it never reached the ground. Even in temperate latitudes virga is a fairly common sight. It looks like a thin, gray curtain below a cloud, and it consists of rain falling from the cloud and then evaporating in the dry air below the cloud base. Even when rain does reach the ground, some of the water will have evaporated during its descent. Cloud forms because air is saturated, compelling some of the water vapor it contains to condense into droplets. Outside the cloud the air is not saturated. If it were, it would be filled with cloud. The base of a cloud marks the height at which the air starts to be saturated. Consequently, rain falling from the cloud enters unsaturated air, and in unsaturated air liquid water starts to vaporize. A desert rose, formed naturally from minerals (Tony and Debbie Henderson)

(CaSO4.2H2O). Hydrated calcium sulfate is better known as the mineral gypsum—hence the name of this type of soil. Other soils, called Calcisols, are similar except that they contain calcium carbonate (CaCO3) rather than calcium sulfate or in addition to it. When water trickles through these soils, the calcium carbonate tends to form crystals of the mineral calcite, and gypsum also forms crystals. Mineral crystals form first on a particle of suitable size and shape but of any composition and then on the surfaces of crystals that formed earlier. As crystallization continues the mineral forms a solid mass. Calcite and gypsum that crystallize in desert soils often produce pebbles or larger stones, but sometimes they form masses with shapes resembling flower petals radiating from the center and overlapping one another. The beautiful, delicate result is known as a desert rose.

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DESERTS AND WATER

Sprinkle a few drops of water onto a dry sponge, and the water will quickly soak downward into the sponge. Before long the surface will be dry again. Leave the damp sponge in a dry place, and soon it will be completely dry once more. Place a dry sponge onto a wet surface, and water will rise into the sponge, leaving the underlying surface drier than it was before. Soils, even desert soils, behave in very much the same way. After it has rained the ground dries again quickly. The water has disappeared, but obviously water molecules cannot simply have vanished. They must have gone somewhere.

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How Fast Do Raindrops Fall? How much of the rain fails to reach the ground depends on the dryness of the air and the length of time the rain takes to pass through it. Drizzle, for example, consists of droplets less than 0.02 inch (0.05 cm) in diameter, and although desert rainfall more often takes the form of heavy storms, drizzle and light showers do sometimes occur. In a severe storm the raindrops may be about 0.12 inch (0.4 cm) in diameter. This is close to the upper size limit for a falling water drop. Drops larger than this break into two or more smaller drops. When a drop of water, or any other body, falls through the air, it will accelerate until the gravitational force pulling it toward the center of the Earth is balanced by the resistance to its fall caused by the frictional force exerted by the air. When this point is reached, the drop can fall no faster. It has reached its terminal velocity, and it will maintain this rate of fall until it strikes the ground. The terminal velocity of a falling body is related to its size and shape, because these are what determine the frictional resistance it experiences. Gravitational acceleration is a constant, symbolized by g and with a value of 32.12 feet per second per second (9.80665 m/s2), so in a vacuum all bodies accelerate at the same rate. A droplet of drizzle has a terminal velocity of about 8 MPH (3.5 m/s) and that of a large raindrop is about 20 MPH (9 m/s). In both cases the drops attain their terminal velocity after they have fallen just a few feet. If the cloud base is at, say, 800 feet (244 m), a droplet of drizzle will take about 70 seconds to reach the ground, and a large raindrop will take about 27 seconds. Both are losing water by evaporation during the whole of their fall, so it is much more likely that the drizzle will fail to reach the ground than that the big raindrops will. Not only does the drizzle have longer to evaporate, its droplets are much smaller.

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Geology of Deserts

Evaporation A water molecule consists of two hydrogen atoms (H2 or H + H) and one oxygen atom (O). The molecule is arranged with both hydrogen atoms on one side of the oxygen atom. The hydrogen side of the molecule carries a small positive electromagnetic charge, and the oxygen side carries a small negative charge. Molecules of this type are said to be polar (see the sidebar “Polar Molecules” on page 68), and in liquid water the molecules form small groups linked by hydrogen bonds between the O- of one molecule and the H+ of another. The drawing shows three water molecules linked by hydrogen bonds. Molecules of liquid water move freely in their small groups, and the more energy they possess, the faster they travel. At a water surface—the boundary between liquid water and air—water molecules are constantly breaking the hydrogen bonds holding them to their groups and escaping into the air. At the same time molecules of gaseous water—water vapor—present in the air are colliding and merging with the liquid, and forming hydrogen bonds with other molecules. If air in the boundary layer, about 0.04 inch (0.1 cm) thick, above the water surface is holding as many water molecules as it can, molecules escaping from the liquid will immediately be replaced by molecules entering the liquid. The molecules in the layer of air exert a vapor pressure on the liquid, and if that pressure is sufficient to prevent any net

Water molecule. The diagram shows how water molecules are linked by hydrogen bonds. The hydrogen and oxygen atoms in a water molecule are bound together because they share electrons. This leaves a small charge on either side of the molecule, so there is an attractive force between a hydrogen atom in one molecule and the oxygen atom in another.

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migration of molecules from the liquid to the air, the volume of liquid will remain constant. The vapor above the surface is then saturated, although it is more usual to talk of the air being saturated, and the vapor pressure is then known as the saturation vapor pressure. If the vapor pressure is less than the saturation vapor pressure, molecules are able to leave the liquid surface, pass through the boundary layer, and enter the air. As molecules continue to escape, the amount of remaining liquid will decrease. The water will have evaporated, or vaporized. If the vapor pressure is equal to the saturation vapor pressure, water will not evaporate because molecules will enter the liquid at the same rate as molecules leave it. The amount of water vapor air can hold is proportional to the air temperature: The warmer the air, the more water it can hold. A given volume of air at 85°F (29°C) can hold more than three times more water vapor than a similar volume of air at 50°F (10°C). This is why water evaporates rapidly into hot desert air.

Water Always Flows Downhill Big raindrops, falling at about 20 MPH (9 m/s or 32 km/h), strike the ground with considerable force. Small mineral particles, such as those of silt and clay, can be packed tightly together by the repeated impact of raindrops so they form an impervious cap over the surface. The rainwater then flows downhill across the impervious surface cap. It will find its way into a channel if there is one and otherwise, it will spread, dispersing and at the same time slowing, as it soaks into the ground. Gentler rain or drizzle falling onto finely particulate material soaks directly downward with little or no horizontal flow. Some desert soils contain significant proportions of silt and clay, acquired in the past when the climate was wetter, but desert winds tend to blow away the finer particles. Most desert soils are sandy, and even the heaviest rain will soak into sand because sand grains are too big and too irregular in shape to be packed together into a cap. On a desert pavement most or all of the water flows horizontally across the surface until it reaches material it can penetrate. Water that is flowing across the surface is also evaporating at the same time. Whether it flows horizontally or vertically, water always moves downhill.

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Groundwater and the Water Table It is gravitational force that makes water soak vertically downward into the ground. The water is continuing to fall, but more slowly than it fell through the air because its movement is impeded by the soil particles. The water flows through the tiny spaces between mineral grains. Its downward movement continues until it reaches a layer that

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it cannot penetrate because there are no spaces through which it can pass. Descending water accumulates at the top of this impermeable layer, filling all the spaces between particles and thus saturating the ground. Water in the saturated region is known as groundwater, and the upper margin of the saturated zone is the water table.

Except When It Moves Upward, by Capillarity Although water always flows downhill across open surfaces, it is able to move upward between the mineral particles in soil. There is a partly saturated region known as the capillary fringe immediately above the water table where water has moved upward from the groundwater and into the small spaces between soil particles. Capillary is from Latin capillus, meaning “hair,” and it describes a very fine tube or channel. Water is able to move upward through capillaries because of the shape of the water molecule. The water molecule is polar (see the sidebar below), but unlike most polar molecules

it consists of hydrogen bonded covalently with oxygen, a combination that allows it to form hydrogen bonds. While it is liquid, hydrogen bonds link water molecules into small groups that can move freely and slide past one another. Hydrogen bonds that link identical molecules can also form between water molecules and the molecules at solid surfaces. Where the liquid and solid meet, those hydrogen bonds draw the water upward along the solid surface, so the water surface slopes. If the water is held in a tube, its surface will be concave (curved downward), as shown by A in the facing drawing. This is not a stable configuration, however, because energy must be expended to sustain it. The most stable shape for a liquid—stable because it requires the least energy to maintain—is a sphere, so water in the confined space moves to adopt a spherical shape. The water surface becomes convex (curved upward), as shown in drawing B. The change in shape brings more water molecules close enough to the solid surface for hydrogen bonds to form, drawing the water a little farther upward. This once again produces a concave surface that gives way to a convex surface. The process continues, hydrogen bonding drawing water vertically upward through the confined space.

Polar Molecules An atom consists of a nucleus surrounded by a cloud of electrons occupying shells at varying distances from it. The nucleus of a hydrogen atom contains just one proton; all other atomic nuclei contain protons and also neutrons. A proton carries a positive electromagnetic charge, a neutron carries no charge (it is neutral), and an electron carries a negative charge that balances precisely the positive charge on a proton. Molecules are groups of atoms held together by chemical bonds. Ionic bonds occur when electrons move from one atom to another, leaving the donor atom with a positive charge, the recipient atom with a negative charge, and the two atoms bound together by the attraction of opposite charges. Covalent bonds occur when two atoms share electrons. Two hydrogen atoms form a hydrogen molecule by each sharing the other atom’s single electron, so that each hydrogen nucleus controls two electrons. When hydrogen molecules bond to oxygen atoms to form water, the outer electron shell of the oxygen atom gains control of the two hydrogen electrons, and each of the hydrogen atoms gains control of one oxygen electron. It often happens that one atom in a molecule attracts electrons more strongly than does the other;

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it is said to possess greater electronegativity. Electrons are held closer to the nucleus of the more strongly electronegative atom, making that end of the bond slightly electronegative and the other end slightly electropositive. A bond of this type is said to be polar, or a dipole, because it has two poles. If the bonds linking atoms in a molecule are polar but one partial charge is distributed evenly around the center carrying the opposite charge, the molecule itself is neutral overall. In certain molecules, however, the charges are not distributed evenly, so one side of the molecule is electropositive and the other side is electronegative. The molecule itself is then polar, or a dipole, as well as its bonds. Polar molecules attract one another, and dipole–dipole bonds form between them. Dipole– dipole bonds link molecules, not atoms. When hydrogen is bonded covalently to oxygen, nitrogen, or fluorine, all of which are very strongly electronegative, the molecule is highly polarized. A special kind of dipole–dipole bond then forms between the electropositive hydrogen of one molecule and the electronegative oxygen, nitrogen, or fluorine of the other. This is called a hydrogen bond, and it is stronger than other dipole–dipole bonds.

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Geology of Deserts A

B

© Infobase Publishing

Capillarity. A) Water is drawn up the sides of the tube by the hydrogen bonds between water molecules and molecules on the tube surface, forming a concave surface on the water. B) The concave shape is unstable, and water at the center moves upward to form a stable convex shape.

Water continues to rise until the weight of the water equals the strength of the hydrogen bonds holding the water to the sides of the tube or solid particles. The narrower the space, the less water it can contain, and, therefore, the less the water weighs and the higher it can rise before the hydrogen bonds can pull it no farther. Capillary refers to the confined space through which the water moves. The force causing it to move is called capillarity, or capillary attraction. Capillarity is the result of hydrogen bonding, and, consequently, it occurs wherever water is in contact with a solid surface. Water will rise up the surface only if the strength of the hydrogen bonds is greater than the gravitational force however. That is why water moves by capillarity through very narrow spaces, where the weight of water is low and there is a large area of solid surface in relation to the number of water molecules.

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AQUIFERS, OASES, AND WELLS

Rainwater that drains vertically and avoids evaporating immediately will sink through the soil fairly slowly. If there are plants growing where the rain falls, their roots will capture some of the water as it passes and return it to

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the air (see “Transpiration and Why Plants Need Water” on pages 101–103). Water that is not captured will move all the way down to the saturated zone and become part of the groundwater. In the world as a whole the water that is aboveground as rivers, lakes, and inland seas amounts to about 55,200 cubic miles (230,000 km3). Belowground there is about 2,016,000 cubic miles (8,398,000 km3) of freshwater. This volume vastly exceeds that of the surface water, although ice sheets and glaciers contain very much more water than surface liquid water and groundwater combined, and the oceans hold more than 97 percent of all the water on Earth. Groundwater is not static. It flows downhill, although it moves very slowly as it makes its way between mineral particles that are packed together fairly tightly. Its speed varies according to the porosity and permeability of the material through which it travels. Through most materials it moves at speeds ranging from a few feet per day to a few feet per month, but through clay it may travel no more than one foot (30 cm) in a century. Hydrologists (scientists who study the movement of water) classify groundwater movement as slow, moderate, or rapid. The table shows the wide differences between these.

Porous or Permeable? Porosity and permeability are not the same thing, although the words are often used interchangeably. Porosity is the

Rate of Groundwater Flow CLASS

RATE OF MOVEMENT (inches per hour)

(centimeters per hour)

Very slow

less than 0.05

less than 0.125

Slow

0.05–0.20

0.125–0.508

Moderately slow

0.20–0.80

0.508–2.032

Moderate

0.80–2.50

2.032–6.35

Moderately rapid

2.50–5.00

6.35–12.7

Rapid

5.00–10.00

12.7–25.4

Very rapid

more than 10.00

more than 25.4

Slow:

Moderate:

Rapid:

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proportion of a volume of material that consists of spaces, or pores. This can range from 0.5 percent, in the form of fractures in a hard rock such as granite, to 50 percent in some soils. It is simple to demonstrate that in a particulate material such as soil, porosity has little to do with the size of the particles. The same measured volume of gravel, sand, and fine soil is placed in separate jars, and sufficient water is added to saturate each material, so water lies on the surface. The volume of water added to each jar is a measure of the total pore space, the porosity of the material. Expressed as a percentage of the volume of each type of particle, the porosity is approximately the same for each material. This does not mean water will flow with equal ease through each material. The bigger the particles, the bigger the spaces between them, and water flows more easily through a few big spaces than it does through many small spaces, even though the total volume of space is the same. Permeability is a measure of the ease with which water moves through a material. This depends on both the porosity and the size of the pore spaces. The shape of particles also affects permeability. Sand grains are very angular, so they do not pack together very closely. This means the spaces between them are bigger than they are in other materials with similar porosity, such as clay. Clay particles are extremely small and flat, like tiny flakes, so they tend to stack on top of each other. Clay and sand are equally porous, but sand is by far the more permeable. In the three examples used for the demonstration, each with the same porosity, gravel is the most permeable, followed by sand, with fine soil the least permeable.

Aquifers Permeable and porous material through which water flows is called an aquifer. Aquifers can be huge. Sandstones form a vast aquifer beneath northeastern Africa and part of the Sahara. The Great Artesian Basin is an Australian aquifer lying beneath much of Queensland and extending into the Northern Territory, South Australia, and the northern part of New South Wales. Materials such as some types of clay, shale, and silt slow the movement of water, and a mass of such material is called an aquitard—it retards the aquifer. Impermeable material that halts the flow altogether is called an aquifuge, or aquiclude. The lower boundary of an aquifer consists of a layer of impermeable material. If all the material between the water table and the ground surface is permeable, the aquifer is said to be unconfined. A confined aquifer is bounded by impermeable layers both above and below. There is a third type of aquifer that forms above the impermeable layer forming the upper boundary of a confined aquifer. This second aquifer, completely isolated from the aquifer below,

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1 Unconfined

ground surface

2 Confined

ground surface

impermeable layer aquifer

aquifer

impermeable layer

impermeable layer

3 Perched

4 Artesian

ground surface

ground surface

aquifer impermeable layer impermeable layer aquifer impermeable layer impermeable layer © Infobase Publishing

Aquifers. 1) An unconfined aquifer lies above a layer of impermeable material. 2) A confined aquifer lies between two impermeable layers. 3) A perched aquifer lies above the upper impermeable layer of a confined aquifer. 4) An artesian aquifer is part of a confined aquifer lying in a natural hollow; the water in the aquifer to either side of the hollow is at a higher level than the water in the hollow, so piercing the upper impermeable layer causes water to flow to the surface.

is called a perched aquifer. The illustration shows the three types of aquifer. Water finds its own level. That is to say, if two bodies of water are connected and unconfined, the surfaces of both will be at the same level in respect of some datum, usually sea level. Consequently, the water table—the upper margin of the groundwater—tends to be at the same height above sea level over the entire area of an unconfined aquifer. Ground level, on the other hand, is determined by quite different forces, and it varies greatly. There are places, therefore, where the water table intersects the ground surface. At these places, depending on the terrain, water either seeps out, wetting the ground, or emerges more vigorously as a spring.

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Using modern equipment, some wells in Australia have been sunk to about 6,000 feet (1,830 m). Digging a well is extremely hard work, and it is also dangerous. The excavated material has to be lifted to the surface, a task that becomes increasingly arduous as the well deepens, and the hole must be large enough for at least one and often more than one man to work. This means it must be at least three feet (90 cm) in diameter, and the diameter is more likely to be up to eight feet (2.4 m); some dug wells are 30 or even 40 feet (9–12 m) across. The first danger is that the sides may collapse. The second is that when the diggers reach the water table, water may rise in the well so rapidly that the workers have no time to escape. Once the well has been dug, water is removed from it either by lowering a bucket on a rope or, nowadays more commonly, by pumping it to the surface. A modern alternative is to drill a narrow borehole with pumps installed at intervals down it to raise the water. This is quicker, simpler, and safer than digging by hand and usually cheaper. A tube well is probably the cheapest type of well. It consists of a steel tube with a pump to raise the water. Millions, perhaps billions, of people throughout the world depend on wells, and sinking wells, is the most obvious remedy for communities lacking a reliable supply of water that is safe to drink. This is almost always successful, but it is essential to examine well water very carefully before allowing people to drink it or cook with it. A scheme to provide tube wells in Bangladesh and West Bengal, India, led to mass poisoning (see the sidebar).

Wells and Oases Where a natural depression in the ground surface penetrates the aquifer, water will lie on the surface as a lake. Such depressions occur in deserts, and if their water is not too salty they form oases, a word derived from the two Coptic words oueh, meaning “to dwell,” and saa, meaning “to drink.” The diagram shows how an oasis of this type is formed, and, as the drawing suggests, the provision of water is all that is required to stimulate the growth of plants. Rivers flowing down from adjacent mountains supply water for another type of oasis. Oases of this kind are found in the Atacama Desert in the foothills of the Andes and near the edge of Asian deserts, where they are fed by rivers flowing from the Himalayas and the Karakorum Mountains. Dig a hole vertically downward all the way to below the water table, and the bottom of the hole will fill with water. That is a well, and the water in it usually rises to a height above the level of the water table. This is because most aquifers flow across an incline, so wells fill to approximately the level of the highest point in the aquifer. It is another example of water finding its own level, although the water may be a long way below ground level. Most wells are less than 100 feet (30 m) deep, but some have been sunk to 500 feet (150 m). Drilling equipment for making wells was being used in China more than 2,000 years ago to sink shallow wells to obtain drinking water and also to recover saline water from depths of about 5,000 feet (1,525 m)—salt was obtained from the salt water.

Artesian Wells There are places where no pumping is necessary, because as soon as the well hole crosses the water table water gushes to

Oasis. An oasis is a place where the bottom of a natural depression is below the water table.

lake

aquifer © Infobase Publishing

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Poisoned Wells Many people living in rural areas obtain their drinking water from wells, ponds, and rivers. This is especially true in less-developed countries. Well water is often safe to drink, but pond and river water is often contaminated. Until about 1970 periodic outbreaks of cholera and diarrhea in rural areas of Bangladesh and West Bengal, across the border in India, were killing up to 250,000 children a year. Cholera and diarrhea are caused by organisms in water contaminated with sewage. With the help of international aid organizations, in the 1970s the government of Bangladesh began drilling deeper wells to provide clean, safe water. The aim was to tap the groundwater flowing through sediment washed from the Himalayas into the Ganga (formerly Ganges) Delta. The government drilled between 6 million and 10 million tube wells—steel pipes that extend below the water table fitted with hand pumps to bring water to the surface. The tube wells tapped into water 50–300 feet (15–100 m) below the surface and supplied water for 97 percent of the rural population. Outbreaks of disease ceased. People were much healthier, and the rate of infant mortality fell by half. There was also water for irrigation, allowing farmers to grow rice during the six months of the dry winter monsoon, so communities were more prosperous and better fed. After about 10 years villagers began to complain of ill health. They had sores on their chests and discolored

the surface of its own accord. The first well of this kind was made, probably by drilling, in 1126 at the small town of Lillers, to the west of Lille in northwestern France. In those days that part of France formed the province of Artois—in modern France it now forms part of two départements (a département is the largest unit of local government), and the name Artois is no longer used. The Latin name for Artois was Artesium, so the well at Lillers was said to be of an artesian type. There are many artesian wells in the Great Artesian Basin, in Australia.

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hands. Cancers of the skin, lung, liver, pancreas, and bladder became common. Eventually, people were dying at a rate of about 3,000 a year. The victims were suffering from arsenic poisoning. Arsenic is found in many rocks. It is widespread in the environment, but no one had checked the groundwater for arsenic before drilling the tube wells. Geologists thought that water draining from the Himalayas would be safe, and scientists still do not know why water in this area should contain up to five times more arsenic than the safe limit for drinking water recommended by the World Health Organization. It is possible that in some areas the contamination began when pumping water for irrigation drew into the aquifer microbes that triggered chemical reactions releasing arsenic into the water. In 1998 the Bangladesh government launched the Bangladesh Arsenic Mitigation and Water Supply Project with backing from the World Bank. It is working to provide safe water, starting in the most seriously contaminated areas. This may be possible by replacing the existing tube wells with deeper wells. Arsenic contamination is beginning to affect wells in the Mekong Delta in Vietnam, Laos, and Cambodia. Solving the problem in Bangladesh and West Bengal will also help people in those countries.

An artesian well taps into water that is held under pressure. Drawing 4 in the illustration of aquifers (on page 70) shows how this pressure arises in a confined aquifer that is depressed into a hollow. Outside the depression the aquifer is at a higher level than the ground surface inside the depression. If a well is drilled from ground level through the upper impermeable layer, water will rise to find its own level, and, because that level—where the water table would be were the aquifer unconfined—is above ground level, water will flow from the well without pumping.

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3 Atmosphere and Desert Movements of air and ocean currents transport heat from equatorial regions to high latitudes. Air gathers moisture as it crosses oceans and deposits moisture as it rises over mountains and crosses continents. Air movements produce weather phenomena such as winds, clouds, and precipitation—or lack of it—and the weather conditions typical of a place over a long period constitute the climate of that place. This chapter explains how, in particular parts of the world, the circulation of air and water produces deserts. It then describes the desert climate. The chapter begins with a story. Modern understanding of the way air moves on a global scale began when scientists sought to unravel the puzzle of why winds in the Tropics are so reliable. Their ideas led to detailed descriptions of the general circulation of the atmosphere. Having described the general circulation and explained why moving air does not travel in straight lines, the chapter explains how the general circulation produces the dry and rainy seasons of the subtropics and also the jet stream that generates the weather systems of middle latitudes. Ocean currents also influence climates, and the chapter explains those effects. Having outlined the broad background to desert climates, the chapter ends by describing desert weather. Apart from the heat and aridity, sandstorms, dust devils, and whirlwinds are the most famous features of the subtropical deserts. The chapter explains what these are and what causes them.

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HADLEY CELLS

Trade used to mean path, way, or track. Derived from trada, the Old Saxon word for “footstep” or “track,” it implied a constant direction, and its modern meaning of “commerce” is derived from the older meaning. When European sailing ships began exploring the world in search of exotic goods to sell at home that the ship owners could buy with goods produced in their own countries, mariners noticed a curious phenomenon. The winds, notoriously variable in northern

waters, were very dependable close to the equator. To the north of the equator the winds almost always blew from the northeast, and south of the equator they blew from the southeast. Using the word in its old sense, sailors called them the trade winds. While the sailors simply exploited the trade winds, back in Europe there were people who found them a puzzle and sought explanations for their regularity. Why should the wind blow from these two directions for so much of the time, and why only in that part of the world? It took a long time to find the answers, and those answers explained much more than the trade winds, interesting though that explanation was. The answers also explained why a belt of deserts extends through the Tropics and subtropics, encircling the Earth in both hemispheres, and it provided the first clues about what it is that produces the weather.

Explaining the Trade Winds It was not until 1686 that the first serious attempt at an explanation was made. Edmund Halley (1656–1742), the English astronomer, proposed that air is heated more strongly at the equator than anywhere else. The warmed air rises, and cooler air flows toward the equator to take its place. This movement of cooler air from higher latitudes accounts for the trade winds. Edmund Halley had experienced the trade winds, and his interests were wide—two years later he drew the first meteorological map. In November 1676 he sailed in a ship of the East India Company to St. Helena, a remote island in the South Atlantic, on an expedition to catalog stars (he cataloged more than 300). That was the first of several voyages he undertook for purely scientific purposes. In 1682 a comet appeared prominently. Halley calculated its orbit and predicted it would return in December 1758. That comet now bears his name. In 1703 he was appointed professor of geometry at the University of Oxford, and in 1720 he became Astronomer Royal. Eminent though he was, Halley was only partly right about the trade winds. The movement he suggested would

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produce winds blowing from due north and south, not from the northeast and southeast. He had failed to explain this easterly component. It was an honorable failure, because no one else did any better for nearly half a century. Then in 1735, a reason was advanced for the easterly component. Warmed air rises at the equator, just as Halley thought, and cooler air from higher latitudes replaces it, but while the cool air is flowing toward the equator the Earth itself is also turning. The land and sea surface is moving beneath the wind, and it is this motion that swings the moving air, making it reach the equator from the northeast and southeast. This was the idea of the British meteorologist George Hadley (see the sidebar). Hadley had trained to become a lawyer but found science much more interesting. His proposal set out to do more than explain the trade winds. He thought about where the rising equatorial air went and discovered a way heat might be transferred from the equator to the poles. The warm, rising air must move away from the equator in order to make room for more air that is rising to follow it. It flows all the way to the poles. There it sinks once more and flows back toward the equator. This means that warm, equatorial air moves into high latitudes, and as it does so it makes the regions it crosses warmer than they would be

otherwise. By the time it reaches the poles the air is cold, and it is cold, polar air that moves back to the equator, making tropical regions cooler than they would be otherwise. This is convection, the transport of heat through a fluid (gas or liquid) by the movement of the fluid itself.

Convection and Convection Cells In fact, convection is caused by gravity. When a fluid is heated its molecules acquire energy. They move faster and space themselves farther apart. This causes the fluid to expand, so a given mass of the fluid occupies a greater volume, expanding as its temperature increases. To put it another way, a given volume of warm fluid contains fewer molecules and therefore has less mass than a similar volume of cold fluid. The volume with the smaller mass weighs less. Fluid around it, which has not been warmed and therefore is heavier, sinks beneath the lighter volume, pushing it upward. That is why warm fluid rises. As it rises its molecules lose energy, and the fluid cools. Its molecules move more slowly, they move closer together, and the fluid becomes denser—the mass of a given volume increases. Being heavier, the fluid now sinks, displacing fluid that is less dense.

George Hadley (1685–1768) George Hadley was born in London on February 12, 1685, the son of George Hadley and Katherine Fitzjames. The family was prosperous and socially influential. George senior owned an estate near East Barnet, Hertfordshire (East Barnet is now part of London), and became high sheriff of Hertfordshire in 1691. (In England, a high sheriff was originally an important and powerful official, but by the 17th century the office was largely ceremonial.) George junior had an older brother and sister and a younger brother. George studied law and became a barrister. Under the English legal system this is a lawyer who is permitted to appear as an advocate in the higher courts. He became increasingly interested in physics, however. In 1719 and 1720 George and Henry, the two younger brothers, collaborated with John, the eldest, in building the first reflecting telescope that worked efficiently. In 1730 the three brothers invented and built the reflecting octant, an instrument that measured the height of the Sun or a star above the horizon. This allowed mariners to calculate their longitude to within 1°. Many scientists were involved in studies of the atmosphere and weather, and the Royal Society of Lon-

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don, England’s first and still most important scientific academy, kept records of meteorological observations. George Hadley was placed in charge of these and held the office for at least seven years. Edmund Halley, the Astronomer Royal, had proposed an explanation for the constancy of the trade winds in 1686. In 1735 George Hadley developed Halley’s idea by also explaining their direction, presenting his results to the Royal Society in a paper, “Concerning the Cause of the General Trade Winds.” His explanation was not complete, but despite this Hadley had devised one of the first credible explanations for the general circulation of the atmosphere. Unfortunately, his paper aroused little interest. It was not until 1793 that the English meteorologist and chemist John Dalton (1766–1844) recognized the importance of Hadley’s work. By then Hadley had long been dead. He died at Flitton, Bedfordshire, on June 28, 1768, at the age of 83. His name is remembered in the Hadley cells that make up the tropical section of the general circulation of the atmosphere.

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Atmosphere and Desert If the fluid is being heated from below, rising warm fluid and sinking cold fluid will establish a vertical circulation. This is called a convection cell, and Hadley had described a convection cell operating on a huge scale. To this day it is called a Hadley cell. George Hadley made a very important contribution to our understanding of the atmosphere and weather. Unfortunately, he was only partly correct. Hadley assumed that his convection cell accounted for all air movement and that the atmosphere was otherwise unperturbed. In fact, local variations due to a host of factors mean no single convection cell could remain stable. In fact, there is not one Hadley cell, but several. More seriously, his cells do not extend all the way from the equator to the poles. This means the air movement generating the trade winds operates over a shorter distance than Hadley supposed, and over that shorter distance the effect of the rotation of the Earth is insufficient to account for the easterly deflection. The deflection was not explained until more than 120 years later by the American meteorologist William Ferrel (1817–91). Any fluid moving over the surface of the Earth tends to follow a circular path about a vertical axis, as water does when it flows out of a bathtub, and once it starts to swing the rotational motion will be maintained. Ferrel proposed in 1856 that it is this rotational movement that swings the air moving toward the equator and produces the northeasterly and southeasterly trade winds. Scientists now accept this as the correct explanation. In all it took 170 years!

Hadley Cells, Equatorial Rain, and Hot Deserts As warm air rises it cools, and as air cools its capacity to hold water vapor decreases (see “Why a Rise in Temperature Makes Air Drier” on page 7). Equatorial air is very moist. Oceans cover most of the equator, and the high equatorial surface temperature allows large amounts of water to evaporate. So it is not simply warm air that rises at the equator, it is warm, moist air. As it rises it cools, and as it cools its water vapor condenses. Clouds form, and the water returns to the surface as rain. Equatorial regions have a wet climate. Now cold and dry, the equatorial air moves away from the equator, but not all the way to the poles. Between latitudes 25° and 30° in both hemispheres it encounters warmer, less dense air and sinks beneath it, subsiding all the way to the surface. Just as rising air expands and cools, so descending air is compressed and becomes warmer (see the sidebar on page 76). By the time extremely cold air at an altitude of about 50,000 feet (15,250 m) has sunk to ground level its temperature has risen considerably. It is now warm air, but no water has evaporated into it, and so it is still very dry.

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divergence

convergence

rising air

subsiding air

convergence

divergence

low pressure

high pressure

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© Infobase Publishing

Divergence and convergence

Imagine a column of air extending from an area on the ground all the way to the top of the atmosphere. If air is subsiding, it must spread out when it reaches the ground, and at high altitude air must be entering the column, drawn downward by the divergence at low level. This produces high atmospheric pressure at ground level. Conversely, rising air produces convergence at low level, divergence at high level where air is leaving the column, and low pressure at the surface. The diagram above illustrates this. Dry, warm air subsides over the Tropics and diverges at low level to complete the Hadley cell. Its divergence generates a general low-level air movement away from the Tropics, preventing air from entering the region except from above. Moist maritime air cannot penetrate. Consequently, the Hadley cell produces both the humid equatorial climate and the arid tropical climate. In seeking to account for the trade winds, George Hadley also explained why there are deserts in the Tropics and subtropics of all continents and both hemispheres.

Walker Cells and El Niño Sir Gilbert Walker (1868–1958) was one of the most distinguished meteorologists of his generation, and in the early years of the 20th century he headed the Indian Meteorological Service. Failures of the monsoon (see “Monsoons” on pages 81–82) had caused severe famine in 1877 and 1899, and in 1904 the British authorities, who then ruled India, asked Walker to see if there was any pattern in the timing and

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Why Sinking Air Is Dry Imagine a balloon partly inflated with air and made from some substance that totally insulates the air inside. No matter what the temperature outside the balloon, the temperature of the air inside remains the same. Air presses down on the Earth’s surface with the entire weight of the atmosphere, extending upward all the way to interplanetary space. Barometers measure air pressure, and they measure the force exerted by the weight of the atmosphere acting on a unit area, such as a square inch or square centimeter. The air inside the balloon is squeezed by the weight of air above it. If the air inside the balloon is less dense than the air above it, denser air will push beneath it and the balloon will rise. As it rises, the distance to the top of the atmosphere becomes smaller, so there is less air above to weigh down on the air in the balloon. This causes the air in the balloon to expand. When air (or any gas) expands, its molecules move farther apart. The amount of air remains the same, but it occupies a bigger volume. This expansion uses energy, so as the air expands its molecules lose energy. Because they have less energy, the molecules move more slowly. When a moving molecule strikes something, some of its energy is transferred to whatever it strikes, and a portion of that energy is converted into heat. This raises the temperature of the struck object by an amount related to the number of molecules striking it and their speed. In expanding air, the molecules are moving farther apart, so a smaller number of them strike an object each second. They are also traveling more slowly, so they strike with less force. This means the temperature of the air decreases. As it expands, air cools. If the air in the balloon is denser than the air below, it will sink. The pressure on it will increase, its volume will decrease, and its molecules will acquire more energy as they move closer together. Consequently, the temperature of the air will rise. The amount of water vapor air can hold depends on the temperature. The warmer the air, the more water vapor it can contain (see “Why a Rise in Temperature Makes Air Drier” on page 7). When air sinks its temperature rises, and therefore the air becomes drier.

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intensity of the monsoon. If failures of the monsoon could be predicted, it might be possible to stockpile emergency food supplies in advance. In those days meteorologists believed that weather was a fairly local phenomenon—the weather in one place was not linked to the weather in another, far distant place. Nevertheless, Walker set about his task by studying climate records from all over the world. These revealed that particular weather in one place sometimes coincided with certain, but different, conditions a long way away. Walker noticed, for example, that when the Indian monsoon failed, Canada enjoyed a mild winter, and when the surface air pressure was low at Tahiti, it was often high at Darwin, Australia. Scientists now know of other links of this kind. They are called teleconnections. From time to time the relative pressures at Tahiti and Darwin changed. Tahiti had high pressure and Darwin low pressure. Walker found that these oscillations were linked to the Indian monsoon and to the amount of rainfall in Africa. He called the periodic changes in pressure the southern oscillation. In 1923 Walker published a description of a tropical air circulation that forms a series of vertical cells through which the air moves latitudinally (parallel to the equator) rather than away from and toward the equator, as in the Hadley cells. This movement is known as the Walker circulation. The Walker circulation ordinarily generates a flow of air from east to west across the equatorial Pacific Ocean. It was not until 1960 that the Norwegian-American meteorologist Jacob Bjerknes (1897–1975) spotted that the southern oscillation periodically alters the wind strength and even direction across the Pacific and that this alteration is linked to another phenomenon, known as El Niño. While the wind blows from east to west across the Pacific just south of the equator, it generates a surface ocean current that drives water away from South America and toward Indonesia. Heated by the equatorial sunshine, this surface water is warm, and it accumulates as a deep pool of warm water around Indonesia. Evaporation from the warm pool gives Indonesia a wet climate. On the opposite side of the ocean the layer of warm water is much thinner, and the cool Peru Current flows through it, parallel to the coast. The easterly flow of air across the continent gives the western coast of South America its arid climate (see “West Coasts and Rain Shadows” on pages 5–6). At intervals of two to seven years the southern oscillation alters the distribution of pressure across the South Pacific. The winds weaken or reverse direction, causing the current flowing parallel to the equator to weaken, and, if the change in pressure distribution is strong enough, it may even reverse its direction. The warm pool around Indonesia becomes shallow, and a deep warm pool accumulates off the South American coast. At the same time the

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Atmosphere and Desert winds blowing from the east across South America weaken, allowing moist air to penetrate inland. The arid coastal strip of South America receives abundant rain, sometimes too much. Farmers are able to grow bumper crops. The rain usually arrives in the middle of summer—December in the Southern Hemisphere—so people think of it as a Christmas gift. They call it El Niño, which means the child, specifically the Christ child. At other times the southern oscillation intensifies the usual distribution of pressure. This accelerates the ocean current, deepening the warm pool around Indonesia and making the water colder off South America. Rainfall increases over Indonesia, and western South America is even drier than usual. This is known as La Niña. The complete cycle of El Niño and La Niña is known as an El Niño–Southern Oscillation, or ENSO, event.

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Subsiding air and divergence at the surface produce the hot desert climates of the Tropics and subtropics. There, surface atmospheric pressure is usually higher than the global average sea-level pressure of 14.7 pounds per square inch, sometimes expressed as 30 inches (760 mm) of mercury, one atmosphere (1 atm), 1013.25 millibars (mb), or in international scientific units as 101.325 pascals (Pa). Where the subsiding air reaches the surface, air flows outward from this high-pressure region. Some of it flows back toward the equator as the easterly trade winds. The remainder flows away from the equator, also at low level. This air produces generally westerly winds in middle latitudes. Air is also subsiding at the North and South Poles, where it flows away from the polar regions at low level. This movement produces polar easterly winds. This air is dry, like the subsiding air in the Tropics, and it produces the cold deserts of the Arctic and Antarctic. As the air subsides it is compressed and warmed in the same way as tropical air, but polar climates remain cold because the solar radiation they receive is much more diffuse than that which warms the Tropics (see “Subtropical Deserts” on pages 1–5). Nevertheless, harsh though they are, polar climates are somewhat milder than they would be without that warming. Arctic and Antarctic air flows toward middle latitudes, where it meets tropical air moving in the opposite direction from the descending side of the Hadley cell. Air approaching from opposite directions converges and rises, producing a belt of generally low surface pressure. The rising air diverges, some flowing toward the poles and some toward the equator. In this way the two main systems of convection cells—the low-latitude Hadley cells and the Arctic and

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© Infobase Publishing

General circulation of the atmosphere. Air rises over the equator, producing low pressure at the surface, and subsides in the subtropics, producing high pressure. Air subsides over polar regions, producing high surface pressure. In middle latitudes air flowing away from the polar regions at low level meets air flowing away from the subtropics, causing air to rise and producing low pressure at the surface.

Antarctic polar cells—drive a third, midlatitude system. This was first described by William Ferrel (see “Convection and Convection Cells” on pages 74–75), so these are called Ferrel cells. Together, these three systems of cells provide a very approximate description of the way air circulates over the Earth or, to give it its scientific name, the general circulation of the atmosphere. It is called the three-cell model because there are three cells in each hemisphere. The diagram illustrates it together with the belts of surface pressure and wind systems they produce.

Troposphere and Stratosphere The illustration of the three-cell model greatly exaggerates the height of the cells in order to make them easier to see. In fact, there is a ceiling that air rising by convection can penetrate only if it is ascending extremely vigorously, as it does at the top of some very violent storm clouds— the kind that produce fierce tropical storms or tornadic (tornado-producing) storms on the Great Plains. The lower part of the atmosphere, in which all the weather systems reside, is called the troposphere; the Greek

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word tropos means “turning.” In the troposphere air temperature decreases with increasing height by an average 3.6°F every 12,000 feet (6.5°C per 1,000 m). Above the troposphere lies the stratosphere—strato- is from stratum, the past participle of the Latin verb sternere, meaning “to strew.” Throughout most of the stratosphere the temperature remains constant or even increases with height. Air rising through the troposphere encounters air at the base of the stratosphere that is at the same temperature as itself or warmer. This means that the density of the overlying air is equal to or lower than that of the rising air, so the air can rise no further. The boundary between the troposphere and stratosphere is called the tropopause. Its altitude varies seasonally and with latitude, averaging about 10 miles (16 km) at the equator and five miles (8 km) at the poles. Over middle latitudes, the region that includes most of North America and all of Europe, on average it is seven miles (11 km) above the surface. Because air temperature decreases with height in the troposphere, and the tropopause is higher over the equator than it is over the poles, the temperature at the equatorial tropopause is lower than that at the polar tropopause. Over the equator it is usually between -95°F and -120°F (-71°C and -84°C), and over the poles the temperature is between -60°F and -75°F (-51°C and -59°C); over middle latitudes the temperature at the tropopause is about -22°F (-30°C). The wind systems balance, with the energy of the easterly winds equal to that of the westerlies. This is simple to prove. Winds blowing across the surface of land and sea exert pressure, the pressure people feel when walking on a windy day. If over a long period and over the world as a whole either westerly or easterly winds predominated, that pressure would accelerate or slow the Earth’s rotation. Unusual wind patterns do cause the rotational speed to vary slightly for short periods, but over longer periods it remains constant. Since the winds have no long-term influence, their westerly and easterly pressures must cancel each other, and the atmosphere as a whole must move with the Earth, not at a different speed.

Transporting Heat by Air Incoming solar radiation is not distributed evenly over the Earth. In fact, the equator receives about 2.5 times more energy from the Sun than do the North and South Poles. Were it not for the fact that the general circulation of the atmosphere transfers heat from low to high latitudes and so has a moderating influence on climate, the temperature difference between low and high latitudes would be much greater than it is. There is evidence that this is the case. When an object is heated it begins to radiate heat. This is called blackbody

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radiation. The Earth is a blackbody in this sense, and it radiates into space all the radiant heat it receives from the Sun. If this were not so, the world would grow warmer or cooler, whereas in fact its temperature remains constant over long periods. This is true for the planet, but it is not true for particular regions of the planet. The amount of solar radiation received is at a maximum at the equator and a minimum at the poles, but there is much less difference in the amount of outgoing radiation. Equatorial regions radiate less than they receive and polar regions radiate more, yet equatorial regions do not grow steadily warmer, nor do polar regions grow steadily colder. This distribution is possible only if heat is being transported from warmer to cooler regions. In fact, though, the atmosphere transfers heat very inefficiently. Solar radiation warms the surface of the Earth, and air is warmed by contact with the warmed surface. In the air convective movement (see “Convection and Convection Cells” on pages 74–75) converts heat energy into kinetic energy—the energy of motion—but kinetic energy eventually dissipates through friction with the surface and in small eddies. Heat is also transferred through the horizontal movement of air masses, by ocean currents, and through the evaporation and condensation of water.

Why Air Does Not Move in Straight Lines It was the American meteorologist William Ferrel who discovered it is the tendency of moving fluids to rotate about a vertical axis that accounts for the easterly component of the trade winds. Known as vorticity, this is also the reason for the westerly component in midlatitude winds. Both arise because the Earth and its atmosphere are rotating. A body that is rotating about an axis possesses angular momentum. Providing no outside force accelerates or slows the rotation, angular momentum remains constant—it is said to be conserved (see the facing sidebar). In other words, if one component changes, one or both of the others will also change to ensure the angular momentum remains the same. Earth rotates, and the atmosphere rotates with it. If a mass of air moves into a different latitude, its distance from the axis of the Earth’s rotation changes, because Earth is a sphere. The mass of the air cannot change, so as its rotational radius changes, its angular velocity must change in order to conserve its angular momentum. If the air moves away from the equator, it will rotate faster, and if it moves toward the equator, it will turn more slowly. Changes in the angular velocity of moving air deflect the wind systems associated with the convection cells to the east if the winds are moving away from the equator and to the west if they are blowing toward it. These directions are the same in both hemispheres.

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Conservation of Angular Momentum A body that is spinning about its own axis possesses three properties: its mass, the radius of the circle it describes, and the speed of its rotation. Its speed of rotation is known as its angular velocity and is measured as the number of degrees through which it turns in a given time. The Earth, for example, completes one revolution in 24 hours. One full turn takes it through 360°, so the Earth’s angular velocity is (360 ÷ 24 =) 15° per hour. Angular velocity is usually expressed in radians per hour or per second (rad/hr or rad/s). A radian is the angle between two radii of a circle that marks out on the circumference an arc that is equal in length to the radius. Therefore, the circumference of a circle is 2π radians, and 1 rad = 57.296°. Multiply these three values together and the product, called angular momentum, is a constant. Call the mass M, the radius R, and the angular velocity V, and M × R × V = a constant. M, R, and V are variables. They can be altered, but the constant must remain the same. This is called the conservation of angular momentum,

The Coriolis Effect Vorticity is not the same thing as the Coriolis effect, the phenomenon first explained in 1835 by the French engineer and mathematician Gaspard Gustave de Coriolis (1792– 1843). The Coriolis effect (abbreviated as CorF, because it used to be known as the Coriolis force, although no force is involved) arises because we measure positions in relation to points on the surface of the Earth. A person might be in New York, for example, or Cape Town. New York, Cape Town, and every other place on the surface remains stationary with respect to the remainder of the Earth’s surface: Places do not move around. Seen from a position away from Earth, however, the entire surface is moving because the Earth is a rotating sphere. The planet turns from west to east, completing one rotation every 24 hours. To achieve this, a point on the equator is moving eastward at approximately 1,038 MPH (1,670 km/h). New York is traveling eastward at about 785 MPH (1,263 km/h), and Cape Town is moving at about 859 MPH (1,383 km/h). Suppose an airplane were to take off from Cape Town and fly due north to somewhere in the latitude of New York—just off the coast of Albania—on a journey that takes, say, 15 hours. Suppose also that the pilot steers by a coordinate system outside the Earth, ignoring completely the position of the airplane with regard to the ground below. As well as traveling north, the airplane will travel east, because

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and it means that if one of the variables changes, one or two of the others must also change in order that the constant remains the same. No one needs to do anything to make this happen; it is entirely automatic. Dancers and ice skaters make use of the conservation of angular momentum when they perform pirouettes. The dancer starts spinning with her arms fully outstretched. The distance from the center of her body (the axis of her rotation) to her fingertips is the diameter of the circle her body describes; the radius is half of this. Then she slowly draws her arms inward to her body. This reduces her radius of spin. She has reduced one of the three variables, and so one or both of the others must increase in order to compensate. Her mass cannot change (she cannot suddenly become heavier), and so the remaining variable, her angular velocity, has to change. It increases as her radius of spin decreases. In other words, she spins faster, but without making any additional effort beyond withdrawing her arms.

it is still moving at the rotational speed of Cape Town. During its 15-hour flight the airplane will move 12,885 miles (20,732 km) eastward in relation to the Earth’s surface. During that same period, its destination also travels east, by 11,775 miles (18,946 km). At the end of the journey, therefore, instead of being between Greece and Albania, the airplane arrives 12,885 - 11,775 = 1,110 miles (1,786 km) to the east of that position (no doubt with the captain trying hard to explain to the passengers how they come to be in eastern Turkey rather than the eastern Adriatic). This is the Coriolis Effect.

INTERTROPICAL CONVERGENCE ZONE, MONSOONS, AND JET STREAMS

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Air converges where the northeasterly and southeasterly trade winds meet in an area called the Intertropical Convergence Zone (ITCZ). Air that is rising strongly produces low air pressure near the surface, and, because the rising air is very moist, this is the region of intense tropical rainfall. The low-pressure region is not like the depressions that bring wet weather to middle latitudes. Air does not flow around them because the Coriolis effect (see “The

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Intertropical Convergence Zone (ITCZ). The map shows the average positions of the ITCZ in July and January.

Coriolis Effect” on page 79) is very weak in the Tropics, and at the equator it does not exist at all. As the map shows, the ITCZ is not always in the same place. Because the Hadley-cell circulation is driven by warmth from the Sun that is absorbed at the surface, its center moves with the seasons as the Earth’s tilted axis turns first one hemisphere and then the other toward the Sun. Despite its seasonal movement, however, the ITCZ and its associated rainfall never move far enough to reach the subtropical deserts, much less to bring rain to the deserts of the cool continental interiors. The contrast between the humid equatorial and arid tropical climates is intensified by a situation that develops as the air crosses the ocean toward the equator. Initially dry, the trade winds collect moisture, and its evaporation from the sea surface cools the air at low level. Above this layer of cool air, subsiding air is warmed as it descends (see the sidebar “Why Sinking Air Is Dry” on page 76). This results in a layer of cool, moist air lying beneath a layer of dry, warm air. The moist

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air cannot rise through the warmer air above. It is trapped beneath a temperature inversion, the situation in which warm air lies above cool air. This is called the trade wind inversion. Because the moist air cannot rise, it cannot cool below its dew point temperature, which is the temperature at which its water vapor will condense, so its relative humidity remains below 100 percent. It is not until the low-level air reaches the ITCZ and is forced to rise through the inversion that it cools, becomes saturated, and releases its moisture. Local conditions can alter this pattern. Off the coast of Namibia, for example, the trade wind air crosses the cool waters of the Benguela Current, and off the cost of Peru and northern Chile it crosses the cool Peru (or Humboldt) Current. The small cooling that this causes is enough for water vapor to condense into fog offshore. Sometimes, especially in the Namib Desert, the fog drifts ashore, but the amount of moisture it delivers is very small. The inversion is also broken where the air crosses high mountains. The amount of water vapor air can contain varies according to the temperature. The amount of moisture present in the air is known as the humidity of the air (see “Why a Rise in Temperature Makes Air Drier” on page 7 and “Saturation and Humidity” on pages 7–8).

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Monsoons On a regional scale convergence and divergence produce climates that are markedly different from those determined purely by the Hadley-cell regime. It is because of these variations that central and southern India and the mainland of South Asia are not deserts despite being in the same latitude as the Sahara. The difference is known by the name that reached English from the Dutch monssoen and may have reached Dutch from an Arabic word, mausim, which means “season.” We call it the monsoon. In fact, there are two monsoons, marked by a reversal in the direction of the prevailing wind. Both monsoons are caused by differences in the way land and sea absorb heat (see “Specific Heat Capacity” on pages 90–91). Parts of West Africa and northeastern Brazil experience monsoon weather, but the effect it most marked in India and South Asia. When people mention the monsoon, it is the Indian monsoon that usually springs to mind. Land warms and cools quickly, the sea warms and cools slowly, and India and South Asia form large landmasses projecting into the ocean. In winter the land cools, reducing the difference in temperature between land and sea. At the same time the trade winds shift to the south with the ITCZ. As the land cools, the subsiding air produces high surface pressure and an outward airflow, while at high altitude the pressure is low because air is moving downward (see the illustration accompanying “Hadley Cells, Equatorial Rain, and Hot Deserts” on page 75). Air rises over the oceans to either side of the land and converges at high altitude, drawn inward by the low pressure. Surface pressure over the oceans is low, the air that is rising is warm, and there is high pressure aloft. By the middle of winter, high surface air pressure is centered over Siberia and extends almost to the equator, producing an outward flow of air at low level and northeasterly winds that bring extremely dry air from Central Asia. In the eight months from October through May, Hyderabad receives an average 7.4 inches (188 mm) of rain, and Mumbai (Bombay) receives 4.1 inches (104 mm). This is the winter monsoon, which is also known as the dry monsoon and because of the direction of the prevailing winds as the northeasterly monsoon. During spring the ITCZ moves northward, and behind it the prevailing winds change from northeasterlies to southwesterlies because although India and southern Asia are in the Northern Hemisphere, at this time of year they lie to the south of the ITCZ. The ITCZ moves all the way to the northern side of the Himalaya. The map shows the summer and winter positions of the ITCZ and the direction of the prevailing winds. In summer the pressure distribution and winds reverse. The land is warmed strongly by the Sun, the air in contact with the surface warms and rises, and air pressure over the land surface is low. This produces high pressure

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The Asian monsoons. During the winter or northeasterly monsoon, air flows across Asia toward the Indian Ocean, producing very dry weather. During the summer or southwesterly monsoon the wind reverses direction, bringing heavy rain.

aloft. Air then flows outward at high altitude and inward near ground level. Over the ocean the opposite happens. Inward-flowing air aloft causes air to subside, producing

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high pressure and outward-flowing air at the surface. This air crosses the ocean, gathering water vapor as it does so, and passes over the land, where it rises and loses its moisture, bringing heavy rain. The mountains form a barrier preventing the incoming air from moving any farther northward, so while the region to their north, in Central Asia, remains dry, south of the ITCZ the winds bring heavy rain. Hyderabad receives 22.2 inches (564 mm) of rain between June and September, and 67.2 inches (1,707 mm) falls in Bombay. The Asian monsoons make Cherrapunji one of the wettest places in the world, with 10 times the average annual rainfall of New York City. Cherrapunji is located in the hills of the Indian state of Assam at an elevation of 4,309 feet (1,313 m), and it receives an average of 425 inches (10,797 mm) of rain a year, of which 316 inches (8,021 mm) fall between June and September. This is the summer monsoon, which is also known as the wet monsoon and from the wind direction as the southwesterly monsoon.

The Easterly Jet The summer position of the ITCZ also produces a highlevel wind known as the easterly jet, which blows from east to west and extends from the South China Sea all the way to the southeastern Sahara. The easterly jet intensifies the monsoon winds and rain. A belt of low surface pressure called the equatorial trough is associated with the ITCZ and moves northward with it in the spring, reaching about latitude 25°N by the middle of summer. The thermal equator, the line marking the highest temperature, lies along the equatorial trough. Warm air is rising vigorously to the south of the ITCZ. The rising air deepens the low pressure centered over the land to the south of the ITCZ, increasing the north–south pressure gradient, and the air is cool between the thermal equator and the geographic equator to its south. Together these produce a thermal wind (see the sidebar “Thermal Wind” on page 83) that is strongest near the tropopause, at an altitude of about nine miles (15 km). In the Northern Hemisphere thermal winds always blow with the cooler air to the left, in this case to the south, between the thermal and geographic equators. Consequently, unlike the other jet streams, this high-level wind blows from east to west. The easterly jet draws air upward and into it, sweeping the air away across the Arabian Sea. That is how it deepens the low pressure at the surface, and the deepening low pressure accelerates the winds circulating around it. These winds approach from over the ocean. Consequently, it is the easterly jet that makes the rainfall during the summer monsoon so much heavier over southern Asia, especially over the Indian subcontinent, than it is in any other part of the Tropics.

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Jet Streams Where air in a Hadley cell meets air in a Ferrel cell in the subtropics, and where air on the other side of the Ferrel cell meets air in a polar cell much farther from the equator, there are clearly defined boundaries between warm and cold air. Warm and cold air do not mix very readily, and the boundary between them is known as a front (see “Air Masses and Fronts” below). These large-scale fronts are called the subtropical front and polar front, respectively. Above the surface wind blows parallel to the isobars (lines joining points of equal air pressure) with a speed proportional to the pressure gradient, which is the rate at which the pressure changes with horizontal distance; it appears on weather maps as the distance between isobars. The pressure gradient can be likened to the gradient on a landscape and the isobars to the contour lines indicating elevation on a map. A region of high pressure then resembles a hill and a region of low pressure a hollow, but on a surface defined by atmospheric pressure rather than land and sea. Across the subtropical and polar fronts the temperature difference increases the pressure gradient with increasing height. This is because cold, dense air is compressed by the weight of overlying air more than warm, less dense air, so the surface pressure is higher, but pressure decreases more rapidly with height. Consequently, wind speed increases with height, but wind direction remains constant. Winds associated with a temperature gradient are called thermal winds (see the facing sidebar). At the tropopause, above which the air temperature remains constant with height, the pressure gradient across the fronts is at its most extreme. It produces ribbons of air moving at great speed. These are thermal winds known as the subtropical and polar front jet streams, and they blow from west to east in both hemispheres. The polar jet stream is the stronger of the two but less continuous, so the term jet stream usually refers to the subtropical jet stream. In winter the Northern Hemisphere jet stream lies approximately along the line of the tropic of Cancer and is at its strongest, with winds sometimes reaching 300 MPH (483 km/h). In summer it is at about latitude 50°N and weaker, with speeds seldom exceeding about 50 MPH (80 km/h). Local areas of relatively low and high pressure form beneath the jet stream and travel with it as frontal depressions (see “Frontal Depressions” on pages 83–85) that bring changeable weather to regions in middle latitudes.

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Warm air and cold air do not mix readily. Warm water and cold water do not mix readily, either, and it is simpler to demonstrate the phenomenon with water than with air.

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Atmosphere and Desert

Thermal Wind Wind is generated by a difference in air pressure over a horizontal distance, which produces a pressure gradient. Air pressure varies according to the density of the air in a column extending upward to the top of the atmosphere, because the denser the air the more air molecules a given volume contains. Air density varies according to temperature, because molecules in warm air move faster and occupy a greater volume than do molecules in cold air. Cold air is denser than warm air. This means that in the lower atmosphere cold air is more compressed than warm air by the weight of air above. Consequently, air pressure decreases with increasing altitude more rapidly in cold air than in warm air because compression ensures that a greater proportion of the total mass of air is held at low level. Where masses of cold and warm air lie side by side, pressure decreases with increasing altitude more rapidly in the cold air than in the warm air. This produces a difference in pressure—a pressure gradient—between the two air masses that becomes greater with increasing height. The pressure gradient generates a wind. The Coriolis effect deflects the wind, and the balance between the pressuregradient force and the Coriolis effect makes the wind blow at right angles to the gradient, rather than directly from high pressure to low pressure. The pressure gradient results from a difference in temperature. Consequently, the resulting wind is known as a thermal wind. Because the pressure gradient increases with increasing altitude, the thermal wind also strengthens. The strongest thermal winds are the jet streams, occurring near the tropopause at the front between tropical and polar air. Thermal winds blow with the cold air to the left in the Northern Hemisphere and with the cold air to the right in the Southern Hemisphere. That is why (with the exception of the easterly jet stream associated with the Asian monsoon) the jet streams blow from west to east in both hemispheres.

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cold water. Then, slowly and carefully to cause the least disturbance, stand the jar upright. The colored hot water will lie above the cold water, and it will stay there until the hot and cold water masses both reach the same temperature. Only then will the color start to spread. The boundary between the hot and cold water is clearly visible, and a similar boundary forms where cold air and warm air lie side by side. Air is invisible, of course, but the position of the boundary can sometimes be seen because of the clouds that form along it. It is not the temperature difference as such that prevents the two bodies of air from mixing, but the difference in their densities. Where they meet the denser air sinks and pushes beneath the less dense air like a wedge, lifting the warmer air. A boundary can exist only if there are two adjoining bodies of air with different characteristics. Nowadays everyone takes this for granted, but it is not at all obvious. Until the fact was discovered, people did not suppose air is exactly the same everywhere, because obviously it is not. Sometimes and in some places it is wet, or dry, or hot, or cold, or clean and fresh, or smoky, or stuffy. Air quality can vary in many ways, but these always seemed to be very local. Leave the smoky city, and a few miles away the country air is clean and fresh. Sooner or later dry air will become moist, and moist air dry. Changes were gradual from one kind of air to another. As a person moved out of the city the air did not improve suddenly, but little by little. It all seemed selfevident, a matter of simple observation and common sense. The challenge to this mental image of the atmosphere began during the early part of the 20th century, and it was mounted in Norway.

The Bergen School In 1917 a Norwegian physicist and meteorologist named Vilhelm Bjerknes (see the sidebar on page 84) founded the Bergen Geophysical Institute at the Bergen Museum (now part of Bergen University) and became its director. There, with the help of his team of collaborators, he established the Bergen School of meteorologists to study the behavior of the atmosphere. His principal collaborators were his son, Jacob Aall Bonnevie Bjerknes (1897–1975) and Tor Harold Percival Bergeron (1891–1977).

Frontal Depressions Leave a half-filled jar of water in a refrigerator long enough for it to become very cold. Heat a small quantity of water almost to boiling temperature, then mix in a few drops of food coloring to make the water easy to see. Take the cold water from the refrigerator. Tilt the jar and pour the hot water very carefully and gently into the jar on top of the

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Polar and tropical air masses meet over the Atlantic at the polar front, and the scientists of the Bergen School showed how local areas of low pressure, called depressions or cyclones (not to be confused with tropical cyclones), develop along it. The illustration on page 85 shows the sequence of events. At first (1) a front lies between polar air to the north and tropical air to the south, with air flowing

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in opposite directions on either side of the front. A wave develops in the front (2). Cold air starts moving around the warm air. The front is now in the process of becoming two fronts. Where the warm air presses against cold air, to the east in the diagram, the front is a warm front. To the west, where cold air is advancing into warm air, it is a cold front. A front is called warm or cold according to the relative temperature of the air behind it. If the front brings air that is warmer than the air it displaces, it is a warm front; if it brings cooler air, it is a cold front. These names refer to the relative temperatures of air on either side of a front and not to any absolute temperature. As the wave becomes sharper (3) a local region of low pressure forms at the crest. This is the depression, or cyclone, or simply low. The low intensifies (4), and the frontal system now comprises a wedge of warm air surrounded by cold air. The diagram is very similar to those in the weather maps printed in newspapers. Like any map, it depicts a plan view, as seen from above, but this does not give a very clear

impression of what is really happening. All the air is moving to the east, but at stage 4 the cold, dense air is moving faster than the warm air, which is less dense. Advancing cold air undercuts the warm air, raising it above the ground, and consequently the front slopes. The diagram shows its position at surface level, but it slopes back, away from the warm air all the way to the tropopause, at an angle of about 2°. The warm front also slopes as it is pushed over the cold air ahead of it. Its slope is much gentler, usually no more than 0.5°–1°. As the cold air continues to advance, some of the warm air is lifted completely clear of the surface (5). The two fronts begin to merge at ground level and are said to be occluded. Finally, a fragment of the occluded front remains (6) well clear of the ground, and the original front returns ready for the cycle to repeat. When moist air is lifted along a front, it cools and cloud may form. Along the gentler slope of the warm front the cloud forms horizontal layers of the stratus type. Viewed from the ground, the highest cloud, at the top

Vilhelm Bjerknes (1862–1951) Vilhelm Frimann Koren Bjerknes was a Norwegian physicist and meteorologist born in Oslo on March 14, 1862. His father, Carl Anton Bjerknes, was professor of mathematics at Christiania (now Oslo) University and a very influential geophysicist who guided his son’s studies. Vilhelm assisted with some of his father’s experiments in hydrodynamics before leaving home to spend 1890 and 1891 working as an assistant to and collaborator with the German physicist Heinrich Hertz. He then worked for two years as a lecturer at the School of Engineering in Stockholm. In 1895 he was appointed professor of applied mathematics and physics at the University of Stockholm. In 1897 while at Stockholm Bjerknes developed a system that made it possible to forecast the weather scientifically. The Carnegie Institution supported his work, allowing Bjerknes to employ a series of “Carnegie assistants,” some of whom stayed with him for many years. Bjerknes returned to Norway in 1907 to take up a professorship at Kristiania (the spelling had been changed) University. In 1912 he was appointed professor of geophysics at the University of Leipzig, Germany. While there he founded the Leipzig Geophysical Institute, with a research school (a school in this sense means a carefully chosen team of colleagues). In 1917 Bjerknes moved to Bergen, Norway, to found the Bergen Geophysical Institute (now part of

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the University of Bergen). It was there that he did his most important work. He and his colleagues of the Bergen School established a network of weather stations throughout Norway from which observers sent measurements and observations. The team at Bergen assembled this information to produce general descriptions of atmospheric conditions over a large area. Studying these, they concluded that large bodies of air exist as distinct air masses. Conditions of temperature, pressure, density, and humidity remain constant at each level within an air mass but differ between adjacent masses. World War I was at its height, and the newspapers were full of war reports. Likening air masses to opposing armies, the Bergen scientists called the boundaries separating them fronts and developed a frontal theory to describe the development and disappearance of fronts and the weather associated with them. This formed the basis of modern meteorology. Bjerknes returned to Oslo University in 1926 and remained there until his retirement in 1932. He died in Oslo on April 9, 1951. His son, Jacob Aall Bonnevie Bjerknes (1897–1975) helped him organize the network of weather stations and continued his work. He was among the first scientists to study the jet streams. Jacob moved to the United States in 1939 and became professor of meteorology at the University of California, Los Angeles. He became an American citizen in 1946.

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The life cycle of a frontal depression. The cycle begins 1) as a front between cold and warm air. A wave develops along the front 2) and becomes steeper 3). 4) An area of low pressure (the depression) develops in the cyclonic flow of air. Cold air pushes beneath the warm air, raising it from the ground 5). The front is then occluded. Finally, all the warm air is lifted clear of the surface 6), and the system dissipates.

of the front, is the first to appear. This consists of wispy strands of cirrus and thin, semitransparent sheets of cirrostratus. As the front continues to approach, the cloud thickens with the arrival of altostratus, followed by nimbostratus and stratus, often accompanied by steady rain or snow. The rain ceases as the front passes, but the sky often remains cloudy. Then the cold front arrives, its lower edge first. Because it slopes more steeply, warm air is lifted more abruptly, and some heaped, cumulus cloud forms. Finally, the cold front passes, and the sky clears. The cold air is subsiding, and compression causes its temperature to rise, making the air drier (see the sidebar “Why Sinking Air Is Dry” on page 76). Any cloud droplets it carries evaporate. Behind a cold front the weather is usually fine, sunny, and dry.

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Rossby Waves and Vorticity The polar front marks the high-latitude margin of the Hadley cells, and the westerly jet stream is located at its top, near the tropopause. The Swedish-American meteorologist CarlGustav Arvid Rossby (1898–1957) is usually credited with having discovered the existence of the jet stream. Another worker from the Bergen School, Rossby moved to the United States in 1926, and it was there that he did his most important work. In 1947 he established the International Institute of Meteorology at the University of Stockholm with financial backing from American and Swedish foundations as well as support from international bodies, including UNESCO (the United Nations Educational, Scientific, and Cultural Organization). In 1940 Rossby showed that undulations form in the high-altitude westerly flow of air. At any one time there are usually between three and five of these waves in each hemisphere, with wavelengths (the distance between one wave crest and the next) of up to 1,200 miles (1,930 km). Over cycles lasting a few weeks these undulations, now known as Rossby waves, become more extreme until the airflow breaks down into isolated pockets of air circulating around centers of high and low pressure. The sequence is

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(left) The index cycle. The sequence of diagrams shows the change in the direction of the jet stream. After the flow has broken down into discrete cells 4), the westerly flow 1) establishes itself once more.

ized by f ) is the rotation due to the rotation of the Earth and is always equal in magnitude to the Coriolis effect (see “The Coriolis Effect” on page 79). The absolute vorticity of the fluid is the sum of the relative vorticity and the planetary vorticity (ξ + f ). Because of the conservation of angular momentum (see the sidebar on page 79), absolute vorticity is a constant. Planetary vorticity, one of the components of absolute vorticity, is proportional to the Earth’s angular velocity—the planet’s rate of rotation—and to latitude. Its magnitude is greatest at the North and South Poles and is zero at the equator. Consequently, if the moving body of fluid enters a different latitude, its planetary vorticity changes, and one of the other components of its absolute vorticity must also change in order to compensate. In the Northern Hemisphere if the fluid moves northward, planetary vorticity (f ) increases and relative vorticity (ξ) decreases to compensate. The effect of this is to turn the moving fluid in a southerly direction, decreasing f and increasing ξ. The resulting movement usually carries the fluid beyond its original latitude, however, and the situation is reversed. Now ξ increases, f decreases to compensate, and the fluid swings northward again. Consequently, once its movement carries the fluid into a higher or lower latitude, changes in its vorticity swing it back again, but overcompensation results in a wave motion that can grow increasingly extreme. It is the development of the Rossby waves that cause the formation of frontal depressions, and it is the eastward movement of the Rossby wave pattern that drags the depressions from west to east.

Blocking

© Infobase Publishing

known as the index cycle, and the drawing illustrates its four stages. Any fluid (gas or liquid) that is moving over the Earth’s surface tends to rotate—this tendency is known as vorticity. It has two components. Relative vorticity (symbolized by ξ, the lower-case Greek letter zeta) is the rotation of the fluid relative to the Earth’s surface. Planetary vorticity (symbol-

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The polar front jet stream carries air from west to east, as indicated in diagram 1 of the illustration of the index cycle. Cold air lies to the north of the jet stream and warm air to the south. As waves develop (stage 2 in the cycle), cold air is drawn farther south, behind the polar front, and warm air penetrates farther north. By stage 3 the waves have grown extreme due to vorticity, and by stage 4 cells of air have become isolated. In the Northern Hemisphere air circulates in a clockwise direction around regions of high surface pressure, called anticyclones, and counterclockwise around regions of low surface pressure, or cyclones. The anticyclones usually form with their centers between latitudes 50°N and 70°N, and there is often a cyclone on either side of each anticy-

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Atmosphere and Desert clone and to its south. (The same situation occurs in the Southern Hemisphere, but air circulates clockwise around cyclones and counterclockwise around anticyclones; the cyclones form to the north of the anticyclones.) The anticyclones and cyclones then remain stationary until the easterly flow reestablishes itself. This usually takes about two weeks, but it can take up to one month. The jet stream often flows around the cells on the side nearer the pole, as indicated in diagram 4, but sometimes it divides, with one section passing the cells to the north and the other to the south. Weather systems continue to be drawn along by the jet stream, but as they approach a stationary anticyclone or cyclone they slow and then move around it. The stationary pressure systems block the passage of frontal weather systems. This phenomenon is called blocking; a stationary anticyclone of this kind is a blocking high, and a cyclone is a blocking low. While blocking continues the weather remains constant within the areas covered by the anticyclones and cyclones. Blocking highs bring spells of fine, settled weather to middle latitudes, and the weather is warm for the time of year because the anticyclones contain tropical air. Farther south (or north in the Southern Hemisphere), blocking lows bring polar air with unseasonably cool, cloudy, and often wet weather.

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OCEANS AND CLIMATES

Winds blowing over the surface of the ocean push the water. This makes waves at the surface, and it also drives surface currents. Since these are wind-driven, not surprisingly tropical currents flow from east to west, following the direction of the prevailing trade winds. Midlatitude currents flow from west to east, which is the direction of the prevailing midlatitude westerlies, and inside the Arctic and Antarctic Circles the winds, and therefore currents, travel from east to west. This makes it sound as though the winds and currents move in parallel belts as streams flowing in opposite directions, so that a jellyfish drifting with the surface current would abruptly change direction if chance should carry it across from one stream and into the adjacent one. In the Southern Ocean the current does flow in this way. It is called the Antarctic Circumpolar Current, or West Wind Drift, and it flows from west to east all the way around the world, driven by the Southern Hemisphere midlatitude westerlies and forming a broad belt of moving water approximately between latitudes 60°S and 66.5°S—the Antarctic Circle. It is able to do so because no continental landmass extends south of 60°S to interrupt the flow. Cape Horn, the southernmost point of South America, is at 55.78°S. Everywhere else, both easterly and westerly currents are deflected by the continents in a direction taking them

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away from the equator. At the same time, on the very large scale of an ocean flowing water does not move in straight lines. Because of the rotation of the Earth, it tends to flow in circles (see “The Coriolis Effect” on pages 79). Once it has moved far enough from the equator, where the Coriolis effect does not exist, ocean currents swing to the right in the Northern Hemisphere and to the left in the Southern Hemisphere. Eventually they are crossing the ocean in the opposite direction. Then, as they approach the continent on the other side of the ocean, they are deflected once more, this time toward the equator, while the Coriolis effect continues to deflect them. The overall result is that in each of the oceans and larger seas there is an approximately circular system of currents, called a gyre.

Ocean Gyres In the tropical North Atlantic the combined Guiana Current and Atlantic North Equatorial Current, flowing westward, enter the Caribbean and Gulf of Mexico. The current passes Florida as the Florida Current and then follows the coast of the Carolinas before the influence of the Coriolis effect sends it heading across the ocean in a northeasterly direction as the Gulf Stream. The current originated in the Tropics, and it carries warm water. In about the latitude of Boston part of the current swings farther to the right as it approaches Europe, heading south along the North African coast as the Canary Current. Its water is now cooler than that of the more southerly ocean, but the Canary Current joins the eastern end of the Atlantic North Equatorial Current, and its water warms under the tropical Sun as it moves westward parallel to the equator to complete the North Atlantic gyre. Not all of the Gulf Stream turns south. A branch of it continues flowing in a northeastly direction as the North Atlantic Current or North Atlantic Drift, passing the western shores of the British Isles and then Norway, where it becomes the Norway Current that rounds the North Cape and ends in the Barents Sea. Another, smaller branch flows along the western coast of Greenland as the West Greenland Current. All these branches of the Gulf Stream are warm currents. There is a similar gyre in the South Atlantic. The warm Brazil Current flows southward down the eastern coast of South America, turns to the east and becomes the cold South Atlantic Current, then heads northward along the African coast as the cold Benguela Current. In the North Pacific gyre, the Pacific North Equatorial and Kuroshio Currents carry warm water from the Tropics along the eastern coast of Asia and return to the Tropics as the cold California Current, flowing past the western coast of North America. In the South Pacific the Pacific South Equatorial and East Australian Currents carry warm water southward past Australia, and the South Pacific and Peru (or

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Humboldt) Currents carry cold water north, past the western coast of South America. Obviously, the Atlantic and Pacific Ocean gyres transport heat from low to high latitudes, from equatorial regions where in late summer the sea-surface temperature commonly reaches 85°F (29°C) to the very edge of the permanent sea ice. This transport greatly moderates global climates. Without it the polar regions would be much colder than they are and the equatorial regions much warmer.

The Great Conveyor It is not only the wind that drives the movement of ocean water. There is another “machine” at work on an even bigger scale. It is called the Great Conveyor. As water cools its molecules move closer together, causing the density of the water to increase. This continues until the density reaches a maximum. In seawater the temperature at which density is greatest depends on the salinity, but for water with the average ocean salinity of 35‰ (parts per thousand, pronounced “per mil”) the maximum density is reached at 32°F (0°C). Seawater with this salinity freezes at 28.6°F (-1.9°C). As the temperature continues to fall, water molecules start forming ice crystals. These have an open structure— they form hexagonal shapes with a space at the center—so the density decreases. Ice is less dense than water that is just above freezing temperature, which is why ice floats. The water at the edge of the sea ice is therefore at its greatest density due to its temperature. It is also very saline. This is because when ice crystals form, only water molecules are involved. Substances dissolved in the water are left behind. The addition of salt further increases the density of the liquid water surrounding the ice. The dense water sinks, and in the North Atlantic, between Iceland and Greenland, dense surface water sinks all the way to the ocean floor, forming the North Atlantic Deep Water (NADW). This drives a thermohaline circulation (see the sidebar). The NADW flows close to the ocean floor, across the equator and all the way to the edge of the Antarctic Circle, where it rises and joins the Antarctic Circumpolar Current flowing from west to east. Part of the Antarctic Circumpolar Current breaks away northward, into the Indian Ocean, turning southward to the south of Sri Lanka. It passes the eastern coast of Africa and Madagascar, rounds the southern tip of Africa, then travels diagonally across the Atlantic until it reaches the North American coast close to Nova Scotia. The current turns eastward then northward, passing around Iceland to the place where the NADW forms. The Antarctic Circumpolar Current continues eastward, passing to the south of Australia and New Zealand before turning northward, flowing about 3,500 feet (1,070 m) below

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Thermohaline Circulation As the Sun shines on the surface of the ocean, water at the ocean surface absorbs its energy and grows warmer. Warm water is very slightly less dense than cold water. One cubic inch of water at 86°F and at standard sea-level air pressure, for example, weighs 0.571 ounce (0.99565 g/cm3 at 30°C). At 50°F one cubic inch of water weighs 0.573 ounce (0.9997 g/cm3 at 10°C). The difference is small, but it is sufficient to ensure that the warm surface water floats above the colder water below. There is very little mixing between the warm surface water and the cold deep water. It is only in low latitudes, where the sunshine is most intense, that the surface water is ever warm enough to maintain this separation, however, and in Arctic and Antarctic waters the situation is very different. In these latitudes water at the ocean surface is often warmer than the air above it. The surface water loses heat to the air and becomes cooler, and therefore denser, than the water below. It may grow cold enough for ice crystals to form. As seawater freezes the salt dissolved in the water is expelled, so the ice crystals that form consist of pure water (although small amounts of salt water may be trapped between crystals). The salt expelled by the freezing process enters the water adjacent to the ice, increasing its density by adding salt molecules to the water molecules without altering the volume of water. This water is, therefore, denser than the water below because it is colder and also because it is saltier. The most abundant salt in seawater is sodium chloride, but the water also contains other chloride salts. The saltiness of water is commonly measured by the amount of chloride it contains, ignoring the metals bonded to the chloride. This measure is known as the halinity of the water. The denser water sinks, and less-dense water replaces it at the surface. This process establishes a vertical circulation of ocean water. Because it is driven by differences in temperature and halinity, it is known as the thermohaline circulation.

the surface. It crosses the equator, makes a clockwise loop in the North Pacific, then flows westward past the Philippines between the islands of Indonesia and into the Indian Ocean, where it joins the part of the current that broke away earlier.

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The Great Conveyor. The map shows the path followed by this closed system of ocean currents.

This continuous system of currents forms a closed loop. This is the Great Conveyor. The map shows its route. The NADW transports very cold water away from the North Atlantic. The water remains cold until it rises in the South Pacific. It grows warmer as it passes through the Tropics and finally reaches Iceland and Greenland as warm water. The conveyor transports heat from the equator to the North Atlantic. The Great Conveyor has a powerful influence on the global climate. When it flows strongly, there are more hurricanes in the Caribbean and Atlantic, rainfall is heavy along the southern edge of the Sahara, there are few El Niño events, and global temperatures fall. When it flows weakly, there are fewer hurricanes, El Niño events are more frequent, rainfall along the southern Sahara is at or below average, and global temperatures are higher.

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Ocean Currents and the Weather They Bring As an air mass crosses an ocean, the temperature of the water beneath it modifies its characteristics. This produces marked differences between the climates of the eastern and western coasts of continents. The differences affect average temperatures, but they affect precipitation much more dramatically. The rate of evaporation is higher from warm water than from cool water because the water molecules have more energy, so more of them can break free from the hydrogen bonds that hold them to their neighbors. The amount of water vapor air can hold increases with the temperature of the air. Air that is warm because of its contact with warm water is able to hold a large amount of water vapor. In the course of its long journey over a warm ocean, air becomes very moist. Less water evaporates from cold water, and air chilled by contact with it can hold less water, so an air mass passing over cold water loses much of the water it was carrying and becomes dry. Ocean gyres circulate in such a way as to produce currents of warm water that flow parallel to the eastern coasts

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of continents and currents of cool water that flow along the western coasts (see the sidebar “Boundary Currents” on page 22). Consequently, the western coasts of midlatitude continents usually have drier climates than the eastern coasts. Northwestern Europe is an exception, because the North Atlantic Drift brings warm water to its shores. Farther south, the cool Canary Current washes Portuguese shores. The difference can be seen by comparing the climates of Brisbane, on the eastern coast of Queensland, Australia, and Antofagasta, Chile, on the western coast of South America. Brisbane experiences the warm East Australian Current, Antofagasta the cold Peru Current. In Brisbane, at 27°S, the average daytime temperature in the four warmest months is 84°F (29°C) and the average nighttime temperature in the four coolest months is 51°F (11°C). The comparable figures for Antofagasta, somewhat closer to the equator at 23°S, are 62°F (17°C) and 52°F (11°C). It seldom rains in Antofagasta, the annual average being 0.5 inch (13 mm). Brisbane receives an average of 45 inches (1,135 mm) a year, with rather more rain in summer than in winter but no month without rain. Behind Brisbane there is rich farmland and pastureland. Antofagasta is in the Atacama Desert. The South Atlantic gyre produces similar contrasts. Rio de Janeiro, Brazil, and Walvis Bay, Namibia, are both in almost precisely the same latitude, Rio (22.1°S) on the eastern coast of South America and Walvis Bay (22.7°S) on the western coast of Africa. At Rio the average daytime temperature in the four warmest months is 84°F (29°C), and the average nighttime temperature in the four coolest months is 64°F (18°C). The equivalent temperatures in Walvis Bay are 74°F (23°C) and 47°F (8°C). Rio receives 43 inches (1,085 mm) of rain a year, distributed fairly evenly. Walvis Bay has an average of 0.8 inch (20 mm) of rain a year. There are wooded hills behind Rio. Walvis Bay lies in the Namib Desert. In the Northern Hemisphere it is the cold California Current that brings dry weather to southern California. San Diego receives only 10 inches (259 mm) of rain a year compared with the 48 inches (1,209 mm) that falls in Charleston, South Carolina, in the same latitude but close to the warm Gulf Stream. Ocean currents, therefore, contribute to the climates of the subtropical deserts.

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DESERT WEATHER

Deserts are dry. In particular, their soils are dry. Just how dry depends on the temperature and winds as well as on the amount of precipitation. A desert climate is one in which more water evaporates from the ground over the course of a year than the ground receives as rain or snow in the same period. Obviously, over any extended period the ground cannot lose more water than it receives, so instead of evapo-

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ration what matters is potential evapotranspiration. That is the rate at which water would evaporate from the ground surface and be transpired by plants (see “Transpiration” on page 102) if there were an unlimited supply of water. It is close to the rate at which water would evaporate from an open water surface and can be measured using an evaporation pan. This is a container of a standard size that is placed in the open exposed to the air and filled with water. The water depth is measured at the beginning and end of a useful period. The rate of evaporation is calculated from the change in depth after allowance has been made for the small difference between evaporation from open water and from wet ground and the amount of any precipitation that fell during the period has been deducted. In the cool, moist climate of northern Europe about eight inches (203 mm) of water evaporates in a year. In parts of the Sahara the potential evaporation exceeds 90 inches (2,286 mm) a year. This vastly exceeds the annual precipitation, and it defines the region as desert. The concept also has another significance. If the land is to grow crops, the amount of water supplied by irrigation must exceed the potential evapotranspiration during the growing season. Although water evaporates faster the higher the temperature, a cold climate can also be a desert climate if the potential evapotranspiration exceeds precipitation. Water evaporates so readily in hot deserts because the temperature is high and the air very dry. For much of the time the sky is cloudless, and by day the Sun beats down mercilessly. More than one-third of the incoming radiation is reflected. Dry sand has an albedo (reflectivity) of 35–40 percent (see “Why Antarctica Is Cold and Dry” on pages 36–37). The reflected radiation does not heat the ground. It goes directly back into space. Once the absorbed heat has raised the surface temperature, the surface itself begins radiating heat back into space. During the morning the air temperature over the desert rises steadily. Noon passes, and the air continues to grow hotter. At In Salah, Algeria, the temperature in July averages 113°F (45°C), and it has been known to reach 127°F (53°C). It is not until the middle of the afternoon, between 3:00 and 4:00, that the temperature reaches its maximum, after which it starts falling, slowly at first and more rapidly after sunset. The night is cool, but in summer it is cool only by comparison with the daytime heat. At In Salah the July average minimum temperature is 83°F (28°C), and it never falls below 70°F (21°C).

Specific Heat Capacity This is only part of the story, however, because so far as living organisms are concerned what matters is the climate at ground level, and in the middle of the day the ground is much hotter than the air. On a hot summer day even in

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Atmosphere and Desert temperate latitudes dry sand can be hot enough to hurt bare feet. In the Sahara it can burn them. Sand and rock can reach 170°F (77°C). Sand and rock heat up much more rapidly than water, and they also lose heat much faster. When a substance is exposed to heat, its temperature rises, but different substances warm at different rates. The amount of heat that must be applied to a substance to make its temperature rise by one degree is called the specific heat capacity of that substance, and with most substances it varies slightly at different temperatures. The specific heat capacity of water at 59°F (15°C) is 4.19 (the units are joules per gram per degree Celsius, but this is not important for the purpose of comparing substances). The specific heat capacity of dry sand and most types of solid rock, at temperatures between 68°F and 212°F (20–100°C), is about 0.8. This means it requires more than 5 times more heat energy to raise the temperature of water by one degree than it does to raise the temperature of rock or sand by the same amount. Consequently, as the Sun climbs higher in the sky, the dry ground heats much faster than water and much faster than the ground would if it were wet, because then its specific heat capacity would be about 1.48. By the time the Sun passes its zenith and begins to sink, the dry ground has reached a much higher temperature than nearby water. Then both dry ground and water start to cool. Just as its specific heat capacity measures the amount of heat needed to raise the temperature of a substance, it also measures the rate at which heat is lost. Sand and rock warm more quickly than water, and for precisely the same reason they also cool more quickly. The ground temperature falls sharply at night, reaching a minimum an hour or two before dawn, but the temperature of water falls much more slowly. Air is heated by the surface beneath it, so the air temperature depends on the surface temperature. Over the ocean in subtropical latitudes the difference in day and night air temperatures (the diurnal temperature range) is about 0.4°F (0.2°C). Over the desert in the same latitude the diurnal temperature range is about 72°F (40°C).

Conductivity and Damping Depth Its low specific heat capacity is only part of the reason why sand heats so fast and reaches such a high temperature. Dry sand consists of irregularly shaped grains that are stacked together loosely, with air filling the spaces between them. Sand is a poor conductor of heat, and air is an even poorer one. Radiant heat raises the surface temperature, but the heat is not conducted very far below the surface. The upper, heated layer just goes on getting hotter, but a little way below the surface the temperature hardly changes. As the surface temperature rises during the morning, heat is conducted below the surface, but it happens slowly and reaches only a certain depth before the daily peak tem-

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perature is passed. Beyond this depth, therefore, the temperature does not alter. It is called the damping depth, and in dry sand it is about three inches (7.6 cm) below the surface. Because heat penetrates slowly, the peak temperature is reached at the damping depth several hours later than it is reached at the surface. Subtropical deserts experience seasons, so there is a seasonal cycle of rising and falling temperature as well as a daily one. The gradual warming during spring and summer also heats the ground, but in many deserts the seasonal temperature range is smaller than the diurnal range, so the effect is not great. The damping depth for the annual cycle is about 0.6 inch (1.5 cm). This is true only in subtropical deserts, however. Deserts in higher latitudes are found in the interior of continents or in the rain shadow of mountains. In these places the climate is arid either because approaching air loses its moisture crossing a mountain range or because of the distance from the nearest ocean. In such continental climates seasonal temperature differences can be extreme, with scorching summers and very severe winters. In Death Valley, California, for instance, summer temperatures sometimes exceed 130°F (54°F), but in January they have been known to fall to 15°F (-9°C). The seasonal variation may increase the annual damping depth, but not beyond 3 inches (7.6 cm), because this depth is determined by a physical property of sand. Conditions below ground, therefore, are very different from those at the surface. The ground may be blisteringly hot, but just a few inches down the temperature is quite tolerable and remains almost constant throughout the day.

Microclimates and Macroclimates In other words, the climate a few inches below the ground differs markedly from the climate at the surface. A local climate of this kind is called a microclimate within the macroclimate of the region as a whole. Many living organisms exploit the advantages offered by microclimates. It is not the only desert microclimate. There are several immediately above the ground surface. Hollows that are most of the time in shade will be cooler than exposed surfaces, for example, and exposure to and shelter from prevailing winds also makes a difference. Above ground the air is much cooler than the surface of the sand or bare rock. At a height of 6.5 feet (2 m) the midday temperature may be 55°F (30°C) lower than the temperature at ground level. This produces another microclimate. Early in the morning the ground is cold. As the Sun rises and its warmth intensifies, heat is absorbed by the ground. During the hottest part of the day energy is transferred from the ground to the air by convection. Air is heated by contact with the very hot ground and rises strongly, creating extremely turbulent conditions. At night the ground

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surface is warmed by heat from the layers above the damping depth. A desert has one macroclimate but many microclimates that change in the course of the day and in higher latitudes in the course of the year. There is one distinct region below the damping depth. There, about 12 inches (30 cm) below ground level, the temperature hardly changes. It is warm when temperatures above the surface are low and cool when they are high. The ground surface is the place of greatest extremes, but even there conditions are more moderate in the microclimates found in sheltered places. Higher still, the temperature decreases with height.

Rain Shadows A macroclimate affects a very large area, such as an entire continent. A microclimate affects a very small area, such as a depression in a desert surface that is in shadow for most of the day. Between these two there are mesoclimates. A mesoclimate affects an area, such as the Great Plains of North America, that is much bigger than the area covered by a microclimate but smaller than that covered by a macroclimate. Death Valley, California, on the eastern side of the Sierra Nevada, has a mesoclimate. It is a desert at least partly because of the mountains. Its mesoclimate is the result of a rain shadow. The Atacama Desert also lies in a rain shadow. Weather systems approach California from the west carried by the midlatitude westerly movement of air, and they approach tropical Chile from the east carried by the southeasterly trade winds. In both cases the weather systems must cross a range of mountains. When air is forced to rise over a mountain range—orographic lifting—its temperature decreases adiabatically (see the sidebar “Adiabatic Cooling and Warming” on page 45). The rate at which the temperature falls in rising air, known as the lapse rate, depends on how much moisture the air contains. If the air is not saturated, it cools at the dry adiabatic lapse rate (DALR) of 5.4°F for every 1,000 feet (9.8°C/1,000 m). As its temperature decreases, the air’s capacity for holding moisture also decreases and continues to do so until the air reaches its dew point temperature, at which atmospheric water vapor starts condensing into liquid droplets that form cloud. Condensation releases latent heat, which warms the surrounding air and reduces the rate at which the temperature decreases as the air continues to rise. The air then cools at the saturated adiabatic lapse rate (SALR). This varies depending on the amount of condensation and, therefore, of the amount of latent heat being released, but it averages about 3°F for every 1,000 feet (5.5°C/1,000 m). As the air temperature continues to fall, more and more moisture condenses. The clouds grow bigger, and the moisture falls as rain or snow.

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Suppose the air approaching the foot of the mountains at sea level is at 80°F (27°C), that its dew point temperature is 54°F (12°C), and that the mountains are 10,000 feet (3,000 m) high. The rising air cools at the DALR, reaching its dew point temperature at an elevation of about 4,700 feet (1,430 m). It then continues to cool at the SALR. When it reaches the summit the temperature of the air is 38°F (3.3°C). Water vapor is still condensing at the summit. This means that the relative humidity (see “Saturation and Humidity” on pages 7–8) is 100 percent. Therefore, at the summit the dew point temperature and actual air temperature must be the same: 38°F (3.3°C). The air now descends the lee side of the mountains. As it descends, the air warms adiabatically. At once its temperature rises above its dew point temperature of 38°F (3.3°C). The air is then unsaturated (its relative humidity is less than 100 percent), so as it sinks it warms at the DALR. By the time the air reaches sea level its temperature has risen to 93°F (34°C). Air that was warm and moist as it approached the mountains is hot and extremely dry by the time it reaches the plain on the lee side. The plain lies in a rain shadow. Wind speed often increases with increasing elevation up a mountainside and is strongest at the summit. The wind is caused by approaching air pushing against the side of the mountain and producing vertical air movements. These continue over the summit, where the wind accelerates, but a further effect of this movement is to greatly reduce wind speed at the base of the mountains on both sides. Rain shadow deserts are often calm places where winds are light. This is far from true in deserts of other types.

Why Hot Deserts Are Windy Deserts are often windy places, but the reason the wind blows varies from one type of desert to another. The wind that blows for most of the year across the central Sahara is known as the harmattan. It is the northeasterly trade wind, in summer blowing from the Mediterranean and in winter, when the ITCZ is close to its northerly limit, from southern Europe at about 30°N. The harmattan is a warm, dry wind. It becomes a hot and still drier wind as it sweeps across the sandy desert. This wind is so dry that in daytime it will harden leather and make wood warp until it splits. It is a daytime wind, however. Saharan nights are calm. During the day the ground surface grows extremely hot, but the temperature decreases rapidly with increasing height. Hot air rises and cools (see the sidebar “Adiabatic Cooling and Warming” on page 45), but its temperature falls more slowly than the temperature of the surrounding air. As it rises the air always remains warmer than the surrounding air, so it continues to rise. Air that keeps on rising in this way is said to be unstable. There is a temperature inversion—a layer of warm air overlying cooler air—about 5,000–6,000 feet (1,500–2,000 m)

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Atmosphere and Desert above the surface of the northern Sahara. Known as the trade wind inversion, it is produced by cooler air moving toward the equator beneath the warm air subsiding at the edge of the Hadley cell (see “Hadley Cells” on pages 73–77). The rising air cannot penetrate the inversion, and its temperature does not fall low enough for the little water vapor it carries to condense and form cloud. The result is that the air trapped beneath the inversion grows hotter and hotter. This is the air that constitutes the harmattan wind. In summer weather conditions in the Gobi and Takla Makan Deserts of central Asia are similar to those in the Sahara. There is a temperature inversion that keeps the lower air hot and dry and a wind, called the karaburan, that blows in daytime from early spring until the end of summer. Nights are calm, but by day the east-northeasterly karaburan often blows with gale force. The karaburan is generated by a change in the distribution of pressure. The anticyclone that sits over central Asia in winter breaks down as the ground begins to warm in spring. As the surface pressure decreases, air is drawn into the region. By the middle of summer, when the high pressure of winter has given way to low pressure, the wind becomes very strong around the center of the low. This is the basis for the karaburan. In the Namib Desert the predominant winds are the southeasterly trade winds, but these are overwhelmed by the much stronger land and sea breezes (see the sidebar “Land and Sea Breezes” on page 22). The sea breeze, commencing on most days at about 10 a.m., is the stronger of these, often blowing at more than 30 MPH (48 km/h) during the afternoon in the southern part of the desert. Winds are generally stronger over the sea than they are over land. This is because obstructions such buildings and trees cause friction that slows the wind crossing land. There is much less friction between the wind and the surface of water, so there the wind speed is higher. Desert surfaces, too, are free from trees and buildings. There are fewer obstructions to slow the wind than there are outside the deserts. In this respect the desert surface is more like that of the sea than of the land, and with less friction the wind is able to blow with greater force.

Why Antarctica Is the Windiest Continent Antarctica is surrounded by the Southern Ocean, across which the weather systems travel from west to east. There is very little land to obstruct the winds, and the weather systems tend to be more intense than those at similar latitudes in the Northern Hemisphere. The frequent gales give the region around latitude 40°S its name of “roaring forties.” Farther south, however, there are the even more violent “furious fifties” and “screaming sixties.” The Antarctic Peninsula

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and parts of Wilkes Land in East Antarctica extend to about 65°S and lie within the screaming sixties. Gales are fierce and frequent, and the storms bring heavy snow. The coasts of Antarctica do not have a desert climate. Farther south the prevailing winds are from the east. Blizzards are common, but these are blizzards of light, powdery snow that is blown from the ground, like the sand in a desert sandstorm. Byrd Station, at approximately 80°S in Marie Byrd Land, experiences blizzard conditions about 65 percent of the time, and the blizzards often reduce visibility to zero. This type of weather is very common, especially in winter, when winds can reach 90 MPH (145 km/h) and blizzards can last for several days and occasionally for more than two weeks. In summer average wind speeds fall to about 20 MPH (32 km/h). These winter winds, called katabatic winds from the Greek verb katabainō, meaning “go down,” are caused by gravity. The West and East Antarctic ice sheets are domeshaped, and the air near the center of each of the two ice sheets is extremely cold and dense, producing high surface pressure. Dense air flows outward from the anticyclone, moving downhill under the influence of gravity, pushing beneath less-dense air at lower levels, and accelerating all the way like a ball rolling down a very smooth hillside. The combination of the pressure gradient, from high pressure near the center of the continent to lower pressure near the coast, and the topographic gradient providing slopes down which the dense air rolls, generates ferocious winds. The winds are strongest where the pressure gradient and topographic gradient both slope in the same direction. The Greenland ice sheet is also dome-shaped. It, too, causes katabatic winds to flow down the sides of the dome. These are less severe than the Antarctic winds, however, because the ice sheet is smaller and the air has a shorter distance to travel.

DUST STORMS, SANDSTORMS, DUST DEVILS, AND WHIRLWINDS

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Weather systems in middle latitudes travel from west to east. They bring areas of low and high atmospheric pressure, and, associated with the highs and lows, they bring winds. Sometimes they bring very strong winds. Air flows from an area of high pressure toward an area of low pressure in order to equalize the pressures in the two areas. The difference in pressure between the centers of high and low pressure constitutes a pressure gradient, and the movement of air is driven by a pressure gradient force, or PGF. The pressure gradient can be depicted on a weather chart in the same way that a hillside can be depicted on a map. A map contour is a line joining points all of which are

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at the same elevation, so the distance between contour lines shows the rate at which elevation changes with horizontal distance—the gradient. On a weather map a line joining points at which the atmospheric pressure is the same is called an isobar. The Greek isos means “equal,” baros means “weight,” and so isobarēs means “of equal weight.”

Why the Wind Flows in Circles One might expect the wind to flow at right angles to the isobars, following the most direct route from the center of high pressure to the center of low pressure. That would equalize the pressure very rapidly. Then there would be no winds, because pressure differences would vanish the moment they started to develop. This is not what happens. There are winds and they can last for hours or days on end, although their speed is still proportional to the pressure gradient and it is still the PGF that drives them. What happens, though, is that instead of blowing across the isobars, well clear of the ground the wind blows parallel to them. A planetary boundary layer extends from the surface to about 1,500–3,000 feet (460–900 m), depending on the type of surface. In this region of the atmosphere friction between the moving air and the surface slows the wind, causing it to blow at an angle to the isobars, in the direction of the low-pressure center. Winds are driven by the PGF, but as soon as air begins to move on a large scale across the surface of the Earth, it is subject to the Coriolis effect, or CorF (see “The Coriolis Effect” on page 79). The CorF acts at right angles to the direction of flow, with a magnitude proportional to the speed, deflecting the flow to the right in the Northern Hemisphere and to the left in the Southern. The drawing illustrates what

Why the wind flows parallel to the isobars. The pressuregradient force (PGF) drives air from the region of high pressure toward the region of low pressure. The Coriolis effect (CorF) deflects the air, and the PGF and CorF reach a balance, with the wind flowing parallel to the isobars.

low pressure

PGF

PGF PGF

isobar wind dire

ction

CorF high pressure © Infobase Publishing

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isobar CorF

isobar isobar

happens. The parallel lines in the diagram are isobars. The PGF acts at right angles to the isobars. As the air starts to move, the CorF, acting at right angles to the direction of motion, swings it to the right. Between them the PGF and CorF produce a resultant force that accelerates the wind, but acceleration increases the CorF, swinging the wind farther to the right until it flows parallel to the isobars. The PGF and CorF are then acting in opposite directions, the PGF at right angles to the isobars, the CorF at right angles to the direction of flow, and the situation is stable. If the PGF increases, it will swing the wind to the left and accelerate it. This will increase the CorF, which will pull it back. If the wind accelerates, the CorF will swing it farther to the right, but it will encounter increased resistance from the PGF. This will slow it, reducing the CorF and turning it back again.

Dust Storms and Sandstorms Weather systems and their associated winds take several days to develop and several more days to dissipate, and in middle latitudes they are usually traveling eastward for the whole of this time. They bring no rain to the desert, because desert air is very dry. It is possible for the relative humidity (RH) to exceed 80 percent near coasts, but inland the RH is usually 10–30 percent, and it can be as low as 2 percent. A relative humidity of 80 percent is high enough for some cloud to form if the air also contains a plentiful supply of salt crystals. Cloud droplets form when water vapor condenses onto microscopic particles called cloud condensation nuclei (CCN). Air in the lower atmosphere contains large numbers of suitable particles. In very clean air with no particles, droplets do not form until the RH exceeds about 300 percent. Salt crystals, which enter the air when sea spray evaporates, are among the most effective CCN. They are hygroscopic, which means they readily combine with water vapor to form liquid droplets. With other types of CCN the RH must be close to 100 percent before cloud will develop. Cloud that forms along the coast can then be carried inland on the prevailing wind. The weather seldom brings rain to the desert, but it often brings winds blowing at 20 MPH (32 km/h) or more. Wind blowing at this speed will raise dust particles and sand grains from the ground. Not all deserts are sandy, of course, but sandstorms are common in those that are, and other desert surfaces lack sand because the wind has swept them clean. The khamsin (see “Shifting Dunes” on page 14) and harmattan are local winds that often produce dust storms and sandstorms—the two types of storm differ only in the size of particles they carry. Both can be severe, and people shelter from them, sealing all windows and doors and covering exposed parts of their bodies. The hot wind produces huge

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Atmosphere and Desert

People try to remain indoors during a sandstorm. Visibility is much reduced, and sand penetrates eyes, ears, noses, and clothing. This sandstorm is over Khartoum, Sudan. (Jan Portonk)

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ing the heated air. As air converges on what is then a very small area of low surface pressure, vorticity sets it rotating (see “Rossby Waves and Vorticity” on pages 85–86). In the Northern Hemisphere it turns counterclockwise. The air turns around the low-pressure center, but friction with the ground prevents it from moving parallel to the isobars. It flows at an angle across the isobars, spiraling inward, and, to conserve its angular momentum, the smaller its spiraling radius the faster the wind blows. Convergence means that the air must also rise, so it spirals inward and upward. It blows fast enough to gather dust and sand, which it carries with it and which make the moving air visible. Small, spiraling columns of dust and sand are called dust devils, and they are very common, especially in the afternoon when the ground is at its hottest. They are common but harmless. Dust devils have enough energy to rise only a short distance from the ground, and they last for only a few seconds before the movement of air eliminates the pressure difference that causes them.

Whirlwinds clouds of swirling dust and sand, dense enough to reduce visibility almost to zero. Storms of this kind are known as andhis in India and simoom in the Sahara. The particles are harmful to the eyes. The Egyptians have several proverbs referring to blindness and failing eyesight that are distressingly common and due mainly to dust. Strong surface heating can also generate windstorms. When the surface temperature is very high, the rate at which temperature decreases with height—the lapse rate—increases in the lower part of the atmosphere. At night, when the surface has cooled to below the temperature of the layer of air some distance above the ground, the situation is reversed. From the late morning until late in the afternoon, air that is being heated strongly from below is expanding and becoming less dense. Denser air is sinking beneath it, lifting it from the ground. This generates considerable turbulence, with strong low-level winds that raise dust and sand. The resulting storms are commonest in the afternoon, and they can make any outdoor activity difficult.

Dust Devils Dust storms and sandstorms are big. They darken the sky, and their approach can be seen from afar. The desert also has other winds that produce effects that are smaller but no less dramatic. Surface heating can be very local. Because of its surface texture, composition, or the way it is angled in relation to the rays of the Sun, a patch of sand or rock can become much hotter than its surroundings. Air over the hot patch expands, and cooler, denser surrounding air rushes in, lift-

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There are also much larger versions, however, and those are much more alarming. They are the whirlwinds that were very familiar to the authors of the Old Testament. “When your fear cometh as desolation, And your destruction cometh as a whirlwind” (Proverbs 1:27). “Behold, a whirlwind of the Lord is gone forth in fury, even a grievous whirlwind” (Jeremiah 23:19). Whirlwinds are generated in the same way as dust devils. The names are often used interchangeably, but whirlwind in the biblical sense means something much more terrifying than the swirl of air that raises dust, dry leaves, and scraps of paper. A whirlwind funnel can reach to a height of more than 6,000 feet (1,830 m), and its winds are strong enough to demolish flimsy buildings and the tents of desert nomads. They look like tornadoes but never generate wind speeds approaching those of a genuine tornado. Unlike tornadoes, they grow from the ground up rather than descending from a cloud. There is no black cloud above a whirlwind. The lack of a storm cloud makes them more frightening, not less. Predicting tornadoes is very difficult, but at least meteorologists have no trouble identifying tornadic storms—storms capable of producing tornadoes—and these can be seen approaching long before they arrive. Whirlwinds rise without warning, like screaming wraiths, and very often they occur in families so several are active at the same time, all wandering about erratically and unpredictably. No individual lasts for more than a few minutes, but as one dies down another arises nearby. It is little wonder that in ancient times people familiar with the desert compared them to armies descending with destructive fury on hapless and helpless villages.

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4 Biology of Deserts Deserts often appear lifeless. In some areas the ground seems devoid of plants of any kind, and with no plants there can be no animal life. Elsewhere there may be scattered shrubs or tough grasses. Appearances are often deceptive, and despite their barren appearance deserts do support living organisms. This chapter describes some of them and the way they live. Plants form the basis of all life on land, because it is only plants (apart from some bacteria) that are able to harness the energy of sunlight and use it to manufacture carbohydrates. Photosynthesis is the process by which they achieve this, and the chapter begins by explaining photosynthesis. Plants need water to supply the hydrogen needed for photosynthesis, as a medium for the transport of nutrients, and to provide physical support to their cells. The chapter explains how they obtain water from the soil and how they use it and conserve it. Desert plants are adapted to the conditions in which they live, and the chapter continues by describing a number of typical desert plants. This leads on to a description of desert animals and the ways in which animals cope with high and low desert temperatures. Some animals become dormant when conditions are harsh, and the chapter describes what happens to an animal that avoids extreme cold by hibernating. Certain animals such as the camel and locust are described in more detail. Having described the life of hot deserts, the chapter turns to the cold deserts of the Arctic and Antarctic. It describes the species found in these regions. No animals bigger than small insects live on land in Antarctica, but Arctic and Antarctic seas teem with life. The chapter ends by describing whales, seals, and walruses.

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With the midday Sun beating down mercilessly, there is little sign of life. No animal crawls or scuttles across the ground. No bird crosses the sky or circles slowly, searching for a meal that does not exist. The desert seems dead.

It is not dead. A closer look reveals a few plants. There are not many, they are scattered widely, but they are present, and if there are plants, somewhere there will be animals that feed on plants. If there are herbivorous—plant-eating—animals there will also be carnivores—meat eaters—preying on them. Apart from some species living near volcanic vents on the ocean floor, all animals depend on plants for food, directly or indirectly. Without plants there can be no animals, and there is one substance, chlorophyll, without which there can be no plants. True, there are parasitic plants that lack chlorophyll and live by taking the nutrients they need from other plants, but those other plants possess chlorophyll. Indeed, without chlorophyll there could be no land-dwelling organism bigger or more complicated than a bacterium. Chlorophyll is a chemical compound that absorbs units of light energy called photons. Only photons with a particular amount of energy, corresponding to particular wavelengths of light, can be absorbed. These photons possess precisely the amount of energy needed to raise an electron in the chlorophyll molecule to a higher energy level—from its ground state to its excited state. A neighboring molecule, called the primary electron acceptor, then captures that electron, after which electrons pass from molecule to molecule along an electron transport chain. As electrons move along the chain their energy is used to drive a series of chemical reactions. These reactions convert carbon dioxide, taken from the air, and water, taken from the ground, into sugars. The process is called photosynthesis, and it can be summarized as: 6CO2 + 6H2O + light energy → C6H1206 + 6O2↑ In the products of the equation, C6H1206 is glucose, a sugar (though not actually the sugar produced by photosynthesis, which is a more complex substance). The oxygen is released into the air, as indicated by the arrow pointing upward. Chlorophyll is the pigment that gives plants their green color. Plants contain two types of chlorophyll, and there are

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Biology of Deserts other, accessory pigments that contribute to photosynthesis. Chlorophyll a is blue-green, chlorophyll b is yellow-green, and carotenoids are various shades of yellow and orange. Carotenoids can be oxidized into the brightly colored red, orange, and yellow xanthophylls that produce the colors of fall in temperate forests. Visible colors are those of the light that is reflected from objects. A green plant looks green because it reflects green light. Obviously, light that is reflected cannot be absorbed, so although natural greenery appears wholesome, green is the one color for which plants have no use.

Stomata Carbon dioxide, one of the raw materials for photosynthesis, is taken from the air. Oxygen, the waste product of the photosynthetic reactions, returns to the air. For this to be possible, plants must have openings to allow carbon dioxide to enter and oxygen to leave. These openings are called stomata (the singular is stoma, the Greek word for “mouth”). Photosynthesis takes place in the cells just below the protective outer layer of cells, called the cuticle, and the stomata are tiny pores in the leaf cuticle. There are usually more on the underside of leaves than on the upper side. Plant stems also need to absorb and release gases; they do so through pores called lenticels, which are constructed differently from stomata. Stomata are able to open and close by means of two guard cells on either side of each stoma. When the guard cells absorb water from the adjacent plant tissue, they swell. This makes the cells buckle outward, pulling their stoma open. When they lose water, the guard cells become limp and sag toward each other, closing their stoma. Gases are able to diffuse into and from the leaves when the stomata are open. There is one drawback to this system. Plant tissues contain water (see “Transpiration and Why Plants Need Water” on pages 101–103), and when the stomata are open water evaporates through them and is lost to the plant. The plant needs to exchange waste oxygen for carbon dioxide, but in doing so it cannot avoid losing moisture. Plants minimize water loss in several ways (see “C3 and C4 Plants” on pages 99–100 and “CAM” on page 100). Their first and most basic technique, however, is to open their stomata during the day and close them at night. This avoids losing moisture at night, when photosynthesis has shut down. The plant has at least three ways of “knowing” when to open its stomata. The first is its own internal clock that regulates processes according to the passing of time. All plants and animals, including humans, possess such a clock. It controls cycles of activity that repeat at intervals of approximately 24 hours, known as the circadian rhythm, from Latin circa, “about,” and dies, “day.”

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Osmosis When two solutions at different concentrations are separated by a membrane that allows the passage of water molecules but not of the larger solute molecules, water moves from the weaker solution to the stronger. This movement dilutes the stronger solution, and it continues until the concentrations are equal on each side of the selectively permeable membrane. The process is called osmosis. Osmosis exerts a pressure across the selectively permeable membrane. Osmotic pressure is proportional to the difference in concentration on each side of the membrane. Freezing damages living tissue because of osmosis. As the temperature falls below the freezing temperature, ice crystals start to form in the intercellular solution—the liquid surrounding cells. Ice crystals consist of pure water, so the concentration of the intercellular solution increases. Water then moves out of cells under osmotic pressure, leading to dehydration of the cells, which can be fatal.

In addition, each guard cell is sensitive to blue light. When it detects blue light—meaning the Sun is rising—the guard cell absorbs potassium from adjacent cells. As the potassium concentration inside the cell increases, water enters the cell to equalize the concentration inside and outside the cell by a process called osmosis (see the sidebar). The cell swells, and the stoma opens. The guard cells also open the stomata when the photosynthesizing cells are depleted of carbon dioxide. The lightindependent photosynthetic reactions continue for as long as the cell has carbon dioxide. Consequently, the concentration of carbon dioxide decreases during the night.

Chloroplasts, Where Photosynthesis Happens The photosynthetic pigments are located inside lens-shaped structures called chloroplasts. Beneath the outer layer of leaf cells is a layer of tissue called mesophyll, and each mesophyll cell contains an average of 30 to 40 chloroplasts. Each chloroplast contains an assembly of a few hundred pigment molecules, including both types of chlorophyll and carotenoids. All but two of these molecules collect light photons and pass them to the central pair of chlorophyll a molecules. These are the only ones to trigger the chain of photosynthetic reactions, and they are known as the reaction center.

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The entire assembly constitutes a photosystem. There are two types of photosystems, called photosystem I and photosystem II because of the order in which they were discovered. In photosystem I, the chlorophyll a molecules at the reaction center respond most strongly to light at the far red end of the spectrum, with a wavelength of 700 nanometers (nm), so they are called P700. The reaction center in photosystem II responds most strongly to red light at 680 nm and is called P680. Carotenoids also absorb light energy at other wavelengths and pass on their excited electrons to the reaction center.

Light-dependent Reactions Photosynthesis proceeds in two stages. In the first, light energy is used to split water into hydrogen and oxygen. The hydrogen is stored temporarily by being attached to NADP (nicotinamide adenine dinucleotide phosphate), converting it to NADPH, and the oxygen is released into the air as a by-product. These are the reactions that convert light energy into the chemical energy used in the second chain of reactions. In the course of the light-dependent reactions, some of the excited electrons in photosystem I provide energy to add a phosphate group to adenosine diphosphate (ADP), converting it to adenosine triphosphate (ATP). In doing so the electrons lose energy, revert to their ground state, and eventually return to the chlorophyll. The addition of a phosphate group is called phosphorylation, phosphorylation by means of light energy is called photophosphorylation, and because the electrons are cycled back to the chlorophyll from which they came, this version of the reaction is known as cyclic photophosphorylation. Other excited electrons are passed along an electron transport chain from photosystem I to photosystem II. These electrons are also used to convert ADP to ATP. In this case the electrons are not returned, so the process is called noncyclic photophosphorylation. Phosphorylation is the means by which energy is stored. Whenever ATP loses a phosphate group, energy is released. ATP can be transported to wherever it is needed, and so it can supply energy to any cell needing it.

Light-independent Reactions NADPH and ATP from the light reactions then drive the light-independent reactions. These form a cycle in which the starting material is regenerated. These reactions proceed at the same time as the light-dependent reactions, but they also continue when it is too dark for those reactions. The second stage in photosynthesis does not require light. The light-independent reactions constitute what is often known as the Calvin cycle, because the details of its steps were discovered by a team at the University of California at

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Berkeley led by the American chemist Melvin Calvin (see the sidebar). The Calvin cycle begins by attaching a molecule of carbon dioxide (CO2) to a molecule of ribulose biphosphate, abbreviated as RuBP, by a reaction catalyzed by the enzyme RuBP carboxylase, or rubisco. RuBP is a sugar containing five carbon atoms, so the addition of CO2 (the reaction is called carboxylation) produces a six-carbon sugar. This is unstable and immediately divides into two molecules of 3-phosphoglycerate. Each 3-phosphoglycerate molecule then receives a phosphate group, obtained from ATP, and becomes 1,3-diphosphoglycerate. NADPH then donates two electrons, reducing the 1,3-diphospho-

Melvin Calvin (1911–1997) Melvin Calvin, the chemist who discovered the sequence of reactions by which green plants convert carbon dioxide and water into carbohydrates and oxygen, was born in St. Paul, Minnesota, on April 8, 1911, the son of immigrants from Russia. Calvin was educated at the Michigan College of Mining and Technology, from which he graduated in 1931, and he obtained his Ph.D. in 1935 from the University of Minnesota. He then spent two years working at the University of Manchester, England, before moving to the University of California, where he spent the rest of his career. He was appointed university professor of chemistry in 1971. Calvin began to study photosynthesis in 1949, while he was director of the bioorganic chemistry group at the university’s Lawrence Radiation Laboratory. He used radioactive carbon-14 to trace the steps by which carbon dioxide is converted to starch. Using the single-celled green alga Chlorella, Calvin showed that the light-independent reactions are a cycle, now known as the Calvin cycle. It took him almost 15 years to work out all the reactions in the cycle. It was for this work that Calvin was awarded the 1961 Nobel Prize in chemistry. He won many prizes in addition to the Nobel. From 1960 until 1980 Calvin was director of the Laboratory of Chemical Biodynamics at Berkeley. During this time he turned his attention to the origins of life on Earth and to the possibility of life elsewhere in the universe. He also worked on the development of alternatives to fossil fuels. Melvin Calvin died at Berkeley, California, on January 8, 1997.

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Biology of Deserts glycerate to glyceraldehyde 3-phosphate, the sugar that is the end product of the cycle. Three complete turns of the cycle are needed to produce six molecules of glyceraldehyde 3-phosphate. One glyceraldehyde 3-phosphate molecule leaves the cycle, and the remaining five are converted into three molecules of RuBP. In synthesizing one molecule of glyceraldehyde 3-phosphate that can be released, the Calvin cycle uses nine molecules of ATP and six of NADPH. These are immediately replaced by the light-dependent reactions. Glyceraldehyde 3-phosphate then enters other metabolic reactions within plant cells. It is used to build other sugars, starches for storage, fats, and proteins.

Photorespiration Rubisco, the enzyme that catalyzes the carboxylation of RuBP, is also involved in another set of reactions, called photorespiration. As well as catalyzing carboxylation, it is also able to catalyze the oxidation of RuBP (in other words, as well as being a carboxylase, it is also an oxidase). Rubisco has an affinity for both carbon dioxide and oxygen and will attach itself to whichever is the more plentiful. Consequently, if the CO2 concentration in the cells is low, oxygen wins. Unless the air contains more than about 50 parts per million of CO2, most plants cannot photosynthesize at all, and at the present average CO2 concentration of about 350 parts per million, a proportion of the energy absorbed by chlorophyll is lost through photorespiration. In many plants this halves the amount of carbon leaving the Calvin cycle. When RuBP is oxidized, the product splits into one molecule of 3-phosphoglycerate and one of a two-carbon compound, phosphoglycolate. The phosphoglycolate is converted to glycolate, leaves the cycle, enters a small cell body (organelle) called a peroxisome, and from there enters a mitochondrion. Eventually it is broken down, releasing CO2. Because the reactions use oxygen and release CO2, they are called respiration, but they produce neither ATP nor NADPH to supply energy to the plant. So far as scientists can tell, there is no way in which photorespiration benefits the plant. It may be that photorespiration evolved at a time when the Earth’s atmosphere contained much more CO2 than it does now. It would not have mattered then, because in the competition for rubisco, CO2, being more abundant, would always have won.

C3 and C4 Plants An early step in the Calvin cycle involves 3-phosphoglycerate. This is a molecule with three carbon atoms, and so this version of the photosynthetic light-independent reaction is known as the C3 pathway. It is the version of photosynthe-

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sis used by most plants, but there is another one, in which the first product is oxaloacetate, a compound with four carbon atoms. This is known as the C4 pathway, and plants that use it are called C4 plants. These are mainly tropical and subtropical grasses. Sugarcane and corn (maize) are C4 plants. C4 plants minimize photorespiration by pumping CO2 into specialized bundle-sheath cells and so maintain a concentration high enough to ensure that rubisco attaches to CO2 rather than oxygen. The C4 pathway begins in the mesophyll cells with the addition of CO2 to phosphoenolpyruvate (PEP). This reaction is catalyzed by the enzyme PEP carboxylase, and it produces the four-carbon compound oxaloacetate. Unlike rubisco, PEP carboxylase has no affinity for oxygen. It cannot behave as an oxidase. Consequently, it is able to seize CO2 even when concentrations are very low. Oxaloacetate is then converted into another four-carbon compound; in many plants this is malate. The mesophyll cells are packed together fairly loosely, but there are passageways, called plasmodesmata, between them and the bundle-sheath cells. The bundle-sheath cells are packed much more tightly around the veins of leaves, with the mesophyll cells outside them. They give the plant a very distinctive anatomy, called a Kranz anatomy. Kranz is a German word that means “crown” or “garland,” and Kranz anatomy refers to the way a layer of mesophyll cells surrounds the bundle-sheath cells, which in turn surround the leaf vein. Any plant with leaves of this type is a C4 plant. Fossils have been found of leaves more than 5 million years old with Kranz anatomy, showing that the C4 pathway evolved at least that long ago. Malate, or whatever other four-carbon compound the plant uses, is passed through the plasmodesmata from the mesophyll cells to the bundle-sheath cells. There the malate releases its CO2 to combine with rubisco and enter the ordinary Calvin cycle. The mesophyll cells pump CO2 into the bundle-sheath cells, so the CO2 concentration at the start of the Calvin cycle is much higher than it is in the mesophyll cells. Pumping CO2 against a concentration gradient requires energy, and the C4 pathway uses more ATP than the C3 pathway. Whereas a C3 plant uses 18 molecules of ATP to synthesize one molecule of glyceraldehyde 3-phosphate, a C4 plant uses 30. Nevertheless, C4 plants are able to thrive in conditions under which photorespiration would exceed photosynthesis in C3 plants. They photosynthesize faster than C3 plants, and so they grow faster. In temperate climates C4 plants are often at a disadvantage compared with C3 plants. It is in the Tropics, and especially in the dry Tropics, that the C4 pathway comes into its own, because C4 plants are more economical in their use of water than are C3 plants. Photosynthesis requires CO2 and water. The CO2 is taken from the air and the water from the ground. Water

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enters the plant through the roots and is transported to the mesophyll cells, ready for the photosynthetic light reactions. To obtain the necessary CO2, the leaf stomata must open. Carbon dioxide enters the leaf and oxygen departs, but water can also escape, evaporating through the open stomata (see “Stomata” on page 97). The loss of water presents no problem so long as water is plentiful. If water is in short supply, however, the stomata close to conserve it. Closing the stomata prevents CO2 from entering the leaf and oxygen from leaving, so within the mesophyll cells the CO2 concentration falls and the oxygen concentration rises, resulting in photorespiration. This amount of photorespiration presents no real difficulty for plants adapted to temperate climates. Most of the time water is plentiful, and the plant can afford to lose it through its open stomata. When the ground is dry, the stomata close, and for a time photorespiration increases and photosynthesis decreases. A fall of rain replenishes the ground before this can cause harm, but should a drought continue plants will cease to grow and eventually will die. In the arid Tropics, and most of all in deserts, drought is permanent, and plants keep their stomata closed for much of the time. These are the conditions under which the C4 pathway is preferable.

CAM Efficient though the C4 pathway is, plants that use it must nevertheless open their stomata to allow the exchange of gases, and while stomata are open some loss of water is inevitable. Cacti, ice plants, pineapples, and a variety of other plants have evolved a strategy to avoid even that loss. The method they use involves incorporating CO2 into an organic acid, and it was first identified among plants such as stonecrops (Sedum species) and houseleeks (Sempervivum species). These belong to the family Crassulaceae, of herbs and small shrubs that have thick, succulent leaves in which they store water. The family has given its name to the photosynthetic method. It is called the crassulacean acid metabolism (CAM), but it is not restricted to the Crassulaceae. At least 17 other plant families employ it. Not all those plants are succulents—plants with fleshy leaves—and not all succulents have a CAM. CAM plants fully open their stomata at night, which is when other plants keep them closed. With their stomata open, CO2 enters mesophyll cells in the leaves and oxygen departs from the plant, but because the temperature is relatively low during the cool desert night, very little water is lost by evaporation. During the day, when water would be lost rapidly through open stomata, CAM plants keep their stomata firmly closed. Photosynthesis is impossible during the hours of darkness, of course, but the CO2 absorbed during the night com-

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bines with malic, isocitric, and certain other organic acids and is stored in this form in the small spaces (vacuoles) inside mesophyll cells. In the morning the organic acids give up their CO2, which enters the Calvin cycle. All plants use the Calvin cycle to synthesize sugar, but C3, C4, and CAM plants differ in the ways they capture the CO2 that is the essential raw material. In C3 plants the entry and fixation of CO2 both take place at the same time in the same cells. In C4 plants they take place at the same time but in different cells. In CAM plants they take place in the same cells but at different times. The C4 and CAM pathways evolved independently of each other as two solutions to the same environmental problem.

Respiration Chemically, respiration is the opposite of photosynthesis. Photosynthesis is the production of sugar, a carbohydrate, with the release of oxygen. Respiration is the oxidation of carbohydrates with the release of energy and carbon dioxide as a by-product. The reaction can be summarized as: C6H12O6 + 6O2 → 6CO2 + 6H2O + energy All plants and animals respire. In vertebrate animals, respiration also describes the act of breathing, which is the mechanism by which respiratory gases are exchanged through lungs or gills. Green plants are autotrophs, a word from the Greek autos, meaning “self,” and trophos, meaning “feed.” Autotrophs are able to manufacture their own food from simple, inorganic compounds. Organisms that must obtain their food by consuming other organisms are heterotrophs; heteros means “other.”

Essential Nutrients Carbon dioxide and water are the raw materials for photosynthesis, but by themselves they are insufficient. Each chlorophyll molecule, for example, contains a single atom of magnesium (Mg); chlorophyll a is C55H72MgN4O5, and chlorophyll b is C55H70MgN4O6 (N is nitrogen). If magnesium is scarce, the plant may be unable to manufacture enough chlorophyll to maintain itself. All proteins contain nitrogen and sulfur, so these are also necessary. In addition to carbon, hydrogen, and oxygen, all plants require a range of other chemical elements, known as essential nutrients. Some, called major nutrients, or macronutrients, are needed in relatively large amounts—in a soil suitable for farming they should be present in amounts of at least one pound per acre (more than one kg/ha). Minor nutrients, or micronutrients, should be present in amounts about 1,000 times smaller. The major nutrients are nitrogen, potassium,

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Biology of Deserts calcium, phosphorus, magnesium, and sulfur. The minor nutrients are iron, manganese, zinc, boron, copper, molybdenum, chlorine, and cobalt. Without an adequate supply of all the essential nutrients, plants cannot thrive and may die. These elements are obtained from the soil, and plant growth will be limited if the supply of any one of them is restricted.

Heat and Light Warmth and light, two of the ingredients that are essential for photosynthesis, are plentiful in subtropical deserts. As with most biochemical reactions catalyzed by enzymes, the speed with which the reactions involved in photosynthesis take place approximately doubles with every 18°F (10°C) rise in temperature between 32°F (0°C) and 95°F (35°C). Photosynthesis does take place, but very slowly, at a temperature as low as 21°F (-6°C), but the optimum temperature is about 85°F (29°C). Although increasing the temperature above the optimum increases the rate of photosynthesis, that increase is sustained for only a short time, after which the reaction rate falls back to its previous level. At temperatures higher than about 105°F (40°C) photosynthesis slows rapidly, and most plants will die if the temperature remains above 113°F (45°C) for more than a very short time. The temperature affecting photosynthesis is not the air temperature, of course, but the temperature on the surface of the leaves, and the transpiration of water from the leaves may help to keep them cool. Desert sunshine is extremely bright, and the rate of photosynthesis is also directly proportional to the intensity of light. Beyond a certain threshold, however, increasing light intensity starts slowing the rate of photosynthesis, probably because the plant is exposed to so much energy that other reactions take place that break down the chlorophyll or other compounds involved in the photosynthetic process. The phenomenon is called solarization, or heliosis. In a desert there may be times when the threshold is crossed and the sunshine is too bright. During the hottest part of the day the high light intensity and temperature often combine to inhibit photosynthesis, and at all times the scarcity of water—and mineral nutrients dissolved in it—imposes a further constraint. That is why few plants can tolerate desert conditions, and those that do are so tough.

TRANSPIRATION AND WHY PLANTS NEED WATER

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Centuries ago people wondered how it could be that a small seed grows into a plant the size of a tree. They argued about what substances plants were made of and

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where they obtain them. Some thought that trees were made from soil. A Flemish chemist named Jan Baptista van Helmont (1579–1644) disagreed. He believed that plants were made from water and air. To test this experimentally, he planted a willow seedling in a container holding 200 pounds (90.9 kg) of soil and grew it there for five years, adding only water. At the end of five years he weighed both tree and soil. The tree had gained 163 pounds (74 kg), but the weight of soil had decreased by only two ounces (60 grams). This, he said, proved the plant was made from the water he had added. He was almost right. Water accounts for 80–85 percent of the weight of a nonwoody plant (but less in woody plants, because wood never contains more than about 30 percent water by weight). Organic compounds made from carbon, hydrogen, and oxygen account for about 95 percent of the dry weight of a plant, and mineral nutrients, such as nitrogen, sulfur, phosphorus, potassium, calcium, and about 40 other elements (see “Essential Nutrients” on pages 100–101) make up the remaining 5 percent. All plants need water. Even desert plants, growing in soil that is dry as dust, contain water and will die if they lose too much of it. Plants can be grown without soil—the method is called hydroponics—but not without water. Water is an essential nutrient because plants use hydrogen obtained from it in photosynthesis (see “Photosynthesis” on pages 96–101), although only a tiny fraction of the water absorbed is used in this way. Most of the dry weight of a plant is derived from carbon dioxide. Deprive a plant of water, and after a time its leaves and then its stem become limp and start to droop. This is called wilting, and it shows that plants use water to give their tissues rigidity—the scientific term is turgor. Water the plant, and provided it has not been left without water for too long, it will quickly recover its turgor. Inside the plant water enters and fills cells, making their walls rigid. So plants need water to allow them to hold out their leaves to the sunlight and in the case of nonwoody plants to stand upright.

Roots and Root Hairs Water enters plants through their roots and never through any of the parts of the plant that are above ground. There are two main types of plant roots: fibrous roots such as those of grasses and taproots such as those of dandelions. Grasses have fibrous roots; carrots and parsnips are edible taproots. Roots have many branches, and near the tip of each branch are fine hairs. The roots extend these hairs through a large volume of soil, and the total surface area of all the hairs is huge. This was measured experimentally for a rye plant (Secale cereale) that was grown from seed for four months. At the end of that time the total length of all its fibrous roots and root hairs was nearly 7,000 miles

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(11,260 km), and their total surface area was about 7,000 square feet (650 m2). Water enters the plant through its root hairs and passes from them into cells in the root itself, where it is drawn into a central structure called the stele. This constitutes one end of a system of tubes, called the vascular system, linking every part of the plant. The tubes are of two types, known as xylem and phloem. Xylem vessels convey water and the mineral nutrients dissolved in it. Phloem cells convey the products of photosynthesis dissolved in water from the leaves to all parts of the plant. Plants need water, therefore, to deliver the mineral nutrients their cells need.

Xylem and Phloem Xylem vessels consist of dead cells. In the evolutionarily more ancient gymnosperms—the group of plants that includes coniferous trees such as firs, pines, and redwoods— the individual xylem cells are called tracheids. These are cylindrical, hollow, and have tapered ends, each tracheid overlapping its neighbor to form a continuous tube. Pits on the sides of the tracheid are regions where the cell wall is thin enough to allow molecules to cross, and the pits of one tracheid are aligned with those of its neighbor, so the pairs of pits are the route by which water travels. Some pits have extensions of the cell wall rolled around their edges. These are called bordered pits. Others, lacking borders, are called simple pits, and a pair of pits are said to be half bordered if one pit is bordered and the other simple. In flowering plants (angiosperms) the xylem cells are called vessel elements. Vessel elements are shorter and usually wider than tracheids, and they are joined end to end rather than overlapping. They have pits, but ordinarily water crosses from one vessel element to the next at the ends. These are perforated or in some plant groups open. Water entering the root passes through the stele, where energy is expended to pump mineral ions (molecules carrying an electrical charge) into the xylem. This increases the concentration of dissolved minerals on one side of the membrane, causing water to follow by osmosis to equalize the concentration on either side. Consequently, the roots are pumping water into the xylem.

Transpiration It is not pumping from below that pushes water all the way from the roots to the top of the plant, however, but pulling from above. Mesophyll cells in the leaves are surrounded by air spaces in which the humidity is very high. It is from the air in these spaces that the cells absorb the carbon dioxide they need for photosynthesis. A film of water coats the cells, and when the stomata are open to allow gases to enter and leave, water vapor leaves the

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plant from the air spaces around the cells. Water evaporates from the film to replace it. This loss of water vapor from plant leaves is called transpiration, and although it causes difficulties for plants when the soil is very dry, it does serve a useful purpose. Evaporation absorbs latent heat, the energy water molecules must absorb to allow them to break free. This is taken from the surrounding tissue, and so transpiration cools the leaves in the same way that the evaporation of sweat cools human skin. The cooling can be enough to prevent the leaf temperature from rising so high that photosynthesis is slowed (see “Heat and Light” on page 101).

How Transpiration Pulls Water from the Ground Evaporation of some of the water from the walls of the mesophyll cells causes the water that remains to adhere strongly to the hydrophilic (water-attracting) walls of depressions in the cell walls. At the same time hydrogen bonds (see “Evaporation” on page 67) in the water itself pull the water into the shape with the smallest possible surface area. The combined effect of the two forces is to pull the water into a meniscus, which is a concave surface in each depression. The water is then subjected to a pressure lower than atmospheric pressure, and under this negative pressure it draws water out of the leaf xylem and into the mesophyll layer. Tracheids and vessel elements are very narrow, and the water inside them forms a continuous column. Water that is drawn out of the xylem at the top remains in contact with water still in the xylem and pulls this behind it. The pressure, starting at the stomata, is felt through the entire column of water molecules, and water rises by capillarity, just as it does through pore spaces in the soil (see “Except When It Moves Upward, by Capillarity” on pages 68–69). It sounds incredible, but the transpiration of water through leaf stomata exerts sufficient pressure to draw water from the roots up to the very top of the tallest tree, holding it against the weight of the water in the column. The pressure is so strong that it pulls the sides of the xylem cells inward, and on a hot day, when the transpiration pressure is at its greatest, the trunk of a tree becomes narrower by an amount that can be measured. Water is then flowing quite fast, at up to 130 feet (40 m) per hour or more. On a warm day in summer a fully grown broad-leaved tree may transpire more than 53 gallons (200 l) of water, and each of its leaves will replace all its water once every hour. Beyond the lower end of the xylem, the pressure is felt in the soil provided the root hairs are in contact with a continuous chain of water molecules outside. Water flows toward the root and then into it under pressure from the top of the plant.

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Biology of Deserts When the rate of transpiration is very low, or at night when most plants close their stomata and transpiration ceases, water continues to enter the plant because of the pressure caused by the pumping of mineral ions into the xylem. This draws in water and pushes it up through the xylem, and the plant may rid itself of the excess by guttation, which is the exuding of drops from the tips of its leaves. It can happen that some of the water evaporates somewhere along a xylem vessel. This is called cavitation, and it breaks the continuous chain of water molecules on which transport depends. A small plant can overcome this. The osmotic pressure (see the sidebar “Osmosis” on page 97) that draws water into its roots is sufficient to push the pocket of water vapor all the way to the mesophyll layer and so restore the continuous column. Bigger plants, such as trees, cannot generate enough root pressure to do this. Instead, they allow water to pass through the pits of the blocked xylem tube and into an adjacent tube, in effect constructing a bypass.

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ROOTS, STEMS, AND LEAVES THAT CONSERVE WATER

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Whenever it rains, water drains by gravity through very large spaces between stones and large soil particles such as coarse sand grains. Capillarity, or capillary attraction (see “Except When It Moves Upward, by Capillarity” on pages 68–69), is what causes it to move through the very small pore spaces. Just as it is capillarity that pulls water upward through the xylem of a plant, so it is capillarity that pulls water downward through the soil and into the ground water. Water often sinks gravitationally through the upper layers and by capillarity through the lower layers. If a layer of soil consisting of fine particles lies above a layer of coarser material, however, the water will drain only so far, leaving moist soil lying above dry soil. Despite capillarity and gravity, the water sinks no farther. This is because there is insufficient soil moisture tension, the force that pulls water through capillaries.

Soil Moisture Tension Imagine a beaker filled with water, like the one in the illustration. At the bottom of the beaker the water is under a pressure due to the weight of water above it. The pressure decreases with height in the beaker because the amount of overlying water decreases. At the surface there is no pressure. It is zero. In the drawing the pressure at the bottom has a value of 20 and the pressure at half way a value of 10. If a tube with a very narrow bore is inserted vertically into the water, water will rise up the tube by capillarity. Level

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© Infobase Publishing

Soil moisture tension. The weight of water in the beaker exerts a pressure that increases with depth. Counting upward from the bottom, the pressure at the surface is 0 and the pressure above the surface has a negative value, so it acts in an upward rather than downward direction.

with the surface of the water in the beaker, the pressure exerted by the weight of overlying water will be zero because there is no overlying water. Above that level, therefore, the pressure will be less than zero. At a level as far above the surface (0) as the +10 level is below it the pressure will be -10, and at a level above the surface equal to the depth of water below the surface it will be -20. Higher up the tube the pressure will be still lower. Water rises up the tube because it is under a negative pressure. Is it pulled or pushed? The pressure below the surface can be ignored, because it is not important and working with negative quantities is inconvenient, so scientists simply eliminate the minus sign in values above the surface. The resulting value, which is now positive, has been changed from a pushing force (pressure) into a pulling force (tension). This is the force known as soil moisture tension, or SMT.

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The magnitude of SMT is inversely proportional to the size of pore spaces in the soil. In other words, the smaller the pore spaces are, the greater is the SMT. It requires force to draw water through capillary spaces, and if the water reaches a region where that force is insufficient, it will move no farther. This is what happens when water drains through fine particles to a layer where the particles are larger. Bigger particles have bigger spaces between them, so coarse-grained soil exerts a lower SMT than does a soil made from fine particles. A layer of coarse material cannot pull water out of a layer of fine-grained material.

Field Capacity Soil from which all the gravitational water has drained out of the large spaces is said to be at field capacity. About half the pore spaces will be filled with water, and the SMT will be fairly low (about five pounds per square inch, or 33 kPa). Plant roots have no difficulty pulling water toward them against this pressure. In unsaturated soil the water is present as a film coating soil particles. As a root draws in water by osmosis (see the sidebar “Osmosis” on page 97), the film on the soil particles adjusts in a way that causes the water in the vicinity of the root to move slowly toward it. The SMT is higher in soil that is below field capacity, however, and the drier the soil the greater the SMT. As the SMT increases, the movement of water toward and into the root slows. As the soil dries, the SMT increases first in the upper layers, and plants absorb more and more of their water from lower layers, where the soil remains moist and the SMT is relatively low. There is also more oxygen in the upper soil because water has drained from the large spaces between particles, and water absorption becomes less efficient as it shifts to lower levels, where there is less oxygen. Oxygen is necessary for cell respiration, and when the supply is curtailed, the root works more slowly and the pressure with which it draws water weakens. When a point is reached at which water enters the root more slowly than it is being lost from the leaves of the plant by transpiration, the plant will start to wilt. In a temperate climate, where transpiration is moderate, wilting may not begin until the SMT reaches about 225 pounds per square inch (1.5 Mpa). Under the hot desert sun and drying wind the transpiration rate is much higher, however, and some plants may start to wilt at an SMT of about 30 pounds per square inch (0.2 Mpa).

Roots of Desert Plants Perennial desert plants—plants that do not die down and disappear completely after one or two seasons—have adapted to soil aridity in various ways. In most the root

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xylem is very well developed and strong, ensuring that when water is available it is transported quickly to the rest of the plant. Some plants have roots that can store water, and many have roots with a thick bark that minimizes the loss of water through the root epidermis (skin). Many desert grasses achieve the same result in a different way. Their root hairs secrete mucilage to which sand grains adhere, eventually coating the hairs in a kind of artificial bark called a rhizosheath. Several groups of plants produce small rootlets on their roots that grow rapidly into full-sized roots when there is water for them to absorb. These are known as proteoid roots in evergreen trees and shrubs belonging to the family Proteaceae, most of which grow naturally in Australia and South Africa. Retema (Retama raetum), the shrub known as juniper in the Bible, grows in sand and dry river beds (wadis) and has horizontal roots up to 33 feet (10 m) long growing from its ordinary roots. The vessel elements in these roots are shorter and narrower than vessel elements in the ordinary roots, and the xylem is able to take up water very efficiently. After a shower gravitational water drains rapidly through sand. These additional roots are positioned where they can capture water before it is lost.

Conserving Moisture Modifications to the roots increase the efficiency with which water is absorbed and transported with the least possible wastage. The other adaptation involves reducing the loss of water by transpiration. CAM plants achieve this by their method of photosynthesis and, because it is more efficient in a desert climate, the C4 photosynthetic pathway also conserves water (see “C3 and C4 Plants” on pages 99–100 and “CAM” on page 100). Other plants have leaves that are succulent. Succulent leaves contain large cells that store water. When the photosynthesizing cells are short of water they absorb water by osmosis from the water-storage cells. These cells then shrink, but they swell rapidly to their former size as soon as water reaches them from the roots. In some plants it is the stem that is succulent. Most desert plants have leaves with a small surface area in relation to their volume. The needle and scale leaves of coniferous trees are a familiar example of leaf reduction as an adaptation to dry conditions. Conifers grow in high latitudes, where water remains frozen throughout a long winter and therefore is unavailable to plants and also in climates of the Mediterranean type, where little rain falls in summer. It is not simply the overall size of the leaves that is reduced. The internal leaf structures and even the cells are small. Relatives of the sage-brush (Artemisia species) and salt-bush (Atriplex species) grow ordinary leaves during the rainy season. They shed these at the end of the season and

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Biology of Deserts replace them with reduced leaves, which they retain through the dry season. There are also plants that shed their leaves at the start of the dry season but retain the leaf stalks (petioles). These contain chloroplasts and continue to carry out photosynthesis. In retema and Calligonum comosum, a shrub resembling broom, photosynthesis takes place in the young branches, which are green, and in the dry season these branches may be shed. Reduced leaves often have a thick, waxy cuticle (outer skin) that reduces water loss and many have trichomes— outgrowths from the leaf in the form of hairs or scales. Trichomes are probably adaptations less to aridity than to temperature and light intensity: They reflect light and heat. Plants must open their stomata, but these can be located where the transpiration rate is lowest, in positions sheltered from the wind and full sun. Many grasses have leaves that are rolled almost into tubes, with the stomata on the inside. Oleanders (Nerium species) are among the plants that have their stomata sunk in grooves or depressions. Retema has its stomata along the bottom of grooves running the length of the branches, where photosynthesis occurs. A few plants of seasonal climates, including the caper shrub (Capparis spinosa), close their stomata altogether during the summer dry season. These extreme adaptations do not reduce transpiration, they stop it altogether, with the result that the leafless plants

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become dry and brittle. They look dead, and yet as soon as it rains they produce new shoots. Most of them survive the drought by storing water inside their stems to supply cells containing chloroplasts. Their stomata are in deep furrows, in permanent shade, and photosynthesis proceeds at a level that is just sufficient to keep the plant cells alive until they are able to end their period of dormancy.

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Few plants are able to survive in the desert climate, but the creosote bush (Larrea divaricata) of the North and South American deserts is one plant that thrives there (see “Creosote Bush” on page 110). Other plants deal with the effects of drought by avoiding them. These include plants with roots, stems, and leaves that are modified to maximize the efficiency with which they use water. Drought happens, but it does not harm them because they need very little water. There is another way. It is possible for plants to evade the desert climate entirely. They thrive in the desert, but they appear only when water is plentiful. In fact, they are desert plants that grow only during those brief spells when the desert ceases to be a desert. These are the most spectacular of desert plants. Invisible almost all the time, they are the species that emerge after rain, the plants that make the desert

The creosote bush (Larrea divaricata) grows in the deserts of North and South America. (Washington University in St. Louis)

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bloom with foliage and especially with flowers that produce a riot of color. Then, as the ground dries, they are gone, vanishing as quickly and mysteriously as they came. Not only are these plants visually spectacular, they are also spectacularly successful. In some of the harshest deserts they are the only plants that can survive. A few are geophytes, plants that spend periods when conditions are unfavorable underground as bulbs, tubers, corms, or fleshy rhizomes. As soon as they are moistened they produce shoots. This is a risky strategy, however, because underground storage organs cannot survive indefinitely without dehydrating.

Surviving as Seeds Most of the plants that emerge after rain are annuals— plants that complete their life cycle, from the germination of seed to production of seed, in a single season. Th ey spend the unfavorable times as seed. This is much more satisfactory. Seeds consist of a plant embryo together with a supply of nutrients to sustain the young plant until it can start feeding itself, contained within a tough, highly protective coat. Seeds are well equipped for survival, and they can wait a long time, in some cases for many years, for an opportunity to germinate. During their dormancy, their metabolism is slowed almost to a standstill. They can tolerate extremes of temperature and their coats are waterproof, so they face no risk of desiccation. Seeds do deteriorate, however, due to the breakdown of RNA (ribonucleic acid). RNA molecules control and regulate the production of proteins, but RNA is somewhat unstable and must be renewed frequently. Consequently, the largest seeds remain viable longest because they contain more RNA than do small seeds. After a long period of dormancy big seeds are more likely to retain sufficient RNA to start up the metabolic processes. Desert plants that use this strategy produce seeds that can remain viable while they wait for rain, even if they have to wait for years. The next challenge comes when the rain finally arrives and the seeds germinate. The water will soon disappear, and when it does the plants will die. By the time that happens, the plants must have produced a new crop of seed, but in order to do that they must grow reproductive structures, eggs must be fertilized, and this growth must be sustained by photosynthesis and the absorption of mineral nutrients from the soil. In other words, the germinating seed must lead to a plant that grows stem and leaves, flowers and seeds, all in the brief time available to it. Plants that grow rapidly and then vanish are called ephemerals. Most small nonwoody plants rely on insects to pollinate their flowers, and they produce brightly colored and often strongly scented flowers to attract pollinators. When desert plants emerge so many appear at the same time, all of them

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in a desperate hurry to be pollinated and set seed before they dry out, that competition for insect pollinators is extreme. It leads to a kind of visual clamor, with plants producing the biggest and brightest flowers possible in an effort to “shout down” their neighbors. When the desert blooms the effect is truly spectacular.

When the Desert Blooms A seed’s response to its environment is very precise. In the Californian desert evening primrose (Oenothera species) seeds can survive half a century or more waiting for the particular combination of temperature and moisture that will allow them to grow fast enough to complete their life cycle in the time available. When they receive the signal the desert is suddenly covered in huge patches of their pink, perfumed flowers. Growth can be very fast. Boerhavia repens, a plant related to bougainvillea that grows in the deserts of southern Africa, produces big, bright flowers and sets seed within eight to 10 days of germinating. This is probably a record,

It seldom rains, but when it does the desert is rapidly transformed by a blanket of bright flowers. (NOAA/ Department of Commerce)

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Biology of Deserts

but the pillow cushion plant (Fredolia aretioides) of Algeria commences photosynthesis within 10 hours of its seeds germinating, and annual species of Convolvulus, which also grow in the northern Sahara, complete their life cycle within six weeks and sometimes in as little as three weeks. These plants grow so fast it is almost possible to watch them grow. African pompano (Blepharis ciliaris), an herb of the Sahara, keeps its seeds stored in a protective capsule. When rain wets the capsule it bursts violently, scattering its seeds. These are covered with hairs that swell when they are wet, and as they swell they drag the seed into a favorable position, so that when it germinates the radicle, which is the precursor of the root, enters the soil immediately. The seeds germinate within one hour of escaping from the capsule. Speedy growth is the secret of the success of such plants, but in itself it is not enough, because the moisture that triggers germination may not last long enough for even the fastestgrowing plant to produce a new crop of seed. Desert plants have evolved ways to avoid germinating at the wrong time. There are two rainy seasons in the Mojave Desert. One occurs in winter, the other in summer, and the annual plants

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Spring in the Mojave Desert is a season when plants react speedily to warmth and moisture. (Ronald and Charlotte Williams)

of the desert are of two kinds: those that germinate in winter and those that germinate in summer. Germination is triggered partly by temperature. Warm temperatures affect the summer germinators, and the winter germinators react to cool temperatures, each seed type responding only when the temperature passes a certain threshold. In order to germinate the seed coats must be wet, so moisture provides the other part of the trigger, but moisture may quickly disappear. Onetenth of an inch (2.5 mm) of rain will moisten the soil, but the Mojave annuals fail to germinate unless at least six-tenths of an inch (15 mm) of rain falls over a short period. These seeds as well as those of many other plants, not all of which are confined to deserts, have chemical substances in their coats that inhibit germination. The compounds are soluble in water, however, so if the seeds are made wet

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enough for long enough the substances drain from them, allowing the seeds to germinate. There are also plants with seeds that will germinate only after bacteria have altered their coats. This can happen only after the seeds have been soaked thoroughly for long enough to allow the bacteria to multiply. In all these cases the effect is to prevent germination unless an adequate amount of water is present. Sicklepod, or wild senna (Cassia obtusifolia), a Saharan plant used in herbal medicine, solves the problem differently. It produces two types of seeds. Some germinate as soon as their coats are moistened. This gives them a quick start, so if the rains continue the young plants will appear before their rivals. If the rain does not continue, the young seedlings will die. This does not harm the plant, however, because the second type of seed will germinate only if moisture soaks through the seed coat. This requires the seeds to be in moist soil for longer. Another herb, Neurada procumbens, has fruits containing seeds that germinate one at a time. When the fruit and its seeds are moistened, only one seed germinates. The second time it rains another seed germinates, and so on. Neurada procumbens grows in deserts from North Africa to India. Not surprisingly, perhaps, when people sow these desert plants in temperate regions they often escape to become highly invasive, troublesome weeds.

Dispersing Seeds Even good timing may not guarantee survival. Seeds must also be in the right place, and ensuring this is not so simple as it may seem. The fact that the parent plant has managed to produce seed apparently proves that it has found a place where plants of its species can grow, but if it releases all its seeds onto the ground around itself, once the seeds germinate the resulting competition for resources may kill them all. On the other hand, scattering seeds may waste them, with seeds landing on bare rock or other places where seedlings have no chance of survival. In the deserts of the Near East Plantago cretica, a relative of the garden weed plantain, keeps all its seeds in one place. After it has produced them the plant dies, and as it dries out its flower stalks bend lower and lower until they are pressed against the ground. There they remain until rain wets them. Then they straighten up, and the raindrops wash the seeds from the stalks. Once the seeds are wet they produce mucilage from their coats, which glues them to the ground. Thus secure, even the heaviest rain cannot dislodge them. Some plant species produce different types of fruit on the same plant. One such plant is Gymnarrhena micrantha, of the aster family (Asteraceae), which grows in the Near East. The two types of fruit develop from different types of flower. One type grows in clusters just above the ground. Its seeds have long hairs, and as they mature they are car-

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ried away by the wind. A second type of inflorescence grows belowground. It has only a few flowers, with petals that form a tube opening just above the ground surface. Its seeds remain buried, and when it rains they germinate where they are. There is one plant with a method of seed dispersal that has made it a movie star. Tumble grass (Schedonnardus paniculatus), which grows in the North American desert, dies once it has produced seed, but as the dead plant dries its stems and leaves curl into a ball with the seeds held on the inside. The ball is then blown about by the wind, scattering the seed as it goes. No ghost town in a Hollywood western would appear adequately windswept and desolate without at least a few balls of tumble grass.

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TYPICAL DESERT PLANTS

Tumble grass, a plant typical of the North American desert, has its equivalent in deserts from Morocco to southern Iran. This is the rose of Jericho, or resurrection plant (Anastatica hierochuntica), which is a member of the cabbage family (Brassicaceae) and not a rose at all. The rose of Jericho is an annual that sheds its leaves as its seeds mature. As the bare stem and branches dry, they fold inward so the dead plant ends as a ball that is blown about by the desert wind, shedding seeds as it goes. When moistened, the plant uncurls. Dead plants are sometimes sold as curiosities to be kept as house ornaments that curl and uncurl according to how dry they are. The rose of Jericho is not the only plant to distribute its seeds in this way. A vine called the colocynth, bitter apple, or vine of Sodom (Citrullus colocynthis) produces spherical fruits about the size of oranges (the watermelon, C. lanatus, is a close relative) that are blown about until eventually they

Desert flowers are brightly colored to attract pollinators. (Ronald and Charlotte Williams)

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become buried in the sand or lodged firmly against rocks. Then the fruit rots, leaving the seeds that remain there until rain moistens them and they sprout. The dried pulp of the colocynth is a laxative, and for thousands of years the plant has been cultivated for medicinal use. There are usually some perennial plants to be seen in even the harshest deserts. It is only where the sands are perpetually shifting or the desert pavement consists of bare rock, swept clear of sand by the wind, that the landscape is utterly devoid of visible life.

Desert Grasses Some grasses can grow in sand. Indeed, grasses are widely used to stabilize sand dunes along coasts in temperate regions as well as in deserts. The species most often planted on coastal dunes is marram grass (Ammophila arenaria), also known as beach grass and mel grass. Not only does it bind sand grains together, its tough stems are used to make mats, bags, chair seats, and roof thatch. The equivalent grass in hot deserts is esparto, Algerian or alfa grass (Stipa tenacissima). Its stems are used to make mats, paper, and ropes. All grasses belong to the family Poaceae. Species vary in many ways—wheat, rice, corn (maize), bamboo, and sugar cane are all grasses—but there are important features they all share, and it is these that allow perennial grasses to thrive on sand dunes. Grass roots form a fibrous mat. This binds soil particles together, and the root system can be extensive. A grass stem has swellings, called nodes, at intervals along it. The stem grows from cells just above each node. Nodes that are in contact with the ground often produce roots called adventitious roots because they grow from an unusual position. In many species the stem itself is hollow. As well as adventitious roots, a number of stems often arise from the lowest node. They are called tillers, and tillering (the production of tillers) causes some grasses to grow as dense clumps. If the stem is cut, for example by being grazed, it will simply continue growing from the node below the cut, and since there are nodes all the way down to ground level and often to just below ground level, grazing cannot injure grass. Adventitious roots, nodes, and the stem of a single grass plant are shown in the illustration. As the picture also shows, a leaf sheath encloses the stem, supporting and protecting it. At each node the sheath grows away from the stem to become a leaf, or blade; blades arise on alternate sides of the stem. The stomata are borne on one side of the blade, and in very hot weather many species can roll their blades into tubes, with the stomata on the inside to minimize the loss of water by transpiration (see “Conserving Moisture” on pages 104–105). Many perennial grasses have stems that grow horizontally. Some, called stolons, lie along the ground, but most

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leaf blade

node

node

leaf sheath

adventitious root © Infobase Publishing

The grass plant

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grow just beneath the surface. These are called rhizomes, and they are typical of Ammophila and Stipa species. Both vertical stems and adventitious roots arise from the nodes below ground, so a complete plant can develop from each node and then produce a rhizome of its own. It is the combination of rhizomes and fibrous root mats that binds loose soil. Grasses are wind-pollinated. Their flowers have no petals, but the fruits often bear stiff hairs called awns that remain with the seed when it leaves the plant. In many species the awns twist and bend with changes in humidity in such a way as to drag the seed down into the ground. Awns on cram-cram (Cenchrus biflorus) seeds stiffen and curl, turning the seeds into burs that cling to the coats of passing animals. Grass seeds remain viable (capable of germinating) for a long time, in some species for 30 years.

Creosote Bush Over large parts of the deserts of the southwestern United States and Mexico as well as in some parts of South American deserts, the creosote bush (Larrea divaricata subspecies tridentata), also called greasewood, is the most abundant large plant. Its small, olive-green leaves contain resin and smell of creosote, giving the plant its usual common name. An antiseptic lotion is obtained by steeping the twigs in boiling water. The creosote bush is a shrub up to about five feet (1.5 m) tall, with a tangled mass of branches, which can survive without rain for more than a year. The plant looks dead during a drought. Its branches are dry and brittle, but it recovers rapidly when it does rain. During the dry period the plant loses all its leaves but protects those leaves that remain in their buds. These become dormant but revive when water reaches them. Its tolerance for extreme drought allows the creosote bush to grow where few other plants are found. The bushes themselves grow as moderately large but scattered clumps, the distance separating them being related to the availability of water. When water is present the roots of each plant draw moisture from the volume of soil around them, and there is a region where root growth is inhibited by competition for limited water between the roots of adjacent plants. The moister the soil the closer together the plants are, and, conversely, the more widely spaced the plants are the more arid is the ground. Except in very dry years, creosote bushes produce bright yellow flowers in spring followed by seeds in capsules, rather like those of poppies. People who grow creosote bushes in their desert gardens sometimes pick the unopened flower buds and use them like capers (which are also flower buds, but of an unrelated plant) to flavor food. This is not the only way the creosote bush reproduces. It also reproduces vegetatively. Shoots grow at intervals along its horizontal roots. These emerge above the surface as suck-

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ers, and at the same time they start to grow roots of their own. The suckers are still attached to the root of the original plant, but if that root should die or be severed the new plant would not be harmed. Even when it is separated from its parent, however, the new plant is not different from it. It cannot be, because it is part of it, no different than a branch or a plant grown from a cutting. Genetically, the parent and offspring are identical. They form a clone. (Strictly speaking, a clone is a group of genetically identical individuals, each member of which is known as a ramet, although people often use clone, incorrectly, as a synonym of ramet.) The creosote bush that stands alone in the desert, therefore, is likely to be a clone, and its clones often cover an area up to 25 feet (7.6 m) across. Individual plants grow old and die, but clones are different. They can continue to produce new ramets as long as there are horizontal roots strong enough to produce suckers. Roots die, too, of course, but since each ramet produces new roots of its own, the roots and suckers are constantly being renewed. This means that what appears to be a single plant, but in fact is a clone, can live for a very long time. Some creosote bush clones in the Mojave Desert are more than 11,500 years old. When they were young the last ice age was just coming to an end.

Agaves Agaves (family Agavaceae) are plants that grow in or close to deserts but that are now cultivated in many parts of the world. Most agaves have short, thick stems and stiff, narrow, pointed leaves that are often crowded around the base of the stem. Many species have succulent leaves. Agave leaves can be up to 10 feet (3 m) long, and they often have prickles along the edges. Plants with big, succulent leaves and

The century plant (Agave Americana) in flower; it flowers every 10–20 years, despite its name. (Marfa Gliders)

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prickles are often called cacti, but agaves are not cacti. They are monocots—plants with seeds that produce a single seed leaf (cotyledon)—and they are placed with the lilies (family Liliaceae) and irises (Iridaceae) in the order Liliales. Not all agaves are originally American, but two of the best-known species are. Agave americana is the century plant, a name that refers to the mistaken belief that it flowers only once every century. In fact, it flowers every 10 to 20 years. Sap, released in copious amounts when its stem is cut, is fermented to make the Mexican drink pulque.

Yuccas and Yucca Moths Yuccas are also members of the Agavaceae. There are about 30 species. Most are found in the southwestern United States and Mexico, but some occur farther south in Central America. Most species of yucca rely for pollination on nocturnal moths belonging to the genus Tegeticula, and the close relationship between the insects and the plants provides a good example of the mutually beneficial relationship between species known as mutualism. Each yucca species has its own species of pollinating moth and can be pollinated by no other, and each species of moth is able to feed on only its own yucca. All the moths are known as yucca moths. They must work fast because many yucca flowers last only one night. The female moth climbs up a stamen and uses her long tongue to scrape up pollen that she rolls it into a ball and holds beneath her head. After she has visited about four stamens she flies to another plant. She inspects the flower to see whether its ovary is at the right stage of development. If it is, and if it contains no moth eggs, she lays a few eggs one at a time in the flower and pushes in her ball of pollen. When the eggs hatch the larvae feed on the seeds, consuming about half of them but leaving enough for the plant to reproduce. Cheats can take advantage of arrangements of this kind. As well as true yucca moths, there are also bogus ones (Prodoxus species). They lay their eggs in the ovaries of yucca flowers, but they take no part in pollination.

Tall Yuccas and the Joshua Tree Some yuccas are the size of small trees. Trecul yucca (Yucca treculeana) has a thick, straight stem with a crown of leaves at the top. Individuals can grow to a height of 15 feet (4.5 m), although most are smaller, and the Mojave yucca (Y. schidigera) reaches between eight feet and 15 feet (2.4 and 4.5 m). The biggest of all the yuccas and the one most often photographed is the Joshua tree (Y. brevifolia) of the southwestern United States and especially of the Mojave Desert. It can reach a height of 35 feet (10.7 m), and it grows at a rate of four inches (10 cm) a year. Its single thick stem divides into

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The biggest and most spectacular of all the yuccas, the Joshua tree (Yucca brevifolia) is up to 35 feet (10.7 m) tall. (Mark Griffith)

a few stout branches, and these also divide. The crown of the tree is composed of dense clusters of straplike leaves that last for up to 20 years. Many yuccas yield useful fiber. Fiber from the Joshua tree is sometimes used to make paper.

Plants That Trap Sand Plants tend to trap sand, and sand that is trapped by plants sometimes forms mounds. In the Sahara these mounds are called nebkas. Grasses play an important part in building nebkas, but so do other plants, especially certain shrubs and small trees. Ziziphus lotus is the shrub or small tree that was known in ancient Greece as the lotus tree. As sand accumulates against it the plant produces additional lateral branches along the ground. These grow adventitious roots and shoots, causing the plant to spread over a wide area. The crown of thorns, or Christ’s thorn (Z. spina-christi), a close relative of the lotus tree, is another desert shrub that traps sand. As its name suggests, it is believed to be the plant that was used to make the crown of thorns worn by Jesus. Tamarisk shrubs also trap sand, and branches that become buried produce adventitious roots. These plants are also of historical interest. Tamarix mannifera, which grows from Iran south through Arabia, is the manna tree and the source of manna. Woody plants are dispersed fairly widely across the desert. Most of them are small, and there is less variety among them than among plants that grow in more favored regions. Not many species can tolerate the severe conditions. Saudi Arabia, for example, contains only about 3,500 native plant species. The distance between woody shrubs is an indication of the aridity of the ground. Roots spread to gather such

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Desert plants sometimes stand alone, with no other plant in sight. This palm tree is growing in the dunes of the Sahara.

moisture as is available, and the roots of an individual plant occupy a volume of soil, drawing water toward them. Once a shrub is established, its roots will command all the moisture within the volume they occupy and for some distance around. No competitor can grow within that radius. Consequently, the drier the soil the greater is the size of the area dominated by each plant.

Date Palms Where moisture is available, however, the land can be made fertile, and the oases of the Sahara and Arabian Desert are very productive. Their most famous crop plant is the date palm (Phoenix dactylifera), a tree that has been cultivated since about 4000 b.c.e. Dates are grown commercially from Morocco to India, Iraq and Saudi Arabia being the most important producing countries. Date palms are trees 60–80 feet (18–24 m) tall. Where they grow naturally the trees often have several stems, but trees grown commercially have only one, the others having been removed as soon as they appeared. Male and female flowers are borne on separate trees, one male tree being able to pollinate up to 100 females. Commercial growers usually make certain of pollination by cutting off bunches of male flowers and hanging them among bunches of female flowers. The fruits are produced in bunches at the crown of the tree, a bunch containing 1,000–1,400 dates and each tree bearing several bunches. A well-tended palm yields more than 100 pounds (45 kg) of dates a year.

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Ripe dates are yellow. They turn brown as they dry, and it is dried dates that are exported. There are three principal types. Soft dates are used in confectionery and are sold pitted (stoned) and pressed together into blocks. Dessert dates that are sold in boxes, often with the dried fruits still attached to the strands on which they grew, are of the semidry varieties. The most popular semidry variety is called Deglet Noor. Dry dates are traded extensively between Arab countries but are rarely seen elsewhere. They keep for a long time and are an important item of diet. Dry dates are quite hard and can be ground into flour, although they soften when soaked in water. The food value of dates consists mainly of sugar with some vitamins. Some varieties of dry dates contain up to 70 percent sugar by weight.

Figs The fig (Ficus carica) also originated in the oases of Middle Eastern deserts, although it is now grown in most countries that have a warm climate. Figs were being cultivated in Egypt 6,000 years ago and possibly earlier than that in Jericho. The fig is the fruit of a small, broad-leaved, deciduous tree up to about 30 feet (9 m) tall belonging to the mulberry family (Moraceae). There are approximately 800 species in the genus Ficus. Some are trees and others are shrubs, lianas, or epiphytes—plants that grow on the surface of other plants using their hosts only for support. Most species grow in tropical forests, where some are stranglers, which germinate

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Biology of Deserts in the crown of a tree and produce roots that hang downward until they reach the ground. Then the roots merge, the top of the plant produces a crown of its own, and the host tree is completely encased and dies. All figs have a unique relationship with the insects that pollinate their flowers. Masses of very tiny fig flowers develop on the inside of a structure called a synconium. A tiny female wasp belonging to the family Agaonidae lays an egg inside one of the flowers. This makes the flower produce a gall-like structure instead of a seed. If the egg hatches into a male wasp, the blind, wingless male emerges from its gall and searches until it finds another gall containing a female egg. It burrows into the gall and mates with the female before she has even hatched. In some wasp species the male then bores an escape tunnel for the female. The male then leaves the gall and dies. After hatching, the female emerges from her gall and makes her way to the end of the synconium opposite the stalk, brushing against many male flowers as she does so. She emerges from the synconium covered in pollen and flies away in search of another fig in which to lay her egg, fertilizing the female flowers at the same time. The synconium becomes the fruit. There is only one wasp species that can pollinate each species of fig, and only one fig species in which each species of wasp can reproduce.

Mulberries Mulberries are also natives of the Middle East. The species cultivated for its fruit is Morus nigra, the common, black, or Persian mulberry. It was introduced to southern Europe long ago and was familiar to the ancient Greeks and Romans. A native of Morocco, crushed seeds of the argan tree (Argania spinosa) yield an oil similar to olive oil, its leaves are fed to livestock, and its timber is used for fuel. It is now being grown in rows, with rows of cereals between the rows of trees in a farming system called alley cropping that is particularly well suited to arid climates. The ye’eb, or jeheb, nut tree (Cordeauxia edulis) is an evergreen with foliage that is browsed by camels and goats and nuts resembling the sweet chestnut. It grows naturally in Ethiopia and Somalia, and it, too, is now being cultivated.

Almonds The almond (Prunus dulcis), now grown in many parts of the world, is another native of the Middle Eastern desert. There are two important varieties, the bitter almond (var. amara) from which almond oil is obtained, and the sweet almond (var. dulcis), which is the edible nut. Bitter almonds contain prussic acid (hydrogen cyanide, HCN), and this gives them a taste so bitter that it would be very difficult for a human to eat enough of them to ingest a

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Desert flowers are renowned for their beauty. Sturt’s desert rose (Gossypium sturtianum), seen here, is the emblem of Australia’s Northern Territories, and it was shown on a platinum coin minted in 2006. (Richard Hill)

lethal dose. Any animal tempted to try eating them would receive a very unpleasant surprise and would probably feel very unwell for some time, but it would survive the experience. The cyanide protects the plant by ensuring no animal tastes its nuts twice, so the seeds have a chance to germinate. Many plants produce poisons to protect themselves from herbivorous animals. Another is the desert rose (Adenium obesum), a shrub that grows naturally in East Africa and southern Arabia and is cultivated in other parts of the world as an ornamental. Its sap is used to make poison arrows, and at one time it was employed in ordeals to which young men were subjected. The bitter, poisonous sap of the Sodom apple (Solanum aculeatissimum) allows it to thrive where other plants are stripped of their leaves by browsing animals. One species of grasshopper feeds on its leaves, but mammals will not touch it. Even goats and camels leave it strictly alone.

Thorn Trees Plants need protection in an environment where they are widely scattered and food for animals is scarce. Many of those that are not poisonous have sharp thorns, and the most common trees in the deserts of Africa, Asia, and Australia are thorn trees of the genus Acacia. In Australia, which is where the largest number of species occur, they are known as wattles, and they have many local names. They also have many uses. A. aneura is mulga, an Australian tree with edible seeds and wood that is used to make boomerangs, and blue or silver wattle (A. dealbata) is florists’ mimosa (not a true mimosa). Many acacias produce

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gum, and the tree known on the southern side of the Sahara as hashab (A. senegal) is one source of true gum arabic, which is used in lozenges and fruit gums as well as in adhesives, inks, and watercolors. Gum arabic is also obtained from the babul, or Egyptian thorn (A. nilotica), and the tahl gum tree (A. seyal). Tannin, used in dyeing, is obtained from A. catechu, a tree known as the cutch, black cutch, black catechu, or khair, found in southern Asia. Its dye is said to be the source of the original khaki. Many Acacia species have no true leaves. Instead, the petioles (leaf stalks) are expanded and flattened and serve as leaves. The modified petioles are called phyllodes. Thorn trees of this type are especially common among those species native to Australia. Acacias are legumes (family Fabaceae), which means colonies of bacteria form white or gray nodules attached to their roots. The bacteria convert gaseous nitrogen into soluble nitrogen compounds that dissolve in soil water, thus supplying nitrogen in a form the roots can absorb. The thorns that give acacias their common name are long, very sharp, and often swollen at the base. They are usually modified stipules, which are structures that grow from the base of the petioles. The South African karroo thorn (A. karroo) and the cape gum tree (A. horrida) of East Africa and India, for example, both have thorns up to four inches (10 cm) long. Acacias growing in deserts have more and bigger thorns than those that grow in moister climates where they are surrounded by more abundant vegetation. Australian acacias grow in areas that receive rain occasionally, and so they are more closely spaced. They are also less thorny than those of the Sahara, suggesting the thorns really do protect plants that occur as isolated individuals. Most acacias are broad-leaved evergreen trees or shrubs. Some grow up to 100 feet (30 m) tall, but most are smaller. Many have a distinctive flat-topped shape. They are thoroughly adapted to hot, dry conditions. For some the optimum temperature for photosynthesis is almost 100°F (38°C), a temperature at which the rate of photosynthesis slows markedly in most plants. Over large parts of the dry Indian plain the babul is the only tree to be seen.

The ants also bite off the shoots of any neighboring plant that grows into the crown of the acacia and threatens to shade it. Obviously, the ants demand “payment.” At the base of the petioles the acacia has nectaries, organs that secrete a sugary liquid called nectar. This high-energy food attracts the ants, which feed on it and then drill into the base of the nearest thorn. They hollow out the thorn and establish their nest inside with a convenient nectary just by the door. Each thorn has its own colony of ants. Payment does not end there, though, because ants cannot subsist on a diet of only sugar. At the tip of each of the leaflets that grow on either side of a central stem to form the compound (bipinnate) acacia leaf, like the one shown in the illustration, there is a tiny store, shaped like a sausage, containing food rich in oils and proteins. The ants collect this food and take it into the nest. A naturalist named Thomas Belt was the first person to describe these food stores in a book called The Naturalist in Nicaragua, published in 1874, so they are known as Beltian bodies. This type of very close and mutually benefi-

Bipinnate leaves

Bull Horn Acacia and Its Ants Most acacias grow in Africa and Australia, but not all of them, and the thorns of the bull horn acacia (A cornigera), originally from Mexico and Central America, are not as they seem. The thorns are about one inch (2.5 cm) long and very swollen near the base. Inside each spine is a colony of ants (the species is Pseudomyrmex ferruginea), and should any passing animal so much as touch the plant the nearest ants will launch an immediate and very painful attack. The acacia’s thorns provide a defense that is augmented by the ants, and it is not only hungry animals that are repelled.

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cial relationship between a plant and ants is known as myrmecophily, and it occurs in many acacia species.

Ice Plants and Living Stones In all hot deserts there are succulent plants, which are plants that store water in their tissues to provide a supply for maintaining photosynthesis. Ice plants, such as Mesembryanthemum crystallinum, are succulents and also CAM plants, photosynthesizing at night (see “CAM” on page 100). Their common name refers to the tiny white pimples (papillae) that cover their leaves and give them a frosted appearance. These probably help keep the plant cool by reflecting light. Ice plants grow in the Kalahari and Namib Deserts, as does another group of about 40 species of plants of the genus Lithops, belonging to the same family (Aizoaceae) as ice plants. Lithops species are widely cultivated as living stones or pebble plants, plants that resemble the small pebbles among which they grow so closely they are very difficult to see except when they produce their attractive, showy flowers. Their thick, stonelike leaves disguise them as well as store water.

Euphorbias Euphorbias are also succulents, and some of them are the size of small trees. One of the biggest is the candelabra tree (Euphorbia candelabrum) of East Africa. It grows to a height of about 35 feet (11 m), and there are often small groups of candelabra trees growing close together. They are curious plants, with multiple trunks that diverge about 10 feet (3 m) above the ground to become branches that are almost vertical. These branch again to give the tree a bushy appearance, but with dense, succulent stems that grow erect, like candles in a candelabra—hence the name. Other euphorbias, such as E. aphylla, which grows in the Canary Islands, are low-growing and form cushions. The euphorbia, or spurge family, Euphorbiaceae, is huge, comprising more than 7,000 species. They include the popular house plant poinsettia (E. pulcherrima), the rubber tree (Hevea brasiliensis), and manioc, or tapioca (Manihot esculenta), and they are probably the most adaptable of all plants. What is remarkable about the African euphorbias is their similarity to cacti. Cacti grow naturally only in the American deserts, but desert euphorbias have adapted to the climate by developing succulent stems, many of them with ridges, and spines. E. canariensis and E. echinus grow as clumps of upright, ridged, green stems that look very much like some of the smaller Cereus species of cactus. This is an example of convergent evolution, in which similar environmental challenges produce similar responses among unrelated species (see “Convergent Evolution and Parallel Evolution” on pages 142–145).

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The candelabra tree (Euphorbia candelabrum) of the African deserts is unmistakable. (Nabeel Farah)

Despite the similarities, it is not difficult to distinguish a cactus from a euphorbia. Euphorbia spines occur in pairs, whereas cactus spines occur singly or in bunches. Cut a euphorbia and it yields a milky latex. In most species this is poisonous and forms part of the plant’s protection against grazing. The liquid that seeps from a cut cactus is also likely to be poisonous for the same reason, but it is not milky. There is a further difference between the two types of plants that is not so obvious. Cacti are CAM plants. Euphorbias are C3 plants, engaging in the most common version of photosynthesis (see “C3 and C4 Plants” on pages 99–100 and “CAM” on page 100). This means that euphorbias are less suited than cacti to extreme aridity and high temperatures. The C3 photosynthetic pathway requires them to open their stomata during the day, and although in some species the stomata are located in the grooves between ridges, more water is lost through them than is lost by cacti, which keep their stomata closed during the day. They also photosynthesize less efficiently, because the C3 pathway does not inhibit photorespiration (see “Photorespiration”

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on page 99). Nevertheless, euphorbias thrive in places where conditions are too harsh for most other plants.

Welwitschia Welwitschia mirabilis (also known as W. bainesii), the strangest of all desert plants, is found only in the Namib Desert and in adjacent montane woodland. It is a gymnosperm, but one that is quite unlike all others. There is only one species, and it is the only member of its family, Welwitschiaceae. Welwitschia is its only English common name. Welwitschia has a thick, short stem with a taproot. The stem, which is almost completely buried in the ground, divides into two lobes near the top, and each part produces a single, leathery, straplike leaf. The two leaves grow from the base and continue to grow at a rate of about five inches (13 cm) a year throughout the lifetime of the plant. The leaves grow longer and longer but wear away at the ends. Uneven wear combines with wind action to break the leaves into thonglike segments. Welwitschia lives for several centuries. The average age of living plants is 500–600 years, but some are thought to be more than 1,000 years old. The climate is very dry, with an average annual rainfall of 0.4–4 inches (10–100 mm) and no rain at all in some years, but Welwitschia does not need rain. It obtains all the moisture it needs from fogs that roll in from the sea. The plant reproduces by means of cones borne at the center, on each of its lobes. Being a gymnosperm, Welwitschia does not produce true flowers, but both the male and female cones are red. Male and female cones are produced on separate plants and are pollinated by insects, probably wasps, although scientists are unsure. The female cones disintegrate in spring, releasing the large seeds—1.4 × 1.0 inch (3.5 × 2.5 cm). These have paper wings and drift on the wind. When a new plant starts to grow, it relies on photosynthesis by its cotyledons (seed leaves) for about one and a half years, which is far longer than any other plant.

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At one time prickly pears (Opuntia species) were grown to feed cochineal insects from which a red dye was obtained, used mainly for coloring soldiers’ tunics, and it was for this purpose that they were introduced to the Mediterranean region. Some plants quickly established themselves on uncultivated land, where they were able to grow naturally, and the edible fruits of some species proved popular. From southern Europe prickly pear plants were taken to various parts of Asia, South Africa, and Australia. They became naturalized—able to grow in the wild with no help from humans—in all these places, but it was only in Australia that they proved harmful.

Prickly Pears and the Cactus Moth In Australia the prickly pears invaded land used for grazing cattle and became troublesome weeds. Thousands of square miles of good pasture in Queensland and New South Wales became densely infested with them and useless for

Prickly pear (Opuntia)

(Eric Praetzel)

CACTI

Cacti are the most famous of all plants of the American deserts. Indeed, prickly pears and giant saguaro cacti with their strange, upright branches typify deserts. Quite apart from their frequent appearances in illustrations of deserts, most people have seen real cacti. They are grown in most botanic gardens, but our familiarity with them is partly due to their popularity as cultivated ornamental plants, a popularity that began soon after Europeans first saw them. Specimens were sent back to Europe, and from there they were taken to most parts of the world. Some reached the desert countries of the Near East, where they were able to grow outdoors, and later they were carried to Australia.

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Biology of Deserts farming. At one time the area occupied by prickly pears in Queensland was increasing at a rate of about 1 million acres (405,000 ha) a year. Eventually, they were brought under control by an operation that is still hailed as the greatest success ever achieved by biological pest control—the control of a pest or weed by introducing organisms that feed on it or by exposing it to disease organisms. Two species were causing the trouble, Opuntia inermis and O. stricta, and it arose because no native Australian animals would eat them. Prickly pears had no Australian enemies to keep them in check, so an American enemy was imported. Cactus moths (Cactoblastis cactorum) live in South America, and their caterpillars feed on prickly pear. Moths from South America were released in Australia between 1928 and 1930, and, finding themselves among a vast food supply, they multiplied rapidly. By 1932 the area occupied by the cacti was decreasing fast, and by 1940 the prickly pears were fully under control. Today both plants and moths live together in fairly small numbers, and a stable equilibrium is maintained.

Shapes and Sizes of Cacti There are cacti of all shapes and sizes. The pear-shaped stems of prickly pears are among the most familiar, but the saguaro (Carnegiea gigantea) is probably the cactus most closely identified with the American West. That is the cactus with a main, vertical stem from which branches grow outward all around the main stem and all at the same level, and then vertically upward, like the arms of a candelabra, although in some individuals the branches grow in all directions or are twisted. The saguaro is also known as the giant cactus—it can reach a height of 50 feet (15 m)—and

Saguaro cacti (Carnegiea gigantea)

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(Gene Hanson)

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A barrel cactus (Ferocactus cylindraceus) (Gene Hanson)

monument cactus because of its tall, straight, stem. The stem and branches are all strongly ribbed. There are climbing cacti, such as Hylocereus undulatus, which is one of several species known as night-blooming cereus. There are barrel-shaped cacti, including Ferocactus and Echinocactus species. E. acanthodes, the barrel cactus, grows to about 3.5 feet (1 m) tall, but the biggest of these cacti can grow to 6 feet (1.8 m) in height. Others, such as the Mammillaria species, are quite small. There are also cacti that grow like cushions of many ribbed but unjointed stems. The six species of Ariocarpus from southern Texas and Mexico are of this type. A few of the approximately 50 species of cacti belonging to the genus Rhipsalis are believed to occur naturally in Africa and Sri Lanka, although some botanists think they may have been introduced and then become naturalized there, no record of the introduction having survived. Most Rhipsalis species are found in Brazil. Apart from the possibly African and Sri Lankan species, all cacti are American. They belong to the family Cactaceae, and there are more than 2,000 species. Some are trees, but most are succulents. Although they are typical of hot, dry deserts, they occur naturally as far north as British Columbia and as far south as Patagonia, and in the Andes they can be found up to 12,000 feet (3,660 m) above sea level. Scientists find cacti difficult to classify. They are not closely related to any other plant family, despite similarities resulting from convergent evolution (see “Convergent Evolution and Parallel Evolution” on pages 142–145), for example with the succulent euphorbias, and the cacti themselves seem to be actively evolving. This makes it difficult to

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A cactus coming into flower in the Mojave Desert and Charlotte Williams)

(Ronald

compare species with the dried herbarium specimens that in most plant families provide standards against which known species are identified and previously undiscovered species are recognized as such and classified.

Adaptations to Desert Life All cacti are CAM plants (see “CAM” on page 100), and they also have shallow roots that take up water rapidly whenever it is available. These features allow some species to live as epiphytes, which are plants that grow on the surface of other plants. Some Rhipsalis species are epiphytes. Although they have no leaves and their green stems have many branches, the branches are thin, flattened, and lack spines, so a Rhipsalis growing from the side of a tree is not at all like most cacti.

Saguaro cacti silhouetted against the setting Sun (Gene Hanson)

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The Christmas cactus (Schlumbergera bridgesii), also an epiphyte, is grown as a popular houseplant because it flowers in winter—hence the name. Its succulent stem sections are shaped very much like leaves and are joined into many long, spreading branches, so the plant does not conform to the usual idea of a cactus. Succulent plants store water in their tissues (see “Roots, Stems, and Leaves That Conserve Water” on pages 103– 105), and cacti use their stems for this purpose. Some species have true leaves, but these are usually small and often shed fairly quickly. Cacti rely on their stems for photosynthesis. Although the swollen structures of prickly pears look like leaves, in fact they are stems known as pads. Like many cacti, prickly pears have stems that grow in sections, each jointed to the next, so one pad grows on the tip of another, but not all are of this type, even among the Opuntia. Chollas have cylindrical stems, and some are small trees, O. fulgida growing to a height of 10 feet (3 m) or more. Species belonging to the subgenus Brasiliopuntia also resemble trees and have unjointed main stems. As well as storing water, the swollen stems or leaves of succulents have a smaller surface area in relation to their volume than the stems or leaves of other plants. This is a matter of geometry. Imagine two leaves, one succulent and one not, both of which (for convenience) are cylindrical in shape. Both have a radius r, diameter d (= 2r), and heights of h1 and h2. If r = 3, d = 6, h1 = 2, and h2 = 4, then the ratio of the surface area (πdh1 + 2πr2) to volume (πr2h1) of the smaller leaf is 1.17 to 1, and the ratio for the larger leaf (height h2) is 1.7 to 1. The ratio of surface area to volume helps equip succulent plants to survive, because the amount of moisture they can hold is directly proportional to their volume, whereas the amount of warmth they can absorb and water they can lose by transpiration is directly proportional to their surface area. Maximizing volume while minimizing surface area increases the amount of moisture they can store while reducing the rate at which it is lost and the extent to which the plant is heated by the Sun. Like many desert plants, cacti have spines. In Opuntia species and other members of the subfamily Opuntioideae to which they belong, these take the form of short, barbed hairs called glochids that grow in bunches. In other cacti the spines are modified leaves. Both spines and glochids grow from sunken cushions called areoles. These are modified shoots. In some species areoles occur singly on raised, wartlike structures called tubercles, and in others they grow in rows along raised ridges. Their possession of areoles is one of the features by which cacti can be distinguished from plants of any other kind. Spines and glochids deter herbivorous animals. They also trap a layer of relatively cool air next to the plant, and dew often condenses on them. As the water runs down the spines it also cools the plant.

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Biology of Deserts

A tree flowering in the Mojave Desert (Ronald and Charlotte Williams)

PLANTS OF CONTINENTAL AND POLAR DESERTS

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Tundra is a Lapp word used to describe land that is barren. By its very name, then, the tundra is a hostile place, an environment as harsh as any tropical or continental desert, but one of strange contrasts. Although the annual precipitation is very low, for a short time in summer large areas are waterlogged, and extensive pools of water lie on the surface. This is the result of severely impeded drainage. Below the surface the ground is permanently frozen—it is permafrost. Rising temperatures in spring and summer thaw the top layer, known as the active layer, which has a depth of one to 10 feet (0.3–3.0 m) depending on the type of soil. The ice in the active layer melts, but the water cannot drain away because of the thick permafrost layer below the active layer. The surface is uneven, however, so some drain-

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age can take place as water moves from higher to lower ground. This leaves small hillocks, ridges, and mounds extremely dry. The environment therefore consists of very arid islands, called fell-field, or fjeldmark, surrounded by ground that is sodden. Trees cannot grow there, for the active layer is too thin for their roots—and no plant roots can penetrate permafrost—and the climate is too cold and too dry. In some places and over very large areas lichens are the only plants that can survive.

Lichens A lichen is not one organism but a pair of organisms living in an intimate relationship. A fungus, called the mycobiont, sends its fine network of hyphae into the tiniest of cracks in search of mineral nutrients it can absorb. The fungus gives the lichen its shape and structure. Embedded in the fungus are phycobionts comprising millions of single-celled organisms that conduct photosynthesis. Depending on the type of lichen, these are either green algae or cyanobacteria, which are photosynthesizing bacteria. The phycobionts supply carbohydrates to the fungus.

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Lichens are well equipped for life in a cold climate. When the temperature falls below the minimum needed for photosynthesis (see “Heat and Light” on page 101), they simply become inactive, and they can remain in this condition almost indefinitely. As soon as the temperature rises above the minimum threshold, photosynthesis resumes at once.

Dry Valleys and Nunataks Lichens are the only form of life in the dry valleys, or oases, of Antarctica. These are areas where the ground is free from ice, and they cover about 2,200 square miles (5,600 km2) of the continent. Lichens also grow elsewhere on exposed rock that emerges above the snow and ice—such an outcrop is called a nunatak—as far south as 86.15°S, where they occur at an elevation of 6,500 feet (1,980 m), and Antarctica supports about 350 species of them. There are also regions near the North Pole where only lichens can survive, though these areas are smaller than their counterparts in the Southern Hemisphere because the North Pole is surrounded by sea, not land. Mosses can also grow under harsh conditions and are found in Antarctica at about 84°S and at an elevation of 2,490 feet (760 m). The most extensive area of unbroken vegetation in Antarctica is believed to consist of moss growing on top of a layer of peat three feet (1 m) thick. It is on Green Island, off the Antarctic Peninsula, and the vegetation covers nearly four acres (1.6 ha).

Sunshine and Flowers Polar landscapes present scenery on a grand scale, but much of the life they support is tiny. Down near the icefree surface lichens and mosses form forests and jungles within which minute animals graze and hunt. There, where they are sheltered from the drying wind, plants and the animals that feed on them are able to benefit from the one advantage of living in such a high latitude—the long hours of intense sunshine in summer. The importance of sunshine to tundra plants is demonstrated by Arctic relatives of the foxgloves (family Scrophulariaceae) that are known as compass flowers because the flowers on the south-facing side of the shoot develop faster than those on the other side, facing away from the Sun. A hair grass (Deschampsia antarctica) and Antarctic pearlwort (Colobanthus crassifolius), a member of the carnation family (Caryophyllaceae), grow close to sea level along the Antarctic Peninsula as far south as about 68°S. They produce flowers but reproduce vegetatively and rarely set seed. Antarctic pearlwort produces colorless flowers, and the plant grows as a cushion. These two species are the only vascular plants (plants with xylem and phloem tis-

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sues) that occur naturally on the mainland of Antarctica, although a few plants have been introduced onto the peninsula, including two species of meadow grass, Poa pratensis and P. annua. Plants of polar deserts, like those of hot deserts, must be quick to exploit brief improvements in their generally inhospitable environment, but they have one advantage. In a polar desert that brief episode is predictable because it occurs in summer, and it allows plants to respond to changes in the length of day. Flowering herbs often start to produce flower buds as the days are shortening. Alpine, or mountain, sorrel (Oxyria digyna), for example, forms buds when the day length shortens from 18 hours to 15 hours. The buds remain dormant through the winter, but as soon as the days start lengthening once more, they are ready to flower immediately. Purple saxifrage (Saxifraga oppositifolia) and an Arctic buttercup, Ranunculus nivalis, are in full flower no more than four days after the snow has started to melt.

Reproduction in a Cold Climate Like the flowering plants of hot deserts, those of polar deserts that rely on insects for pollination compete strongly for attention. They produce big, showy flowers, and some of them go even further by providing warmth for the benefit of visiting insects. Their flowers focus warmth onto their reproductive organs in the same way a parabolic dish for receiving satellite TV focuses radio waves onto its aerial, and, also like some satellite dishes, the whole flower turns as the Sun crosses the sky. It can be up to 18°F (10°C) warmer inside the flower than it is in the air outside. Despite their bright flowers, however, most of the plants reproduce from shoots that grow from rhizomes (underground stems). Like the creosote bush of hot deserts (see “Creosote Bush” on page 110), this method of reproduction leads to the formation of a clone. Old individuals die, new ones emerge, and they are all part of the same plant joined below ground. Reproduction from seeds is even more difficult for a plant in a polar desert than it is for one in a hot desert. If its flower is pollinated and if the temperature remains above freezing long enough for its eggs to be fertilized and for seeds to form, then the seeds may survive the winter on the ground. Next spring they will germinate as soon as the temperature rises above a critical threshold. The growing season is very short, so seeds must germinate early in the year if they are to grow into seedlings that are strong enough to withstand their first winter. This means they cannot afford any delay in the coming of the spring thaw. Once the seeds have germinated, the young plants must be able to grow steadily through the spring and summer. Anything that interrupts their growth, such as a drought, will kill them. They must also survive the drying effect of

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Biology of Deserts the wind and being battered by ice crystals and debris the wind throws at them, as well as the needle-sharp ice crystals that can form just below ground and pierce stems and roots. Not surprisingly, annual plants are uncommon in cold deserts.

Biennials and Woody Perennials In the northern tundra there are biennials—plants that live for two years, producing seed in the second—such as moss campion (Silene acaulis). It forms dense green cushions and has solitary pink flowers about 0.4 inch (10 mm) across. Apart from having colored flowers, moss campion closely resembles Antarctic pearlwort. Some of the cushions found on the Arctic fell-fields are of shrubs, such as arctic white heather (Cassiope tetragona), trailing azalea (Loiseleuria procumbens), and bearberry (Arctostaphylos uva-ursi), all of which are members of the heath family (Ericaceae). Like plants of hot deserts, they have small, waxy leaves that reduce water loss. The dense mats and cushions they form shelter the ground, reducing evaporation, and also protect the plants themselves from the drying effect of the wind. Mountain avens (Dryas octopetala), widely cultivated as an alpine ornamental plant, is also a low-growing, cushion-forming shrub. It belongs to the rose family (Rosaceae), and its roots have nodules containing bacteria that fix nitrogen. All of these are perennial plants—plants that live for more than two years—with woody stems and branches. They grow very slowly in the cold, dry climate. It may take mountain avens 100 years to spread to the sides by three feet (1 m). Here and there on the moister ground are also woody plants that in a warmer climate would be trees but that grow in the Arctic as spreading shrubs, never more than three feet (1 m) tall. Dwarf birch (Betula nana) is common, as are reticulate willow (Salix reticulata) and Arctic willow (S. arctica). Juniper (Juniperus communis) grows as a tree up to 40 feet (12 m) tall in temperate regions, but in the Arctic the dwarf variety, J. communis var. nana, is only one foot (30 cm) tall.

Plants in Central Asian Deserts Although the Gobi is a desert, it is not devoid of plants. Pasture grasses grow in the warmer areas, especially Echinochloa species, a type of wild millet known as jungle rice and also as barnyard grass in North America, where it has been introduced. Elsewhere on the Gobi plateau are feather grasses (Stipa species), with blades folded into tubes so the stomata are on the inside. These include Gobi feather grass (S. glareosa) and timuriya (S. villosa). Around the desert margins are snakeweeds (Cleistogenes species), including Dzungarian bridlegrass (C. soongorica).

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The soils are salty in some places, so many of the woody plants are halophytes (tolerant of salt) as well as xerophytes (tolerant of drought). Yellow khotir, also called yellow wood beancaper (Zygophyllum xanthophyllum), has edible flower buds that are used as a substitute for capers. The nitre bush (Nitraria sibirica), another member of the same family (Zygophyllaceae), has berries that some animals eat because they are rich in salt. Tamarisks are shrubs or small trees that often grow on sandy soils near coasts. Dzungarian reaumuria (Reaumuria soongorica), a member of the tamarisk family (Tamaricaceae), is a small halophytic shrub found in the low-lying Dzungaria basin of northwestern China. Yellow ephedra (Ephedra prezewalskii) is a xerophytic shrub or small tree. Winter fat (Krascheninnikovia lanata), a member of the sugar beet family (Chenopodiaceae), also grows in North America, where it is known as white sage. It is a small bush covered with a dense mat of pale-colored hairs. Gobi kumarchik (Agriophyllum gobicum) belongs to the same family but is an annual herb with seeds that are an important Mongolian food. Saxaul (Haloxylon ammodendron) is a shrub about 10 feet (3 m) tall that grows in the sandy parts of the desert where the sand is stable. In many of the less arid places saxaul plants grow so close together they form forests that bind the soil together and prevent erosion. There are also peashrubs (Caragana bungei and C. leucocephala), small shrubs that are the dominant plants over large areas. In contrast, the Taklimakan Desert of western China is extremely dry, and plants are very few and far between. In depressions where there is groundwater within 10–15 feet (3–4.5 m) of the surface, there are sparse thickets of tamarisk shrubs and nitre bushes, but over most of the desert the shifting sands make it impossible for plants to gain a secure anchorage. There are more plants near the edges of the desert.

Plants of Patagonia Patagonia has a much richer plant life. There are forests along the western border. On the tableland to the east of the Andes Mountains the plants are xerophytic, adapted to the combination of low rainfall and constant drying winds. Few plants grow in the driest areas, but there are wetter areas where the ground is completely covered by plants. In the desert proper, where plants cover up to 15 percent of the ground surface, there are halophytes such as saltbushes (Atriplex species) and other members of the sugar beet family (Chenopodiaceae). In the moister areas, where almost half the ground is covered in plants, the predominant species are cushion plants, often growing between rocks, and dwarf shrubs. Many of these are widely cultivated as alpines. Among the

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shrubs and cushion plants are tufts of feather grass (Stipa species) and meadow grass (Poa species). Where the desert merges into the pampas grassland, shrubs cover more than half the ground. They include Chuquiraga species, up to 20 inches (50 cm) tall, and Berberis species, which grow as small trees up to 10 feet (3 m) tall.

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Birds and mammals are warm-blooded. Fishes, amphibians, reptiles, and all invertebrate animals are cold-blooded. This division of animals into two types, warm-blooded and cold-blooded, is based upon real physiological differences, but nevertheless it is misleading. While it is active a lizard or snake is quite warm despite being cold-blooded. In fact, its body temperature is not much different from that of a warm-blooded mammal, and it may even be higher. A bird also feels warm, and its body is several degrees warmer than a mammal’s. So if mammals are warm-blooded, perhaps birds—and possibly lizards—should be labeled “warmer-blooded.” Fish, on the other hand, actually do feel cold, as do most invertebrates unless their surroundings are warm. Oddly enough, cold-blooded animals originally earned their name not because they were thought to function at a lower temperature than warm-blooded animals, but because they were thought to tolerate very high temperatures. Lizards can often be seen basking on rocks that are uncomfortably hot to the touch. Clearly, they would be burned unless they had very cool blood—or so people believed for so many centuries that eventually they came to take it for granted. It is only in this century that scientists have discovered how and why animals regulate their body temperatures. It is much more complicated than ideas of warm or cold blood suggest.

Exotherms and Homeotherms A more useful distinction is based not on body temperature itself, but on how it is regulated. This requires animals to be placed into several categories. The first contains what are called exotherms, or poikilotherms. Exo- is from the Greek exo, “outside,” poikilo- is from poikilos, which means “changeable,” and thermē means “heat.” Exotherms are animals in which the body temperature varies with that of their surroundings. Fish and invertebrates are exotherms. Exothermy does not mean that their temperatures necessarily fluctuate widely. Fish live in water, and because of its high heat capacity (see “Specific Heat Capacity” on pages 90–91), the temperature of water changes only slowly and within fairly narrow limits. Most fish will die if the temperature of their water changes by more than about 25°F

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(14°C) over a short period of time, although some can survive such a change if it happens over several hours. Many invertebrates survive times when the weather is too hot or too cold by becoming dormant (see “Estivation and Hibernation” on pages 132–136). Animals that maintain a fairly constant body temperature that may be warmer or cooler than their surroundings are called homeotherms; Greek homoios means “like.” The ability to remain warm when the air temperature falls and cool when it rises is essential for most land-dwelling animals. Temperatures fluctuate much more widely on land than they do in water. Changes can be rapid, especially in hot deserts, and the temperature may rise or fall outside the range most animals find tolerable. In all homeotherms the internal body temperature, known as the core temperature, is held within fairly narrow limits. There are two ways in which an animal can achieve this regulation, so homeotherms are of two types.

Those That Bask and Those That Shiver Some animals, such as amphibians and reptiles, use behavioral means. They bask in order to warm their bodies, for example, and seek shade to cool them. These are known as ectotherms, ecto- being from Greek ekto, “outside.” Others have internal, physiological ways to regulate temperature. When they are cold, for example, blood vessels in the skin contract to restrict the flow of blood and loss of heat to the outside, and animals may shiver to generate warmth by moving their muscles rapidly. When they are hot, blood vessels in the skin dilate, and animals may sweat. The evaporation of sweat absorbs latent heat from the skin, thereby cooling it. A mammal can make its fur more erect to trap a layer of air that is warmed by its body and provides insulation. Animals of this kind are called endotherms, endo- coming from the Greek endon, meaning “within.” Only mammals and birds are endotherms. The distinction is not quite so clear as the definitions make it seem, because most endotherms respond behaviorally to extreme temperatures. Humans are the clearest example. When the weather is cold people turn up the heat and put on warmer clothes. When it is hot they wear lighter clothes. Many other mammals grow thicker coats in winter and shed them in summer, which is the equivalent, although it is not a behavioral response. Animals do modify their behavior, however. They bask in the warm sunshine to warm themselves—sunbathing—and lie in the shade when they feel too hot. Only humans light fires to keep warm, of course. Both endothermy and ectothermy allow animals to function in an environment where the temperature is not constant, but a price must be paid for this freedom. An ectotherm living in a hot desert has to spend part of every day warming its body and part of the day sheltering to prevent

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Biology of Deserts it from overheating. This reduces the time it has available for other activities, such as feeding. Vertebrate ectotherms cannot live in cold deserts because the low temperatures make it impossible for them to maintain body temperatures high enough to be active long enough for them to feed and reproduce, although there are two snakes that live inside the Arctic Circle. The adder (Vipera berus) occurs at 68°N in Scandinavia and the common garter snake (Thamnophis sirtalis) at 67°N in North America. Invertebrates can inhabit cold deserts by emerging only during favorable periods and because, being much smaller, they can warm themselves more quickly. Endotherms are not constrained in this way. They can live anywhere there is food for them, but that is where they meet a different constraint. Maintaining a constant core temperature by physiological means involves a considerable expenditure of energy. An endotherm must eat much more food than an ectotherm of similar size.

Metabolic Rate The energy that different animals require can be compared from measurements of their oxygen consumption. Bodies derive the energy they need from the oxidation of carbohydrates in the process of cellular respiration. This consumes oxygen, and oxygen consumption is easy to measure under controlled conditions. Each liter of oxygen consumed by respiration liberates about 20.2 kJ (4.83 kcal) of energy, so from the amount of oxygen consumed in a day it is simple to calculate the amount of energy an animal uses in a day. The result of this calculation is a measure of the rate at which an animal’s metabolism functions and is often expressed as the basal metabolic rate (BMR). This is the metabolic rate of an animal that is lying completely at rest in surroundings where it does not need to warm or cool its body. The BMR for an average human adult male resting at 68°F (20°C) is about 1,600–1,800 kcal (6.7–7.5 MJ), for example, and for an adult female about 1,300–1,500 kcal (1.5–6.3 MJ). The numbers sound large, but this is about as much energy as would keep a 100-watt lightbulb shining. When the amount of energy the animal uses is divided by the surface area of its body or by its body mass (conventionally in kilograms), the result is a metabolic rate per unit of surface area or per mass per day for that species. This is called the standard metabolic rate (SMR). Once metabolic rates have been corrected for body size, those of different species can be compared. Comparisons show a direct relationship between metabolic rate and body size. The smaller an animal, the greater is its metabolic rate. This is true for all vertebrates, regardless of whether they are exotherms, ectotherms, or endotherms, and the ratio of metabolic rate to body size is the same for them all.

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Cost of Regulating Body Temperature When BMR and body size are plotted on a graph for a number of species, the result is a straight line, with the smallest animals at the top of the slope and the biggest at the bottom. If the species include both ectotherms and endotherms, however, the graph contains two parallel straight lines, one for each type. The two lines demonstrate that although the BMR to body size relationship is sustained, all endotherms have a higher BMR than all ectotherms. The fact that the relationship holds equally for ectotherms and endotherms also demonstrates that it is not due solely to temperature regulation. Some other factor must partly account for it, but at present scientists have not discovered what that factor is. Nevertheless, part of the difference between the two arises from the energy that must be expended to maintain a constant temperature by physiological means. Lying on a warm rock in the sun or on a cool rock in the shade consumes much less energy than shivering, sweating, and constricting or dilating blood vessels. Since metabolic rates can be compared, it is possible to measure the extent of this difference. At 68°F (20°C) the BMR for an adult male human is about 1,800 kcal (7.5 MJ), and that for an American alligator, which is an ectothermic reptile of about the same size, is about 60 kcal (2.50 kJ). Energy is released by the oxidation of carbohydrates, and animals obtain their carbohydrates from the food they eat. The amount of food an animal must eat, therefore, is proportional to the amount of energy it needs to maintain its metabolism. The higher its BMR, the more food it requires, so endotherms—birds and mammals—are obliged to eat much more than ectotherms—amphibians and reptiles—of similar size.

HOW HEAT CAN KILL AND HOW ANIMALS KEEP COOL

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Animals adapt to the conditions under which they live. These conditions include climate, and animals adapted to different climates tolerate different temperature ranges. A polar bear could not survive long in the Sahara, any more than a sidewinder rattlesnake could survive in Greenland. Their tolerance of different climates reflects the way animals regulate their body temperatures. Mammals and birds have a core body temperature of around 97–104°F (36–40°C) regardless of where they live. The body of a wolf living in the tundra is at the same temperature as that of a jackal living in a subtropical desert. An ability to control its internal body temperature does not mean an animal can remain unaffected by the air

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temperature outside its body. The regulatory mechanisms can be overwhelmed, and extreme heat can kill. In “The Day the Sands Caught Fire,” an article in the November 1998 edition of Scientific American, Jeffrey C. Wynn and the late Eugene M. Shoemaker described an expedition they led into the Empty Quarter, the Rub’ al-Khali, of Saudi Arabia. One day Wynn went out to conduct a geomagnetic survey when the temperature in the shade under a tarpaulin was 142°F (61°C) and the relative humidity was 2 percent. “By the time he returned he was staggering and speaking an incoherent mixture of Arabic and English. Only some time later, after water was poured on his head and cool air was blasted in his face, did his mind clear.” Dr. Wynn had come perilously close to death. Endothermy evolved as an adaptation to cold climates. It is much more efficient at minimizing and compensating for heat loss than it is at dissipating heat to prevent the body temperature from rising. Many endotherms find it more difficult to hold their body temperature 18°F (10°C) below the air temperature than 18°F (10°C) above it.

Keeping Cool Mammals cool themselves by dilating blood vessels near the body surface. This increases the blood flow, and provided the outside temperature is lower than the core temperature, the blood will be cooled. This is an efficient way to transport heat from the interior of the body and then to lose it. Mammals also allow water to evaporate, deriving the latent heat of vaporization from their bodies. Humans sweat, secreting water onto the skin, from which it evaporates. Other mammals, dogs being the most familiar, pant. Panting allows moisture to evaporate from the inside of the mouth and respiratory passages. Evaporation is an effective way to keep cool, but there is a cost. Water that is lost from the body must be replaced, and in a desert that may not be easy. Cooling by evaporation is not satisfactory except as a short-term measure.

Heat Exhaustion and Heatstroke Athletes competing in events that require prolonged effort, such as marathon races and long-distance cycle races, drink frequently. Cyclists carry bottles of liquid, and assistants pass bottles to marathon runners. The athletes are not drinking merely to slake their thirst and make themselves feel more comfortable. Without regular intakes of liquid they would collapse. Humans lose heat by sweating. If the liquid that evaporates is not replaced, the body gradually becomes dehydrated. The first symptom of dehydration may be painful muscular contractions—heat cramps. Prolonged exposure to high temperatures does not always cause heat cramps, but it may produce heat exhaus-

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tion, also called heat prostration. In this condition lack of liquid reduces the blood circulation, and the individual collapses. The victim has a normal or only slightly elevated temperature but sweats profusely and has a rapid pulse, and the skin feels cold. She may feel drowsy and nauseous. Victims of heat exhaustion usually recover by resting somewhere cool, but without effective treatment heat exhaustion can lead to a serious type of neurosis. Heatstroke can be fatal. In this condition the body starts to lose its capacity to maintain a normal core temperature. The temperature rises, the pulse is rapid, and the skin is hot, dry, and often flushed. The victim is confused and feels dizzy and weak. This is the state Dr. Wynn was in. Had he not received immediate treatment he might have collapsed, lost consciousness, and sunk into a coma.

Explosive Heat Death As the body loses water the proportion of plasma in the blood decreases. This causes the blood to become more viscous until a point is reached at which the rate of circulation slows. The body is then unable to lose heat by using blood circulation to transport it from the interior of the body to the surface. The core temperature rises rapidly, and death follows quickly. This is known as explosive heat death. Most mammals will die if they lose between 10 percent and 20 percent of the water in their bodies. Body fluids are not pure water but solutions of a variety of salts. Evaporation involves only water, however. When water molecules break the hydrogen bonds that link them and escape as vapor, molecules of other substances are not affected. They remain behind. Consequently, when water evaporates from a solution the concentration of that solution increases. Water that is lost through sweating and panting is taken from the aqueous solution in the spaces between cells. As this water is lost and the solution outside cells becomes more concentrated than that inside cells, water may start to move across cell walls by osmosis (see the sidebar “Osmosis” on page 97). This increases the concentration of the solution inside the cell. At first this accelerates the activity of the cell. This is because the metabolism—of a cell or of a body consisting of billions of cells—is a series of chemical reactions catalyzed by enzymes. Enzyme molecules have locations called active sites, at which other molecules, called substrate molecules, become attached in the first stage of a chemical reaction. The greater the concentration of substrate molecules, the more often they will collide with the active sites of enzyme molecules and so the faster the reactions will proceed. Eventually, though, all the active sites are occupied, and as soon as one is vacated at the completion of the reaction it is filled again. The metabolism can move no faster unless more enzymes are manufactured, which sometimes hap-

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Biology of Deserts pens. If dehydration continues, the function of the cell is disrupted, and it dies. There is an optimum temperature above and below which metabolic reactions slow down. This is determined by the optimum working temperature for enzymes, and in vertebrates that temperature is about 104°F (40°C). In turn, this sets the optimum body temperature—in humans, 98.6°F (37°C). Should the core temperature exceed this, enzymecatalyzed reactions start to slow. At temperatures not much higher than 104°F (40°C) enzymes start to degrade as the bonds holding together the constituent parts of their molecules begin to break. Most mammals suffer brain damage if their body temperature remains above 109°F (43°C) for more than a few minutes.

Living in Burrows Small mammals avoid exposing themselves to the full rigors of the desert climate. They spend their days below ground in burrows, where the temperature is much lower than it is at the surface, and emerge at night to feed. If an animal the size of a rat spent the hottest part of the day above ground, it would have to evaporate about 13 percent of the water in its body every hour in order to avoid overheating. Rodents do not sweat, but when its body temperature rises a kangaroo rat (Dipodomys species) salivates copiously and licks its fur, cooling itself by the evaporation of saliva. A substantial proportion of the food a bird or mammal eats is used to provide the energy for maintaining a constant body temperature. Some desert rodents, such as the California pocket mouse (Perognathus californicus) and cactus mouse (Peromyscus eremicus), exploit this fact. They become torpid. Usually they do so in response to a scarcity of food, but sometimes they become torpid when water is scarce. Safe in its burrow, the animal ceases to move, and its metabolism slows and with it its heart beat and rate of breathing. Its temperature then falls approximately to the air temperature in the burrow. A pocket mouse can become torpid for just a few hours and then arouse itself. Torpor greatly reduces the need for food and water.

The Camel Large animals cannot shelter in burrows, but because they are large their bodies absorb heat more slowly than do those of smaller animals. This does not prevent them heating, of course, and desert species have various ways of surviving. Surprisingly, thick fur can help by shading and insulating the skin. The single-humped camel, or dromedary (Camelus dromedarius), has fur on its back. During the middle of the day the top of the fur can be at 160–175°F (71–79°C) and the skin beneath the fur at 104°F (40°C). Dromedaries sweat, but their fur also increases the efficiency of sweating. Their sweat

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evaporates in the shade beneath the fur, using latent heat absorbed from the skin. If the skin were naked, the solar heat to which it was exposed would provide most of the latent heat, and sweating would have very little cooling effect. At night the temperature of a dromedary’s body falls, sometimes to 95°F (35°C). Then during the day it rises to about 104°F (40°C). This fluctuation reduces the amount of energy and water the dromedary needs to expend. Its behavior also helps it keep cool. In the early morning, when the ground is cold, dromedaries lie down with their legs folded beneath them. The sun warms their backs, but they lose heat by contact with the ground. During the hottest part of the day dromedaries sometimes lie close together, their sides touching. This ensures their skins are all at the same 104°F (40°C) rather than being heated further by the Sun. (See “The Ship of the Desert” on pages 138–142.)

Antelopes and Gazelles Antelopes and gazelles also allow their temperatures to fluctuate. The temperature of a gemsbok (Oryx gazella), an inhabitant of the deserts of southern Africa, can rise to 113°F (45°C), and a Grant’s gazelle (Gazella granti) can tolerate a body temperature of 115°F (46°C) for six hours. At these temperatures the animals should suffer brain damage. They avoid this by means of a circulatory system in which the small arteries to the brain lie adjacent to veins returning from the nasal passages and therefore carrying blood that has been cooled. This ensures a supply of cooled blood to the brain.

Reptiles Reptiles have none of these physiological mechanisms. They avoid excess heat either by seeking shade or by spending much of the day in burrows. Basking and sheltering is a method of regulation that permits very fine tuning. Most lizards are fully active within a temperature range of about 7°F (4°C). Other reptiles have a wider range of up to 18°F (10°C). When its temperature approaches the upper end of the range, the animal seeks shade or buries itself, and when it approaches the lower end, it seeks sunshine and a warm surface on which to bask.

HOW TOLERATING A SLIGHTLY HIGHER TEMPERATURE PAYS DIVIDENDS

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Life in a hot desert is hard, and not many plants have been able to adapt to the aridity and high daytime temperatures.

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Consequently, plants are few and widely scattered, and many of them are ephemeral, emerging only after rain and spending most of the time as seed (see “Surviving as Seeds” on page 106). Heat and drought are not the only problems plants must face. There are also hungry animals. The fact that each perennial plant is likely to grow in isolation increases its vulnerability to attack from herbivorous animals. An isolated plant is clearly visible because there are no other plants among which it can be hidden, and the surrounding area offers no alternative food for herbivores. Everywhere in the world plants have evolved ways to deter herbivores from eating their leaves, stems, and roots. Many plants are poisonous, and some extremely so. Plant toxins, known as “secondary compounds” because they are formed as by-products of the major metabolic pathways, include strychnine from Strychnos species, mescaline from the peyote cactus (Lophophora williamsii), and morphine from the poppy Papaver somniferum. Other plants have spines or thorns or tough, unpalatable leaves. Desert plants are not unique in being generally difficult or dangerous to eat, but isolation has exposed them to strong natural selection that has intensified these features. Quite simply, the less palatable the plant is, the more seeds it is likely to produce. Consequently, the pressure of natural selection ensures that the palatability of desert plants decreases generation after generation. Even grasses are less edible than they seem. Most perennial grasses have rhizomes—stems that run along or just beneath the ground surface, out of the reach of most animals. The leaves can be removed without harming the plant, but they have tough cell walls containing silica, the mineral from which quartz sand is made. This will wear away the teeth of animals that seek to obtain the rather small amount of nutritious material contained inside cells. Many animals eat grass, but it is poor fare, especially the clumps of coarse grasses found in deserts.

Facing Challenges As though the daytime heat, nighttime cold, lack of water, and scarcity of food were not sufficient obstacles, the selfprotection of plants adds another challenge for desert herbivores to overcome. Herbivores are also under selective pressure, of course, and can evolve ways to render plant toxins harmless and to bypass physical defenses, such as thorns and tough leaves. Rodents have teeth that grow continually, so they remain the same size despite being worn down, and grazing mammals have large, strong teeth that can crush tough material without being damaged. A few animals have even found ways to cooperate with plants, like the ant Pseudomyrmex ferruginea and the bull horn acacia (see “Bullhorn Acacia and Its Ants” on pages 114–115).

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No single herbivorous species can evolve ways to counter all the threats presented by all plants, however. Plants cannot achieve total protection, but they can compel herbivores to be selective in their diet. There are only certain parts of certain plants that a desert herbivore can eat. If any herbivore were able to eat any plant, an animal could approach the nearest plant of any species. Food would be easy to find. When it arrived, however, the animal would find itself competing with other animals, all feeding on the same parts of the same plant. It would have to assert its claim to a portion of the plant, perhaps by driving away rivals, and this would reduce the time it could spend feeding. Dietary specialization avoids much of such competition. That is to the advantage of each herbivorous species. The disadvantage is that an animal must spend more time searching for a plant it can eat. The time it can spend feeding is still restricted.

More Time for Feeding Whatever metabolic or behavioral strategy an animal adopts, its survival in the desert depends on maximizing its mobility and foraging time. Endotherms are highly mobile. Birds can and do fly long distances, and mammals can and do run. Nor do endotherms have difficulty devoting sufficient time to foraging. Birds fly to wherever food is available, traveling by air where the temperature is much lower than it is at ground level, and their aerial view helps them to locate food. Small mammals shelter by day and forage in the cool of the night, a habit that also affords them some protection from predators. Against these advantages there is a disadvantage, however. Endotherms need much more food than do ectotherms. Insects cannot fly far in search of food, and reptiles— ectothermic vertebrates—cannot fly at all, nor can most of them forage at night, because then their muscles are too cold. A reptile will die quite rapidly if it is exposed to a temperature below a certain minimum or above a certain maximum. Within these extremes there is a low temperature at which a reptile can survive but is unable to move or can move only very slowly, and a high temperature at which it is also rendered immobile and will die with continued exposure to the heat. For as long as it is immobile or moves only with difficulty, an animal is vulnerable to predators.

Controlling Temperature by Basking A reptile is fully active only when its body is within a few degrees of 98°F (37°C). There are two ways for it to maximize its foraging time. The first is to warm to its active temperature rapidly and cool slowly. This will allow it to start foraging earlier in the day and continue foraging later into the evening. Alternatively, it can adapt to tolerate higher temperatures, which will allow it to continue foraging for

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Biology of Deserts a little longer in the hottest part of the day. Reptiles have done both. While it is true that reptiles must bask to warm their bodies and seek shade to cool them, this is only part of the story. For one thing, basking and seeking shade are more sophisticated activities than they sound. As well as the solar radiation reaching it directly from the sky, a basking reptile also seeks reflected radiation, choosing a spot to bask where it is exposed to radiation reflected from nearby surfaces, such as rocks. To speed up the warming, many reptiles lie with their cold body pressed against a warm surface. Later in the day, when the ground is hot, a lizard that has long legs (some lizards have short legs or are legless) stands with its legs fully extended to raise its body and tail as far above the ground as it can. Many desert insects also have long legs and hold their bodies as high as they can to minimize contact with the surface. On hot ground a lizard may raise each foot in turn so it is always standing on three legs, with the fourth held clear of the surface. Similarly, some shaded places are cooler than others, and ectotherms select the place with the temperature they need.

Regulating the Blood Flow Small reptiles reach their optimum temperature quickly as the warmth of the sunshine heats the whole of their bodies, but when a large lizard, snake, or turtle lies basking and its skin begins to warm, blood vessels in its skin dilate. This is an automatic response to rising temperature, and it accelerates the blood flow, carrying warmed blood to the interior of the body. In the early morning, when its body is cold, a large reptile basks with its body oriented so its back is fully exposed to the Sun. As its blood vessels dilate the animal warms rapidly. In the late afternoon, as the Sun sinks lower in the sky and the air temperature starts to fall, the mechanism goes into reverse. When the skin temperature falls, its blood vessels cease to be dilated. This slows the circulation and thus reduces the rate at which warm blood from inside the body

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travels close to the skin and is cooled. The overall effect is that the reptile warms up much more quickly than it cools down. This extends the time it can devote to foraging.

Changing Shape and Changing Color Reptiles are also able to control the amount of solar radiation their bodies absorb. When a reptile lies with its back exposed to the Sun, it absorbs much more radiation than it does when facing into the Sun. The drawing illustrates this. Lizards and many snakes—but not turtles, of course—are able to increase this effect by spreading their ribs in a way that alters the shape of their bodies. Probably the most extreme example of this ability is found in horned lizards, such as the Texas horned lizard (Phrynosoma cornutum). By altering the shape and orientation of its body it can achieve a sixfold difference in the amount of solar radiation to which it is exposed. Many lizards, including the Texas horned lizard, can also change their body color by spreading the dark pigment melanin throughout specialized skin cells called melanophores. Some snakes can also do this, but to a lesser extent. Rattlesnakes can make their bodies lighter and darker in color, but turtles and crocodilians (crocodiles and alligators) cannot change their color. Darkening the skin increases the amount of radiation the body absorbs, and lightening it reduces absorption. Some lizards can alter the amount of radiation their skin absorbs by as much as 75 percent. All these strategies increase the efficacy of basking and seeking shade as a means of regulating body temperature. They allow ectotherms to remain active for longer, and so

Basking and temperature control. When it stands with its back to the Sun, the lizard exposes a large area of its body to absorb heat. When it stands facing the Sun it minimizes the body area exposed to the Sun’s rays, preventing its body from becoming too hot.

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The desert iguana, or crested lizard (Dipsosaurus dorsalis), lives in the deserts of the United States and Mexico. (NASA)

they increase the time the animals can spend in search of food. During the hottest part of the day, however, even these mechanisms are insufficient, and all desert animals must seek shade.

Tolerating the Heat A very few of the desert inhabitants have an advantage over all the others: They are active at a temperature a little higher than most animals can tolerate. The desert iguana, or crested lizard (Dipsosaurus dorsalis), of the southwestern United States and Mexico is active when its body temperature is between 104°F and 108°F (40°C and 42°C). When its temperature threatens to rise above the upper threshold, the lizard shelters beneath a creosote bush. Until then, however, it can move around while most other animals are out of sight and inactive. During this period it has no competitors and can claim for itself all the food it can find. It can claim only plant food, of course. With other animals out of sight there is even less food than usual for carnivores, and most lizards are carnivores. The desert iguana will eat insects and small vertebrates, but it enjoys its thermal advantage because its diet is mainly vegetarian.

HOW FREEZING KILLS AND HOW ANIMALS AVOID IT

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No one likes to feel cold. The sensation is very uncomfortable, and if the air temperature is below about 84°F (29°C), a naked person who is not permitted to move around soon

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starts to feel the chill. We are not alone. No animal enjoys feeling cold—and for a very good reason. Cold can kill. This is not the same thing as saying we do not enjoy living in a cold climate. Many people do enjoy cold weather and the outdoor pursuits it makes possible, such as skiing and skating, and people have been living in the Arctic for thousands of years. When Scandinavians arrived to settle in Greenland in about 980 c.e., they found Inuit people already living there. We know now that the Inuits had been in Greenland since about 2500 b.c.e., and they had been living for much longer than that in Arctic North America. They obtained their food by hunting and fishing, and the game they pursued included bears, moose, caribou, bison, musk oxen, walrus, seals, and whales. All these are mammals, and they also inhabit the far north. Clearly, low temperatures have never deterred humans or other mammals. Inuit people are acclimatized to the cold climate in which they live. That is to say, over many generations they have grown accustomed to it and learned how to exploit its opportunities and avoid its dangers. They are also adapted to it physiologically. Their physical build helps their bodies retain warmth, and the body of an Inuit has a core temperature no different from that of anyone else. Inuits are able to enjoy their environment because they do not feel cold. They keep warm mainly by wearing a thick layer of clothes to provide insulation.

What Happens as the Temperature Falls Animal bodies function because of countless biochemical reactions catalyzed by enzymes. Most of these reactions proceed at a rate that varies according to the temperature, but their degree of sensitivity to temperature varies. Move far from the optimum temperature and the reactions involved in respiration and digestion start to become uncoordinated, some accelerating or slowing more than others. Nervous responses also slow as temperature falls because they depend on the diffusion of neurotransmitter substances, such as acetylcholine and norepinephrine, across the synaptic junctions between neurons, and the rate of diffusion is directly proportional to temperature. At low temperatures muscles become stiffer, and more energy is needed to make them contract. Suppose a naked person were made to remain immobile while the air temperature slowly fell from a starting point of about 84°F (29°C). All sweating would cease immediately. Then the individual would start to look pale as blood vessels near the surface were constricted to prevent loss of heat from the interior of the body. Hair would rise, although because humans have little body hair this would be evident mainly as goosebumps caused by contraction of the skin muscles that

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metabolic rate

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efficiency range base metabolic rate temperature of environment © Infobase Publishing

Temperature and metabolic rate. For every animal there is a range of temperature within which its body operates most efficiently. Outside that range its metabolic rate increases until its body can no longer cope with the high or low temperature.

would raise hairs were there any to raise. The person would start to shiver uncontrollably. As the air temperature fell below about 79°F (26°C) these mechanisms would no longer be sufficient to maintain the body’s core temperature. This is the critical temperature below which the individual’s metabolic rate would begin to rise, oxidizing carbon faster to release more heat energy. This increase in the rate of cell respiration would necessitate an increase in the oxygen supply. The metabolic rate would continue to increase as the temperature fell until the chemical reactions began to fail. The individual would then become confused, feel dizzy, and suffer from cramps. A person will die if the core temperature falls below about 90°F (32°C). This is the low lethal temperature. It varies a little from one person to another, depending on age and physical condition, but it does not vary with nationality or ethnic origin. The graph shows how these thresholds are related to the basal metabolic rate (see “Metabolic Rate” on page 123). It also shows the corresponding high critical temperature, at which the metabolic rate increases, and the high lethal temperature.

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Maintaining the Core Temperature Animals that have adapted to very different climates have different low critical temperatures. The low critical temperature for a kangaroo rat (Dipodomys species) is 88°F (31°C), for example, and that for an arctic fox (Alopex lagopus) is -40°F (-40°C). What is more, as the air temperature falls below the low critical temperature the metabolic rate of an arctic fox—or any other Arctic mammal—increases more slowly than does the metabolic rate of a tropical species, and its low lethal temperature is much lower. The core temperature for kangaroo rats, arctic foxes, humans, and indeed all mammals is about 100°F (38°C). It is not difficult to maintain this temperature in a warm climate, where the air temperature is within a few degrees of body temperature, but the situation is very different in a cold climate. Air at -40°F (-40°C) is 140°F (78°C) below core temperature. A mammal living in the Arctic needs to generate 10 times more heat than a mammal of similar size living in the Tropics. It is not possible for any mammal to generate this much warmth by metabolic means, and the gap between the low critical temperature and the low lethal temperature is small. There is a limit to the metabolic rate that can be achieved. The metabolism is fueled by food, and this must be digested, which takes a certain amount of time. Even if the food supply were limitless and an animal could eat incessantly, its digestive system simply could not process the food fast enough. Arctic mammals must have some other way of maintaining

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their core temperatures, and the clue is provided by the large difference in low critical temperatures between the kangaroo rat and the arctic fox. The arctic fox does not increase its metabolic rate until the outside temperature falls to -40°F (-40°C) because it is not until then that the cold penetrates its body sufficiently deeply to trigger the acceleration. The fox does not feel the cold because a layer of insulation protects the inside of its body. The size of an animal also makes a difference. Mammals living in very cold climates tend to be physically larger than closely related mammals living in warm climates. This is because a large body retains warmth better than a small body (see the sidebar).

Insulation Like other Arctic mammals, the arctic fox is insulated by its thick coat, although it is only the bigger mammals that have a coat thick enough to keep them warm through the winter. Small rodents, such as lemmings, are not big enough to carry a very thick coat, and they are prone to losing heat rapidly because of their large surface area in relation to volume (see the sidebar “Why Small Animals Tolerate Heat and Large Animals Tolerate Cold” below).

Consequently, many small rodents must spend the coldest part of the winter in hibernation (see “Estivation and Hibernation” on pages 132–136). Fur provides very little insulation when it is wet, and marine mammals, such as seals, whales, and walrus, have very short fur. Their insulation takes the form of a thick layer of fat, called blubber, just below the skin. An animal’s coat cannot be of the same thickness over the whole of its body, however. Its lower legs and paws must be free to allow it to move, the tips of its toes are naked, and its nose and eyes must not be covered. Nor can its extremities be at the same temperature as its internal organs. If an animal’s feet were at its core temperature, they would melt the snow and ice over which it walks, which would be dangerous. Consequently, the bodily extremities are much colder than the internal organs. The illustration shows the temperature on various parts of the body of an arctic fox when the air temperature is -22°F (-30°C). Its working muscles, in its shoulders and hips, are warm, its nose is cool, and its paws are at freezing temperature. The difference in temperature at the bodily extremities is even more obvious in a bird. In most bird species the thin legs and feet have no covering of feathers at all. The extremities contain muscles and nerves, so they receive a blood

Why Small Animals Tolerate Heat and Large Animals Tolerate Cold In 1847 the German physiologist Karl Georg Bergmann (1814–65) proposed the idea that the body sizes of closely related species of homeothermic animals—animals that maintain a fairly constant body temperature either by behavioral or physiological means—vary according to the temperature in which they live. Animals living in warm climates are smaller than their close relatives living in cool climates. For example, the pale fox (Vulpes pallida), which inhabits North Africa, measures an average 17 inches (43 cm) from its nose to the base of its tail and weighs six pounds (2.7 kg). The red fox (V. vulpes), found from the Arctic Circle to about 40°N (and introduced into Australia) measures an average 27 inches (68 cm) and weighs 13 pounds (5.9 kg). This relationship between average temperature and body size is known as Bergmann’s rule. It is due to the relationship between the volume of a body and its surface area. Animals lose body heat through their skin. Consequently, an animal with a large skin area in relation to the volume of its body loses heat rapidly and finds it

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relatively easy to remain at a comfortable temperature in hot weather. An animal with a small skin area in relation to its body volume loses heat more slowly. In hot weather it may be in danger of overheating, but it is better equipped for cold weather because it needs to expend less energy to maintain a comfortably warm body temperature. Small animals have a large skin area in relation to their volume, and large animals have a small skin area in relation to their volume. This is a matter of simple geometry. Consider a cube measuring two units along each edge. The volume of the cube is 8 units cubed, and its surface area is 24 units squared, a ratio of 1 to 3. Compare this with a cube measuring 4 units along each edge. Its volume is 64 units cubed, and its surface area is 96 units squared, a ratio of 1 to 1.5. The bigger the body is, the smaller is its surface area in relation to its volume. That is why big animals cope well with cold weather but are uncomfortable in hot weather and why small animals cope well with hot weather but not with cold weather.

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-22°F air temperature 73°F 46°F 41°F 91°F 95°F 99°F

57°F 46°F 32°F © Infobase Publishing

Temperature of an arctic fox’s body surface when the air temperature is -22°F (-30°C)

supply. This suggests that they may steadily lower the temperature in the rest of the body. Warm blood flows from the body to the extremities, where it loses heat to the outside— blood in the fox’s paw is 68°F (38°C) below the animal’s core temperature. As the cold blood flows from the extremities back to the heart it will cool the body, and before long the core temperature will be falling much faster than the animal can generate warmth. It sounds as though life should be impossible in climates where the air temperature is so low that animals cannot generate enough metabolic energy to maintain their core temperature. If that were true, the Arctic would be uninhabited. Clearly it is not, so animals must possess some other means to stay warm—and they do.

Countercurrent Exchange Arctic animals do not freeze because there is a network of small blood vessels—both arteries and veins—situated close to where each extremity joins the main part of the body. The network is called the rete mirabile, or “wonderful net.” Warm arterial blood flowing toward the extremity raises the temperature of the cold venous blood flowing back into the body, losing some of its own warmth as it does

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so. This exchange of heat is called countercurrent exchange because heat is exchanged between blood flowing in opposite directions. The outgoing blood is chilled, so the extremity remains cold, and the incoming blood is warmed, so the core temperature is maintained.

Insects and Reptiles in Cold Climates Exotherms and ectotherms are less able to survive cold, but nevertheless they, too, have established themselves in the far north and south. Indeed, in summer people working in the tundra must keep every part of their bodies covered, using mosquito netting over their faces, to protect them from clouds of biting insects. Insects—exotherms—emerge and breed during the short summer but spend most of the year as eggs. There are even two snakes, the viper, or common adder (Vipera berus), and the common garter snake (Thamnophis sirtalis), and one lizard, the European common, or viviparous, lizard (Lacerta vivipara), that live inside the Arctic Circle. It would be difficult and probably impossible for an ectotherm to incubate eggs in a cold climate, and both species avoid the problem by giving birth to live young. Even then, however, reptiles are active only during the brief summer. At low temperatures the metabolism slows, and the animal becomes incapable of obtaining food or avoiding predators. If its body temperature falls below about 45°F (7°C), a reptile becomes unable to move at all, and if it falls below freezing, the animal dies.

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ESTIVATION AND HIBERNATION

Not many mammals inhabit the Mojave Desert, but in one corner of the desert lives the Mojave ground squirrel (Citellus mohavensis), a small, brown-furred animal about 6.5 inches (16.5 cm) long not counting its tail and weighing about 5.25 ounces (150 g). It is a calm, relaxed little animal that does not fight others and rarely wanders far from home. The best place to look for it is around its favorite food plant, the cholla cactus, a relative of the prickly pear (Opuntia species), but with a cylindrical stem. Cholla fruit is its main food. Even then, though, the squirrel is seldom seen because it spends most of its time in its burrow, dug in the loose sand in the shade of the cactus. The Mojave ground squirrel emerges from its burrow in March, when the cholla fruits start to form, and from then until August it feeds and mates. In August it returns to its burrow and remains there until the following March. When the squirrel first emerges in spring it is very thin, but after several months of eating voraciously it grows very fat, adding about 3.5 ounces (100 g) to its weight. Back inside its burrow in late summer the squirrel goes to sleep. This is no ordinary sleep, however. In the course of about six hours its core temperature drops until it is just a degree or two above the temperature of its environment. And that is where its temperature remains, rising and falling with the temperature of its surroundings. At the same time, the squirrel’s metabolism slows to its basal metabolic rate (see “Metabolic Rate” on page 123). Its heartbeat and breathing slow, and for quite long periods it does not breathe at all. When it is time to awaken, the squirrel is able to arouse itself fully within an hour. The first sign of arousal is an acceleration of its rate of breathing. This increases the supply of oxygen to its tissues, and within 15 to 20 minutes its oxygen consumption reaches the level needed for ordinary activity. Its heartbeat accelerates, and the squirrel shivers. Shivering generates body heat. Within about half an hour its core temperature can rise from 68°F (20°C) to 86°F (30°C).

Resting to Save Energy During the first part of this period of dormancy the outside temperature is high. Dormancy during hot weather is known as estivation, from the Latin word aestus, meaning “heat.” The latter part of the period covers the Mojave winter, when temperatures sometimes fall below freezing. Dormancy during cold weather is called hibernation. The Latin word hibernus means “wintry.” Many animals engage in one or the other, but for the Mojave ground squirrel there is no difference. Its physiological changes are the same regardless of the outside temperature. Both estivation

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and hibernation allow the squirrel to save energy. Above or below its critical temperatures (see “What Happens as the Temperature Falls” on pages 128–129), the squirrel’s metabolic rate would ordinarily increase, but in its torpid state there is no acceleration. An animal that is not eating relies for energy mainly on the oxidation of body fat, and oxidizing one ounce of fat consumes about 32.4 pints of oxygen (two liters of oxygen to oxidize one gram of fat). When the air temperature is 68°F (20°C), a torpid Mojave ground squirrel consumes one-tenth the amount of oxygen it would consume if it were active. Measuring an animal’s oxygen consumption is not difficult. In laboratory studies the squirrel has been found to consume 0.12 cubic inches of oxygen per ounce of its body weight per hour (0.08 cm3/g/h). At this rate a fat squirrel, weighing 10.5 ounces (300 g), consumes enough oxygen to oxidize 0.01 ounce (0.29 g) of fat a day. It accumulated about 3.5 ounces (100 g) of fat before it became torpid, so this energy store is enough to sustain it for almost a full year (3.5 ÷ 0.01 = 350 days). Its fat reserve would not really last quite that long, because now and then during its dormant period the squirrel wakes. In laboratory studies it has been observed to wake up every three to five days, and it will eat and drink during these episodes if food and water are available, although it seems unperturbed if they are not and just goes back to sleep. No one knows whether it stores food in its burrow under natural conditions or how often it wakes, but it very likely consumes more energy than its basal metabolic rate requires. The Mojave ground squirrel lives economically even when it is active. Most vertebrate animals can tolerate only small variations in core temperature, but the squirrel remains relaxed even about that. It suffers no obvious harm when its core temperature falls as low as 88°F (31°C) or when it rises to 107°F (42°C). Allowing this wide a fluctuation reduces the animal’s energy requirement, because until these limits are exceeded it need not expend food energy responding behaviorally or physiologically. Tolerating a wide temperature range combined with estivation also reduce the amount of water the Mojave ground squirrel needs. Most animals keep themselves cool by allowing water to evaporate, and the squirrel is no exception. This uses a considerable amount of water, so reducing the need to cool the body also reduces the water requirement. Like some other desert mammals (see “Finding Water and Conserving Water” on pages 136–138), the Mojave ground squirrel never drinks, obtaining all its water from the food it eats.

Estivation and Diurnation The Mojave ground squirrel is not the only animal that estivates, and estivation is not confined to desert inhabit-

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Biology of Deserts ants. Many temperate species become torpid in hot or dry weather. Some species of European earthworms, such as Eisenia foetida, spend the summer in this state. It is especially common among insects, and many bats estivate to survive periods when insects are not flying, although they do so for only short periods, often for less than a day. There are also birds that briefly enter a torpid state, with a body temperature close to that of their surroundings. Some hummingbirds (family Trochilidae) do so at night, for example, and the white-throated poorwill (Phalaenoptilus nuttallii), a bird of the Californian desert, enters a torpid state when the air temperature falls below about 64°F (18°C). A temperature of 77°F (25°C) is needed to trigger estivation in most mammals. Becoming torpid overnight or for a short period during the day is called diurnation (Latin dies means “day”). Many species of birds enter diurnation, but it is doubtful whether any bird enters true estivation. Among desert species estivation is usually a way to conserve water, and its effects can be dramatic. Snails are soft-bodied and prone to desiccation, but they have a means of protection. They can retreat into their shells and seal the shell opening with a watertight cover called an operculum, retaining all the moisture inside. Even so, most snails live in humid climates, but not all of them. Being exotherms, they expend no energy on maintaining a constant body temperature and can survive long periods without food. Some snails live in deserts, where they can remain dormant for years on end, reviving rapidly when it rains.

Estivating Toads Frogs and toads are amphibians. As with all amphibians, their eggs have no shells and must be laid in water, and a significant proportion of their respiratory gas exchange takes place through their skins. This is called cutaneous respiration, and it means the amphibian skin is highly permeable. Its wearer is liable to become desiccated if the skin dries. Nevertheless, Couch’s spadefoot toad (Scaphiopus couchii) lives in the North American desert. It spends 10 or even 11 months of the year estivating below ground and revives only when the vibration of falling rain arouses it. On emerging it enters the nearest pool of water. It mates, and its eggs are laid and fertilized at once. Within nine days the eggs have developed into tiny toadlets. This toad’s adaptation to desert conditions is an extreme version of the way of life of most spadefoot toads—54 species comprising the family Pelobatidae. There are two genera, Pelobates found in Eurasia and the North American Scaphiopus, and their common name refers to a modification of their hind feet, which they use for digging. They breed in the pools that appear after heavy rain, and their young mature rapidly, before their pool vanishes.

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Fishes That Survive Drought As well as toads that can live in a desert, there are even fishes that live in a semiarid environment. African lungfishes (several Protopterus species) inhabit freshwater, where they grow up to 6.5 feet (2 m) long. While there is water they live as predators, but as drought begins they burrow into the soft mud. Each fish makes a chamber big enough for it to turn round inside. When the water level falls below the entrance to the chamber, the fish seals this with mud, curls up in its chamber, and secretes mucus that completely envelopes it, leaving only an opening for its mouth. The mucus then sets hard, forming a cocoon. As the name lungfish indicates, these fishes breathe with lungs as well as gills, so the fish can continue to breathe in the absence of water. Once inside its cocoon the fish allows its metabolic rate to fall, and it enters estivation. In most of the places where they live, estivation needs to last only a few months, but African lungfishes have been known to remain in this state for several years. When the rains return the water level rises, soaking and dissolving the cocoon and reviving the fish, no worse for its experience. South American lungfish (Lepidosiren species) are very similar, but they do not make cocoons.

Why Freezing Is Dangerous People preparing to spend time outdoors in subzero temperatures are advised to carry a small mirror. From time to time they should examine their faces in the mirror, watching for the small patches of abnormally white skin that are an early indication of frostbite. They should also wiggle their toes and move their fingers now and then to make sure they are able to do so and that they have feeling in them. Mild frostbite must be treated at once. Severe frostbite destroys the frozen tissue. It is an extremely serious medical condition. As the air temperature falls the body continues to maintain its normal core temperature, but the temperature falls in exposed parts of the body such as the ears, nose, and fingers. Frostbite occurs if the temperature falls low enough for ice crystals to form. When tissue freezes crystals of ice form in the liquid inside and between the cells. This liquid is a weak solution of many chemical compounds, but when water freezes, it is only the water molecules that lock together into solid crystals. Substances that were dissolved in the water become concentrated in the remaining liquid. Consequently, the solutions become more concentrated until eventually they reach different concentrations on either side of cell walls. At that stage water will move into or out of the cells by osmosis (see the sidebar “Osmosis” on page 97). Under these conditions the cells can become severely

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dehydrated and may die or may absorb so much water that they burst. Water expands when it freezes. As ice crystals grow, they rupture cell walls. Then, when the ice melts, the contents of the cell are lost. Freezing destroys tissues, and when the dead tissues thaw their site is liable to become infected. The infection can kill an animal, even if it manages to survive the freezing of some of its tissues. Despite the risks, however, there are animals that thrive in extreme cold. Some of them can avoid freezing, and a few can even survive it.

arctic ground squirrel (Citellus undulatus) hibernates, and so does the long-tailed souslik (Spermophilus undulatus) of northern Eurasia, a close relative of the ground squirrels. Marmots (Marmota species) weigh an average of 11 pounds (5 kg) and are the largest mammals that hibernate. The times it takes an animal to enter hibernation and for a hibernating mammal to become fully aroused depend on its body size. The bigger the animal, the longer it takes and the more energy arousal requires. This is what imposes the size limit for hibernation. Bears remain inactive through the winter, but their dormancy does not amount to true hibernation (see facing sidebar).

Animals that Survive Freezing

Entering Hibernation

Estivation is a response to high temperatures, and it occurs among some inhabitants of hot deserts. Low temperatures are the problem in cold deserts, and for those unable to migrate to warmer regions the equivalent response is hibernation. The two are fairly similar physiologically, but hibernation is the more difficult because of the risk of freezing. In hot weather an animal can fall deeply unconscious and allow its core temperature to fall with no risk that the air temperature will fall below its lethal temperature, but this is not the case in the high-latitude cold deserts. Consequently, a hibernating animal must have some means of preventing its temperature from falling dangerously low. Some invertebrate animals and a few ectothermic vertebrates such as the wood frog (Rana sylvatica) tolerate freezing conditions. There are fish, for example, that have chemicals such as glycerol in their body fluids. These substances act as antifreeze, lowering the freezing point of the fluids and preventing the formation of ice crystals even when the temperature falls below freezing. A Canadian wasp, Bracon cephi, carries enough glycerol to prevent it from freezing until its body temperature falls below -51°F (-46°C), and some butterfly and moth caterpillars survive being frozen completely solid. Other animals have substances in their body fluids that act as ice-nucleating agents. These encourage the formation of ice crystals in the fluid outside cells. This is believed to reduce the risk of ice formation inside and between cells, which is usually fatal. Reptiles and amphibians are also immobilized in winter, and even if they were active, they would find very little food. They burrow or shelter in rock crevices.

An animal prepares for hibernation over a period of several weeks. It chooses a safe, sheltered place to make the nest in which it will spend the winter, and it gathers material for bedding. Some species feed voraciously during this time, and their hormonal system makes their body convert much of the food into a thick layer of body fat. Other hibernators accumulate stores of food inside the nests they have prepared. Most animals do both, accumulating both body fat and food stores. Then, as its normal food supply diminishes, indicating that the time to hibernate is approaching, the animal enters its nest and goes to sleep. It lies in the same body position it adopts for ordinary sleep, and its nest resembles its ordinary sleeping quarters, although it is made in a more sheltered location. As it sleeps the blood vessels in its skin constrict, and its heartbeat slows. Its blood pressure remains high internally, but the heart beats only a few times each minute. In ground squirrels the heart beats 200 times a minute when the animal is active and 10 times a minute during hibernation. Then the body temperature falls. The animal does not abandon endothermy—its physiological control of its core temperature—but resets its temperature control, usually at about 40°F (4.5°C). At this temperature its basal metabolic rate (see “Metabolic Rate” on page 123) drops to about 1 percent of its active rate, and the animal generates around 2 percent of the body heat it produces when active. With the reduced oxygen requirement of its lowered metabolic rate breathing also slows, in a ground squirrel from about 100 breaths per minute to about four. Changes then take place in the composition of the blood plasma. This is necessary because the reduced heartbeat pumps so slowly that without those changes blood would start to clot. The brain and nervous system continue to function because chemical impulses are able to cross synapses even though the temperature falls below the minimum needed for chemical diffusion in other animals. This allows the heart to remain under nervous control and the brain to remain in a state from which it can be aroused by an appropriate stimulus.

True Hibernation True hibernation is not common among mammals and occurs only in those with small bodies, such as bats and some rodents, few of which live in the polar deserts. The

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Do Bears Hibernate? Bears living in high latitudes are inactive through much of the winter. Many people believe they hibernate, but their long winter rest is not true hibernation. The body temperature of a hibernating mammal falls to within a degree or two of the air temperature and at times may be close to freezing temperature. A bear’s normal body temperature averages 100°F (38°C). During its winter rest its temperature falls only to about 93°F (34°C) and is rarely lower than 88°F (31°C). Its core body temperature falls, but it remains much higher than the surrounding air temperature. To maintain this high temperature the bear must eat at intervals, arousing itself from a sleep that is deep but not so deep as the sleep of hibernation. The bear cannot allow its temperature to fall as low as that of a true hibernator because its body could not generate the energy it would need to arouse itself from hibernation. If a bear weighing 440 pounds (200 kg) were to hibernate, allowing its temperature to fall to 40°F (4°C), then raising its temperature to 100°F (38°C) would take several days and require as much energy as the bear uses in approximately three and a half days of ordinary activity. It would be impossible for the bear to eat sufficient food prior to hibernation for it to lay down a layer of body fat that was thick enough to sustain it through the winter and then allow it to revive. There is an additional risk. Hibernating animals never freeze. If its body temperature falls below a certain danger threshold, a hibernating animal will arouse itself. It wakes, warms itself by shivering and moving about, and then eats food from its winter store to replace the energy this activity used. Small animals wake several times during the winter. Under these circumstances it is unlikely that an animal the size of a bear could arouse itself quickly enough to prevent its temperature from falling so low as to cause serious harm, and it could not possibly replace the energy this used by snacking from its winter hoard.

Arousal A body temperature of 40°F (4.5°C) is much higher than the outside air temperature during the polar winter, and even inside its nest the hibernating animal may become

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chilled. When it cools below a certain threshold the nervous system detects the cold and the animal starts shivering. This generates heat, but in doing so it consumes a large amount of energy, which the body obtains by oxidizing stored body fat. Shivering also wakes the animal, and that is when it needs some of the food it hoarded prior to the onset of winter. While it is awake it must eat enough to replenish the fat its body just used. Only if it accumulated enough body fat and hoarded enough food to replenish it when necessary will the animal survive the winter. Its bodily reserves must also be sufficient to supply the energy that is needed to rouse the animal in spring. Then it may have no more than about four hours to raise its core temperature from 40°F (4.5°C) to 95°F (35°C). Arousal begins with a rapid acceleration of the heartbeat, followed by violent shivering in the front part of the body, which warms much faster than the rest of the body. At the same time the breathing rate increases, and the blood vessels dilate, first in the front of the body and then in the hind parts. The blood flow accelerates, and that causes a corresponding increase in the metabolic rate. Only then, when its body has returned to its fully active state, is the animal able to leave its nest and start searching for food.

Spending Winter Beneath the Snow Hibernation involves such radical changes in the way the body works that it is hardly surprising so few mammals have adopted it. The commonest small mammals of the far north are voles and lemmings, constituting a subfamily (Microtinae) of rats and mice (family Muridae). Microtines are tiny rodents, none of them more than about 4.5 inches (11 cm) long with a tail up to 1.6 inches (4 cm) long, and they weigh up to 0.7 ounce (20 g). Despite their diminutive size, they do not hibernate. They have an alternative means of surviving the harsh winter. They live beneath the snow. Snow provides good thermal insulation and, most important in high latitudes, complete shelter from the biting wind. Voles and lemmings make tunnels through the snow along which they move around in search of food, and they dig chambers in the snow in which they build nests. Food and nesting material are plentiful on the ground among the plants buried by the snow. The winter ends when the snow begins to melt, flooding the passages and chambers and forcing their occupants into the open. These animals breed during the winter, and by the time they emerge in spring their young are strong enough to look after themselves. The snow also protects them from most predators, although weasels and stoats can follow them through their tunnels. Arctic foxes and other larger animals remain aboveground and active throughout the winter.

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A female polar bear (Ursus maritimus) with her two cubs. She may be bringing them into the open for the first time. (Alastair Rae)

How the Polar Bear Passes the Winter The polar bear (Ursus maritimus) is the biggest animal of the northern cold desert and the largest of all bears. Males, which are somewhat bigger than females, can measure 10 feet (3 m) from nose to tail and weigh more than 1,400 pounds (635 kg). If an animal of this size were to hibernate, raising its temperature from 50°F (10°C) to 99°F (37°C) as it aroused itself from hibernation would take several days and consume more energy than the animal would use during three days of normal activity. It is a demand that cannot be met, and consequently hibernation is not an option available to polar bears (see the sidebar “Do Bears Hibernate?” on page 135). Instead, polar bears dig dens, usually in snowdrifts, and spend part of the winter in them, lying dormant. Males remain in their dens for varying periods and are active at other times, by day or night. Pregnant females enter their dens in November or December and remain there until late March or early April. Their cubs are born in the den in December or January. While they are dormant polar bears do not eat, obtaining the energy they need by metabolizing body fat. This is not hibernation, but it does cause a small drop in body temperature of about 9°F (5°C) and a 50 percent slowing of their metabolic rate.

FINDING WATER AND CONSERVING WATER

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Off the western coast of southern Africa the Benguela Current flows northward, carrying cool water from the

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Southern Ocean. Air crossing the current is chilled, and its water vapor condenses, bringing fog—but not rain—to the Namib Desert. On nights when the fog rolls in from the sea large numbers of darkling beetles (family Tenebrionidae) make their way to the crests of the Namib sand dunes. There they stand in rows and stretch their long legs so their wing cases—the technical name is elytra—are raised almost vertically and their heads are close to the ground. As the fog arrives water droplets collect on the elytra, trickle down to the insects’ mouths, and are ingested. This is the only water the beetles drink, and their food consists entirely of seeds and other scraps of dry plant material. Extraordinary though this behavior may seem, in fact it is no more than a minor modification of ordinary tenebrionid behavior. All darkling beetles have long legs, and many species raise their bodies, standing upright on their front legs as a defensive measure to deter predators. In most species the elytra are fused together so the beetles cannot fly, and there is an air space between their bodies and elytra that provides insulation and reduces water loss. Most darkling beetles are nocturnal, although some, such as Adesmia antiqua of the Sahara, forage by day, sheltering in a burrow only when the heat is extreme. Its long legs hold its body clear of the hot ground.

Storing Food in Burrows Many desert animals retreat into burrows to escape the heat. This also conserves water, because less body moisture is lost by evaporation into the cooler air belowground. Water vapor exhaled by its occupant makes air in a burrow more humid than the air aboveground, and this also reduces evaporative losses. The exhaled moisture does not always soak into the dry sand to be lost. The bannertail kangaroo rat (Dipodomys spectabilis), which lives in the Arizona desert and among the sand dunes of Death Valley, California, spends its days in its burrow, emerging to forage only at night. It stores much of the food it finds in its burrow, where the relative humidity is much higher than it is in the air aboveground. Indeed, the humidity can approach saturation. Under these conditions the very dry food taken into the burrow absorbs moisture from the air. This can increase the water content of the food from 4 percent to 18 percent, and the rodent can live indefinitely on a diet of seeds and other dry plant material without ever drinking. The related but smaller Merriam’s kangaroo rat (D. merriami) has a different way of exploiting this method of water conservation. Rather than gathering dry food to stock its larder, it steals food from the burrows of bannertail kangaroo rats. The bannertail kangaroo rat is not the only desert rodent to store food in a humid burrow. Kangaroo rats are a genus

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Biology of Deserts of pocket mice (family Heteromyidae), found only in North and Central America and the northwest of Colombia. Their equivalents in the deserts of Africa, Arabia, and Asia are the gerbils, a subfamily (Gerbillinae) of the Old World rats and mice (family Muridae). Gerbils are also nocturnal seed eaters. The seeds they collect have often been moistened by dew, but storing them in their burrows increases their moisture content still further. Dry seeds contain 4–7 percent water by weight, but seeds taken from gerbil burrows have been found to contain 30 percent water. Larger animals, such as antelopes and gazelles, cannot burrow, so they are unable to store food in this way. This does not prevent them from choosing food items that have been moistened, however. They obtain water by feeding at night, when dew often collects on leaves and is absorbed by them, in the extreme case of some African desert bushes increasing the moisture content of the leaves from about 1 percent to 40 percent. At their driest the leaves will crumble to dust when rubbed between the fingers.

Finding Water in Plants Wherever there are cacti or other succulents, some animals will find ways through the plants’ defenses to get at the water stored in their tissues. In North Africa the fat sand rat (Psammomys obesus) feeds on succulent plants that grow in dry riverbeds. These plants are very salty. The rat rids its body of excess salt by excreting urine that is four times saltier than seawater. The Mojave ground squirrel (Citellus mohavensis) of the North American desert feeds on cholla fruit (see “Estivation and Hibernation” on pages 132–136), and some species of pack rats (Neotoma species) feed on cholla and other cacti. These plants contain up to 90 percent water by weight. Carnivorous animals obtain moisture from the body fluids of their prey.

Making Water from Dry Food There is no mystery about where these animals find the water they need, and even the driest food can be made to yield water. This is not water contained inside the food but water that is produced chemically when the food is metabolized to provide energy. Carbohydrates, fats, and proteins consist of chemical compounds containing hydrogen. The oxidation of hydrogen yields energy, and the chemical product of the reaction is water (2H2 + O2 → energy + 2H2O). Respiration (see “Respiration” on page 100) causes a series of chemical reactions that release energy through the oxidation of food. As well as energy, oxidizing one ounce of carbohydrate produces 0.6 ounce of water, oxidizing one ounce of protein yields 0.3 ounce of water, but oxidizing 1 ounce of fat yields 1.1 ounces

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of water, the differences being due to the chemical reactions involved. This provides all the water some desert animals need, and they can gain weight on a diet of nothing but dry seeds. They can do so, however, only because they are also able to reduce to a minimum the amount of water that leaves their bodies. There are two routes by which water is lost: respiration and the excretion of feces and urine.

Reducing Moisture Losses Due to Breathing Lungs are made from very delicate tissues that are easily damaged, and they could not survive prolonged exposure to cool, dry air. Consequently, the respiratory passages through which air passes on its way to the lungs are moist, and contact with them saturates the air entering the lungs. Air leaving the lungs is saturated, which means that all airbreathing vertebrate animals exhale saturated air. This is the moisture that condenses when the air is exhaled into very cold, dry air, becoming visible as a cloud of steam. Burrowing rodents collect some of the moisture from this exhaled air, but all mammals, birds, and lizards minimize the loss of water before it leaves the body by means of a system of countercurrent exchange. This is not an adaptation to desert life but arises inevitably from the way the nasal passages are constructed. Moisture enters the inhaled air by evaporating from the walls of the respiratory passages. This cools the passages through the loss of latent heat of vaporization, but as the air approaches the lungs the cooling effect decreases. By the time it reaches the lungs the air has been raised to the core body temperature of about 100°F (38°C), and it has also absorbed more water as its temperature rose. Exhaled air is saturated, but moisture from the warm air condenses onto the sides of the respiratory passages that were cooled by the inhaled air, at the same time warming them and cooling the outgoing air. A large amount of energy is needed to vaporize water, so using air leaving the lungs to warm the respiratory tissues saves energy. Despite the fact that the exhaled water is saturated, this countercurrent exchange also saves water. This saving is due to the fact that the amount of water vapor air can hold varies according to its temperature, and air cools as it travels from the lungs to the nostrils. Suppose the inhaled air is at 86°F (30°C) and 80 percent relative humidity. One quart of this air contains 0.00074 ounce of water (24 mg/l). The air is then warmed to 100°F (38°C) and saturated. It then contains 0.0014 ounce per quart (46 mg/l), so it has absorbed an additional 0.00068 ounce of water per quart of air (22 mg/l) from the body. As the air is exhaled, suppose it is cooled to 88°F (31°C) by the time it leaves the nostrils. It remains saturated, but at this temperature saturated air contains 0.00095 ounce of water per quart (31 mg/l).

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The difference between the water content of inhaled and exhaled air, both saturated but at different temperatures, is 0.00045 ounce per quart (15 mg/l), and this moisture condenses and remains within the body. At night, when the air is cooler and many of the small desert animals are active, the saving is even greater because more water evaporates as the air is warmed to core temperature. This greatly cools the nasal passages, increasing the amount of condensation from the exhaled air.

Urine and Kidney Efficiency Urine is produced by the kidneys of vertebrates. It consists of metabolic waste products filtered from the blood and carried in water. Minimizing the amount of water that is excreted involves improving the efficiency of the kidneys. Mammalian kidneys are highly efficient, but there is wide variation among them. Those of desert animals are the most efficient of all. Kidney efficiency can be measured by the concentration of the urine excreted—the more concentrated the urine, the less water it contains and, therefore, the less water the animal needs to drink to replace it. If humans have a kidney efficiency value of 1, that of a dromedary (see “The Ship of the Desert” at right) is about 1.96, so a camel’s kidneys are almost twice as efficient as a human’s. A pack rat, however, has a kidney efficiency of 2.97, Merriam’s kangaroo rat of about 3.25, and a gerbil of 3.85. A kangaroo rat has kidneys that are so efficient it can drink seawater without coming to any harm. It may be the only mammal that can do this.

The Australian equivalent of the kangaroo rat is the hopping mouse (Notomys alexis), an inhabitant of the Australian desert. It has what are probably the most efficient kidneys of all, producing urine that is about 6.55 times more concentrated than human urine.

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THE SHIP OF THE DESERT

The camel—the “ship of the desert”—is by far the most famous of all desert animals. Part of its fame derives from its economic importance as a beast of burden as well as being a source of meat, milk, wool, and hides. Camels are also used for riding, and there are clear differences between animals that have been bred for riding and those bred for carrying loads, just as there are between different breeds of horses. Because they are so widely used and highly valued, camels exist in large numbers and are a familiar sight in the Sahara, the Arabian, Middle Eastern, and Indian Deserts, and in parts of Afghanistan. Their uses have not always been peaceful and commercial. Dromedaries—single-humped camels—with two riders, one of them an archer, were used in battle by the Assyrians around 650 b.c.e., and they were also used in wars between the Greeks and Persians. When Alexander the Great sacked the Persian city of Persepolis in 330 b.c.e., he used 5,000 Bactrian camels—two-humped camels—to carry away the loot. The Romans used camels as pack animals and may even have introduced them to northern France. Napoleon used them in his North African campaign of 1798.

The desert camel, or dromedary (Camelus dromedarius), is superbly adapted to life in a dry climate and thrives under conditions that would kill most animals. (Hartmut Mueleberg; edenpics.com)

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Biology of Deserts Between 1840 and 1907 camels were also taken to Australia, where it was thought they would be as useful as they are in Africa. It was not long before the spread of railroads and road vehicles made the camels redundant, however, and they were released. The Australian desert suited them, and they have thrived. Today there are believed to be about 25,000 of them living wild in Australia, where they are a fairly common sight. In the 1850s the U.S. Army also experimented with camels as beasts of burden in their frontier garrisons in the southwestern United States. That attempt did not succeed, and the animals were turned loose. They survived for a time, but none have been seen since about 1900. At various times camels have also been introduced into Spain and Zanzibar.

One Hump or Two Everyone knows what a camel looks like. There are two species: the Arabian camel, or dromedary (Camelus dromedarius), which has one hump, and the Asian, or Bactrian, camel (C. bactrianus), which has two. They belong to the family Camelidae, which also includes four South American species: the guanaco (Lama guanicöe), llama (L. glama), alpaca (L. pacos), and vicuña (Vicugna vicugna). The camelids evolved in North America during the latter part of the Eocene epoch that ended about 34 million years ago. Between 2 and 3 million years ago, during the Pliocene epoch, one group expanded into South America and another—Camelus species, the ancestors of the dromedary and Bactrian camel—entered Asia. Both groups then died out in North America. Both of the Camelus species were domesticated long ago, although precisely when this happened is uncertain. Camel dung about 4,600 years old has been found at an archaeological site in southern Iran. This is the oldest evidence of domestication, and scientists believe the dung was from a Bactrian camel. What is certain is that the only dromedaries living in the wild today are feral (descended from ancestors that were domesticated). There is no truly wild dromedary. Bactrian camels do live in the wild. There is a small population around Lop Nor, on the eastern edge of the Takla Makan Desert, and a larger one in southwestern Mongolia. Together, these two groups are believed to number about 1,000 animals. Some scientists believe them to be truly wild and the species from which the domesticated Bactrian camel was derived. They differ from domesticated camels in a number of small ways and are sometimes known as C. ferus to distinguish them. Other scientists believe them to be feral (descended from domesticated camels that returned to living in the wild). Both camels are about the same size and weight, but the Bactrian camel has shorter legs and a much thicker coat,

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giving it a shaggy, unkempt appearance, especially in spring when the coat is shed. It lives mainly in cool, rocky regions and on the steppe grasslands of Central Asia, and it is well able to withstand the harsh steppe winters. The dromedary is the true desert animal. It can carry heavy loads over long distances, is able to endure the heat of the Sahara, and has a legendary ability to survive for long periods without water.

Where Does the Camel Store Water? At one time it was thought that the dromedary carried some kind of reservoir in which water was stored. One group of people speculated that this might be in the form of a bag or bladder linked to the stomach. Camels are ruminants, related to cattle and sheep, and, like them, they have a complex stomach that is divided into chambers. In the camels’ case there are three chambers rather than the four of cattle and sheep, because two of them (the abomasum and omasum) are united. They lie below the second chamber, the reticulum, and in the diagram they are labeled abomasum. It has even been suggested that sometimes camels were killed for the water they carried. Other people thought the reservoir might be in the hump. When it was revealed that the hump consists mainly of fat, supporters of this view suggested that the camel might obtain water by oxidizing the hydrogen in the fat. A hump with 50 pounds (22.7 kg) of fat could yield 55 pounds (24.97 kg) of water (see “Making Water from Dry Food” on page 137). That is about 6.6 gallons (24.97 l), a useful amount of water.

The camel’s stomach. The stomach has three chambers. Food that the camel swallows first enters the rumen, where bacteria partly digest it. The food then passes to the reticulum and from there to the abomasum, after which it leaves the stomach through the duodenum.

rumen

esophagus reticulum

pouches (“water cells”) duodenum

abomasum © Infobase Publishing

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That one or the other explanation was true seemed self-evident because of the time a camel can survive without drinking and because of the rate at which it drinks when water is available. In 10 minutes a camel can drink an amount of water weighing about one-third its body weight. That is about 10 times more than a human can drink at one time. A camel has been seen to drink 27 gallons (103 l) in 10 minutes. This is like an average-sized man drinking six gallons (23 l). Anyone attempting to drink this amount in one session would die from water toxicity, because in excessive amounts water is poisonous to humans. Not surprisingly, people thought an animal drinking that much water that fast must be pouring it directly into some kind of storage tank. Both explanations proved incorrect. It is true that in camels the rumen, the first stomach chamber, has pouches (see the diagram on page 139). These are sometimes incorrectly called water cells, but they contain partly chewed food rather than water, and no more than about one gallon (3.8 l) of that. The main part of the rumen contains digestive juices. A person dying of thirst might drink this fluid, but it would be an act of extreme desperation! It is true that the chemical reactions oxidizing fat yield water as a by-product, but there is a problem for the camel. In order to oxidize its hump the animal must supply oxygen, which it obtains from its lungs. As it absorbed the required amount of oxygen, the camel would lose water in its exhaled breath, and calculations show it would lose more water by that route than it could gain by metabolizing the fat in its hump.

Reducing Water Losses Perhaps, then, a camel economizes by conserving water. A camel excretes urine that is about twice as concentrated as human urine. This makes it appear that although the camel’s kidneys are more efficient than those of humans, they are nowhere near so efficient as the kidneys of other desert mammals, such as the kangaroo rat. The appearance is misleading, however, because the camel has a digestive system that deals very efficiently with food containing little protein. In a human proteins are broken into their constituent amino acids in the stomach, and the final waste product of protein metabolism is ammonia, which is converted into urea in the liver. Urea is a poisonous compound containing nitrogen. It is filtered out by the kidneys and passed continuously to the bladder, from where it is excreted. Ruminant diets are deficient in nitrogen, an essential ingredient of the amino acid building blocks of proteins, and so ruminant mammals recycle it. Some of the urea from the liver passes through the bloodstream back to the stomach, where it mixes with cellulose to make new amino acids. The effect is to lower the urea concentration of urine, disguising the true efficiency of the kidneys. In fact, the camel may have

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kidneys as efficient as those of the kangaroo rat, and it is possible that a camel could drink seawater without coming to any harm. Camels that live near the coast are said to eat seaweed, which is very salty. In summer a camel may excrete as little as a quart (1.14 l) of water a day. Camels also conserve water by sweating very little until their body temperature exceeds 105°F (40.5°C). At night the camel allows its temperature to fall as low as 93°F (34°C), so it takes several hours for it to heat up sufficiently to trigger perspiration (see “Keeping Cool” on page 124). Its temperature fluctuates by this amount only when the camel has no access to water in summer. In winter and in summer if it has water to drink, its temperature varies by only about 4°F (2.2°C).

Insulation—and Lack of It The rate of heating is slowed even more by the camel’s insulation. Its thick coat traps a layer of insulated air next to the skin, working in the same way as the many layers of clothing worn by desert peoples. We feel comfortable wearing thin, light-colored clothing in summer, but this is not the best attire for the middle of the day in the Sahara. There, several loose, flowing robes worn on top of each other are much better at keeping the body cool. Like the camel’s coat, desert clothing works in two ways. The outer garments absorb heat, preventing it from reaching the skin, and perspiration evaporates into the layer of air between the skin and the inner garment, so it is evaporated by heat from the body, not from the Sun. The camel’s hump is also an insulating device. The hump is a store of food, of course, not water, and it differs only in its location from the fat all mammals store. Instead of being spread over most of the body, it is concentrated on the back. That is the part of the animal most directly exposed to the heat of the Sun, so the hump provides insulation. At the same time, the remainder of the body lacks an outer layer of fat. This means there is no insulating barrier to prevent the escape of heat from the body.

Avoiding Explosive Heat Death When it loses water the camel does so from its body fluids and not from its blood. This is important. When a human loses body fluid the blood thickens, its rate of flow slows, and the circulatory system becomes less able to carry heat from the inside of the body and lose it from the surface. The body temperature rises, and, for a person exposed in a hot desert, death follows swiftly when the amount of water lost is equal to about 12 percent of body weight. This is known as explosive heat death (see “Explosive Heat Death” on pages 124–125). In the camel the volume of blood remains fairly constant, but the loss of other fluids causes the animal to become thinner and thinner. It can tolerate this, however, and survives a

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Biology of Deserts loss of fluid equal to 25 percent of its body weight. Its emaciation is due entirely to fluid loss, and the animal continues to eat normally even though it has no opportunity to drink. When it drinks the camel is transformed in a few minutes from an emaciated condition in which its ribs and pelvis are clearly visible into a perfectly healthy animal, although the fat in its hump may have been used up, and that takes longer to grow back. By drinking the camel is merely restoring water it has lost, and it never drinks more than that. It is not consigning water to a store or drinking for a thirst to come, about which it obviously can have no knowledge. In fact, the camel is not the world’s fastest drinker. That prize goes to the donkey, which can take in water faster than a gas station pump can fill a car’s tank. A donkey can drink water equal to 25 percent of its body weight—about 18 gallons (70 l)—in about two minutes. Camels and donkeys are able to restore all the fluid they have lost in one drinking session. Other animals, including humans, are incapable of this. We drink more slowly, but because of that we must drink more often. In the desert a person must drink at least once every day. A donkey can go up to four days without drinking.

Endurance A camel has been known to go 17 days without drinking in summer, and in winter some camels do not drink at all. They have been known to refuse water even though they had drunk nothing for two months. This is not quite so remarkable as it sounds. Desert temperatures are much lower in winter than in summer, and although rain is uncommon, it is more likely in winter than in summer. Consequently, the body loses less water, and there are usually shrubs and succulent plants on which the camels feed and from which they obtain as much water as they need. Where there is vegetation, camels feed regularly, just like any other animals. No one chooses to cross the desert in the scorching heat of summer. It is in winter and spring that the long journeys are undertaken, and camels will travel more than 300 miles (480 km) in two to three weeks, drinking nothing along the way. Although they are domesticated and there is no such thing as a truly wild dromedary, desert camels are not tame in the way horses are tame. Their owners leave them to find most of their food for themselves. In winter they must be herded to prevent them wandering off in search of edible plants and failing to return, but in summer this is unnecessary. Every few days camels will visit the human camp or settlement in search of water.

Broad Feet, Loose Limbs Camels move well over loose sand. This is another of their adaptations to desert life. Camels are artiodactyls—

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ungulates (hoofed mammals) possessing an even number of toes. They are related to deer, cattle, sheep, and goats. All of these animals have hooves, but those of camels are different from the hooves of the others. Its two toes are turned forward, and each toe is covered by a thick pad of skin. The hooves are present at the tips of the digits, but they have no function. The camel walks on the skin pads. This makes its feet very broad and spreads the animal’s weight over a larger area than would be the case if it walked on its hooves like a cow or sheep. Being so broad, its feet are less likely to sink into soft sand or snow. The hind limbs of camels are attached only at the top of the thigh, whereas the legs of other artiodactyls are attached to the pelvis by muscles and skin all the way from the knee. This form of attachment gives camels their long-legged appearance and allows them to lie with their hind legs tucked completely beneath their bodies rather than being held to the sides of the body like the legs of a cow when it lies. It is a small difference, but it reduces the area of the body that is exposed to the sun when the camel is lying. Camels, other than the possibly wild Bactrian camels of Asia, which lack them, have pads of horny skin on their chests and knees that cushion the animals when they kneel and insulate them from the hot sand. Camels run with a characteristically loping gait called pacing that is quite different from the gait of a horse. This is because when they are galloping camels move both legs on the same side of the body together—left–left, right–right, rather than front left–hind right, front right–hind left. Their pacing gait causes them to sway from side to side. With this method of walking they cannot travel fast. People do race camels, but only over short distances. A dromedary can carry a rider at up to 10 MPH (16 km/h) and maintain that speed long enough to cover more than 100 miles (160 km) in a day. As a pack animal it can carry a load of about 500 pounds (227 kg) and cover 25 miles (40 km) a day walking at about 5 MPH (8 km/h). Bactrian camels are slower. They walk at only about 2.5 MPH (4 km/h), but nevertheless they can cover about 30 miles (48 km) a day carrying a load of up to 1,000 pounds (454 kg).

Are Camels Lazy and Bad Tempered? Apart from their feats of endurance, camels are renowned for their bad tempers and supercilious expressions. They seem to regard humans with disdain and are rumored to require little provocation to make them kick, bite, and spit a noxious fluid. They cannot help their faces, and their most familiar expression is much misunderstood. The apparent superciliousness is due partly to their long eyelashes and the height of their heads in relation to those of the people handling

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them. They look down at humans because they have to, and as they look down their eyelashes partly cover their eyes, giving them their disdainful look. The lashes are yet another adaptation to the desert. They keep out windblown sand, and for extra protection each row of lashes is double. Camels also have long hairs protecting their ears. As though that were not bad enough, camels also seem to sneer. This is because their upper lips are deeply cleft, and both upper and lower lips are very mobile. Camels use their lips to manipulate food items. In addition, they are able to close their nostrils, and often do so. This keeps out sand and dust, protecting the delicate membranes of their nasal passages, but it makes them look as though they are trying to avoid a disgusting smell. It is true that camels can be bad tempered, especially during the mating season, although they are usually placid provided they are treated well. They are often reluctant to carry riders or loads. This has given them an undeserved reputation for laziness. It, too, is behavior appropriate to their environment. Where food and water are so scarce, animals cannot afford to waste energy by exerting themselves needlessly.

CONVERGENT EVOLUTION AND PARALLEL EVOLUTION

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Ocotilla (Fouquieria splendens), which is also known as the coach whip, Jacob’s staff, and vine cactus, is a shrub that grows naturally in the rocky deserts of the southwestern United States and Mexico. It is a curious plant, producing several slender stems, rather like canes, that grow from the base. These are usually 8–20 feet (2.4–6.1 m) tall and furrowed with many spines. Fleshy leaves 0.5–1.0 inch (13–25 mm) long appear in spring and soon fall, leaving behind their midribs, which harden to become new spines. Between March and July the plant bears branched bunches (panicles) of scarlet, tubular flowers at the tips of its stems. Hedges of ocotilla are grown locally. Euphorbia splendens is another tall, spiny plant that sheds its leaves and bears bright red flowers. It is known as the crown of thorns, and it grows naturally in Madagascar but is cultivated as a houseplant in other parts of the world. It belongs to the spurge family (Euphorbiaceae). Although they are very similar, these two plants are not related to each other. Despite being called the vine cactus, ocotilla is not a cactus. It belongs to the family Fouquieriaceae, along with the boojum tree (Idria columnaris), and merely resembles the cacti among which it grows. Many euphorbias also resemble cacti, although the Euphorbiaceae and Cactaceae are quite unrelated, and there are profound physiological differences between the two families (see “Euphorbias” on pages 115–116).

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Convergent or Parallel? Such similarities between unrelated species are common, especially in deserts. Some occur because evolution tends to repeat itself, so successful structures appear independently in widely separated evolutionary lines. Eyes are the most obvious example. Since these groups diverged from a shared and blind ancestor, eyes have evolved independently many times, in insects, mollusks, other invertebrate animals, and vertebrates. Other similarities evolve because there is a limit to the number of ways in which it is possible for an organism to adapt to its environment. Consequently, unrelated organisms tend to evolve similar solutions to identical problems. It is because deserts present such stark evolutionary challenges that it is in deserts that the few solutions available have led to so many close resemblances among quite unrelated plants and animals. Ordinarily, when two species evolve from a common ancestor they become increasingly dissimilar in subsequent generations. This is called divergent evolution, or evolutionary divergence. Two diverging arrows can be used to represent it, as in the diagram on the facing page. Where natural similarities emerge among organisms that are not closely related, the process is called convergent evolution, or simply convergence. The ancestors of the modern organisms may not have resembled each other, but their descendants appear very similar. Two converging arrows illustrate the process, the point being that although the arrows converge, they do not meet. The species remain quite distinct, and their similarity is superficial, arising simply from the fact that they live in similar ways under similar circumstances. If the similar plants or animals belong to distinct but closely related species, the process is called parallel evolution. The distinction is somewhat arbitrary, since all living organisms are related by connections at some point in their ancestry. Relatedness therefore depends on the time that has elapsed since particular species became distinguishable from their shared ancestor and each other. Parallel evolution occurs when two species continue to inhabit similar environments and to maintain similar ways of life; the similarities in lifestyle mean the species continue to resemble each other physically. In this illustration the two arrows are parallel.

Desert Foxes Similarities between members of the Fouquieriaceae, Euphorbiaceae, and Cactaceae probably result from convergent evolution, because the differences between these plants are considerable and appeared long ago. Widely separated desert foxes have also evolved similar features, but this is an instance of parallel evolution in closely related

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divergent

© Infobase Publishing

Types of evolutionary development

species, both of which inhabit hot deserts to which they have adapted in the same way. Both the kit fox (Vulpes velox) of southwestern North America and northern Mexico and the fennec fox (V. zerda) of the Sahara and the Arabian Desert have yellowish coats, small bodies, and very big ears. The fennec is the smallest of all foxes, measuring about nine to 16 inches (23–41 cm) not counting the tail and weighing 1.75–3 pounds (0.8–1.4 kg), but the kit fox is not much bigger. Its head and body measure 15–20 inches (38–51 cm), and it weighs four to 6.5 pounds (1.8–3 kg). Small size is an advantage to a desert carnivore.

The kit fox (Vulpes velox) of North and Central America closely resembles the fennec fox (V. zerda) of Africa and Arabia. Both species are small and have large ears, adaptations to desert life. (U.S. Fish and Wildlife Service)

It enables it to feed on smaller prey than a larger animal would require and to find shade much more easily. Big ears allow an animal to detect the small, rustling movements of its prey, but ears are also effective heat exchangers. They are richly supplied with blood vessels, and the blood is cooled as it passes through them. In contrast, the arctic fox (Alopex lagopus) has very small ears to conserve warmth (see “Arctic Fox” on pages 163–164). Small animals do not make rustling sounds as they move through snow, so more acute hearing would not help in its search for food. Today both foxes are classified in the genus Vulpes, although the fennec was formerly placed in a genus of its own, Fennecus. The immediate ancestors of modern dogs, including foxes, lived in North America in the late Eocene epoch, which ended about 34 million years ago, and the earliest true dogs were about the size of the fennec fox. All the modern species existed by early in the Miocene epoch, which began 23 million years ago, and physically they have changed little since then. V. velox and V. zerda remain very closely related and have not evolved far from their common ancestor, both foxes having evolved similar adaptations to a similar environment. This represents parallel evolution.

Kangaroo Rats and Jerboas North American kangaroo rats (Dipodomys species) and jerboas (Jaculus species) of the Sahara and the Arabian Desert are much less closely related. Not only are they classified in different genera, they are in different families. Kangaroo rats are pocket mice (family Heteromyidae), and jerboas belong to the family Dipodidae. Their evolutionary lines diverged more than 57 million years ago, during the Paleocene epoch. Both have adapted to life on hot ground by developing very long, kangaroolike hind legs, and both move about by hopping. This gives them a very similar appearance and probably arises from convergent evolution. Some doubt remains, however, because many other small rodents have long hind

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legs and hop, so perhaps kangaroo rats and jerboas inherited a tendency to do so from their shared ancestor—in which case the long legs result from parallel evolution.

Marsupial “Mice” There is no such doubt in the case of similarities between marsupial mammals, the group that includes kangaroos and koala, and the eutherian mammals, such as dogs, cattle, mice, and humans. These two main groups diverged so early in mammalian evolution and are so different from each other that any similarities between species living in similar environments must be due to convergent evolution. Small rodents such as kangaroo rats, jerboas, and gerbils have adapted to life in the deserts of America, Africa, and Asia, respectively. Similar animals inhabit the Australian deserts, but these are marsupials, which raise their young in pouches. The mulgara (Dasycercus cristicauda), for example, looks like a short-legged rat with a thick tail. Its head and body measure about eight inches (20 cm) and its tail about five inches (13 cm). It lives in central Australia, sheltering in its burrow for most of the day. It never drinks and has extremely efficient kidneys that allow it to conserve water by excreting highly concentrated urine (see “Urine and Kidney Efficiency” on page 138). The kowari (Dasyuroides byrnei) looks even more like a rodent and might pass for a close relation of the kangaroo rats, but it is about half their size. Its head and body are about seven inches (18 cm) long, and its bushy tail adds about five inches (13 cm). It has big, mouselike ears, a long muzzle, and hind legs that are longer than its front legs.

head thrown forward

The most mouselike of them all is the pygmy planigale (Planigale maculata). With a head and body only about two inches (5 cm) long and a thin tail about the same length, it is smaller than a house mouse (Mus musculus). Like other small animals of hot deserts, both the kowari and pygmy planigale are nocturnal, spending the day in their burrows. Convergence has produced unrelated animals of startlingly similar appearance, but the similarities are superficial. Desert rodents feed on seeds and other plant material. Mouselike marsupials are carnivores. They belong to the family Dasyuridae, which also includes the quoll, or native cat (Dasyurus viverrinus), and Tasmanian devil (Sarcophilus harrisi). The small marsupial “mice” feed on insects, lizards, small or very young snakes, and birds. Even the tiny pygmy planigale catches small birds.

Sidewinders and Horned Snakes Parallel evolution has also affected snakes. Snakes have difficulty moving through loose sand, and some have taken to sidewinding, a method of locomotion in which the snake moves at an angle of about 45° to the line of its body by throwing forward first its head and then a succession of loops of its body. Only two or three parts of its body are in

Sidewinding. Where the sand is so soft that its usual method of moving would not work, the snake progresses by throwing its body forward, with never more than two sections touching the ground. This efficient method of locomotion has evolved independently in several species of desert snakes.

body loop thrown forward

head thrown forward

part of body touching the ground

J-shaped mark left in the sand

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Biology of Deserts contact with the ground at any time, and because they are placed downward onto the surface, the sand does not slide to the sides and cause the animal to lose its traction. The illustration on page 144 shows the sequence of throws in this type of movement. Most snakes will sidewind if they need to move through material that slides away when they push against it, and sidewinders move in the same way as other snakes when they cross a solid surface. Sidewinding works only for small snakes, but several species have adopted it. The most famous is the sidewinder (Crotalus cerastes), a small rattlesnake of the American deserts, but there are similar snakes in other deserts. The horned viper (Cerastes cerastes) inhabits the Sahara and Middle Eastern deserts, and the carpet viper, also called the saw-scaled viper (Echis carinatus), occurs from North Africa to India. In South Africa there is Peringuey’s, or the desert sidewinding, viper (Bitis peringueyi). All of these are small snakes, Peringuey’s viper being only about 10 inches (25.5 cm) long and the carpet viper only about 30 inches (76 cm). Despite its small size, however, its highly toxic venom, excellent camouflage, and bad temper make the carpet viper possibly the most dangerous snake in the world and the cause of most deaths from snakebite in North Africa. Snakes lack eyelids, and those that live in deserts must protect their eyes from dust and sand. Peringuey’s viper achieves this by having its eyes located on top of its head. The sidewinder and horned viper do so by having small horns over their eyes. This gives these two species a similar appearance, but it is a feature that has arisen by parallel evolution and is shared by other desert snakes. The Sahara (Cerastes vipera) and Persian (Pseudocerastes persicus) horned vipers and McMahon’s viper (Eristicophis macmahonii) of Afghanistan and western Pakistan also possess horns. All three are about the same size as the carpet viper, and all of them are dangerous to humans.

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DESERT INVERTEBRATES

Before dressing in the morning, people who live in the desert shake out their shoes. It is a routine action, and there is a very good reason for it. A shoe might contain a scorpion. Scorpions are arachnids (class Arachnida), related to spiders, mites, and harvestmen, and arachnids belong to the phylum Arthropoda, together with insects and crustaceans. Scorpions (order Scorpiones) are the oldest of them all. There were scorpions on Earth in the Silurian period, more than 420 million years ago, although those scorpions lived in water. Scorpions began living on land during the Carboniferous period, 359.2 to 299 million years ago. Some of the Carboniferous scorpions were more than 30 inches (76 cm) long. That is much bigger than any scorpion alive

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today. Today the biggest scorpion is probably Pandinus imperator, which lives in the Sahara. It is seven inches (18 cm) long. Most are no more than half that size, and the smallest is Microbuthus pusillus, found in the Middle East, which is only 0.5 inch (1.25 cm) long. Not all scorpions live in deserts, but many of the 700 or so species do. Scorpions are well adapted to desert conditions. They are nocturnal, which is how they find their way into the shoes of people who are sleeping, and many live in burrows. These can be quite deep. Hadrurus arizonensis of the Sonoran Desert burrows as much as three feet (90 cm) below the surface. Species that do not dig burrows spend the day sheltering beneath stones. When they are on the surface, some species will raise themselves on tiptoe from time to time as a way to keep cool. This is called stilting, and it allows air to circulate beneath the body. Scorpions are tough. They can survive temperatures as high as 115°F (46°C), and they lose very little water by evaporation through their exoskeletons, which are rendered almost impervious by a protective layer of wax. Although the rate at which water passes through the exoskeleton is low, it often continues for a long time, but a scorpion can lose water equivalent to 40 percent of its body weight without coming to any harm.

The Scorpion’s Sting Scorpions never drink, obtaining all the water they need from their food. All scorpions are entirely carnivorous. They feed on insects and other invertebrate animals. Scorpions do not hunt by sight, and they do not see well. Some cave-dwelling species are blind, but other scorpions have one pair of eyes in the center of the carapace covering the prosoma—the front part of the body—and between two and five pairs of eyes around the edges of the carapace. Most scorpions detect prey by means of hairs, called trichobothria, on their pedipalps—the pair of big claws at the front of the body. Others can sense vibrations in the ground. Some desert scorpions can locate a burrowing cockroach 20 inches (50 cm) away and take just a few seconds to dig it out and capture it. Prey is seized and held by the pedipalps and then killed or subdued by the sting, and it is the sting, borne on the final segment of the postabdomen, or tail, for which scorpions are best known and most feared. All scorpions sting, and they all do so in the same way, by raising the postabdomen over the body and using a stabbing motion to inject venom into prey held in front of their heads by the pedipalps. Scorpion venom will kill most invertebrates, but there are only a few species capable of delivering anything worse than a painful sting to humans. Among those, however, there are some that are dangerous. A sting from Androctonus australis, of the Sahara, delivers venom as potent as that of

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a cobra. It is said to kill a dog within about seven minutes and a human in about seven hours. Centuroides sculpturatus, which lives in Arizona and New Mexico, can also kill, barefoot children usually being the victims.

Sun Spiders Scorpions are not often seen because most of them lie in wait for prey, emerging only far enough to seize it and then retreating again. They are not the only invertebrate predators living in the desert, however. There are also the sun spiders, or solifugids, and they are much more likely to make an appearance. Solifugids make up another order (Solifugae) of arachnids. Their alternative name of sun spiders refers to the fact that some species are active by day. They are also called false spiders and jerrymanders as well as camel spiders and wind scorpions because of the speed with which they run in pursuit of prey. What looks like a ball of thistledown blowing over the ground is most likely to be a wind scorpion in hot pursuit of a meal. Sun spiders are fairly large animals. Galeodes arabs, found in North Africa, is about 2.5 inches (7 cm) long, and its legs are about three inches (7.5 cm) long. They use only three of their four pairs of legs for running. The legs at the front of the body are smaller than the others and are used as sense organs to explore objects by touch. Apart from their size, sun spiders’ main distinguishing feature is the pair of huge chelicerae—pincers—they carry. These are longer than the prosoma and very heavy. Sun spiders do not sting, and they are harmless to humans. They chase their prey, seize it with their pedipalps, which resemble legs, and pass it to the chelicerae, where it is killed. They are carnivores and feed on any animal they can catch and subdue. They will even catch small vertebrates, such as lizards, birds, and mice, and they are voracious. A sun spider will often continue feeding until its body is so swollen it is almost incapable of moving. There are about 800 species, of which more than 100 live in the United States, from the Southwest as far north as Colorado.

The desert also harbors predatory insects. Tiger beetles (family Cicindelidae) are active by day. They are about one inch (2.5 cm) long, often brightly colored in metallic hues with contrasting stripes or bars, and they have large eyes and mandibles, which are the part of their mouth with which they seize prey. They have long legs and run fast in pursuit of prey. Their larvae are also fierce predators, but they use a quite different hunting strategy, one that calls for patience rather than strength and stamina. A tiger beetle larva excavates a vertical hole up to one foot (30 cm) deep, where it lies in wait. Just its mandibles are visible, wide open at the surface. Only the front part of the larva’s body is hard. The rest is soft, but on one of the abdominal segments there are two strong hooks that curve forward. Whenever an insect walks within range, the larva grabs and holds it until it is subdued. The two hooks gripping the sides of the hole make it impossible for the prey to drag the larva out, and when at last it is exhausted the larva drags its victim below ground to be eaten.

Ant-lions The larvae of some species of ant-lions (family Myrmeleontidae) also lie in wait, but the holes they dig in the sand are pitfall traps. The illustration shows how they work. An ant-lion hole is conical in shape, its sides at an angle of about 35°, which is as steep as it is possible for them to be while remaining stable (see “How Dunes Form” on pages 60–61). The larva lies buried at the bottom of its cone with just its mandibles exposed, from time to time moving around the pit so it is always on the shady side.

Ant falling into an ant-lion trap. The sides are so steep that once it is over the edge the ant falls into the mandibles of the waiting ant-lion.

Wolf Spiders and Tiger Beetles Wolf spiders also chase their prey, using their speed and strength to catch and overcome their insect prey. Webspinning spiders lie in wait for prey and do not need good eyesight, but wolf spiders see well. They have fewer eyes than most spiders, but the eyes themselves are better. With a total of six eyes—two large eyes and four smaller ones below them—they are able to detect small movements and form sharp images. Jumping spiders also have big eyes and good vision. They stalk their prey and jump on it when they are within range.

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Biology of Deserts When an insect—most commonly an ant, hence the name ant-lion—tries to explore the hole or cross the top of it, the larva immediately starts throwing sand grains upward. This triggers a minor landslide that carries the prey to the bottom of the pit and into the waiting mandibles. The larva then injects its victim with enzymes that liquefy its organs and sucks out the resulting juices. Ant-lions are also carnivorous as adults. Adult ant-lions are flying insects with remarkably long hind wings that trail behind them like streamers and contribute nothing to the insect’s flight. They patrol in search of prey, in the manner of dragonflies, but when they are mating the males form swarms in which they dance up and down to attract females.

Worm-lions, Robber Flies, and Butterflies Worm-lion larvae also trap prey in pits they have dug. These insects belong to the genus Vermileo of snipe flies (family Rhagionidae), of which most species are carnivorous. Robber flies (family Asilidae) prey on all herbivorous insects. The fact that so many predators can survive indicates that desert insects are plentiful. Butterflies and moths appear whenever plants flourish and flower following a heavy rain shower. Some butterflies are able to fly from flower to flower inside a bush without emerging from the shelter this affords.

Darkling Beetles and Scarabs Beetles are also common, the darkling beetles (family Tenebrionidae) being the most fully adapted to desert life (see “Finding Water and Conserving Water” on pages 136–138). Darkling beetles are vegetarian, and most species are unable to fly. They are active during the day and have few enemies. Predators learn to avoid them because of the offensive smell they release when threatened, and if this does not work they sham death, a behavior that deters the many predators that respond only to live prey. Some American chafers, or dung beetles (family Scarabeidae), have become adapted to feeding on plant material, but most feed on dung. There are species that work on the dung from below and drag it down into their tunnels. Other species make the dung into balls that they roll along the surface until they find somewhere suitable to bury them. This group includes those known as tumblebugs, or scarab beetles. One of the tumblebugs is Scarabaeus sacer, the scarab that in ancient Egypt was sacred to Khepri, a sun god. The commonest type of amulet—a good luck charm—was in the form of a scarab, with a representation of the beetle on one side and often the personal seal of its owner on the other. Scarab amulets were in use for many

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centuries and spread throughout the Mediterranean and Middle East. Egyptians believed that the scarab laids its egg inside its dung ball, and they saw a metaphor for the cycles of life and in particular for the daily reappearance of the Sun in the way the beetle rolled its ball. In a sense they were correct. The scarab contributes to the cycling of nutrients and performs a valuable ecological function. They were wrong about its reproductive habits, however. It does not roll a ball containing its egg but buries the ball on which it then feeds and next to which it lays its eggs, so its larvae also feed on the dung.

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LOCUSTS

There are insects that bite and insects that sting. Some insects transmit serious, occasionally fatal, diseases. Deserts harbor lethally venomous snakes and scorpions. Such animals are treated with respect and are best avoided, but there is one desert animal that cannot be avoided and that is capable of inspiring the deepest dread. It does not bite or sting, and it transmits no disease. Instead, it kills by starvation, its attacks are known as plagues, and those plagues have occurred throughout history. The insects are locusts, and they were well known to the authors of the Old Testament. And the locusts went up over all the land of Egypt, and rested in all the coasts of Egypt: very grievous were they; before them there were no such locusts as they, neither after them shall there be such. For they covered the face of the whole earth, so that the land was darkened; and they did eat every herb of the land, and all the fruit of the trees which the hail had left: and there remained not any green thing in the trees, or in the herbs of the field, through all the land of Egypt. (Exodus 10:14–15)

The story goes on to report that the sky over Egypt was darkened for three days. Scientists now believe that this biblical story of the eighth of the plagues of Egypt describes a real event that took place in about 1470 b.c.e. and affected the Nile Delta. Sadly, the biblical account is wrong in one respect: This was unlikely to have been the most serious locust plague in history, and there have been many since.

The 2004–2005 Plague In fall 2003 locusts began to gather in North Africa. In July and August 2004 the swarms took to the air and swept westward along the southern border of the Sahara, bringing devastation to countries already afflicted by severe drought. By the end of the year the plague was being

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brought under control, but the threat continued into the summer of 2005. It was the worst locust outbreak since one lasting from 1986 through 1989. Bringing the 1986–89 plague under control cost the international community an estimated $300 million. Controlling the 2004–05 plague may have cost more. In September 2004 the Food and Agriculture Organization of the United Nations (FAO) was asking for $100 million in international aid to deal with the crisis. At that time the FAO estimated that 2.5 million rural households could face food shortages. The swarms moved across the continent, from Senegal, Gambia, and Guinea Bissau in the west to Mauritania, Mali, Niger, northern Nigeria, and Chad. Farther north the locusts infested Morocco and Algeria. Swarms also moved westward, reaching the Canary Islands by the end of November. In November 2004 there was a fresh outbreak in the eastern Mediterranean region affecting Egypt, Israel, and Cyprus. Those countries were better equipped to deal with the insects than the poorer countries farther west. Then, in summer 2005, swarms appeared in the Aveyron region of southern France, where they devoured farm crops, including the alfalfa dairy farmers grow to feed the cows that produce the milk to make Roquefort cheese. They even ate flowers growing in window boxes. Mauritania, the country most seriously affected, reported that approximately half its 2004 cereal crop had been lost. In neighboring Senegal the 2004 cereal harvest was lower than the 2003 harvest, but mainly because of drought around sowing time that reduced the areas sown to millet and sorghum. Intensive control measures largely kept the swarms away from the principal crops. In late August 2005 the FAO warned that there were bands of hoppers and groups of immature adults in the Darfur region of Sudan, and small bands of hoppers in Eritrea near the border with Sudan. Hopper infestations had been controlled in Tigray Province, Ethiopia. Locusts were breeding in Yemen and in parts of Chad, where one small swarm had been reported. In general, it seemed that the plague had ended.

Plagues of the Past There is no pattern to the occurrence of locust plagues. In recent times there were plagues in 1926–34, 1940–48, 1949–63, 1967–69, 1986–89, and 2004–05. During each of these periods there were repeated outbreaks, when swarms of locusts descended on cropland and pasture. During the 1949–63 plague, for example, there were several swarms that each covered an area of more than 100 square miles (259 km2). Early in 1954 50 swarms invaded Kenya. One of those covered an area of about 77 square miles (200 km2) with a density of about 19.3 million locusts for every

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square mile (50 million/km2), so the entire swarm consisted of 10 billion locusts. This was larger than most swarms, which contain between 40 million and 80 million insects, but together the 50 swarms covered about 390 square miles (1,010 km2) to a height of 3,000–4,500 feet (900–1,400 m). Although most plagues originate in Africa, they do not always remain there. At various times swarms have crossed the Mediterranean into southern Europe and have even traveled as far north as Russia. A swarm reached the British Isles in 1954, and in October 1988, during the 1986–89 plague, a swarm crossed the Atlantic. It took 10 days to travel 2,800 miles (4,500 km) from North Africa to the Caribbean, landing from St. Croix and the British Virgin Islands southward to the eastern coasts of Suriname and Guyana. During plagues locusts may visit all or part of 60 countries over an area of about 11 million square miles (28.5 million km2). Locusts affect the Middle East, Pakistan and India, and the southern part of central Asia and China. Australia is sometimes plagued. Outbreaks prior to the 20th century are poorly documented, but plagues affected Western Australia in 1982, 1990–91, and 1999–2001. Six major plagues occurred in eastern Australia between 1934 and 2000. A plague in North America lasted from 1874 to 1877. In that plague there were swarms covering 125,000 square miles (324,000 km2), an area bigger than Colorado, to a height of 5,000 feet (1,525 m). The last North American locust plague was in 1938 and affected the midwestern states and Canada.

The Hopper That Changes Its Appearance Historically, the fear that locusts inspire arises only partly from the damage they cause. It is also due to the unpredictability of plagues. Nowadays satellite surveillance and a much better understanding of the biology of locusts allow plagues to be predicted, but this has not been possible throughout most of history. Swarms have appeared suddenly, without warning and seemingly from nowhere, as a cloud darkening the horizon that blacks out more and more of the sky as it draws closer. It is composed of insects that are seen only during the plagues. At other, quiet times, called recessions, the locusts are not simply hard to find— they do not exist at all. Locusts are short-horned grasshoppers belonging to the family Acrididae. There are about a dozen species of them. Unlike ordinary grasshoppers, locusts exist in two forms that are quite different in appearance and behavior. They are so different, in fact, that they were once believed to be two separate species. One form, or species, was familiar in certain places, and it was quite harmless. The other form was the one that appeared in huge, dreadful swarms.

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Biology of Deserts It was not until 1921 that the Russian entomologist Boris P. Uvarov (1889–1970) recognized them as two forms of the same species. Uvarov had been studying the migratory locust (Locusta migratoria), which in 1912 had infested the area around Stavropol, in the Caucasus, and at the same time he examined what was thought to be a harmless species about which scientists knew little, L. danica. Uvarov discovered that if a danica grasshopper came upon other danica insects, it would move away from them, but if a migratoria insect met others like itself, it would join them. Further study revealed, however, that migratoria grasshoppers sometimes produced danica offspring, and some insects were intermediate in form between migratoria and danica, making it difficult to tell which species they belonged to. Then Uvarov began to suspect that L. migratoria, the cause of the Russian plague, was the same species as the African migratory locust, now known as L. migratoria migratorioides, which periodically infested various regions of Africa south of the Sahara. Expanding his study to other species, he finally concluded that migratoria and danica were different versions of the same species, and the name Locusta danica was abandoned. Uvarov also discovered how the confusion had arisen. In a paper published in the September 1921 issue of the Bulletin of Entomological Research he wrote that “The direct cause of [this] ignorance is that injurious insects, and locusts especially, are studied only in the years of maximum development, and nobody cares about them in the minimum years, when the clue to the whole locust problem is likely to be found.”

The Desert Locust Judged by the frequency, severity, and geographic extent of its plagues, the most serious pest is the desert locust (Schistocerca gregaria). It lives in the deserts and semiarid regions of Africa, the Near East, and Southwest Asia wherever the annual rainfall is less than eight inches (200 mm). In addition to the African migratory locust, there are other, less serious, pests. The oriental migratory locust (L. migratoria manilensis) occurs in Southeast Asia. The red locust (Nomadacris septemfasciata) inhabits eastern Africa, the brown locust (Locustana pardalina) southern Africa, and the Moroccan locust (Dociostaurus maroccanus) is found from northwestern Africa to Asia. The Bombay locust (Nomadacris succincta) lives in southern Asia. Most Australian outbreaks are caused by the Australian plague locust (Chortoicetes terminifera), spurthroated locust (Austracris guttulosa), and migratory locust (Locusta migratoria). There are also various tree locusts (Anacridium species) found around the Mediterranean. The Rocky Mountain locust (Melanoplus spretus) was the cause of American plagues.

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The Life Cycle of a Locust In its harmless form, when it is said to be solitarious, the desert locust (Schistocerca gregaria) is gray, brown, or green in color. An insect about two inches (5 cm) long, it is able to fly but does so only occasionally and covers short distances. There are usually large numbers of these locusts wherever food is to be found, but individuals avoid each other, spending their time feeding on the dry, sparse vegetation. They will mate and lay eggs if conditions are suitable. In order to mate they need some moisture 4–6 inches (10–15 cm) belowground in sandy or mixed sand and clay soil, bare ground on which to lay eggs, and some green plants to feed the young. Their eggs hatch after an average of two weeks. The young, known as hoppers, cannot fly. Hoppers grow for about 30 to 40 days, and it takes on average two to four months for the adults to become sexually mature. Locusts are not found everywhere, and no locusts may be present in a particular area even though the ground is moist and some of the vegetation green. That is how the locusts remain unless there is rain. Then green plants will quickly cover the area, and female locusts will lay many more eggs to take advantage of the sudden abundance of food. Hopper numbers will increase, and so will the frequency of encounters between individuals. Then, as dry conditions return and the vegetation dies back, the hoppers find themselves crowded more and more closely together, feeding in an area that is contracting. That is when a change comes over the hoppers. Their color turns pink, and they may be marked with black, yellow, or orange stripes. They are now in the immature stage of their alternative phase. They are ceasing to be solitarious and are becoming gregarious. Individuals no longer avoid each other. Instead, when two hoppers meet they usually stay together, all the time feeding and moving in bands that join each other and merge. In order to grow the hoppers must molt their exoskeletons. After their fifth molt they are adults and are now yellow. At night they roost in trees and shrubs, by day they feed, and before long their food supply is exhausted. They have eaten everything, and it is time to move on. They are now locusts, and they take to the air, moving as a swarm. Solitarious locusts travel alone and by night. Gregarious locusts fly in swarms and by day.

How Swarms Travel Once airborne, the locusts remain in small groups within the swarm. When a group finds itself at the edge of the swarm it turns toward the center. In this way the swarm is maintained, but inside it the locusts are flying in all directions. The swarm is carried by the wind and does not travel in a direction the locusts have chosen.

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DESERTS of Africa and Asia, and the Egyptian cobra (Naja haje) is found over most of Africa. It prefers the desert, however, and its brown coloration camouflages it well. Like most desert snakes, it shelters in crevices and beneath rocks and feeds mainly on small rodents. The Egyptian cobra is a large snake, up to eight feet (2.5 m) long. When disturbed a cobra will rear up, extend its hood by spreading the ribs just behind its head, hiss loudly, and lunge repeatedly. This is a threat, meant to intimidate its attacker, and the lunge is often deliberately short and made with the mouth closed. Cobras are not especially aggressive, but sometimes the lunge is genuine and the strike real. Cobra venom is extremely potent.

Coral Snakes A desert locust (Schistocerca gregaria) in its adult form (Alastair Rae)

Depending on the speed of the wind, locust swarms usually travel at 10–12 MPH (16–19 km/h) and cover up to 100 miles (160 km) a day, although they can travel farther over seas. Often they enter the intertropical convergence zone (see “Intertropical Convergence Zone, Monsoons, and Jet Streams” on pages 79–82), where they can be carried to great heights on rising air currents. Usually the prevailing tropical wind carries them from east to west. In the Exodus account the locusts came from the east. A swarm that took to the air in 1950 over Saudi Arabia was tracked all the way to the Atlantic coast of Africa, in Mauritania, a journey of 3,100 miles (5,000 km) that took it less than two months. Swarms land when the locusts sight vegetation. A desert locust can eat approximately its own weight in food every day. This is about 0.07 ounce (2 g). Consequently, 1 million locusts can eat more than 2 tons (1.8 t), and 1 ton (0.9 t) of locusts—a very small fraction of an average swarm—will eat as much food in one day as about 10 elephants, 25 camels, or 2,500 people. When the locusts leave, no green leaf remains.

REPTILES OF OLD AND NEW WORLD DESERTS

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Snakes have adapted well to desert life, often evolving similar physiological or behavioral solutions to similar environmental conditions (see “Sidewinders and Horned Snakes” on pages 144–145). Where a particular genus is widespread in a region that includes deserts, some of its species are likely to have colonized those deserts. Cobras, for example, occur throughout the Tropics and subtropics

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Obviously, the color of the Egyptian cobra camouflages it, and camouflage helps the snake to pursue prey unobtrusively and avoid predators. This is one use of color, and in the North American deserts there is another member of the cobra family (Elapidae) that uses color quite differently. The Sonoran coral snake (Micruroides euryxanthus) is one of about 40 species of coral snakes distributed over most of the warmer regions of North, Central, and South America. Nearly all coral snakes are marked with brightly colored bands. Those of the Sonoran coral snake are red, black, and yellow. Coral snakes shelter in tunnels belowground or beneath rocks, and they feed mainly on other snakes, especially those that tunnel below ground, and on worm-lizards or wormsnakes, members of an order (Amphisbaenidae) of reptiles that live belowground, feed on invertebrates, and are neither true lizards nor snakes. Coral snakes are not large—the biggest is no more than 3 feet (90 cm) long—but they are extremely venomous. If disturbed they lash out wildly. There are also harmless snakes with body markings very like those of coral snakes. The milk snake (Lampropeltis triangulum), for example, also lives in the desert and is easily mistaken for a coral snake, although the two species are quite unrelated. At one time scientists thought these similarities evolved as a protection. Coral snakes are easily seen and dangerous, and predators that learn to avoid them will not take chances with any snake that mimics their markings. Unfortunately, there is a problem with this apparently plausible explanation. No predator that disturbs a coral snake will learn to avoid it in future for the simple reason that the coral snake will kill it. Consequently, bright bands confer no defense on either coral snakes or their mimics. An alternative explanation proposed by the German biologist Robert Mertens and called Mertensian mimicry centers on a group of mildly venomous banded snakes called false coral snakes. Venom from a false coral snake cannot kill a predator, but the bite will make it feel very

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Biology of Deserts sick. So predators learn to avoid false coral snakes, and both true coral snakes and their mimics benefit. Even then, though, there is a difficulty. False coral snakes are found only in Central and South America. There are none in the North American deserts, so similarities between North American coral snakes and their mimics cannot be due to Mertensian mimicry. Yet another explanation, proposed by the British biologist Chris Mattison in his book Snakes of the World (Blandford Press, 1986), may be the true one. All these banded snakes are nocturnal and secretive. Their bright colors do not impede them when hunting because they do not show up at night. Should a predator dig one from its tunnel during the day or turn over the rock beneath which it is sheltering, it would confront a brightly colored snake thrashing furiously. This would startle the predator, making it retreat or at least stop moving for a moment, and that moment might give the snake as much time as it needed to escape. In other words, the bright bands are startle, also called deimatic, coloration. These are not the only snakes with bright bands. The Californian kingsnake (Lampropeltis getulus californiae) lives in the deserts of Utah, Nevada, and Arizona as well as in California. Its bands are white or cream and brown, except in a small area around San Diego, where, instead of bands, snakes of the same subspecies have stripes running the length of their bodies. These are fairly big snakes, three to six feet (0.9–1.8 m) long, and as well as small mammals and lizards they eat other snakes, including rattlesnakes, copperheads, and coral snakes and are immune to their venom.

Pit Vipers The sidewinder (see “Sidewinders and Horned Snakes” on pages 144–145) is one of several rattlesnakes that live in deserts. These include the Mojave (Crotalus scutulatus) and tiger (C. tigris) rattlesnakes, as well as one of the biggest rattlesnakes, the western diamondback (C. atrox), which can grow to more than 6.5 feet (2 m). Rattlesnakes are pit vipers (family Crotalidae). The pits are a pair of depressions between the eyes and nostrils, each containing an organ that can detect a temperature change of as little as 0.2°C (0.4°F). Its pit organs allow the snake to locate prey in total darkness by comparing the warmth of the prey’s body with the temperature of its surroundings. The copperhead (Agkistrodon contortix) is also a pit viper. It occasionally grows to about 50 inches (1.25 m), but most copperheads are smaller. They feed on small rodents and are not aggressive. Not all desert snakes shelter beneath rocks. Some bury themselves in the sand with just their eyes and nostrils exposed. One of these is McMahon’s viper (Eristocophis macmahonii). It buries itself in loose sand by sweeping its

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body from side to side. This pushes away the sand beneath its body, and it sinks vertically until only its outline in the disturbed sand betrays its position. There it waits to strike at passing lizards and small mammals that are its prey—or at any human who steps on it.

Venomous Lizards There are only two species of venomous lizards in the world, constituting the family Helodermatidae, or beaded lizards, so called because their scales are surrounded by rows of beadlike granules. They are related to the monitor lizards, and both species occur in America. The Gila monster (Heloderma suspectum), named for the Gila River Basin, occurs in the southwestern United States and Mexico, and the Mexican beaded lizard (H. horridum) in western Mexico. The Gila monster grows to about 20 inches (50 cm) in length and the Mexican beaded lizard to about 30 inches (80 cm). Both are stout-bodied animals. The Gila monster is marked with black and pink blotches or bands. The Mexican beaded lizard is darker in color. They hunt by night for any small animal they can catch as well as birds’ eggs and carrion. In the winter they metabolize fat that they store in their tails during the summer. Both lizards move sluggishly, but if disturbed they have a very strong bite. Most of their teeth have grooves along which their venom, a nerve poison, flows from glands in the lower jaw. Their venomous bite is painful, but it is rarely fatal to humans.

Swimming through Sand Not only snakes bury themselves in sand. There are also lizards that behave in the same way. Indeed, some skinks move through loose sand with such a smooth swimming motion that one of them is called the sandfish. This is Scincus philbyi, a lizard about eight inches (20 cm) long that lives in the Arabian Desert. Skinks comprise the family Scincidae, of which there are more than 1,200 species. Many have very small legs and some are legless, but the sandfish has strong legs and feet, with fringes of scales on its flattened toes that improve its grip on loose sand. It eats small invertebrates, especially millipedes, and because it hunts a little distance belowground, swimming through the sand like a fish, it is protected from the heat and can be active during the day. Other lizards dig burrows. The Arabian toad-headed agamid (Phrynocephalus nejdensis), a sandy-colored lizard about five inches (12.5 cm) long, is typical of these. It has long legs that keep its body clear of the ground when it is on the surface and, like most agamid lizards (family Agamidae), a long tail. It buries itself by wriggling from side to side and feeds mainly on insects, although it also eats some plant material.

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The desert spiny lizard (Sceloporus magister) is heavybodied and up to 5.5 inches (14 cm) long. Classified in the family Phrynosomatidae, it is a close relative of the agamids. It occurs throughout the southwestern United States. (Courtesy Gene Hanson)

Thorny Lizards Agamid lizards are quite common in the deserts of Africa, Asia, and Australia, and they have few weapons with which to defend themselves. The Arabian toad-headed agamid raises its tail, which it then repeatedly rolls up and unrolls. Other agamids have evolved exotic armor. In the Australian deserts there is a lizard called the thorny devil (Moloch horridus). Its head, body, tail, and legs are covered with conical scales that look like bristles or thorns, and there is an especially big one over each eye. It needs to terrify its

Despite its fearsome appearance, the thorny devil (Moloch horridus) is small, slow, and feeds on ants and termites. The bristles are meant to deter enemies. (NASA)

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A predator would hesitate before attempting to seize a lizard covered in spines. This is a thorny devil (Moloch horridus). (NASA)

enemies because it is only about six inches (15 cm) long and moves slowly. It feeds mainly on ants and termites. The princely mastigure (Uromastyx princeps) lives in the eastern Sahara. About nine inches (23 cm) long, it has a thick tail covered with sharp spines that it lashes at any animal threatening it. The armadillo lizard (Cordylus cataphractus) is a girdled lizard (family Cordylidae), about 8.25 inches (21 cm) long, that lives in southern Africa. It, too, is covered with spiny scales, but it has an additional means of defense. When threatened, it rolls up like an armadillo, holding the tip of its tail in its mouth so a predator is presented with a solid ball of armor. It is active by day, feeding on insects and other invertebrates. Like many lizards, the armadillo lizard can shed its tail voluntarily when threatened and then regrow it. This is called autotomy. Often the discarded tail will wriggle for a while, distracting a predator long enough for the lizard to escape. Iguanas (family Iguanidae) are the American equivalent of the agamid lizards, and convergent evolution (see “Convergent Evolution and Parallel Evolution” on pages 142–145) has given some of them thorny scales. The Texas horned lizard (Phrynosoma cornutum) is the most spectacular of them all. It has two big horns at the back of its head, with smaller spikes on either side forming a collar, two rows of spines along its back, and spines around its sides. It feeds on ants and is never more than seven inches (18 cm) long. The chuckwalla (Sauromalus obesus) is also an iguanid. One of the bigger lizards, with dark skin and a pale yellow tail, it is about 16 inches (40 cm) long and lives in the arid southwestern United States. Its skin is loose, hanging in folds, and contains glands that can store water. Most lizards are carnivorous, but the chuckwalla is a her-

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mole (Eremitalpa granti) may swim three miles (5 km) in a single night. There are also hedgehogs in the Sahara and in the deserts of Asia. Hedgehogs feed mainly on invertebrates, but some will eat prey as large as mice.

Big Ears

The chuckwalla (Sauromalus obesus) of the southwestern United States is a vegetarian. (Gene Hanson)

bivore, feeding on leaves, flowers, and buds, often of the creosote bush.

Monitors The monitors (family Varanidae) are the biggest of all lizards. They are powerfully built carnivores with long necks and tails and strong limbs, and some species are found in deserts. Gould’s monitor (Varanus gouldi) of Australia is also known as the sand monitor. It is about five feet (1.5 m) long, and when threatened it stands upright on its hind legs, hissing loudly. Gould’s monitor shelters in a burrow or beneath debris but hunts over a wide area for mammals, birds, reptiles, insects, and carrion. Like all monitors, it has a snakelike forked tongue with which it samples the air.

MAMMALS OF OLD AND NEW WORLD DESERTS

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Lizards are not the only animals that swim through the desert sands. In the Kalahari and Namib Deserts there are golden moles that do the same. Although they resemble ordinary moles, golden moles (family Chrysochloridae) are only distantly related to them. They live only in southern Africa. Skin covers their eyes, and fur covers their ears. The bones of their inner ears are large, however, making the moles very sensitive to vibrations. If disturbed on the surface a golden mole makes quickly and unhesitatingly for the entrance to its tunnel and for a secure chamber if it is disturbed while below ground. These desert swimmers move just below the surface, foraging for food. Their diet consists mainly of invertebrates, but they also catch and eat legless lizards. Grant’s golden

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The desert hedgehog (Paraechinus aethiopicus) of the Sahara and Brandt’s hedgehog (P. hypomelas) and the long-eared hedgehog (Hemiechinus auritus), both of Central Asia, have large ears. These act as heat exchangers, helping the animals keep cool, and many desert mammals have them. All hares have big ears, even those living in temperate climates, but those of the desert jackrabbits of North and Central America and desert populations of the Cape hares (Lepus capensis) of Africa and Asia are bigger than most. Jackrabbits are hares (Lepus species), not rabbits, and several species inhabit deserts. Most rabbits dig burrows, but few cottontails or hares do so. In summer the black-tailed jackrabbit (L. californicus) digs short burrows in which to shelter from the desert heat, but otherwise it lives aboveground. As well as helping animals to keep cool, big ears provide their owners with acute hearing. This is valuable for desert animals that live aboveground. Many predators rely on their hearing to locate prey at night, and prey species need to be able to hear an approaching predator. Golden moles can hide belowground, and hedgehogs have spines, but hares and jackrabbits have no defenses or weapons. They rely on their keen hearing to detect approaching danger and escape by running. A black-tailed jackrabbit can run at 45 MPH (72 km/h), but even so it can avoid capture only if it has adequate warning.

Carnivorous Mice Rodents are the most abundant of desert mammals. Most are mainly vegetarian, although they will also eat insects, but the grasshopper mice are carnivores. There are three species (genus Onychomys), and they inhabit the deserts and semiarid regions from central Mexico northward as far as southwestern Canada. Up to five inches (13 cm) long with a two-inch (5-cm) tail, they are bigger than most mice. As their name suggests, they feed mainly on grasshoppers, but they will also eat any other insect they can catch as well as scorpions, lizards, and rodents smaller than themselves. When one grasshopper mouse detects another nearby it utters a series of high-pitched squeaks, each lasting about one second. These cries, uttered more frequently by males than by females, may serve to maintain a minimum distance between individuals or breeding pairs.

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Small Desert Cats Small rodents have many enemies. In the North American deserts snakes and birds of prey are the principal carnivores, although pumas (Felis concolor) sometimes hunt in the desert. They will eat rodents and jackrabbits. The puma—also known as the cougar and mountain lion—is the only cat found in American deserts. Several species of cats live in the deserts of Africa and Asia. The sand cat (F. margarita) inhabits deserts from the Sahara to Central Asia. Its head and body measure about 20 inches (50 cm) in length, and its tail is about 12 inches (30 cm) long, making it rather bigger than a domestic cat. It is sand-colored as its name indicates, but with dark rings on its tail, and the pads of its paws are covered with fur, presumably as insulation. It is a nocturnal hunter. Pallas’s cat (F. manul) lives from Iran to western China on grassland and in mountains as well as in deserts. It is roughly the same size as the sand cat but stockier, with shorter legs and much longer fur. A secretive animal that is rarely seen, it, too, is nocturnal, spending its days in rock crevices, caves, or burrows made by other animals. Cats are conventionally divided into two groups. The sand and Pallas’s cats are both categorized as small cats, all of which are usually included in the genus Felis. Lynxes are the biggest of the Felis cats, and one of them, the caracal (F. caracal), stalks its prey in dry, open country including deserts from Africa to India. The caracal, or caracal lynx, somewhat resembles a small, long-legged puma with very long, pointed ears that end in tufts of hair. It lives alone, hunting mainly at night but some-

times during the day in cooler weather. Most small cats hunt either by waiting to ambush prey or by stalking and pouncing. The caracal also chases animals and runs them down. It will eat anything it can catch, from mice to small deer, including birds, reptiles, and domestic sheep and goats.

Cheetahs Most big cats are placed in the genus Panthera, except for the clouded leopard (Neofelis nebulosa) and the cheetah (Acinonyx jubatus). The cheetah is also a desert hunter, and, like the caracal, it runs down its prey. It is famous for being the fastest mammal. Some people claim that over a short distance it is capable of running at 70 MPH (113 km/h), although others think it more likely that its top speed is closer to 60 MPH (96 km/h). This is still very fast, and its speed is made possible by the way the cheetah’s body is constructed. Its spine is very flexible, and its pectoral (shoulder) and pelvic (hip) girdles are able to move very freely against the spine. The illustration on page 155 shows how this skeletal flexibility gives the cheetah an extremely long stride, allowing it to move in a series of huge bounds. At the same time, its short face—much more like that of a domestic cat than the longer muzzle of other big cats—means that its big eyes look straight ahead, giving it excellent binocular vision with which to judge distances very precisely. Small cats are able to retract their claws. The cheetah is unable to retract its claws completely. This means the claws are blunt through wear, but it also means that they give the cat a firm grip on the ground when it is running.

Prized for its speed and beauty, the cheetah (Acinonyx jubatus) was once used for hunting. (Birmingham Zoo)

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How the cheetah runs. Its flexible skeleton gives the cheetah a very long stride.

over anything from about 10 square miles to 160 square miles (26–414 km2). Ranges overlap, but each pride has a central core area from which other lions are excluded. Sometimes lions collaborate in hunting. One group of females will circle around the prey, taking advantage of whatever cover is available, and then hide. When this group is in position, a second group approaches the prey more openly, driving it into the ambush. Lionesses undertake most of the hunting, but a male lion is quite capable of hunting for itself. Food is shared among the pride, but young cubs can be at a disadvantage. Their small size makes it difficult for them to compete with the adults, and they may go hungry.

An average adult cheetah measures about four feet (1.25 m) from its head to the base of its tail, and the tail, which it uses for balance, is about two feet (60 cm) long. The animal weighs about 130 pounds (59 kg). It hunts gazelles and similar animals, but it will also take smaller prey, such as hares. Its hunting technique is to select a target and then stalk it slowly and patiently—up to four hours if necessary—until it is within about 100 feet (30 m) of its quarry. Then it charges, the prey runs, and the final chase commences. This usually covers about 200 yards (183 m) and lasts less than one minute. Cheetahs probably evolved in Asia. Their name is from the Hindi word chita, which means spotted, and they were once used for hunting. Today most cheetahs live on open grassland and desert in Africa and the Middle East, as far as northern India. There are estimated to be 9,000 to 12,000 cheetahs living in the wild, and they are listed as vulnerable to extinction, due mainly to loss of the habitat they need. Cheetahs are also much more uniform genetically than are most other species. Scientists believe this uniformity, known technically as a bottleneck, results from the loss of large numbers of cheetahs about 10,000 years ago. Their narrow genetic base increases their vulnerability, because it reduces the natural variability that might allow them to adapt to changes in their environment.

Lions, cheetahs, and caracals hunt gazelles. These are hoofed animals related to cattle and sheep, but small, lightly built, and quite at home in the desert. The dorcas gazelle (Gazella dorcas), found from the Sahara to India, is typical. It can obtain all the moisture it needs from the plants on which it feeds, so drinking is not essential, although it loses weight if it subsists on a diet consisting only of dry food. It is a small animal, only two feet (60 cm) tall at the shoulder, but to escape a cat it can run at 50 MPH (80 km/h). The springbok (Antidorcas marsupialis) is also a gazelle. It lives in southern Africa. At one time herds numbering tens of thousands used to migrate across the dry plains, but today it is confined mainly to reserves and does not migrate.

Lion

Addaxes and Oryxes

Lions (Panthera leo) also hunt in the Sahara and Kalahari Desert. They are the most social of all cats, living in family groups called prides. A pride consists of up to about 15 related females and their offspring together with up to about six adult males. Depending on the abundance of game and number of lions to be fed, a pride ranges in search of food

The addax (Addax nasomaculatus) used to live throughout the Sahara. Today it occurs in just some parts of the desert, but it always lives a long way from water in territory that is too barren to support most other herbivores. Its feet are widely splayed, which helps it move across loose sand, and it obtains all its water from the food it eats.

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Gazelles

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The addax has long, spiral horns. Those of the oryx are also long, but straight. The Arabian oryx (Oryx leucoryx), which once ranged over most of the Arabian peninsula, is now found only in Oman and the Sinai peninsula. The scimitar, or white, oryx (O. dammah) occurs along the southern Sahara from Mauritania to the Red Sea, and the gemsbok, or common oryx (O. gazella), lives in northeastern and southwestern Africa.

BIRDS OF OLD AND NEW WORLD DESERTS

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In some ways birds find it easier than other animals to adapt to the hot desert environment. They start with a physiological advantage. They are endotherms, of course, and maintain a fairly constant core temperature by physiological means, but the ordinary core body temperature of a bird is about 104°F (40°C), which is higher than that of a mammal. An endotherm needs to activate its regulatory mechanisms—dilating or constricting blood vessels, raising or lowering hair or feathers, shivering or sweating, and so forth—when the difference between its core temperature and the outside air temperature exceeds a certain threshold. The difference of about 3.6°F (2°C) between the core temperature of a bird and a mammal is small but significant. It means that each day a bird spends a shorter time than a mammal having to prevent its body from overheating. Added to this, birds allow their body temperature to fluctuate more widely than do mammals, and they are not harmed if it rises a few degrees. All birds share this feature, and there is no difference between desert birds and those that live in other environments.

Moving through Cool Air Birds also fly, and this gives them another advantage over mammals, because while they are flying they are moving through air that is much cooler than air near the ground surface. Air temperature decreases with height by an average of 5.4°F per 1,000 feet (9.8°C per km). During the hottest part of the day the temperature of a sand or rock surface may be about 170°F (77°C). If a small mammal ventures from its burrow, this is the temperature it must brave. At a height of about 6.5 feet (2 m), however, the temperature will be about 115°F (46°C). This is the temperature of the air around the head of a large mammal such as a dromedary. A bird flying overhead, say at about 200 feet (60 m), moves through air at about 114°F (45.5°C), and the air flowing over its body as a consequence of its forward motion is the equivalent of a wind exerting an additional chill factor. Many birds fly at around 25 MPH (40 km/h), a speed that

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will have a cooling effect equivalent to a decrease in temperature of a degree or two. The reduction is small, because as the air temperature approaches the body temperature, the chilling effect of a wind decreases. Nevertheless, birds spend a good deal of their time in conditions that are markedly cooler than those experienced by animals living on the ground, and they are physiologically better equipped for high temperatures. Most birds rely on their vision to locate food. Consequently, all but a few birds, such as owls and nightjars, are active during the day. At times, however, they find the air intolerably hot even when they are flying, so, despite the advantages they enjoy over mammals, desert birds often have to seek shade during the hottest hours. They rest with their bills open until the temperature starts to fall, and the only birds remaining airborne are the eagles, hawks, and vultures, circling slowly at high altitudes where the air is much cooler.

Pigeon Milk It is also easier for birds than for mammals to find water. All birds need to do is fly to the nearest river, lake, or water hole, and they are able to travel a long way. Every morning, mourning doves (Zenaida macroura) gather to drink at water holes in the North American desert, some having flown there from 40 miles (64 km) or more away. Then, when they have drunk their fill, they fly off to seek food or return to tend their young. Newly hatched birds and fledglings cannot make such journeys, of course, so a desert bird must have some means of carrying water to its young. Pigeons, including the mourning dove, do so by producing a liquid called pigeon milk or crop milk. Pigeon milk looks like cottage cheese, and its production is stimulated by the hormone prolactin, which also stimulates milk production in mammals. Pigeon milk is similar to mammalian milk in composition, consisting of 13–19 percent protein, 7–13 percent fat, 1–2 percent minerals and vitamins (A, B, and B2), and 65–81 percent water. (Cow’s milk is 50 percent sugars, 35 percent protein, 40 percent fats, and 9 percent minerals and vitamins.) Both males and females produce pigeon milk, and for their first few days after hatching it provides the only nourishment the young birds receive. After that the pigeon milk is augmented with solid foods brought by the parents, but the amount of milk the young receive remains fairly constant almost until they are fully fledged. Obviously, the adults must drink enough water to be able to produce milk in addition to supplying the needs of their own bodies. This imposes a stress on the birds, but one that is far less severe than the stress imposed on a lactating mammal, which must also use some of the water it drinks or obtains from its food to make milk.

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Sand Grouse Only pigeons and flamingoes produce crop milk (and flamingoes are wading birds that live beside water and do not venture into the desert). Other desert birds must find different ways to bring water to their young, and all but one of the 16 species of sand grouse (family Pteroclididae) have evolved a novel method. Sand grouse are small, ground-nesting birds found throughout the deserts of Africa and Asia. They eat dry seeds, and sand grouse chicks can find their own food within a few hours of hatching. Their diet contains very little water, however, and although they can feed themselves the young birds are unable to fly. Their parents deal with this problem by using their breast feathers as a sponge to bring water to them. The only exception is the Tibetan sand grouse (Syrrhaptes tibetanus), which is never far from melting snow. Contour feathers, which cover the body of a bird and give it its shape, consist of a central shaft, or rachis, from which the barbs grow at an angle of about 45°. The illustration shows this arrangement on part of a feather (left), much enlarged in the diagram (right). Each barb is lined on both sides by smaller structures called barbules, which also emerge at about 45°. The feather retains its overall structure because the distal barbules—those farthest from the bird’s body—have hooks with which they engage the smooth proximal (nearest the body) barbules adjacent to them. Sand grouse, especially the males, have modified belly feathers. When the feathers are dry, the proximal part of each barbule is twisted into a spiral and is held flat against the barb. When wetted, the barbules untwist and stand

Structure of a feather

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approximately at right angles to the barbs. This causes the feathers to form a dense mat, about 0.04 inch (1 mm) thick, that holds water. The belly feathers of male sand grouse hold up 20 times their own weight of water and those of females about 12 times. Soon after dawn the sand grouse take off for the water hole, usually a distance of less than about 20 miles (32 km). Females often go first, the males setting off after their mates have returned. When the males arrive they rub their bellies on the dry ground to remove the natural oils in their feathers. Then they drink their fill, after which they soak their belly feathers. Some do this by walking into the water. Others, such as Burchell’s, or the variegated, sand grouse (Pterocles burchelli) of the Kalahari, float while they drink and then take off from the water. Some of the water their feathers absorb is lost by evaporation on the flight back to the nest, but after a journey of 20 miles (32 km) the feathers are still wet when the birds arrive. An arriving bird stands erect, and the young rush to him and strip the water from his feathers. After that he rubs his belly in the sand to dry it, and then all the family starts foraging for seeds.

Desert Falcons Birds that congregate in large numbers attract predators. In addition to jackals and foxes, birds of prey also keep a watchful eye on desert water holes. Falcons, especially, hunt small birds and mammals. The lanner falcon (Falco biarmicus) lives in the deserts of Africa and Arabia, the saker falcon (F. cherrug) in central Asia, the laggar falcon (F. jugger) in India, and the brown falcon (F. berigora) and black falcon (F. subniger) in Australia. All of them are fairly small birds, between about 15 inches (40cm) to 20 inches (51 cm) long. They are fast and very

barbule (distal)

barb

barbule (proximal) shaft (rachis)

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maneuverable at low level. While flying close to the ground, they sometimes catch birds that are still on the ground or just as they are taking off. Falcons are powerfully built, with long, pointed wings and fairly short tails. The lanner and saker falcons were traditionally used for falconry in the Middle East and were highly prized. The saker was especially valuable because it is extremely aggressive and will attack prey much larger than itself. Falconry was not simply a sport for desert people but a means of obtaining food, and Arabs have hunted in this way for about 2,000 years. The birds of prey were trained to kill quail, bustards, hares, and other small game, then return to the falconer’s gloved wrist. Although people are no longer dependent on falconry for their survival, the birds are still highly revered. Falcons hunt by diving onto their prey at great speed, a technique known as stooping. Small birds are usually killed in flight.

Vultures At one time no adventure movie set in a desert was complete without a shot of vultures circling above as they waited for the hero to die from thirst. Vultures are birds very often associated with deserts, although, in fact, most vultures avoid deserts. American vultures belong to the family Cathartidae and African and Asian vultures to the family Aegypiinae. The two families are not closely related, although a similar way of life has led to similarities in appearance—another example of parallel evolution (see “Convergent Evolution and Parallel Evolution” on pages 142–145). Both families comprise large birds with the hooked beaks of birds of prey (raptors) and many with bare heads. They feed by scavenging carcasses. The lappet-faced vulture (Torgos tracheliotus) soars over the deserts of Africa, often alone but sometimes in groups of up to four. No other vulture interrupts its feeding, because it is by far the biggest of them—up to 40 inches (1 m) long and weighing 30 pounds (13.6 kg)—and its huge bill can tear open the hide of an elephant or rhinoceros. In parts of its range the lappet-faced vulture may be joined by griffon (Gyps fulvus), white-headed (Trigonoceps occipitalis), or hooded (Necrosyrtes monachus) vultures. The Egyptian vulture (Neophron percnopterus) ranges from Africa to India. Other vultures feed only on dead animals, but the Egyptian vulture will also eat vegetable matter and garbage and will hunt small animals.

Secretary Birds Other carnivorous birds feed mainly on reptiles. The secretary bird (Sagittarius serpentarius) is perhaps the species

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most committed to this diet, although it also eats insects and small rodents. A tall bird, standing about 3.3 feet (1 m) tall, it has long feathers at the back of its head resembling the quill pens office clerks once used to carry behind their ears. It hunts by walking along and staring at the ground, picking up small food items with its bill. It kills larger animals such as snakes by stomping on them. Although it is a raptor, the secretary bird has long legs, and its feet have short toes for walking rather than talons for seizing prey. It spends much of its time on the ground, but the secretary bird has wings that span 6.5 feet (2 m), and it flies well, sometimes soaring high into the sky. Like many raptors, pairs of secretary birds perform spectacular aerial mating displays. They nest on the tops of acacia trees.

Roadrunners The North American equivalent of the secretary bird is the roadrunner, or chaparral cock (Geococcyx californianus), a member of the cuckoo family (Cuculidae). It stands about 11 inches (14 cm) tall and has a long tail, and it hunts its prey by running along the ground at up to 17 MPH (27 km/h), with its body almost horizontal, then stopping abruptly. It is a weak flier and tires quickly. On occasion it eats venomous snakes, including rattlesnakes, but its diet consists mainly of lizards, scorpions, small mammals, ground-nesting birds, and insects. It seizes larger prey items in its bill and beats them against the ground. The roadrunner was once persecuted because people believed, incorrectly, that it hunted game birds. The roadrunner has adapted to desert conditions in an unusual way. Desert nights can be cold. Most birds would increase their metabolic rate to maintain a constant core temperature, but the roadrunner is different. It allows its body temperature to fall by just a few degrees, becoming torpid and thereby saving energy. The bird would be unable to respond quickly if a predator threatened it, but the roadrunner has few enemies and the risk is slight. Early in the morning it needs to warm up, so it basks, but it has a special adaptation to help. There is an area of dark-colored skin on its back between its wings. The roadrunner stands with its back to the sun and fluffs the feathers on its back to allow the warmth to reach its skin and the blood vessels just beneath its skin more easily. A smaller species, the lesser roadrunner (G. velox), occurs in Mexico and Central America.

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DESERT ECOLOGY

Ecology is the scientific study of the relationships among different species of organisms and between those organisms and their physical and chemical surroundings. It is a fairly new scientific discipline. The word ecology was

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Biology of Deserts coined in 1866 in a book called Generelle Morphologie der Organismen (General Morphology of Organisms) by the German zoologist Ernst Heinrich Haeckel (1834–1919). He made it out of two Greek words, oikos meaning “house” and logos meaning “account.” Haeckel wrote it as Ökologie, for which the English equivalent was oecology. The spelling was changed to ecology following the International Botanical Congress held in 1893 at Madison, Wisconsin. So ecology is an account (or explanation) of house(holds). Just as families keep watch on how the money coming into the home is allocated and the bills are paid, so ecologists are interested in the ways plants, animals, and other types of organisms use the resources available to them. Economy is household (oikos) management (Greek nemo), and ecology is the study of what has sometimes been called the economy of nature. Resources that are important in the economy of nature include shelter, places where young can be raised safely, opportunities for social interaction and mating, a tolerable climate, and, most important of all, a source of food. When a plant or animal succeeds in establishing itself in a place where it finds the resources it needs, it is said to occupy an ecological niche. The organism defines the niche by occupying it, and the niche does not exist until that happens.

What Is an Ecosystem? A discrete unit in which organisms and their chemical and physical surroundings interact to form a coherent system is called an ecosystem, which is short for ecological system, and it can be of any size provided the area in which it occurs can be clearly distinguished from surrounding areas. A forest can be distinguished from adjacent farmland, for example, a single tree with mosses, lichens, insects, birds, and squirrels living in its branches can be distinguished from the surrounding forest, and a small pool of water can be distinguished from the dry land around it. A forest, tree, or pool of water—or even a single drop of water with its microscopic inhabitants—can be regarded as an ecosystem. A desert is also an ecosystem. Food, the most fundamental resource, consists basically of sunlight, water, carbon dioxide, which are the raw materials for photosynthesis, and the energy that powers the reactions, together with mineral nutrient compounds taken from the soil. Plants require adequate amounts of all these, and it is the scarcity of water that limits the abundance of desert plants. Desert vegetation usually consists of widely spaced small shrubs. In the North American deserts, for example, there are sagebrush (Artemisia tridentata) with various grasses in parts of Utah and Nevada, and creosote bush (Larrea divaricata subspecies tridentata) with cacti, including saguaro and Joshua trees (Yucca brevifolia) in the Mojave, Sonoran, and Chihuahuan Deserts. These plants are the organisms

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that form the biological basis of the North American desert ecosystems.

Producers and Consumers Green plants form the biological basis of the desert (or any other) ecosystem, because it is only green plants (and some bacteria) that can convert the raw ingredients of water and carbon dioxide into carbohydrates (see “Photosynthesis” on pages 96–101) and simple chemical compounds into proteins. Animals must obtain the nutrients they require by consuming plants. Consequently, green plants are described ecologically as producers, and all animals are consumers. The total quantity of organisms can be measured as their dry weight per unit area. This is known as their biomass, and it is reported scientifically in grams per square meter (g/m2; 1 g/m2 = 0.005 ounce/foot2). In a desert the total plant, or producer, biomass is commonly 4.7–9.3 ounces per square foot (1,000–2,000 g/m2). This is between 3 percent and 7 percent of the biomass of a temperate broad-leaved forest, and 3.7–7.4 ounces per square foot (800–1600 g/m2) of the desert producer biomass is belowground in the form of roots and rhizomes. There is little vegetable matter above the ground for the animals—consumers—that rely on it for food, and much of the vegetation that is within their reach is poisonous or covered with spines or thorns to prevent them from eating it. It is not surprising, therefore, that a traveler crossing a desert sees scattered plants but may see no animals at all. The vegetation that is visible aboveground may amount to 0.9–1.8 ounces per square foot (200–400 g/m2), but this is not the quantity of food that is available to animals. If the consumers ate all of that, no plants would be left and the animals would starve. Consumers must rely not on the plant biomass, but on the primary, or plant, productivity. The primary productivity of an ecosystem is the amount of material that plant growth adds to the biomass each year. Plants use up in respiration some of the carbohydrates they make by photosynthesis. Deduct this, and the result is the net primary productivity (NPP). NPP is the maximum amount of food available for animals. In deserts the annual NPP aboveground is usually 0.1–0.5 ounce per square foot (30–100 g/m2), and the belowground NPP adds a further 0.9–1 ounce per square foot (200–220 g/m2). This is the potential food supply for animals in the form of leaves, flowers, stems, fruits, seeds, and roots, but the actual food supply is smaller. Some plants and parts of plants are inedible, and a proportion of the material falls from the plants and is lost. Herbivorous animals are known as primary consumers. Their biomass can be measured in the same way, and it is approximately one-tenth of the plant biomass. The primary consumers feed in many different ways, but when

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all the mammals, birds, insects, other herbivorous invertebrates, and herbivorous species that live belowground are included, the total biomass of primary consumers cannot be more than about 0.11–0.14 ounce per square foot (23–32 g/m2). Carnivores, which feed on herbivores, are secondary consumers, and the same rule applies to them. Herbivores use most of the food they eat to provide them with the energy they need to live. Birds and mammals eat almost every day, but once they attain their adult size they grow no larger because all the food they eat is used in maintaining their body temperature, moving around, reproducing, and repairing and renewing body cells. Only about one-tenth of the food they eat becomes part of their bodies and available to meat-eaters. Consequently, the insectivorous lizards and birds and the snakes and cats that prey on small rodents and larger grazing herds can have a total biomass of no more than about 0.011–0.014 ounce per square foot (2.3–3.2 g/m2). Some snakes feed mainly on other snakes. The secretary bird (Sagittarius serpentarius) of Africa and roadrunner (Geococcyx californianus) of California eat lizards and snakes. These carnivore-eating carnivores are top predators, or tertiary consumers, and the rule still applies. There can be no more than about 0.0011 ounce of them per square foot (0.2–0.3 g/m2). The arithmetic explains why animals are scarce. Obviously, there are large animals living in deserts, but each of them needs a very large area within which to find enough food to survive. A pride of lions living in the desert needs up to 155 square miles (400 km2) to supply it with sufficient food.

Feeding Pyramids These relationships can be illustrated by a type of diagram known, because of its shape, as an ecological pyramid, or sometimes as an Eltonian pyramid, after Sir Charles Sutherland Elton (1900–91), the British zoologist and ecologist who invented it. There are three types. The pyramid of numbers shows the number of organisms at each level. This is not very satisfactory because it takes no account of the size of individuals. If the producers are herbs, for example, there will be many more of them than there would be if they were trees, but the NPP would not necessarily be any greater. The pyramid of biomass solves this difficulty by measuring the biomass at each level. This, too, runs into difficulties in some ecosystems, however, because wide seasonal fluctuations distort it, so it is necessary to know the time of year to which it refers. The most useful version is the pyramid of energy. This converts biomass into the energy it represents. Instead of counting or weighing all the plants, the ecologist measures the amount of the

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tertiary consumers (top predators) secondary consumers (carnivores) primary consumers (herbivores) producers (plants)

Ecological pyramid

Sun’s energy the plants captured when they made carbohydrates by photosynthesis. The higher levels then measure the amount of that captured solar energy that passes to consumers. Ecologists measure the amount of energy in the biomass by burning the material and measuring the energy released. They do not need to do this each time, of course, because other scientists have already performed the measurements, and the results for different plant and animal materials are published. All three pyramids look very similar. Each level is represented by a band. All the bands are of the same thickness, but they vary in width. The lowest band, or step, represents the producers, and the bands above it represent the several levels of consumers. The drawing shows what an ecological pyramid looks like, although it is not practicable to draw the steps to scale. Suppose the bottom level, of producers, is 10 inches (25 cm) wide and a little too big to fit on a page of this book. The second step, of primary consumers, would then have to be about one inch (2.5 cm) wide, the third step, of secondary consumer, 0.1 inch (0.25 cm) wide, and the fourth step, of tertiary consumers, 0.01 inch (0.025 cm) wide—which would be difficult to print and to see! Ecological pyramids demonstrate graphically why large predators, such as lions, are uncommon.

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Scavengers and Detritivores All plants and animals produce wastes, eventually every individual dies, and wastes and dead materials provide food for another pyramid of organisms. Vultures scavenge dead animals, dung beetles use animal dung, and snails and slugs as well as ants and other insects feed on leaves. Fungi and bacteria feed on fallen twigs, branches, and whole trees, opening up the structure of the wood and allowing insects and other invertebrate animals to enter. Dead matter lying on the ground is called detritus, and animals that feed on it are detritivores. Scavengers and detritivores break up and scatter detritus, and, of course, they produce detritus of their own. Soil fungi and bacteria finally break down the remaining organic material, releasing it in the form of simple chemical compounds that dissolve in water and can be absorbed into plant roots. There is, therefore, a second set of organisms through which food passes, and it can also be illustrated as a pyramid. In this case the layer of detritus takes the place of the producers, and all the scavengers and detritivores are consumers. The detritus they produce provides food for secondary consumers, which include some of the same organisms as were included as primary consumers, complicating the pyramid. At each level there are predators such as centipedes, scorpions, spiders, and certain microorganisms preying upon the scavengers and detritivores.

Food Chains and Food Webs Useful though they are, the pyramids do not show the complexity of feeding relationships among the members of an ecosystem. Indeed, these are so intricate and so flexible that it is almost impossible to illustrate them at all. When they are shown, it is usually as a food chain or a food web. That is how the relationships are often illustrated in ecology textbooks. A food chain simply shows links between organisms at each level of an ecological pyramid. Grass → gazelle → lion is an example. The arrows indicate that gazelles eat grass and lions eat gazelles, so food and energy pass from grass to gazelles to lions. The difficulty with it is that gazelles are not the only herbivores that eat grass. As well as grass, gazelles eat leaves on the lower branches of trees, lions eat animals other than gazelles, and the same applies to all the organisms in any such chain. Link animals by arrows to all of their sources of food, and the result is a food web. This is a much more complicated diagram. A food web diagram allows for the fact that animals eat more than one type of food. It gives an impression of the extent to which the members of an ecosystem are linked to and dependent on each other, but its drawback is that it is static. Most animals eat differently at different

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times of year, according to the food that is available, and some animals spend only part of their time in the ecosystem—they come and go. Because they can reflect nothing of this dynamism, food web diagrams can easily mislead. Unfortunately, any attempt to construct a complete food web diagram showing all the relationships, the way they change over the year, and the relative importance of each would generate a diagram as complex as the real ecosystem it represented. Ecosystems, even relatively simple ones like the desert ecosystem, are extremely complicated and constantly changing. Ecologists spend their time exploring the relationships among organisms and analyze the information they discover by direct observation in order to understand how ecosystems function. They conduct experiments to produce data they interpret statistically, and they use computers to construct mathematical models of the way ecosystems function. From their studies scientists try to predict how ecosystems are likely to respond to changes imposed from outside.

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ANIMAL LIFE OF THE ARCTIC

Conditions are harsh in the cold desert of the Arctic. Lowlatitude deserts are dry, but at least there is soil on their surfaces in which plants can find nutrients whenever there is a little moisture. Beyond the limit of the Arctic tundra, the desert of the far north is covered with ice and snow for most of the year and in many places for the whole year. Snow and ice contain no nutrients for plants. Nothing can grow on their surface. Farther south the tundra is more densely populated. Tundra animals feed on the low-growing trees and shrubs, sedges, grasses, mosses, and lichens. Yet even in the bitter cold of the high Arctic there are places the ice fails to reach. There, in places sheltered from the wind and thus from the dry snow blown by the wind, lichens survive as well as mosses and a few other low plants that provide a source of food for animals. Even so, the supply is uncertain, and animal populations fluctuate dramatically. The sea is much more plentiful. Life there is abundant.

Do Lemmings Commit Mass Suicide? It is not true that every so often lemmings commit mass suicide by throwing themselves into the sea. What is true is that from time to time Norway lemmings (Lemmus lemmus), one of the nine species of lemmings, undertake mass migrations in the course of which they may panic and rush into a lake or the sea, where many of them drown. Migrations take place at intervals of three to four years, and not all migrations lead to such heavy mortality—it is largely a matter of luck.

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Lemmings are small rodents, closely related to voles in the subfamily Microtinae of the family of rats and mice (Muridae). One species, the wood lemming (Myopus schisticolor), inhabits the northern coniferous forest—the taiga. All the others live in the Arctic tundra. The arctic, or collared, lemming (Dicrostonyx torquatus) grows a white coat in winter, but all the other species remain the same brown color throughout the year. All lemmings and voles undergo large changes in population size on an approximately three-to-four year cycle, though not all of them do so throughout the whole of their range. Brown lemmings (Lemmus sibericus) live in North America, and their population density was measured over a 20-year period at Barrow, Alaska. This varied from fewer than four lemmings per acre (10/ha) to peaks of about 61 per acre (150/ha), and in one year (1960) the density rose to 91 per acre (225/ha). Brown lemmings share their habitat with arctic lemmings, but arctic lemmings are uncommon, with seldom more than about one to every one to 25 acres (10 ha).

Population Fluctuations No one is quite sure why lemming populations fluctuate so widely. Over large areas of the Arctic the ground surface consists of islands averaging 40 feet (12 m) across surrounded by narrow trenches that form above wedges of ice. Lemmings live mainly in the trenches. They spend the winter beneath the snow in the grass and sedges that grow on the bottom of their trenches, and it is there that the young are born. In some years the birth rate is especially high, and by the time the snow starts to melt, the trenches begin to flood. Water forces the animals aboveground, where their food supply is already depleted. The lemmings scatter in search of food, and predators kill many of them. Members of several species migrate to new areas. In those years lemming numbers fall drastically during the spring and summer and take several years to recover. Norway lemmings differ in that they seem to migrate when food is abundant, not when it is scarce. Extremely quarrelsome animals, they are usually solitary, but when their numbers increase, sometimes to more than 121 per acre (300/ha), encounters become unavoidable. Older and stronger lemmings drive away the younger and weaker animals. The outcasts move down from the Scandinavian mountains, where they ordinarily live. They wander in all directions, but eventually natural barriers, such as rivers and lakeshores, force them together. This is how they form small groups that merge eventually into vast crowds, and it is then that the close proximity of so many mutually hostile animals triggers the panic that sends them rushing headlong into forests, onto farms, across glaciers—and sometimes into lakes or the sea. Norway lemmings swim

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well, but the distance across a large lake or the Baltic Sea is too great for these small animals, and they drown. No one witnesses the catastrophe, but some time later millions of bodies are found washed up on the shore. An old Scandinavian belief held that lemmings fall from the clouds during storms. Some Inuit peoples also believe lemmings fall from the sky. Lemmings undergo the most dramatic population fluctuations, but all the Arctic rodents experience them, their numbers varying according to the amount of food they can find. When growing conditions are favorable and plants produce abundant foliage and seeds, rodent populations increase. The number of rodents continues to grow for a few years, until a year when the weather is poor reduces the amount of vegetation. Then large numbers of rodents starve, and their populations fall rapidly. Fluctuations in the rodent population affect all the predators that depend on rodents. Their numbers increase and decrease in response, but in some cases only in the Arctic. The predators do not die out but simply move farther south in search of food. Along with the arctic ground squirrel (Spermophilus parryi), lemmings and voles are the only small herbivorous mammals of the Arctic desert, and the squirrel is the only Arctic mammal that hibernates (see “Estivation and Hibernation” on pages 132–136). Hibernation is safe only if there are nesting sites where the temperature remains a little above freezing, and there are few such refuges in the Arctic.

Animals That Hunt Small Rodents As the most abundant primary consumers (see “Producers and Consumers” on pages 159–160), lemmings and voles represent the food supply for a much larger number of carnivorous species (secondary consumers). Some carnivores even pursue them belowground. With their short legs and slender bodies that are no more than eight to nine inches (20–23 cm) long, weasels (Mustela nivalis) and ermines, or stoats (M. erminea), move easily along the narrow tunnels rodents make beneath the snow. Snowy owls (Nyctea scandiaca) also hunt small rodents. These are large birds, up to 26 inches (66 cm) long, white with dark bars or flecks and long feathers covering their feet. Unlike most owls, they hunt mainly by day. When the rodent populations crash, snowy owls move away in search of food, occasionally traveling as far south as the Great Lakes and Scotland.

Ptarmigans Apart from snow owls, ptarmigans are the only other birds that live in the Arctic all year round, and, like snowy owls, even they head south in years when food is scarce. Ptarmigans are

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Biology of Deserts ground-nesting birds related to grouse (family Tetraonidae). They occur throughout northern Canada and Eurasia and feed on berries and other plant material. There are three species, the white-tailed (Lagopus leucurus), willow (L. lagopus), and rock (L. mutus) ptarmigans. All three molt their brown plumage in the fall and grow pure white feathers to replace it. Other birds are summer visitors, but no migrating species can be relied on to be present every year. Ravens (Corvus corax) are the most regular, traveling to the coast of Greenland and occurring throughout the whole of the American and Eurasian Arctic.

Cranes Permafrost is a layer of soil beneath the ground surface that remains frozen throughout the year. In summer, when the active layer of ground above the permafrost thaws, the surface becomes marshy over parts of the tundra, and there are birds that arrive to nest in the marshes. These include two species of cranes, the sandhill crane (Grus canadensis) and the Siberian, or great white, crane (G. leucogeranus). Cranes are large, long-legged birds that feed on leaves, seeds, and small animals up to the size of lemmings and voles. The sandhill crane breeds in parts of western North America. It is brownish–gray in color. As its alternative name suggests, the Siberian crane is white with black wing tips. It has a patch of bare red skin on the front of its face and a red bill and legs. The Siberian crane is more than 4.5 feet (1.27 m) tall and has a wingspan of 6.5 to eight feet (2–2.4 m). It is a handsome bird and nowadays extremely rare, with a global population of only about 3,000 birds. It breeds in two parts of Siberia: in Yakutia to the west of the Lena River and beside the Ob River, more than 2,000 miles (3,200 km) to the west. The reduction in their population is linked to the discovery of oil in Yakutia and loss of habitat in some of the places where they stop to rest on their migrations. The birds arrive at their breeding grounds toward the end of May, when the ground has started to thaw but is still covered with snow. They build their nests on the ground, and the female lays and incubates two eggs while the male stands guard. A male Siberian crane is more than a match for a hungry arctic fox. The cranes are wary of humans, however, and will take flight if anyone approaches closer than about 900 feet (275 m), so they are difficult to study. In late September the cranes set off on their journey south. Cranes fly higher than do most birds, and people living on the steppes and farmlands of Russia used to watch as flocks of Siberian cranes passed high overhead uttering their wild calls. The flight of the cranes was a harbinger of spring. The birds fly all the way to China, and some of the western population travel to India, flying at more than 30,000 feet (9,150 m) across the Himalayas.

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Seabirds Farther north, in the icy wastes that lie beyond the tundra, huge, noisy breeding colonies of seabirds congregate in summer on rocky coasts. Some build their nests on ledges on the faces of cliffs, others build on more level ground, and all of them feed on fish. The Arctic birds include the only completely white gull, the ivory gull (Pagophila eburnea). Unlike other gulls, it rarely swims, but it can run fast. Black-legged kittiwakes (Rissa tridactyla) are oceandwelling gulls that come ashore only to breed. They nest on rocky cliffs throughout the Arctic, gathering in colonies containing many thousands of birds. As well as gulls, there are auks. These are birds that come ashore only to breed, spending the remainder of the year far out at sea. They include the little auk, also known as the dovekie (Alle alle), which is less than eight inches (20 cm) long, and the razorbill (Alca torda), which is about twice that size. Guillemots are also auks, and there are several species, including the guillemot, or common murre (Uria aalge); Brünnich’s guillemot, or thick-lipped murre (U. lomvia); and the black guillemot (Cepphus grylle). In the west there is also the Atlantic puffin (Fratercula arctica), a bird about 12 inches (30 cm) long with a large, broad bill that grows larger and more brightly colored during the mating season. Puffins nest in burrows and feed on fish. Two other puffins breed along the Arctic coast of the Pacific: the horned puffin (F. corniculata) and tufted puffin (Lunda cirrhata). Ducks such as the eider duck (Somateria mollisima) also live along Arctic coasts. Inevitably, the large breeding colonies attract predators. The great black-backed gull (Larus marinus) and glaucous gull (L. hyperboreas) are the most voracious. They will kill adults or chicks. There are also thieves. The common gull (L. canus) and three species of jaegers, also called skuas (Stercorarius species), harass birds carrying food to their young, forcing them to drop it and then swooping down to catch it in midair.

Arctic Foxes Arctic foxes (Alopex lagopus) take eggs and hunt birds as well as rodents, and they are found throughout the tundra and Arctic in North America and Eurasia. They are smaller than red foxes, measuring about 21 inches (53 cm) with a 12-inch (30-cm) tail, but look bigger because of their long fur—in relation to their body size, it is the longest fur of any Arctic mammal. The fur turns pure white in winter, and in the southern part of the fox’s range there is a variant that remains a blue-gray color all year round. White and blue fox fur has been very valuable, and the animals have been extensively hunted. Detailed records of

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the numbers killed have allowed scientists to trace fluctuations in arctic fox populations. These follow those of the lemmings and voles, which show how dependent the foxes are on the rodents.

Musk Oxen Musk oxen (Ovibos moschatus) are even more superbly adapted than foxes to the harsh Arctic climate. Although they are hoofed animals related to cattle, musk oxen are not true oxen but goat-antelopes, a subfamily (Caprinae) of 26 species that have adapted to extreme environments. Musk oxen are big animals, about 7 feet (2 m) long, protected by a thick, waterproof undercoat covered by an outer coat of long, coarse hairs that reach almost to the ground. This provides such good insulation that snow falling on a musk ox’s back does not melt. The very broad feet of a musk ox distribute its weight, making it easier for the animal to move over soft snow, an adaptation that parallels that of the camel (see “Broad Feet, Loose Limbs” on page 141). Musk oxen feed mainly on grass but will also eat lichens, mosses, and fallen leaves, and they dig through the snow to find food. Both sexes have big, very solid horns. These almost meet at the center of the forehead to form a plate about nine inches (23 cm) thick across the front of the head, from which they curve downward and outward, turning up at the ends like a pair of sharp hooks. These are the formidable weapons with which musk oxen defend themselves against wolves, their principal predators. Musk oxen move in herds of up to 100 animals, although young bulls driven out by older bulls during the mating season may live alone or in small bachelor herds. When wolves threaten, the adult musk oxen form a circle facing outward with their heads down, with the calves inside. The wolves face a circle that is constantly turning to present them with the horns of the biggest and strongest oxen. A wolf that comes within their reach is likely to be impaled and tossed. If it falls inside the circle it will be trampled to death. Despite their defenses, musk oxen became extinct in Eurasia about 3,000 years ago, and by this century they survived only in Canada and Greenland. From there they have now been successfully reintroduced in Alaska, Norway, and Siberia.

Caribou Caribou, known in Europe as reindeer (Rangifer tarandus), also have broad, “snowshoe” feet, and they, too, are hunted by wolves. They are small deer, males standing about four feet (1.2 m) tall at the shoulder and females being somewhat smaller. They are the only deer in which both sexes have antlers. Their bodies are as well insulated as those of musk oxen: Snow falling on their thick coats does not melt.

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There are several populations of caribou, sometimes classified as subspecies, and one of them, the barren ground caribou (R. t. groenlandicus), undertakes long seasonal migrations. Barren ground caribou live in northern Alaska and Canada and in western Greenland. In spring they assemble in large herds and move northward, always following the same routes and traveling 1,000 miles (1,600 km) or more. Some reach the shores of the Arctic Ocean while others stop inland, and they spend the summer grazing the tundra vegetation. That is where their young are born in May and June. In September or October they return south, heading for the food and shelter of the pine forests. In winter caribou and Eurasian reindeer use their broad feet to shovel away the snow in search of food. The word caribou means “shoveller” in the language of the Micmac peoples of northeastern Canada. Their scraping exposes reindeer moss, which is their principal winter food. This is not a moss but a lichen (Cladonia rangiferina and other Cladonia species) with shrublike branches about three inches (7.5 cm) tall.

Saiga When scientists first tried to classify the saiga (Saiga tatarica) they decided it was a goat. Later they changed their minds and identified it as an antelope. Then it became a gazelle. Today scientists consider it is a goat–antelope, related to both the goat and the antelope, and it is placed in the subfamily Antilopinae of the family Bovidae. Clearly, its identity is not at all obvious. The classification difficulty arises partly from the saiga’s face and, in particular, its humped, fleshy nose, which resembles a short trunk or the nose of a tapir and can be moved about like an elephant’s trunk. But inside, the structure of the nose is much more complicated. It contains intricately arranged bones, hairs, and glands that secrete mucus. During summer, when herds of saiga are migrating and the weather is hot and dusty, the nose filters out the dust before inhaled air enters the respiratory passages. In winter air is warmed and moistened as it passes through the long nose, so the animal does not damage the delicate tissues of its lungs by inhaling bitterly cold, very dry air. In common with many animals that live in strongly seasonal climates, the saiga has a thin summer coat and a long, thick winter coat. Its winter coat adds to its peculiar appearance by making its body look much too bulky for its thin legs. The saiga is adapted to a dry climate and is unable to walk through snow that is more than about 16 inches (40 cm) deep. Its legs can be injured if its feet pierce a layer of ice covering snow. Saiga feed on shrubs, grass, herbs, and lichens. Saiga live on the treeless steppes and semideserts from the Volga River eastward to central Asia. They are nomads, constantly on the move in search of pasture. In summer they migrate northward, returning to the south for the win-

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Saving the Saiga Male saiga have horns. These are eight to 10 inches (20–25 cm) long, are slightly curved, and have ridged rings along them. It was its horns that brought the saiga close to extinction. Powdered saiga horn is used in Chinese medicine, and for many years the animals were hunted for their horns. This greatly reduced the number of saiga, and by 1917 very few remained. In 1920 the saiga was given complete protection from hunting, and slowly it recovered. Today there are more than 1 million saiga. The harsh environment imposes its own dangers, and, in common with most animals that live in cold deserts, from time to time its population decreases sharply. The saiga has responded to the environmental challenge in two ways: behavioral and physiological. Its nomadic way of life allows the saiga to travel long distances in search of food; that is its behavioral response. Its young mature very quickly; that is its physiological response. Females become sexually mature when they are eight months old and males when they are 20 months old. Rapid maturation makes it possible for the population to recover fairly rapidly following a period during which many animals die. Despite its recovery, poaching has intensified in recent years, and the saiga is not yet out of danger. The species as a whole is officially classed as vulnerable, which means that its numbers are falling, the area it occupies is decreasing, and there is at least a 10 percent chance that without protection it will become extinct within the next 100 years. The Mongolian saiga (S. t. mongolica), which is one of the two subspecies of saiga, is classed as endangered. This means there is at least a 20 percent chance that it will become extinct within 20 years.

ter. During their migrations the herds cover 50–70 miles (80–113 km) a day, walking with their heads close to the ground. Saiga can run fast. Over a short distance they can attain 50 MPH (80 km/h). In late April the males set off on the migration ahead of the females, moving in groups of up to 2,000 animals. Then all the females give birth within about one week. Between eight and 10 days later the young are ready to move, and the mothers and their young commence their migration, mov-

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ing in herds of 100,000 or more individuals. Herds of 200,000 have sometimes been seen. When they reach the summer grazing grounds the large herds disperse into smaller groups of between 30 and 40 individuals. In autumn the big herds assemble once again for the long journey south. The saiga’s diet and way of life are similar to those of the caribou. The saiga is smaller than most antelopes and even smaller than some sheep. An adult stands two to 2.6 feet (0.6–0.8 m) tall at the shoulder and weighs 46–112 pounds (21–51 kg). Although there are now large herds of saiga, early in the 20th century they came close to extinction, and they continue to be at risk (see the sidebar).

Wolves As the herds of migrating caribou move slowly across the tundra, the wolves are never far away. Caribou are an important source of food for North American populations of the tundra wolf (Canis lupus tundarum). The wolves of Asia follow the migrations of the saiga (Saiga tatarica) as it migrates across the cold, dry steppe. The tundra wolf is one of several subspecies of the gray, or timber, wolf (Canis lupus). It occurs throughout the Arctic and subarctic tundra, and it is the largest of all wolves—up to five feet (1.5 m) long with a tail about 20 inches (50 cm) long. Its long coat is pale in color and often white, although tundra wolves can be brown or even black. All wolves catch ground-nesting birds, hares and other small mammals, and they also eat berries. They will eat carrion, will scavenge from trashcans, and, given the opportunity, will attack domestic animals. Where these foods are available wolves hunt alone, but a single wolf will not attack an animal as large as a caribou. For this wolves hunt in packs, and if a single hungry wolf comes across caribou it will howl to summon other members of the pack to join in the pursuit and share the resulting food.

Social Life of Wolves—and Domesticated Dogs Domesticated dogs are descended from wolves, and they have inherited from their ancestors the social behaviors that allow them to fit so easily into human society. Human and dog societies are very similar. Wolves often mate for life, and wolf society is based on an adult male and female together with their offspring. Cubs are born in early spring, and, provided they are well fed, the young wolves are big and strong enough to travel with the pack by the time they are about four months old. Some of the young leave the pack during the following winter, but others remain, so the pack is an extended family. The young animals act as babysitters, helping to

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raise the next litter of their brothers and sisters. The size of a wolf pack depends on the type of prey on which its members depend. A typical pack of tundra wolves numbers about 10 animals. Living closely together, with a central area in which the cubs are born and to which the pack returns regularly, wolves are constantly interacting with one another. Each individual has a clearly defined status, and there is one dominant male and one dominant female. Usually these are the only animals that mate, and fights can occur when other wolves contest their right to do so. Other pack members sometimes seek to improve their social status by challenging the wolf immediately dominant to them. Young cubs do this to establish their rank among their siblings. However, most of the time life is peaceful, and the wolves use gesture and body language to maintain the hierarchy. A pack occupies a territory of 40–400 square miles (104–1,040 km2), the size varying according to the availability of food, and the dominant male and female mark out its boundaries with urine. There is some overlap with the territories of adjacent packs, and a pack will fight any strange individual or pack that it meets. Fights are dangerous, and wolves do not seek confrontation unless they urgently need access to more resources. From time to time, therefore, a wolf howls, and the rest of the pack joins in. This advertises the pack’s presence and allows any other pack in the area to avoid it. Sometimes another pack will howl in reply. Wolves do not howl very often, however, for fear of attracting a rival pack that is seeking a fight that will allow it to take over the territory. In general, a pack will move away quietly if it hears howling unless it is feeding at a recent kill or has young cubs traveling with it. These it will defend, especially if the pack is a fairly large one. Solitary wolves also howl as a way of keeping in touch with other pack members when they are all hunting alone. On the other hand, a wolf that has left the pack into which it was born and that is living alone very rarely howls or makes scent marks. It wanders in search of a mate with which it can start its own pack, but it is wary of drawing attention to itself.

Wolverines Wolves are not the only large predators of the Arctic. There is also the wolverine (Gulo gulo), a member of the weasel family (Mustelidae). Its head and body measure about three feet (0.9 m) long, its tail is about eight inches (20 cm) long, and it is heavily built, with a long, dark coat and long muzzle that make it look a little like a small bear. It lives mainly on the ground, but wolverines can and do climb trees. A wolverine is a fierce hunter and would have no difficulty defending itself against a solitary wolf, although

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grizzly bears and packs of wolves sometimes kill wolverines. In some countries wolverines are hunted for their fur, for sport, or because they are regarded as pests that prey on farm animals. Wolverines are misunderstood animals. Their hunting prowess allows them to kill animals that are far too big to be eaten in one meal, so wolverines store food. This has given them an undeserved reputation for gluttony, and the wolverine is sometimes known as the “glutton” (see the sidebar).

The “Glutton” For many years the wolverine was seriously misunderstood. It was characterized as the “glutton” because naturalists believed it ate until it could eat no more, then squeezed itself through a narrow space to push the food out of its stomach so it could continue eating until all the food was consumed. This is obviously nonsense, but it is true that the wolverine will eat berries, carcasses of animals killed by other predators, birds, and small mammals, and in winter it hunts caribou. It is quite capable of killing an animal many times its own size. The wolverine has broad feet, and, like several other mustelids (and humans), it walks on the soles of them—this is said to be a plantigrade gait. Its feet end in claws, and the plantigrade gait keeps these clear of the ground so they remain strong and sharp. Caribou also have broad feet, but the caribou is much the heavier animal. The weight pressing on each of its feet is greater, and they sink deeper into soft snow than do those of a wolverine. On a hard surface the caribou can outrun a wolverine, but the wolverine is faster over soft snow. It kills an animal much larger than itself by jumping on its back and holding on with its claws until the prey tires and falls. A wolverine cannot eat prey that is much bigger than itself in a single meal, but in the Arctic desert no animal can afford to waste food. Instead, it exploits a climatic advantage: Food keeps well in subzero temperatures. Having eaten its fill, the wolverine tears what is left of its kill into fragments—its jaws can break even large bones—and hides each piece in a different place. Perhaps it was this somewhat gory activity that gave rise to the myth of gluttony. A female wolverine has been seen to return to one of her stores months later to feed herself and her kits.

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Biology of Deserts Wolverines live in the remotest regions and are generally solitary, although through the summer and fall females are accompanied by their young, called kits. Females mark out and defend large territories. Males have smaller territories that overlap those of two or three females with which they mate. Mating takes place in summer, but the fertilized eggs are not implanted until they have undergone a period of dormancy in the womb.

Polar Bear Both the wolf and wolverine are powerful and efficient hunters, but the polar bear (Ursus maritimus) is the most impressive of all the Arctic predators. It is also the largest of all land-dwelling carnivores. A male can be almost 10 feet (3 m) long and weigh 1,400 pounds (635 kg) or more, and females are only slightly smaller. A polar bear can outrun a caribou over a short distance. Caribou and musk oxen form part of its diet as well as smaller mammals such as hares and lemmings, birds, and some plant matter. It will scavenge carcasses of walrus and whales, but it feeds mainly on seals. Sometimes it catches them by waiting beside a breathing hole or a small area of open water for a seal to surface, then seizes it and drags it onto the ice with tremendous force. At other times, camouflaged by its white coat, it will stalk a seal that is resting on land or an ice floe. Seals can outmaneuver it in the water, so polar bears hunt only on the surface. Polar bears occur throughout the Arctic. In the course of a year an individual will travel more than 600 miles (965 km) in search of food, but most polar bears remain within their own geographic region. They seldom move far from the edge of the sea ice, which is where they find seals, and they are most likely to be found where the ice is kept constantly moving by the wind and sea currents. In summer, when the sea ice retreats, the bears move southward with it and move northward again as winter approaches. They are strong swimmers and can spend hours in the water crossing from one ice floe to another. Their creamy white fur, thicker in winter than in summer, is completely waterproof, and beneath it thick body fat adds a further layer of insulation. Its fur covers the whole of its body except for its nose and the pads of its feet, and its feet are partly webbed. Its small ears also help to conserve body warmth, but such a large animal is well able to remain warm even while bathing in the Arctic Ocean (see the sidebar “Why Small Animals Tolerate Heat and Large Animals Tolerate Cold” on page 130). Most of the time polar bears are solitary, although they sometimes congregate around a large source of food or when the melting of the sea ice forces them onto land. Encounters are usually peaceful at such times, but during the mating season males will fight rivals. Mating takes place

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in early summer, but implantation of the fertilized eggs is delayed, so the gestation period is long—195–265 days. In November and December pregnant females choose where their young will be born and dig out maternity chambers in the deep snow. Cubs are born in December and January and remain with their mothers in the maternity chambers until March or April, being fed on milk that contains 31 percent fat. Their mothers do not eat, relying instead on body fat they accumulated the previous summer. The family stays together until the second summer and occasionally through the second winter.

ANIMAL LIFE OF THE ANTARCTIC

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Visitors to Antarctica will not encounter lemmings or hares or the land-dwelling predatory mammals that hunt them. Drake Passage, the stretch of ocean separating the southernmost tip of South America from the northernmost tip of the Antarctic Peninsula, is about 600 miles (965 km) wide at its narrowest point. That is the shortest distance between Antarctica and any other large landmass, so the continent is isolated behind an ocean barrier too wide for animals to cross. Were any such animal to survive the crossing—carried on a piece of floating wood or ice, perhaps—it would quickly perish in the harsh Antarctic environment. The mainland climate is much more severe than that of the Arctic (see “Why Antarctica Is Colder Than the Arctic” on page 38), and there are no plants bigger than mosses, algae, and lichens—and even those can be found in only a few places. The only native animals are somewhat more than 100 species of invertebrates, half of which are parasites of birds or seals. Mites are the most abundant because they are the most tolerant of the extreme conditions. These are arachnids belonging to the order Acarina. Some are parasites found in the nostrils of seals and penguins, but many of them feed on the algae and lichens or on microscopically small soil organisms. There are also predacious mites that eat other mites or springtails. One mite species, Nanorchestes antarcticus, has been found at 85.53°S at an elevation of 7,365 feet (2,245 m). There are also ticks, lice, and one species of flea (Glaciopsyllus antarcticus), all of which are parasites. The flea lives in the nests of the southern, or silver-gray, fulmar (Fulmarus glacialoides) and snow petrel (Pagodroma nivea). When the breeding season ends and the birds depart, the fleas hibernate until they return. Where there is soil there are springtails, tiny insects of the order Collembola. These are locally plentiful, but of about 2,000 species that occur in the world as a whole, only about 20 are found in Antarctica.

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Sheathbills An ocean crossing presents no difficulty for seabirds, however, and more than 40 species breed in Antarctica. There is even a land bird, the Antarctic pipit (Anthus antarcticus), that breeds in South Georgia and feeds on small invertebrate animals. Sheathbills, comprising the family Chionididae, also breed on Antarctic islands. Sheathbills are thought to be related to both gulls and landdwelling birds, so they form an evolutionary link between the two groups. There are two species. The black-faced sheathbill (Chionis minor) lives all year round on some of the offshore islands. The snowy sheathbill (C. alba) breeds on the Antarctic mainland but migrates in winter to the Falkland Islands (Las Malvinas) and Argentina. Both have pure white plumage and a covering resembling a chicken’s comb over the base of the bill, giving them their common name. Both species live in the same way, running about on the ground, flying only when they must, and feeding on anything they can find. They scavenge around the research stations and grow fat on carrion from stillborn seal pups, and when they need extra food for their young they harass penguins. A pair of sheathbills will establish a territory that they defend vigorously. The territory contains a number of penguin nests and their occupants, and the sheathbills’ aim is to rob the penguin parents of the food—mainly krill (see “Krill” on page 171)—they bring to their chicks. They will also steal penguin eggs and eat penguin droppings.

Penguins Penguins are the most famous Antarctic natives, of course, although they are not confined to Antarctica. There are species, including the little blue, or fairy, penguin (Eudyptula minor), that breed around the shores of South Island, New Zealand, southern Australia, South Africa, and along the western coast of South America all the way to the equator, but penguins do not occur in the Northern Hemisphere. Penguin is the name that was given to the great auk (Pinguinus impennis) in the 16th century. It was a black and white seabird of the North Atlantic that is now extinct. On land it had an upright stance and waddling gait, and when European explorers found similar birds in the Southern Hemisphere the name was also applied to them. There are 18 species of penguins in six genera, constituting the only family (Spheniscidae) in the order Sphenisciformes, and they are instantly recognizable. Species vary in size and detailed markings, but all of them are flightless birds that stand erect, walk with a waddling gait on short legs, and as adults have dark blue or more commonly black backs and wings and white fronts. They range in size from the emperor penguin (Aptenodytes forsteri), standing about four feet (1.2 m) tall and weighing up to 70

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pounds (32 kg), which is the biggest, to the little blue penguin, which is barely 16 inches (41 cm) tall.

How Penguins Keep Warm Penguins are superbly adapted to life in a cold climate. Although some species live far from Antarctica, the coasts and islands they inhabit are bathed by cool currents—the Peru Current that flows along the South American coast, the Benguela Current affecting southern Africa, and the West Wind Drift that passes southern Australia and New Zealand. Penguins do not enter warm tropical waters, and only two species, the Galápagos (Spheniscus mendiculus) and Humboldt (S. humboldti) penguins, breed in tropical latitudes. Most species occur between latitudes 45°S and 60°S. Penguins are aquatic birds, so in addition to tolerating the low air temperatures of the Antarctic, they must also remain active in water that is close to freezing. How do they do it? The ancestors of penguins were able to fly, but as well as losing this ability, penguins have evolved highly modified feathers. These are small and almost scalelike and form a dense covering, three layers thick, that is fully waterproof. Beneath the skin most penguins have a thick layer of fat, and an efficient network of blood vessels where the legs and wings join the body prevents the loss of body warmth to the wings and feet (see “Countercurrent Exchange” on page 131). The overall body shape of the bird also helps it conserve heat. Its feet, wings, and head are small in relation to its body, giving the penguin a small surface area in relation to its volume. The tropical species have much larger wings and patches of bare skin on their faces to help them lose excess heat.

Emperor and King Penguins Some species are better than others at coping with extreme cold. The emperor penguin is probably the best of all. It breeds in the fall (May). Males incubate the eggs, carrying them on their feet covered by a fold of skin. Incubation lasts 60 days and takes place on the open sea ice, where the average air temperature is -4°F (-20°C) but where it can fall to -80°F (-62°C), and the wind blows at an average 16 MPH (26 km/h) but often much harder. To help keep warm, the emperors huddle together in groups of up to 5,000 birds, with about 11 birds to every square yard of ice surface (10/m2). Birds exposed to the wind on the outside of the group are constantly moving forward along the sides and then toward the center, so the whole crowd moves slowly downwind. Individuals that are exposed to the wind move back into the shelter of the crowd before they come to any harm. Once the eggs have hatched, both parents feed the young.

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(Catharacta skua), Antarctic, or brown, jaeger (C. antarctica), and South Polar jaeger (C. maccormicki). They are fairly large birds, with an average wingspan of four feet (1.2 m), that feed on fish, krill, and small birds as well as penguin eggs and chicks. Jaegers are also known for kleptoparasitism—harassing flying birds to make them drop food they are carrying. The South Polar jaeger has been seen flying over the South Pole, making it the world’s most southerly bird.

Leopard Seal

Emperor penguins (Aptenodytes forsteri) carry their chicks on their feet, and, like all penguins, they live in large social groups. This male emperor’s chick is clearly visible. (Alastair Rae)

King penguins (A. patagonicus) are close relatives of emperor penguins, and they are also large, about three feet (0.9 m) tall. Like emperors, they build no nests and carry their eggs on their feet while incubating them, but king penguins breed in spring and summer. Other species build simple nests of sticks and stones, and Humboldt and jackass (Spheniscus demersus) penguins nest in burrows or other sheltered places.

Penguin Society On land penguins move by hopping, waddling, or tobogganing on their fronts. In water they swim fast and are highly maneuverable, using their wings for propulsion and their feet as rudders as they “fly” through the water. They feed on fish, squid, cuttlefish, and krill. Adélie (Pygoscelis adeliae), chinstrap (P. antarctica), gentoo (P. papua), and macaroni (Eudyptes chrysoplophus) penguins feed mainly on krill and other crustaceans. Even while they are feeding at sea penguins move in groups, and at breeding time these highly sociable birds form rookeries numbering many thousands of pairs, often of several species but with each species occupying its own area. Parents defend the small area around their nests. Penguins have elaborate courtship rituals and a repertoire of calls and gestures by which birds returning from feeding locate their partners and are recognized by them. Vast congregations of breeding birds attract predators seeking eggs and chicks, and penguins have enemies, the most dangerous of which are the jaegers. The jaeger, which is the German word for “hunter,” is known in Europe as the skua. There are three Antarctic species: the great jaeger

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Penguins are harassed by jaegers during the breeding season, but a much more terrible enemy awaits them in the sea. Fear of that hunter often halts a party of penguins just before they plunge into the water. Leopard seals (Hydrurga leptonyx) eat fish, krill, squid, and other seals, but penguins constitute about one-quarter of their diet. About 10 feet (3 m) long and weighing about 770 pounds (300 kg), the leopard seal is the biggest of the Antarctic seals and the only one that preys on both birds and mammals. Its body is spotted like that of a leopard and is slender, with a long neck. The leopard seal is built for speed, but it is dangerous only while it is in the water. Like all seals, it moves clumsily on land with no help from its hind flippers. Sea lions and walrus pull their hind flippers forward beneath their bodies when on land and use them in walking, but a seal cannot move its hind limbs in this way. It moves on land with support from its front flippers, but its hind flippers are useless. Even penguins have no difficulty escaping from a leopard seal that has hauled itself onto an ice floe. The seal does not bother to try catching them, and penguins ignore it.

Albatrosses Jaegers have been known to fly from Antarctica to Greenland, and the arctic tern (Sterna paradisaea) has the longest migration of any bird, flying between breeding grounds north of the Arctic Circle and Antarctica, a direct distance of 9,300 miles (15,000 km). But the albatrosses are the most aerial of all Antarctic birds. They occur throughout the Southern Hemisphere, and a few of the 13 species are found in the North Pacific, although most live between 45°S and 70°S. Albatrosses come ashore on remote islands only to breed. They spend the rest of their time far out at sea, riding the wind over the Southern Ocean, sleeping on the sea surface, and drinking seawater. Salt glands in their nostrils accumulate and excrete surplus salt. Albatrosses feed on plankton, fish, squid, and crustaceans. They also follow ships, a habit that has given rise to the sailors’ myth that they are birds of ill omen. Albatrosses have a flying technique known as dynamic soaring that allows them to remain airborne for hours with

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minimum effort. A bird glides downwind, gradually losing height as it does so. Its airspeed—the speed at which air flows over its wings—is equal to its forward speed over the sea minus the speed of the wind, because the wind is moving in the same direction as the bird. Friction with the water slows the wind very close to the sea surface, and when the albatross is almost touching the wave tops it turns into the wind. Its airspeed is then equal to the speed of the bird over the water plus the speed of the wind. This accelerates the air flowing over its wings, which increases the amount of lift the wings generate, allowing the albatross to climb. It climbs in this way through the layer of relatively slow wind and enters the faster wind that is not slowed by friction, further increasing its airspeed and allowing it to climb still higher. In this way it climbs to about 50 feet (15 m), turns downwind, and once more starts its slow descent. Albatrosses form the family Diomedeidae in the order Procellariiformes, or tubenoses. The name refers to their nostrils, which are tubular and are located near the base of the bill rather than on top, as in most birds. With a wingspan of about 11.5 feet (3.5 m), the wandering albatross (Diomedea exulans) has the longest wings of any bird.

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LIFE IN POLAR SEAS

Although the ice-covered land is a barren desert, life is abundant in the Arctic and Southern Oceans. The long hours of summer daylight allow the photosynthesizing marine algae to multiply, and the water is rich in dissolved oxygen because the cooler the water, the greater is the amount of oxygen that will dissolve in it. The algae form the producer level (see “Producers and Consumers” on pages 159–160) in a flourishing ecosystem. The season of abundance is short, however. As winter darkness approaches and temperatures fall, most of the algae disappear and with them the small animals that feed on algae and provide food for the larger animals.

Whales Whales are the most spectacular animals of polar seas. There are 79 species of whales and dolphins, forming the order Cetacea, and apart from five species of river dolphins, they roam freely through all the world’s oceans. Many species spend all or part of their lives among the sea ice of the Arctic and Antarctic. Cetaceans (whales and dolphins) are mammals and therefore endotherms, with a core body temperature of 97–99°F (36–37°C). Their bodies are hairless, and cetaceans have an insulating layer of body fat called blubber immediately beneath the skin. Their blubber allows the animals to remain comfortable in water that is sometimes close to

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freezing. The bowhead whale (Balaena mysticetus) has a blubber layer that is up to 20 inches (50 cm) thick. Whales and dolphins live entirely below the sea surface, although they must break the surface in order to breathe, and many of them dive to great depths. A land mammal could not do this, because the pressure of the surrounding water would crush its lungs. Human divers breathe compressed air, allowing them to equalize the pressure in their lungs with that outside the body, but this has the effect of forcing nitrogen from the air to dissolve in bodily fluids. When a diver rises toward the surface and the pressure decreases, the nitrogen comes out of solution as bubbles that cause great pain in the joints—a condition called the bends. A cetacean has small lungs for its body size, and it does not take a deep breath before diving. Instead, it allows the pressure to compress its lungs, forcing the air into the respiratory and nasal passages, from where it is unable to pass directly into the tissues. As the animal surfaces the lungs expand, collecting the used air. This is expelled explosively through the blowhole when the cetacean surfaces. Dolphins leap clear of the surface to breathe, but the much bigger whales breach the surface only with their backs. There are two groups of cetaceans: the toothed whales (suborder Odontoceti) and baleen whales (suborder Mysticeti). The toothed whales include the river dolphins (family Platanistidae), dolphins (family Delphinidae), porpoises (family Phocoenidae), white whales (family Monodontidae), sperm whales (family Physeteridae), and beaked whales (family Ziphiidae). The killer whale (Orcinus orca) is a member of the dolphin family. The baleen whales include the families Eschrichtidae comprising just one species, the gray whale (Eschrichtius robustus), the rorquals (Balaeonopteridae), and the right whales (Balaenidae). The blue whale (Balaenoptera musculus) is a rorqual. The world’s biggest whale, indeed the largest animal ever to have lived, a blue whale grows to a length of up to 90 feet (27 m) and a weight of up to 165 tons (150 t). The minke whale (Balaeoptera acutirostrata) is the most numerous rorqual species. It is one of the smallest baleen whales, growing to about 36 feet (11 m) in length. The right whale (Balaena glacialis) earned its name because it swims slowly, floats after it has been killed (other whales sink), and contains large amounts of baleen and oil, the most valuable whale products. These features made this the “right” whale to hunt. The other two members of the family are the pygmy right (Caperea marginata) and bowhead whales. As their name suggests, the toothed whales possess teeth. The baleen whales lack teeth. Instead they have baleen consisting of plates of keratin—the substance from which hair and horn are made—fringed with hairs. The whale opens its huge mouth to take in a mouthful of water, lowers its baleen plates, then forces the water out of its mouth through the plates, which filter the water and collect food items.

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Biology of Deserts Most whales roam the world, but the two species of white whales confine themselves to waters off the coasts around the Arctic Ocean. They are often found near pack ice, and the beluga (Delphinapterus leucas) enters river estuaries. The beluga is gray or brown when young, but adults are entirely white. They are 10–16 feet (3–5 m) long and feed on fish, crustaceans, and other invertebrate animals. A herd of them makes an impressive sight. The other member of the family is even more impressive. Groups of narwhal (Monodon monoceros) often move in a line, and as they rise to the surface some of them are seen to possess a single spiral tusk up to 10 feet (3 m) long— on an animal that is 13–16.5 feet (4–5 m) long. Making the animal look even odder, the tusk protrudes from the left side of the mouth and is inclined downward. The narwhal possesses only two teeth, neither of which is used in feeding, and the tusk is the left tooth of the male. Up to about 3 percent of females also have one of their teeth extended into a tusk, and about the same percentage of males have two tusks. The animals are mottled gray-green, cream, and black in color. As it ages the tips of the narwhal’s tail flukes grow forward, so that seen from above it looks as though the tail is on back-to-front.

Krill Baleen whales as well as many other Arctic and Antarctic birds and mammals feed on krill. These are crustaceans related to shrimps, crayfish, lobsters and crabs. They look like shrimps, but there are anatomical differences that set them apart, so they form a separate order, the Euphausiaceae, with 85 species. Krill inhabit most oceans. Some species live near the surface, others live below it at depths down to 6,500 feet (2,000 m), and some migrate between surface and deep waters. Euphausia superba, the most plentiful Antarctic species, is about two inches (5 cm) long and orange or red in color and transparent, with big, black eyes and five pairs of swimming legs. E. superba feed mainly on phytoplankton, which are microscopic green algae that drift in surface waters, and also consume some zooplankton, which are planktonic animals. Euphausia superba is the principal krill species on which many Antarctic animals depend. It lives in surface waters of the Southern Ocean, where it forms huge swarms, often more than 15 feet (4.5 m) thick, that color the water over several square miles. A swarm contains up to 1,800 individuals in every cubic foot (63,500/m3). Individuals take two years to mature and are believed to live for five to 10 years. In all, there are estimated to be at least 550 million tons (500 million t) of krill, or 500 million million (5 × 1014) individuals, so krill may be the most numerous of all animals. A blue whale (Balaenoptera musculus) consumes about four tons (3.6 t) of them every day.

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Krill have been caught commercially since the mid1970s. The current catch, mainly by vessels from Russia, Ukraine, and Japan, is between 165,000 and 220,000 tons (150,000–200,000 t) a year, but biologists suggest it would be possible to catch more. Salmon fishermen from the northeastern United States and eastern Canada have protested at the effect of large-scale krill fishing in the North Atlantic, because salmon also feed on krill. The Antarctic krill fishery is strictly regulated by the Convention on the Conservation of Antarctic Marine Living Resources, which the member nations of the Antarctic Treaty signed in 1981 to protect the animals dependent on the krill.

Crabeater Seal Sometimes a leopard seal will rest peacefully on the ice next to a crabeater seal (Lobodon carcinophagus). Crabeaters are smaller than leopard seals and much faster on the ice, where they may be capable of moving at 15 MPH (24 km/h). The two species are often found together, in the water as well as out of it, but in the water leopard seals prey on crabeaters, especially on their newly weaned pups. Apart from leopard seals, the main predator of crabeater seals is the killer whale (Orcinus orca). The crabeater seal is believed to be the most numerous of all the seals and possibly the most abundant large mammal on the planet, with the exception of humans. There are believed to be between 15 million and 40 million of them distributed throughout a range of about 8.5 million square miles (22 million km2) in winter and about 1.6 million square miles (4 million km2) in summer, when the area of pack ice decreases. Crabeaters feed almost exclusively on krill, crustaceans on which penguins, baleen whales, and other Antarctic animals also depend. Crabeater seals eat even more krill than the whales, consuming up to an estimated 176 million tons (160 million t) a year, which they strain from the water through their premolar and molar teeth, which are shaped like combs with four (premolars) or five (molars) tines.

Weddell and Ross Seals Farther south the Ross seal (Ommatophoca rossi) feeds mainly on squid, which it hunts from remote patches of pack ice. It is fairly uncommon and not well known. Its young are born in November on the pack ice, but Ross seals are not gregarious, so they do not form large breeding colonies. Adults are about 6.5 feet (2 m) long. They have thick necks, short muzzles, and large, round eyes. The Weddell seal (Leptonychotes weddelli) breeds on the permanent ice shelves surrounding Antarctica. It is the most southerly of all seals, although individuals have been seen around New Zealand, Australia, and as far north as

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Uruguay. Adult males are about eight feet (2.5 m) long and females a little larger. The Weddell seal feeds on squid and bottom-dwelling fish and invertebrates, which it catches by diving to depths of up to 1,800 feet (550 m).

Arctic Seals Young ribbon seals (Phoca fasciata) also feed on krill, but they do so in the Bering and Okhotsk Seas. As adults they feed on fish and squid. They are small seals, about five feet (1.5 m) long. Their common name refers to the distinctive bands in their fur around the neck, rear part of the body, and each flipper. Hooded seals (Cystophora cristata) are bigger—about eight feet (2.4 m) long. Altogether there are about 325,000 of them. They spend much of their time far out at sea, feeding on fish and squid, for which they dive to considerable depths, but in summer they migrate to the Arctic, especially to the ice floes near Greenland, where they all haul themselves out of the water and molt their fur. The hood is an enlargement of the nasal cavity of mature males that forms a sac on the front of the head. The male is able to inflate the sac, like a black balloon, and use it to amplify the calls with which he threatens rivals. Alternatively, he can blow the lining of one nostril out through the other nostril and inflate it like a red balloon. The ringed seal (Phoca hispida), an animal about five feet (1.5 m) long when adult, is the most numerous species in the Canadian Arctic. It is less common in Eurasia. The total population probably amounts to about 2.5 million. Its name refers to the pale rings that mark the dark gray fur on its sides and back. At sea the ringed seal feeds on krill and other crustaceans and also eats some fish. Inshore it seeks fish and crustaceans that live on or near the seabed. Ringed seals rarely come to land but haul themselves onto ice shelves or less commonly ice floes, which is where the young are born from February to March in the Baltic Sea and March to April in the Canadian Arctic. In early winter both male and female ringed seals dig lairs in the snow that accumulates around and then over their breathing holes in the ice. Males haul themselves into their lairs to rest, but females give birth to their pups in the lairs they dig. They often make two or three pupping lairs within a short distance of each other, perhaps as alternative refuges to which they can take their young if danger threatens. The lairs provide some protection from polar bears and arctic foxes, but they are not difficult to find. Predators take more than half of all newborn pups. Bearded seals (Erignathus barbatus) have long, bushy whiskers. These are bulky animals, about seven feet (2.1 m) long, that inhabit shallow waters throughout the Arctic Ocean, individuals sometimes straying as far south as Japan. No one knows how many of them there are, but their pop-

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ulation is estimated to be several hundred thousand. They rest and breed on ice floes and feed on bottom-dwelling fish and invertebrates. Spotted seals (Phoca largha) also feed on the seabed and give birth on ice floes. They are a little larger than ringed seals.

Walrus Bigger and heavier than any seal, the walrus (Odobenus rosmarus) is found throughout the Arctic. There are two subspecies, the Atlantic (O.r. rosmarus) and Pacific (O.r. divergens) walrus. Pacific walrus are bigger than those of the Atlantic. An adult Atlantic male is about 10 feet (3 m) long and weighs about 1.4 tons (1.3 t). Females are somewhat smaller. Walrus have been hunted extensively in the past, but this is now controlled. There are estimated to be about 20,000 Atlantic and at least 200,000 Pacific walrus. Walrus are highly social. When they haul themselves onto the ice or land they usually do so in such large numbers they are forced to lie on top of each other. Their colonies are generally peaceful if noisy, but walrus will vigorously defend themselves, their young, and each other. Polar bears and killer whales hunt walrus, and walrus calves are particularly vulnerable. Walrus feed on invertebrate animals such as worms, sea cucumbers, crustaceans, and especially on clams, mussels, and other bivalve mollusks. Occasionally they eat fish and seals. Their food is obtained from the sediment on the sea bottom, and a walrus has been observed to dive to a depth of 370 feet (113 m) and remain submerged for almost 25 minutes. They can probably dive deeper than this. There is little light at such depths, even in summer, and in winter the darkness is total. A walrus uses its strong whiskers to find food and can dig animals from the sediment by rummaging with its snout or spitting a jet of water into their burrows. Walrus use their tusks to haul themselves out of the water, for fighting, and as status symbols. When a dominant animal rears up to display its huge tusks, subordinates move out of its way, allowing it to go where it chooses and occupy the best position. The tusks, which can be 3.5 feet (1 m) long and weigh more than 10 pounds (4.5 kg), are enlarged upper canine teeth. Walrus also have loose skin, lying in folds at every joint, that can be more than two inches (5 cm) thick, and males have lumps like warts on the neck and shoulders. Blood vessels in the skin expand and contract to help walrus maintain a constant core body temperature, a process that makes the animals change color. When walrus are warm the blood vessels in the skin expand to help them lose heat, and the animals look pink. When they are cold—as they often are when surfacing from a dive—the blood vessels are constricted, and the animals are gray.

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5 History and the Desert Although they appear barren and empty today, many of the desert lands were not always so. Crops grown in the northern Sahara were once exported to Rome, and a prosperous civilization flourished along the Indus Valley between India and Pakistan in land that is now dry and infertile. Western civilization began in Mesopotamia, now called Iraq and mainly desert. This chapter describes the history of desert lands and their peoples in Africa, North America, and Asia, as well as in the Middle East. A settled way of life, leading to the growth of cities, becomes possible only when urban populations can rely on farmers to supply their food. The chapter tells of the discovery of agriculture and the domestication of cereal crops and livestock. Agriculture is a precarious undertaking in lands where the rainfall is seasonal and unreliable, and the expansion of farming was necessarily accompanied by the development of irrigation schemes. The chapter describes several ancient irrigation systems, and it also explains the way badly designed irrigation can poison the soil, destroying the farms it was meant to serve. When irrigation succeeds, however, it can sustain a large population and a major culture. Ancient Egypt and its use of the Nile and Mexico with its floating gardens are among the most impressive examples of successful water management. Both systems are explained here. The chapter then moves on to describe the ways of life of the nomadic peoples who are the true inhabitants of the world’s deserts, including the desert of the frozen north. It describes the dwellings desert peoples build. Where a desert lies in their path, traders must cross it, and the chapter explains how a camel caravan travels. The most famous of all trade routes was the Silk Road, linking China with the Mediterranean and bringing silks to Europe. The chapter tells of the origin of the Silk Road and outlines its route. Finally, the chapter describes a few of the many explorers and expeditions that brought reliable information about remote areas of the world to the American and European public. Among other explorers of the Arctic and Antarctic, the chapter includes the stories of Nansen in the Arctic and

Amundsen, Shackleton, and Scott. It then tells of the exploration of the deserts of Africa and the rediscovery of the Silk Road.

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WHEN DESERTS GREW CROPS

Petra is uninhabited today except by wandering tribes and tourists, but from about 300 b.c.e. to 100 c.e. it was a thriving metropolis. After that it began to decline. By then the city lay within the Roman Empire. For a time after its citizens had converted to Christianity it was the seat of a bishopric, but trade routes were shifting. Crusaders built a castle there in the 12th century, but after that Petra lay abandoned until 1812, when it was rediscovered for Europeans by the Swiss traveler Johann Ludwig Burckhardt (1784–1817). Petra is located about 16 miles (26 km) northwest of the town of Ma’an, in Jordan. The map shows its location. Today the ancient city lies in the Jordanian Desert, and other deserts surround it—the Negev Desert to the west, the Syrian Desert to the north, and the Arabian Desert to the south. People do not build cities in the middle of deserts, so clearly the region was not always so arid and inhospitable as it is now. The city the Romans occupied had been standing and prospering for many centuries, and that meant its citizens were supplied with water and with food grown on nearby farms. It did not stand incongruously in the middle of a desert. Movies set in the Near East or Egypt in biblical times depict landscapes that are those of today, and the moviemakers evidently feel it unnecessary to explain how it can be that so many people were able to wrest a living from such barren land. If the scene were to be depicted accurately, however, there would be no mystery. The audience would see fields growing wheat, barley, and other crops, cattle and sheep grazing, and trees bearing citrus and other fruits. In those days this land was fertile, although the desert was never far away, and the biblical writers knew it well.

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DESERTS been possible in earlier times, and in about 300 records of wine imports to Britain cease, possibly because that Roman province was self-sufficient in wine. There were other towns farther east, also in regions that are now desert. Silks, spices, and other luxuries from the Orient were carried to Europe by camel caravans traveling along the Silk Road. Towns and Buddhist shrines were built at points along the route, presumably at places where the caravans paused to rest and graze their animals. The remains of some of them can still be seen. It was during the first decades of the fourth century that conditions began to change. The weather became cooler and drier. There were severe droughts, and the water level fell drastically in the Caspian Sea. In central Asia the increasingly arid climate caused the grasslands to deteriorate. Nomadic tribes found it increasingly difficult to feed themselves and their horses and they began migrating westward. Eventually, they undermined the Roman Empire to such an extent that it retreated and finally collapsed.

TURKEY

Aleppo

SYRIA

Cyprus

Homs

Nicosia Nic Tripoli LEBANON

Beirut

Petra, Jordan, attracts visitors from all over the world. (The New 7 Wonders of the World)

The Granary of Rome In those days North Africa was known as “the granary of Rome.” Large areas were farmed, and the produce was exported. The outlines of fields can still be seen from the air in parts of what is now the Sahara. Ptolemy (Claudius Ptolemaus), the astronomer, geographer, and mathematician who flourished in the second century c.e., kept a record of the weather in Alexandria, the city where he lived from 127 to 145. Alexandria is on the Mediterranean coast of Egypt. Ptolemy recorded that it rained in every month of the year except August, and there was thunder in all the summer months. Today the average annual rainfall in Alexandria, calculated over 45 years, is seven inches (178 mm), and no rain falls between the end of April and the beginning of October. People were migrating northward at the time Petra was flourishing. Olives were being grown farther north than had

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IRAQ Damascus

Me d it e rra n e a n Se a Tel Aviv Jerusalem salem Port Said

ISRAEL

Amman

JORDAN PETRA

Cairo

Suez

Al Aqaba

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© Infobase Publishing

The ancient city of Petra. Now abandoned and an important archaeological and tourist site in southern Jordan, in Roman times Petra was a thriving city.

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History and the Desert

Canoes in the Sahara This was not the only climate change to affect what are now deserts. Much earlier, around 5,500 years ago, people living in what is now Aounrhet, in central Algeria, depicted their way of life in paintings on cave walls. These show people herding cattle and hunting hippopotamuses from canoes. Other still older paintings show a variety of animals, including crocodiles, rhinoceroses, buffalo, and elephants, that live in or beside water. At that time what is now Lake Chad was a large inland sea. Prior to 6000 b.c.e. the annual rainfall in southern Libya was eight to 16 inches (200–400 mm), and rivers flowed all year round from the Tibesti Mountains. Today it rarely rains at all in this region. Australia, too, had a much wetter climate prior to about 2500 b.c.e. It was about then that climates throughout the world started to become cooler and drier, and deserts every-

Emi Koussi is an extinct volcano rising to 11,205 feet (3,415 m) in the arid landscape of Chad. (NASA Earth Observatory)

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where began to expand. This change seems to have marked the peak of the climatic warming that began with the end of the most recent glaciation (see “When Northern America, Europe, and Asia Were Cold Deserts” on pages 45–49). Warm weather is usually wet weather, because the rate of evaporation increases as temperatures rise and so, therefore, does precipitation. Falling temperatures reduce the rate of evaporation and produce a drier climate.

The Indus Valley Between about 2500 b.c.e. and 1700 b.c.e. a civilization flourished in the Indus Valley, centered on the cities of Harappa and Mohenjo-Daro. Farms covering an area that was much greater than the farmed area of the Nile Valley grew cereal crops, dates, melons, and possibly cotton and raised livestock. At that time the annual rainfall was 16–30 inches (400–760 mm), and occasionally crops were ruined by floods. By 1700 b.c.e., however, the climate was becoming drier. Crops failed repeatedly, and the cities were abandoned. The change happened slowly, but it was relentless. Today the Indus Valley marks the western boundary of the Thar Desert (see “The Great Indian, or Thar, Desert” on pages 17–18). The town of Sukkur, in Pakistan, lies close to the ruins of MohenjoDaro. It receives an average of 3.2 inches (81.5 mm) of rain a year. The increasing aridity also affected all the civilizations of the Middle and Near East, Egypt, and the Mediterranean.

The Warm, Wet Middle Ages and the Little Ice Age More recently the climate was warmer and wetter in the Middle Ages, from the 11th to the 14th century. The northern boundary of the Sahara lay then at about 27°N, in the center of Morocco, Algeria, Libya, and Egypt. Most of western Africa lay within the Mali Empire (see “Mali Empire” on pages 177–178), the principal city of which was Timbuktu (also spelled Tombouctou), on the Niger River. Climates throughout the world became colder and drier in the 16th and 17th centuries, during the period known as the Little Ice Age (see “The Little Ice Age” on page 11). During the Little Ice Age the African summer monsoon did not extend so far north as it had done previously. Intense monsoon rains sometimes resulted in flooding of the Niger, carrying excessive water northward from the Tropics, but the change in climate also produced a series of prolonged droughts.

Cahokia North America has also experienced the effects of changing climate. Six miles (10 km) to the east of St. Louis, Missouri,

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Cahokia Mounds State Park preserves the remains of the biggest Native American city north of Mexico. It includes Monks Mound, a four-sided pyramid covering 14 acres (5.65 ha) that rises in four terraces to a height of 100 feet (30.5 m), as well as many smaller mounds. In all, the site occupies six square miles (5.54 km2), and at its peak, between 1050 and 1250, its population is believed to have been as high as 40,000 or even 50,000. Its citizens belonged to the Mississippian culture (see “Peoples of the American Desert” on pages 202–205), and they were fed from farms in the surrounding countryside. Then the climate became drier, until finally, in about 1300, Cahokia was abandoned. When French traders and missionaries arrived in the area they found only small, scattered settlements. The missionaries founded the present town of Cahokia in 1699, naming it after a tribe of the Illinois people. Cahokia was the biggest settlement, but it was far from being the only one. Around 1000 Native American peoples were growing corn extensively in eastern Colorado and western Nebraska, and in the east there were large villages along the forested river valleys. By 1150 practically the whole of the Colorado Plateau and the adjacent parts of Arizona and New Mexico were occupied and being farmed. People grew corn, squashes, and beans, and they hunted game. As the climate deteriorated, the smaller villages in the driest areas were abandoned first, the others followed one by one, and for a time people congregated in the larger towns until they, too, became uninhabitable because their populations could not be fed. The change is recorded in the pollen held in the soil. Trees became more scattered as forests were replaced by prairie, and as the process continued the tall grasses, which require more water, gave way to short grasses. The area around the North Platte and South Platte Rivers, on the border of Colorado and Nebraska, does not have a desert climate today, but that is because the climate has changed again since Cahokia was abandoned. People traveling westward in the gold rush of 1849 found the Midwest was so dry as to be almost a desert. The current annual rainfall averages 16.9 inches (429.1 mm) at Sterling, Colorado, and 20.9 inches (532.1 mm) at North Platte, Nebraska.

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DESERT CIVILIZATIONS

Sharru-ken, the name by which King Sargon was known in Akkadian, the language he himself spoke, was the first of three rulers of the same name. He came to power in about 2370 b.c.e. According to an Akkadian legend, his mother gave birth to him in secret, then hid the baby in a basket made from rushes and floated it on a river. The baby Sharru-ken was found and raised to be a gardener. Ishtar, daughter of the moon god, fell in love with the gardener and made him a king.

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Historically, Sargon had been a vizier at the court of the king of Kish, a city-state in what is now southern Iraq. A vizier was a high official, the equivalent of a modern government minister. Sharru-ken founded a city called Akkad, and from there he came to rule all the Sumerian city-states to the south as well as the region known as Akkad further north. He was styled king of Sumer and Akkad. His empire lay to the south of the modern city of Baghdad.

Akkad, Sumer, and Babylon The expansion of the empire was not by conquest in a military sense, but more like colonization. Sharru-ken and his followers were Semitic desert people. Little by little they moved into Sumerian cities and settled there, their descendants came to hold key positions, and finally they took control. Akkad itself grew into a great city, covering more than 200 acres (80 ha). It is said there were splendid buildings, paved streets, and drains to carry away surplus water and prevent flooding. The citizens were fed from fields kept watered from irrigation canals and protected by fortresses. The empire lived by trade, and it expanded rapidly. Akkadian records have been found in Cyprus, and its territories may have extended as far as Lebanon. It controlled trade in valuable commodities from as far to the east as Afghanistan and all the way south to the Gulf of Oman. Akkad was a port on the banks of either the Tigris or Euphrates—its precise location is unknown, but Akkad was centered on the region where the two rivers are closest together. Babylon later became the capital of the empire occupying the same area. Ships from all over the Near and Middle East and possibly from as far away as India were alleged to lie beside its quays. This mighty empire lasted for little more than 100 years. The rains became irregular, and despite the irrigation system the crops failed repeatedly. The drought continued for more than a century, and little by little the wheat fields were buried beneath windblown sand. Akkad and the other cities were abandoned, and gradually the first of the great empires of the world collapsed. All traces vanished of its magnificent capital. Founded by desert peoples, the desert destroyed it. Its lands now form part of the Syrian Desert.

Failure Due to Changing Climate Akkad fell victim to the general climatic change that also destroyed the cities of Harappa and Mohenjo-daro in the Indus Valley, and these were not the only victims. Civilizations in Crete and Greece also failed at about that time. Towns were abandoned in Palestine. Egypt suffered badly because a reduction in rainfall over the mountains of Ethiopia meant there was a serious decrease

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History and the Desert in the amount of water carried by the Nile. The annual Nile floods that watered Egyptian fields and fertilized them with silt carried down from the mountains became increasingly unreliable (see “The Annual Nile flood” on page 190). Egypt suffered severe famines between about 2180 and 2130 b.c.e., between 2000 and 1950 b.c.e., and again around 1750 b.c.e. People migrated into Egypt from the east at these times, perhaps having been driven from their own lands by famine. This destabilized and finally caused the collapse of the Old and Middle Kingdoms. Greatness eventually returned to what had once been the Empire of Sumer and Akkad, and Baghdad became very powerful centuries later, but by the Middle Ages the climate was becoming drier once again. Mesopotamian (Iraqi) agriculture was in decline. Baghdad received a final blow in 1258 c.e. from which it has never recovered. That was the year when Mongol warriors entered the city and massacred its people.

Rise of the Mongol Empire Ironically, it may have been a period of warmer, wetter weather that started the process leading to the establishment of the Mongol Empire. Pastures flourished on the central Asian steppes at that time, leading to an increase in the populations of the nomadic tribes living there and in the size of their herds. Then, around 1200 c.e., it may be that the climate turned cooler and drier as the region became dominated by weather systems that drew Arctic air southward. Tribal leaders fought for supremacy in the increasingly crowded, deteriorating grasslands around the Gobi Desert. In about the year 1167 a Mongol chief called Yesugei Ba’atur killed a Tatar chief called Temujin. A son was born to Yesugei at about the time he killed Temujin, and, as was the custom, the infant was given the fallen warrior’s name. When he was eight Temujin was betrothed to Burte, a young girl of a different tribe, the Onggirat (or Konkirat), and, in accordance with tradition, Yesugei took Temujin to live in the Onggirat camp. On his way home, however, Tatars revenged themselves by poisoning Yesugei. As Yesugei lay dying he sent for Temujin to return, but his death left the tribe without a chief. The warriors and other subjects deserted Temujin and his mother, and relatives stripped them of their possessions. Temujin and his mother, together with Temujin’s four brothers and two half-brothers, were reduced to the direst poverty. Temujin grew up to be a skilled horseman and brave fighter. At age 15 he proved he was entitled to the status of warrior by recapturing some horses that had been stolen. With his new status he claimed Burte, his bride, and as a dowry her father gave Temujin a cloak made from sable pelts. Temujin gave this cloak to one of his father’s former

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allies. The gift bought Temujin the protection of a powerful tribal chief, and it marked the start of Temujin’s career. An astute politician, he won the loyalty of those around him and always rewarded that loyalty. Over the following years Temujin built an alliance of Mongol tribes. Once they were strong enough, his warriors conquered their tribes’ traditional enemies, such as the Tatars, and in late 1206 or January 1207 their successes prompted the warriors to acclaim Temujin their “wide-encompassing chief,” or Genghis Khan. United, the Mongol tribes began to expand their territories. Beijing fell to them in 1215, and by 1227, the year Genghis Khan died, their empire stretched from the Caspian to the China Seas. Its first capital was Karakorum, but later, under Kublai Khan (1264–94), it was transferred to Beijing. After the death of Genghis further campaigns were launched westward. Kiev, Ukraine, fell in 1240, and advance parties of the Mongol army reached Wrocław, Poland. After soundly defeating a joint force of German and Polish knights, the Mongols then headed south into Hungary. Those who settled on the grasslands of southern Russia remained there as the Golden Horde, an empire within the empire. At its maximum extent, in about 1300, the Mongol Empire extended from the Danube in the west to the China Sea in the east, and from the borders of Lithuania in the north to the Himalayas in the south. In the 16th century a descendant of Genghis Khan, Babur (Zahir ud-Din Mohammed, 1483–1530), conquered part of India and became its first Mogul ruler, bringing India under Mongol influence, although India never formed part of the Mongol Empire. Kublai Khan, the grandson of Genghis, was possibly the greatest of all emperors of China. The Mongol Empire was probably the largest the world has ever seen, and the last traces of it did not disappear in the West until late in the 18th century.

Mali Empire No part of Africa fell under Mongol rule, although the Golden Horde traded with Egypt, but West Africa had a medieval empire of its own. The Sarakolé Empire of Ghana, developed from a Ghanaian Empire founded by immigrant Berber peoples in the fifth century, occupied the region between the Niger and Senegal Rivers. It flourished from the eighth to the 13th century, when it was conquered by Songhai people from the region around Gao and later absorbed into the Mandingo Empire of Mali. This was centered on the middle and upper reaches of the Niger and gave its name to the modern Republic of Mali. An empire based on trading, the Mali Empire reached the pinnacle of its influence and prosperity during the reign of Mansa Musa (ca. 1312–27). Its two principal cities, Gao and Timbuktu, became centers of Islamic learning, and in 1325 a palace and a tower for the mosque were built in

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Timbuktu. By the 1330s the empire extended over most of West Africa except for Upper Volta, where several emerging states remained independent. Malian merchants opened trade routes to the south, importing gold, kola nuts, and slaves that they paid for with salt from the desert. Then in the 15th century, the Mali Empire began to decline. The Songhai regained their independence, but there were disputes over control of the trade routes. In 1590–91 a Moroccan expedition crossed the desert and defeated the archers and cavalry of the Songhai sultan, Ahmad al-Mansur. The Moroccans were unable to restore the trade routes to the south, however, and managed to occupy only the main cities of Gao, Timbuktu, and Djenne. The empire had effectively disappeared. In 1352 large herds of wild cattle were recorded in parts of the Sahara, but already the climate was changing. Areas that had once been fertile had turned to desert, and people living around the Kufra Oasis, in eastern Libya at 25.37°N, where formerly there had been large herds of domesticated cattle, had been compelled to abandon the raising of beef. Pollen records show that water-demanding plants declined sharply between 1300 and 1500. Even so, the climate seems to have been wetter then than it is now, because there were oak woodlands in Mauritania as late as the 17th century. It is impossible to estimate the extent to which changing climate and the resulting encroachment of the desert contributed to the collapse of the Mali Empire. Too many other factors were involved. Nevertheless, it seems highly probable that the desert played an important part. There, as in Central Asia before and throughout the Near and Middle East thousands of years earlier, decreasing rainfall dramatically changed the course of history.

ANATOLIA, MESOPOTAMIA, AND THE BIRTH OF WESTERN CIVILIZATION

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On the Anatolian Plain in southern Turkey, a few miles south of the modern town of Konya, are the sites of some of the oldest cities in the world. In November 1958 the archaeologist James Mellaart discovered one of them and excavated it over the course of four seasons, from 1961 to 1965. It is called Çatal Hüyük (pronounced chatalhOOyook), and it consists of rectangular, mud brick houses built so closely together that their occupants entered them through doors in the roofs, presumably by means of wooden ladders, and crossed their neighbors’ roofs when they wanted to walk from one house to another. Each house had a hearth, oven, and platforms that may have been for sleeping, sitting, or storing goods. There were no doors or windows.

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B l ac k Se a

BULGARIA Sofiya Skopie

Catal Hüyük Istanbul

Tirane

Asikli Hüyük Ankara

Erbaba

TURKEY

GREECE

Adana

Ath Athens Hacilar

Crete © Infobase Publishing

Irakl Iraklion

Süberde Can Hassan III

Cyprus

Med i t er r a n ea n S e a

N Nicosia

SYRIA

Beirut

Damascus

Early agricultural sites in Turkey

Çatal Hüyük was occupied from about 6500 to 5800 b.c.e. It must have prospered, because its inhabitants had sufficient time free from the tasks involved in finding food to make pottery, wooden dishes, and baskets and to weave cloth. Not far away there are archaeological remains of other towns, all of similar date. The map shows their relative locations. Many of the buildings in these towns were shrines containing figures of goddesses and animals modeled in high relief on the walls, frescoes depicting hunting scenes, and statuettes. There were also pictures of vultures devouring headless human corpses, evidently illustrating the method by which these people disposed of their dead.

Early Farmers Like all ancient sites, those in Turkey comprise several layers of occupation. In levels dated at about 6200 b.c.e. at Süberde and Can Hassan III and at about 5600 b.c.e. at Erbaba and Çatal Hüyük, there are the remains of plant crops and domesticated animals. By that date these communities were feeding themselves partly or mainly by farming. Civilization had begun. Our words citizen and civilization are derived from the Latin civitas, which means “city,” and the origin of civilization is dated from the time when people first built and occupied cities. These are not the oldest villages, of course. Villages were springing up throughout the Near East from about 11000 b.c.e. Their inhabitants ate cereal grains, which they ground into flour or meal, but they did not cultivate the cereals. The grains they ate were gathered from wild plants, and the meat they ate was game. Their culture was paleolithic, based on hunting and gathering.

Origin of Wheat To this day there are areas of natural grassland in parts of southern Turkey and northern Syria and Iraq where the pre-

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History and the Desert dominant plants include the grass Triticum boeoticum, one of the wild ancestors of modern wheat. At one time T. boeoticum grew alongside other Triticum species, and the grasslands extended through parts of what are now Lebanon, Israel, and Jordan as far as the Nile Valley, in Egypt. Thousands of years ago people living in that part of the world harvested the seeds of Triticum and related grasses and ground them into a coarse meal or flour. They boiled the meal in water to make a type of porridge, and they made the flour into bread. These highly nutritious foods formed a major part of their diet. About 13000 b.c.e. the climate changed as the world emerged from the most recent ice age. Woodland expanded, but the changes also benefited the grasses, and the wild cereals became more plentiful. People who lived as seminomadic hunters and gatherers were able to settle in permanent villages. There was so much food that they could store grain to last them from one harvest to the next. Populations increased, and after a time people found a way to regulate their food supply. They gathered seeds from the best-tasting and most productive plants, cleared the ground to remove competing plants, and scattered the seeds on the bare soil. These were the first farmers. They lived about 12000 b.c.e. in the valley of the River Jordan and in the northern part of the Fertile Crescent, between the Euphrates and Tigris Rivers in what is now Iraq. Wild wheat plants shed their ripe seeds. That is how the plant reproduces, but it means that many seeds fall to the ground and are lost before they can be gathered. Consequently, people gathered seeds from plants that held on to their seeds for a little longer than the others, and those were the seeds the early farmers sowed. Farmers could harvest all the seeds from these plants, but they also controlled the plants’ reproduction. Over many generations this selective breeding produced wheat plants that retained their seeds. Gradually, the cultivated cereals continued to change. The wheat that farmers grow commercially today has changed so much genetically that it is a different species from its wild ancestor. The wild wheats were called emmer (Triticum turgidum) and einkorn (T. monococcum). Emmer was grown for making bread and is still grown on a small scale in some places, nowadays mainly to feed livestock. Spelt wheat (T. spelta) was an early cultivated species, grown extensively in Roman times and still used on a small scale to make bread. Modern bread wheat is T. vulgare, and durum wheat, used to make pasta, is T. durum.

Barley and Other Cereals The ancestor of wheat was one of several grass species that produced edible seeds. Barley (Hordeum vulgare) grew in the same areas, and its range extended as far westward as

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the highlands of Ethiopia. Farmers were growing barley in Egypt in 5000 b.c.e., in Mesopotamia (modern Iraq) by 3500 b.c.e., by 3000 b.c.e. in northwestern Europe, and by 2000 b.c.e. in China. Barley was the cereal used to make most of the bread eaten in ancient times by the Hebrews, Greeks, and Romans, and many people were still eating barley bread as recently as the 16th century. Wheat needs a fairly long growing season and a period of warm, dry weather to ripen the grain. This limits the regions where it will grow. Barley ripens faster and tolerates cooler, wetter climates. It is also better than wheat in tolerating extreme heat and drought and can be grown along the edge of the Sahara in conditions where wheat would fail. Barley grain contains very little gluten, the combination of two proteins (gliadin and glutenin) that gives dough made from wheat flour the elastic texture that allows it to rise when yeast ferments in it, releasing carbon dioxide. Consequently, barley can be used to make only unleavened bread, although barley meal can also be cooked like porridge. Barley bread has a pleasant flavor and is rich in carbohydrates. Rye (Secale vulgare) grows alongside wheat and may once have been a weed of wheat crops. Eventually, it was domesticated and grown to make bread. Rye flour contains gluten, but less than wheat, making rye bread heavier than bread made from wheat flour. Both barley and rye are also fed to cattle, and both grains are fermented to make beer and their liquor distilled to make whisky. Wheat, rye, and barley were not the only crops early farmers learned to cultivate. They also grew lentils, chickpeas, and peas as well as flax from which they made linen cloth. From its origins in southwestern Asia and northeastern Africa, agriculture then spread westward into Europe and eastward into northern India. Rice farming began independently in India and China, and corn (maize) farming began later in Central America.

Varying Climates Villages grew into towns during a long period of warm, wet weather that occurred as the ice sheets of the last glaciation (see “Not One Ice Age, but Many” on pages 46–47) were disappearing. The evidence for two warm periods—known as interstadials—is taken from two sites in Denmark after which they are named. The first, lasting from about 11000 to 10200 b.c.e., is known as the Bölling Interstadial, and the second, from about 9800 to 9000 b.c.e., as the Allerød Interstadial. Toward the end of the Allerød the climate was becoming colder and drier as it entered a stadial—cold period—known as the Younger Dryas. There were two or perhaps three stadials during the glacial retreat, each of which is identified by pollen from mountain avens (Dryas octopetala), a small flowering herb that grows on mountainsides and on

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low ground in the far north and that gives these stadials their name. The Younger Dryas, from 9000 to 8000 b.c.e., brought Arctic conditions to northern Eurasia and North America. In southwestern Asia the climate became drier and the cereal grasses—ancestors of modern wheat, barley, oats, and rye—became scarcer. After the Younger Dryas ended the climate grew warm again, and forests spread into the region. They broke up what had been open range, dividing the landscape into discrete areas and producing within it a complex pattern of natural habitats. The new setting seems to have encouraged people to settle permanently in one place and to regard the area that they inhabited as territory for their exclusive use. It was around this time that hunting peoples started to herd and corral game animals. Later they started cultivating plant crops, and when the sustained climatic warming allowed cereal grasses to return they began cultivating them. By around 7000 b.c.e. the inhabitants of most of the villages in the Near East were sustaining themselves by farming crops and tending livestock.

From Wolf to Dog The wolf was the first animal to be tamed and domesticated. Domestic dogs are descended from the Arabian (Canis lupus arabs) and Indian (C.l. pallipes) wolves, two subspecies that were once widespread but are now extinct. These were smaller animals than the present timber wolf (C. lupus). There are differences in the lower jaw (the mandible) that distinguish the skeleton of a wolf from that of a domesticated dog. A mandible found in a cave at Palegawra, Iraq, and clearly that of a dog, not a wolf, has been dated at about 10000 b.c.e. Wolves are believed to have competed with human hunters for prey and for carcasses left by other predators. People and wolves would have met often, and similarities in social behavior would have allowed them to tolerate each other and then to become friendly. Orphaned cubs would have been kept as pets, and when they grew up tame wolves would have hunted alongside humans, sharing the resulting food. Skeletons of “wolf-dogs” are fairly common in Upper Paleolithic sites. Dogs were also domesticated in North America. The bones of a domesticated dog that lived between 9,000 and 10,000 years ago have been found at Danger Cave, Utah.

Sheep and Goats Wild sheep inhabit most temperate regions of the world, and there are many species. This made it difficult to trace the ancestry of the domestic sheep until biologists were able to study their DNA. These studies revealed that the most likely ancestor of the domestic sheep (Ovis aries) is the Asiatic mouflon (O. orientalis). Mouflons have dark

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coats, small bodies with long legs, short tails, and long horns marked with rings. They live in the mountains from the eastern Mediterranean to southern Iran. There are also native North American sheep. The thinhorn sheep (O. dalli) lives in the mountains of Alaska and northern British Columbia, and the bighorn sheep (O. canadensis) inhabits western North America from Canada to northern Mexico. Neither species has ever been tamed, however, and all the sheep on American farms and ranches are descended from imported stock, as are the sheep on Australian and New Zealand farms. Sheep are social animals. They thrive best when they live as groups with a leader, grazing within a well-defined range. This behavior made it fairly easy for people to control the flocks, and as they did so they would have rescued orphaned lambs and raised them in their own homes. The flocks would have grown accustomed to people, and the lambs would have grown into adults with no fear of humans. The earliest evidence of sheep under human control comes from a site in northeastern Iraq where the remains are about 10,870 years old. Fully domesticated sheep were common in Asia by 5,000 years ago. Sheep were domesticated as a source of meat and skins. Wild sheep have an outer coat of long, stiff fibers over a woolly undercoat that grows in winter and is shed in spring. It was later that selective breeding produced domesticated sheep lacking the coarse outer coat and with a much thicker undercoat—the fleece. The ancestor of all domesticated goats (Capra hircus) is the bezoar, or pasang (Capra aegagrus), which is probably native to Asia. Today the bezoar survives in the Greek islands and Turkey and across Asia as far as Pakistan and western India. It has long horns, curved like scimitars, those of males being much larger than those of females. Following domestication goats became much smaller, and so did their horns. Then some goats began to grow twisted horns, and these seem to be the goats early farmers preferred. No one knows why this was so. People began keeping goats in western Asia in about 8000 b.c.e. Although sheep were domesticated earlier, in many places goats rather than sheep were kept for meat. In fact, sheep and goats complement each other. Both animals thrive in hilly areas with poor pasture, but sheep prefer to graze on grass, while goats will browse on shrubs and small trees, which they can climb.

Pigs All domestic pigs are descended from the wild boar (Sus scrofa). Farmers probably began keeping pigs at around the same time they domesticated sheep and goats. The oldest remains of a domesticated pig were found at Jericho, near the Dead Sea, and are approximately 9,000 years old. Pigs were also being raised in Sumer more than 4,000 years ago and also in Egypt.

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History and the Desert The Egyptians considered pigs to be unclean animals. According to the Greek historian Herodotus, if an Egyptian man accidentally touched a pig he would immediately rush, fully clothed, into the nearest river. The Egyptians raised pigs, but swineherds were forbidden to enter any temple, although these were open to all other Egyptians. Swineherd families had to marry among themselves, because no other Egyptian would marry one or allow a daughter to marry one. Nevertheless, by 2686 b.c.e. Egyptians were permitted to eat pork on certain days, and pigs were sacrificed. Probably the taboo was not observed very strictly. It seems perverse to raise animals no one is allowed even to touch, far less consume. Wild boar are still fairly common throughout Europe, Asia, and North Africa, although they do not occur in North, Central, or South America. Most are forest animals, but pigs are highly adaptable. Some subspecies have taken to living in drier, more open habitats. S. s. barbarus lives in North Africa, from Tunisia westward to northern Mauritania. A pig found in eastern Sudan and Ethiopia, classified as S. s. sennaarensis, may be feral—a wild pig descended from domesticated ancestors. S. s. lybicus is a native of eastern Turkey and much of the Middle East. Pigs were probably easy to domesticate. They will eat a wide range of foods and remain perfectly healthy on a diet similar to that of a human or a dog. Also, they enjoy the company of other pigs and like to huddle together, although this applies only to members of a family group. Pigs do not naturally form large herds. They are not territorial, so although boars are aggressive in defense of their sows and sows in defense of their piglets, pigs will not try to drive humans away from their feeding grounds. Once they lose their fear, pigs live contentedly in close proximity to humans.

Cattle All domesticated cattle are descended from an animal called the aurochs (plural aurochsen, Bos primigenius), also known as the giant ox. The aurochs became extinct in the 17th century, but herds of aurochsen once roamed Eurasia between latitudes 30°N and 60°N. They lived mainly in the forests, feeding by browsing leaves on branches low enough for them to reach, but they also grazed in more open areas. A subspecies, B. primigenius namadicus, lived farther south in India. It may be the ancestor of the humped zebu cattle (B. indicus) found in India and Africa. Aurochsen bulls were five to 6.5 feet (1.5–2 m) tall at the shoulder, with long horns that curved forward. Most males were black, often with a white line along the center of the back. Cows were much smaller, had smaller horns, and were reddish in color, as were calves. They lived in herds, and if threatened the bulls would defend the cows and calves.

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People hunted these wild cattle, and no one knows how they tamed such a big, aggressive beast. Possibly they exploited their one weakness: a craving for salt. There are people living today in the hills of Assam, India, who lure wild cattle (unrelated to domestic cattle) to their villages by leaving salt and water for them. While they are licking the salt people are able to approach the animals safely and can even stroke them. This may be how early farmers began to tame the aurochs. Partly tamed aurochsen would have been suspicious of people and much too nervous to permit humans to milk them. Indeed, it is unlikely that the cattle were used for food at all. They were brought into villages for religious purposes—as they are today in Assam—and used in religious rituals that involved decorating and venerating them, but not killing them. After a time they came to be used as draft animals to haul wheeled carts and plows. The earliest evidence of domesticated cattle comes from the site of Çatal Hüyük, where as well as bones there is a shrine where aurochsen horns are set in clay. The earliest bones date from about 6400 b.c.e. and the shrine from about 5950 b.c.e. Over many generations the descendants of aurochsen became smaller and more docile. Eventually, they became a different species, Bos taurus, the species to which all European domestic cattle breeds belong.

The Fertile Crescent The region into which agriculture spread first includes the Nile Valley (see “Egypt” on pages 189–192) and extends from the Dead Sea in the south and southern Turkey in the north in a broad curve southeastward to the Persian Gulf. This roughly crescent-shaped stretch of land is often called the Fertile Crescent, a name coined by the American orientalist and historian James Henry Breasted (1865–1935). It is where Western civilization began. It was in the Fertile Crescent that Sargon forged the Empire of Sumer and Akkad (see “Akkad, Sumer, and Babylon” on page 176) and where the Assyrians, Babylonians, and Phoenicians flourished. The Assyrian Empire was centered in what is now northern Iraq and was powerful from about 2000 b.c.e. to 612 b.c.e. Ninevah and Nimrud were two of its most important cities. The earliest Phoenician town was Byblos, a seaport on the coast of Lebanon north of Beirut. Known today as Jubayl, people have lived there since before 3000 b.c.e, making this possibly the oldest continuously inhabited town in the world. Papyrus paper was exported to the lands of the Aegean through the port of Byblos, and byblos is ancient Greek for papyrus. Our word bible means papyrus book— from Byblos. The Phoenicians were great explorers and traders, traveling by sea. One of their exports was a form

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of writing in which symbols represented consonants rather than whole words. It was simpler to learn and to write than either Egyptian hieroglyphics or Mesopotamian cuneiform. The new writing was being used in Byblos by about 1500 b.c.e. It has evolved into the alphabet we use today. These civilizations flourished during the warm period that followed the end of the Younger Dryas Stadial. Then, at least at first, the Fertile Crescent truly was fertile, with reliable rainfall, warm temperatures, and soils that retained moisture. The region enjoyed weather much like that of the Mediterranean countries and southern California today. There were permanent settlements even in what is now the Arabian Desert. Already, though, the climate was becoming cooler and drier. As long ago as 5000 b.c.e. farmers in Mesopotamia were starting to irrigate their fields. Around 2000 b.c.e. farms still occupied a much larger area than they do now, and many more people lived between the Euphrates and Tigris than the land could sustain nowadays. Today the Fertile Crescent is mostly desert. The desert has taken over not because of any failing on the part of the ancient civilizations, but simply because the climate has changed.

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IRRIGATION, RIVERS, AND DAMS

Early farmers soon learned that crops do not thrive in dry ground and that a delay in the arrival of seasonal rains can substantially reduce the harvest. If the rains failed to arrive at all, many people would go hungry, and rulers understood that hunger might trigger civil unrest. A solution to the problem was to provide water where and when it was needed. This is called irrigation, and it has been practiced for thousands of years. In the lands between the Euphrates and Tigris Rivers, where Western civilization began, irrigation was introduced early. The first channels carrying water to the fields may have been constructed as long ago as 4000 b.c.e. Over subsequent centuries the network of channels became very elaborate and strongly influenced the political development of the region.

The First Civil Engineers Within the great empires that ruled the Fertile Crescent were many more or less autonomous city-states. Their rulers were concerned with defense, religion, and the security of the food supply. These concerns favored the emergence of strong governments that were willing and able to invest as much money and labor as were needed in irrigation systems. Irrigation systems and defense works were designed and built under the supervision of engineers who were also priests and high government officials. They were the first civil engineers the world had seen, and their examples showed

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later generations and civilizations what could be achieved in the control of natural forces. There was no attempt to coordinate irrigation schemes throughout the entire area, but together the individual schemes of different cities succeeded in irrigating a very large area of cultivable land. A canal was built in what is now southern Iraq to link Ur, then on the bank of the Euphrates, to the Tigris. The course of the Euphrates has moved over the years, and today the site of Ur is in the desert about 10 miles (16 km) from the river. It lies 140 miles (225 km) south of Babylon, about halfway between that city and the coast of the Persian Gulf. This is the city called Ur of the Chaldees in the Bible because its citizens spoke a dialect of Aramaic called Chaldean, and it is reputed to be where Abraham was born. At its height Ur was the capital of Sumer and immensely wealthy. There was another canal farther north. Its water was carried across a small river by means of an aqueduct.

The Hanging Gardens of Babylon Water was channeled to the fields around Babylon from an artificial lake held behind a dam on the Euphrates. King Hammurabi (ca. 1792–50 b.c.e.) left a written description of canals built from brick with asphalt mortar that fed water into a system covering 10,000 square miles (25,900 km2)— an area the size of Vermont. Babylonian farms fed a population of 15 to 20 million people. That is almost equal to the population of modern Iraq, about 22 million. Babylon was famous for its hanging gardens, which are known as one of the Seven Wonders of the World. The gardens were probably built during the reign of Nebuchadnezzar II (604–562 b.c.e.). Legend has it that the king built them to console his wife, Amytis, who was from Persia and missed the mountains and forests of her homeland. Despite their name, the gardens did not hang; they were roof gardens. The framework for the gardens consisted of a stone structure supporting a series of terraces in the form of platforms, each of which projected over part of the one below. The entire structure was in the shape of a ziggurat—a stepped pyramid. Plants of all kinds, including trees, were grown on each of the terraces as well as on the flat roof at the top, and the structure was hollow, with galleries on the inside through which people could walk. No one knows how water was raised all the way to the top, but the machinery to do so must have been impressive. The Babylonian irrigation system lasted until the 13th century c.e., when it was destroyed during a Tatar invasion.

Flood Control The Tigris and Euphrates Rivers are not dependable. At irregular intervals between April and June they can rise suddenly to produce devastating flash floods that leave behind

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History and the Desert huge amounts of silt. In 1929 archaeologists working at the site of Ur found a layer of deposits eight feet (2.4 m) thick that had been laid down by a major flood in about 3200 b.c.e. There is evidence of other severe floods that occurred between 4000 b.c.e. and 2400 b.c.e. at Kish and Ninevah. Clearly, the early civil engineers faced a formidable task. Not only did they have to supply water where and when it was needed, they also had to protect the cities from flooding. Consequently, they devoted a great deal of time and attention to studying the rivers, the weather, and signs—magical as well as physical—that might allow them to predict the amount of water being carried. As well as being the first civil engineers, they were the first hydrologists, and their studies may have been the beginning of science. The authority that allowed them to recruit and manage a labor force was backed by the first strong, centralized legal system. The Euphrates and Tigris floods came too late in the year to be of use for crops sown in the winter and too early in the year for crops sown in the spring. This meant that the floodwaters had to be kept out of the fields altogether. Embankments called levees were built to contain the rivers as they rose, and large basins were constructed into which excess water from the rivers could be channeled when necessary. Both the basins and channels had to be kept free from silt. The silt could be used as fertilizer, and the stored water could be directed through other channels to the fields for use when the ground was dry. Occasionally, there were exceptionally high river levels that overwhelmed the defenses, but most of the time the cities were protected and the farms flourished. Eventually, the irrigation system fell into disrepair. As the climate became drier and the rivers changed their courses, so gradually the farms were abandoned. Desert finally claimed the area.

The Kingdom of Sheba Mesopotamia was not the only part of the world where engineers built irrigation systems. In what is now Yemen, in the south of the Arabian Peninsula, there was once a kingdom called Saba, known in the Bible as Sheba. Sometime around 500 b.c.e. the Sabaeans built a dam at Ma’rib. Its ruins are still there. The dam, about 50 feet (15.25 m) high and nearly 1,970 feet (601 m) long, controlled the flow of water into the valley below it, allowing that area to be irrigated. The dam broke and was repaired several times, but finally the breach was not mended. Between 542 and 570 c.e. the irrigation system gradually broke down, and the kingdom of Saba, or Sheba, disappeared as the desert reclaimed the fields. Irrigation was also introduced in eastern and southern Asia. In China the Gukow River was dammed in 240 b.c.e. by a structure 98 feet (30 m) high and 985 feet (300 m) long.

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Dams for irrigation were also built in Sri Lanka, the earliest from the fifth century b.c.e.

Farming the Negev Dams were not the only way to capture water. What is now the Negev Desert in southern Israel was once farmed. During the Israelite period, about 950–70 b.c.e., the inhabitants built low stone walls to channel rainwater into terraces of fields in a low-lying area. Even then the Negev had a dry climate, and such rain as it received fell mainly as heavy showers. Water entered the top terrace, soaked the soil until water lay on the surface, then overflowed the low wall and fell into the next terrace, and then the next after that. At the bottom any remaining water drained into a storage tank below ground so it could be used later. The system evidently worked well, and it was extended between about 300 b.c.e. and 630 c.e., during the Nabatean and Roman-Byzantine periods.

The Hohokam Irrigation was also used in North America. The most impressive system was developed in Mexico (see “Chinampas” on pages 192–194), but it was not the only one. In central and southern Arizona, mainly along the Gila and Salt Rivers, a group of Native Americans built what eventually became more than 150 miles (240 km) of irrigation channels in the Salt River Valley alone, some of them 30 feet (9 m) wide and 10 feet (3 m) deep. Pima and Papago people living in the area later, after the canal-diggers had gone, called their predecessors the Hohokam, which is believed to mean “those who have vanished” in the Pima language. The Hohokam arrived in Arizona sometime around 300 b.c.e. They cultivated corn, gathered wild beans and fruits, and hunted game, and they dug a canal three miles (5 km) long to carry water from the Gila River to their fields. Little by little more canals were added to the system, cotton was added as a crop, and later beans and squashes were introduced. Water was being lost through evaporation and by soaking into the ground. In order to reduce losses by around 700 c.e. the canals were being cut deeper and narrower. Hohokam people made distinctive pottery, composed epic poems, and learned to make wax molds in which they cast bells made from copper. Their culture lasted for about 1,700 years, but early in the 15th century it began to disintegrate. No one knows why or what happened to the last of the Hohokam.

The Anasazi The Anasazi lived in the area where New Mexico, Arizona, Utah, and Colorado meet. Their name is the Navajo word

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for “ancient enemy,” but the Hopi knew them as the Hisatsinom, the “ancient people.” Descendants of traders who arrived in the region from Mexico some time after 2000 b.c.e., the Anasazi became farmers growing corn (maize), pumpkins, and beans. At first the Anasazi farms were close to rivers, but as their population increased they moved onto land farther away, where they practiced a form of dry farming (see “Dry Farming” on page 258). As the climate grew warmer in the Middle Ages, after about 900 c.e., summer rains became reliable, and every year brought a good harvest. The Anasazi prospered, and their population eventually reached approximately 100,000. Even so, the farmers had a hard life, and up to 45 percent of their children died from disease or malnutrition. Although food was plentiful, most of it went to the priests who lived in grand houses and conducted the ceremonies that ensured adequate rain and successful harvests. Anasazi prosperity eventually came to an end. There was severe drought in 1090 and again in 1130. By that time the soil was depleted and seriously eroded, and the hillsides had been stripped of the trees that had once cloaked them. The farmers dug irrigation ditches, built dams, and terraced the land, but to no avail, and there was starvation. The priests lost control of the communities, and gradually the farming families moved away. By 1170 they had left the valleys, and by the early 13th century they were building villages in cliffs and on the tops of the mesas. In the middle of the century they returned to the valleys they had abandoned and farmed beside the Rio Grande and similarly reliable sources of water. But another even more severe drought afflicted the entire region from 1246 until 1305, being particularly intense from 1276 to 1299. Nevertheless, descendants of the Anasazi still live in New Mexico and Arizona. Irrigation is more important today than it has ever been. If all the people of the world are to be fed adequately, farming must be made as productive as possible. This means that crops must receive as much water as they need. Irrigation can double or triple crop yields by making it possible to produce two or three crops a year on land that previously produced only one. Such increases have been achieved in many countries. In the world as a whole, some 20–25 percent of all farmland is irrigated, and the irrigated land produces approximately 40 percent of the world’s food.

AQUIFER DEPLETION, WATERLOGGING, AND SALINATION

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Without irrigation there would be no farms in large parts of the southwestern United States. In the Great Basin, the

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basin-and-range region covering 189,000 square miles (489,500 km2) in Nevada, western Utah, and parts of California, Oregon, and Idaho, the average annual rainfall ranges from four to six inches (100 to 150 mm) in the south to 10 to 12 inches (250 to 300 mm) in the low-lying basins. This is sufficient to sustain natural vegetation adapted to semiarid conditions, but not to produce farm crops. With irrigation, supplied from the artificial lakes impounded behind the Hoover, Shasta, and other large dams, the Southwest is highly productive. A total of 82,600 square miles (214,000 km2) of farmland is irrigated in the United States, most of it to the west of longitude 100°W. Plants use water for mechanical support, as a medium in which to transport dissolved nutrients, but mainly as a source of hydrogen for photosynthesis (see “Photosynthesis” on pages 96–101). If the ground provides less water than a plant requires, the plant experiences water stress.

Water Stress In extreme cases the effects of water stress are obvious. Leaves wilt, stems lose their rigidity, and the plant may collapse. This level of stress occurs where the soil is dry all the way from the surface to below the level of plant roots. If it continues, the top of the plant may wither away, and the feeding roots may die. These are the roots through which the plant absorbs nutrients. Loss of its stem, leaves, and feeding roots may not kill the plant, but when its water supply is restored it may not respond quickly. Before it can recover fully it must grow replacements for the parts it has lost. Less severe stress produces subtler symptoms that are not immediately visible. If the soil contains water below the level of the feeding roots, the deeper roots may absorb enough to sustain transpiration. Its xylem vessels will be full and its turgor pressure maintained (see “Transpiration and Why Plants Need Water” on pages 101–103). The plant will not wilt. For some time it will continue to appear healthy, but it is not absorbing nutrients and is unable to grow. Crops can experience water stress even in Britain, where the rainfall is moderate and ordinarily falls throughout the growing season. In summer higher temperatures can increase the rate of evaporation sufficiently to dry soils, and in most years there are periods of a few weeks without rain. Irrigation is used on about 267,000 acres (108,000 ha) of farmland, an area that tripled between 1973 and 1998. Given the advantages, it is not surprising that so many farmers are willing to install irrigation. The increased yields more than pay for the cost, and in some countries, including Mexico, Turkey, and parts of the United States, farmers pay a reduced price for electricity that is used to pump water for irrigation.

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Crops Need a Great Deal of Water Plants need a startling amount of water. Only a minute proportion is used in photosynthesis or held in and between plant cells. Most is lost through transpiration, and to that must be added the amount that evaporates from the ground before it reaches a plant at all. Altogether, it takes approximately 500 tons of water to grow one ton of potatoes, 1,500 tons to grow one ton of wheat, and about 10,000 tons to grow one ton of cotton. To produce one ton of beef from cattle fed on grain requires around 100,000 tons of water. In the United States and Europe approximately two-thirds of all the water consumed nationally is used to irrigate crops, and in the drier areas, such as California, Egypt, and the Murray-Darling basin in Australia, the proportion rises to more than 90 percent. In some regions irrigation water is taken from rivers. Elsewhere it is abstracted from the groundwater (see “Aquifers, Oases, and Wells” on pages 69–72). When water is poured on the land, some of it quickly evaporates, and some enters plants and is returned to the air by transpiration. A small amount becomes incorporated in plant tissues and is removed when the crop is harvested. The remaining water drains through the soil, enters the groundwater, and flows toward the river at the bottom of the drainage basin, but because of evaporation, transpiration, and the incorporation of water in plant tissue, more water is abstracted than returns. This may reduce the amount of water flowing in rivers. If any polluting substances discharge into the river— and these may originate naturally as well as through human

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activities—there will be less water to dilute them. Irrigation can make pollution worse.

The Aral Sea The abstraction of river water has produced spectacularly harmful effects in Central Asia. Two rivers, the Syr Darya and Amy Darya, discharge into the Aral Sea and are the main sources of water entering the sea. The Aral Sea is a large, shallow, saltwater lake on the border between Kazakhstan and Uzbekistan. In 1960 its surface area was 26,300 square miles (68,000 km2), and its average depth was 53 feet (16 m), although it was 226 feet (69 m) deep on its western side. In the 1920s the Soviet government embarked on a scheme to bring previously uncultivated land in Kazakhstan into agricultural production and to make the Soviet Union one of the world’s leading exporters of cotton. The area of farmland expanded, and within a few years the Soviet Union became the world’s third-largest cotton exporter, after the United States and China. By the 1950s the farms needed more water. This was taken from the Syr Darya and Amu Darya. As the area of farmland increased, so did the amount of water fed into it. Much of the water soaked into the ground or evaporated, but enough reached the crops to maintain production. Water levels fell in the two rivers, and by the 1980s both rivers sometimes dried up completely in summer. Deprived of replenishment, the Aral Sea started to shrink as water evaporated from it. By about 1990 the volume of water in the sea was half of its 1960 value, and its level had fallen by approximately 50 feet (15 m). The sea was

The Aral Sea as it appeared from space in 1977, 1989, and 2006. This sequence of images clearly shows the rate at which the sea was shrinking. (NASA)

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reduced to two separate parts, called the Greater and Lesser Aral Seas, with a combined surface area of about 13,000 square miles (33,800 km2). Evaporation concentrated the mineral salts in the water. It became unfit for drinking, and most of the fish died. This destroyed a formerly prosperous fishing industry and the livelihoods dependent on it. What had once been ports were now towns far from the sea. Dry salt, blown as a dust by the wind, fell on a large area of surrounding land, rendering it infertile. There were even fishing boats stranded in what had become a salt desert. Water draining from unlined irrigation channels and excessive amounts applied to the crops combined to raise the water table. This caused salination (see “Salination” on pages 187–188), increasing the damage to the land. Cotton farming continued, but yields fell. Scientists and engineers from many countries are collaborating in schemes to restore the Aral Sea, but it will take a long time. Improving the irrigation system to reduce wastage should eventually allow the water table to fall, and it may then be possible to flush the excess salt from the soil.

The Ogallala Aquifer Abstraction can also deplete the aquifers from which water is obtained. Beneath the Great Plains, for example, the huge Ogallala aquifer holds water that has accumulated from the melting of snow and ice from several ice ages. The aquifer extends from South Dakota to Texas and underlies parts of Wyoming, Nebraska, Colorado, Kansas, Oklahoma, and New Mexico. It is a sandy formation, about 330 feet (100 m) thick, and its water table is close enough to the surface to be easily accessible to shallow wells. Since the 1940s farmers have been encouraged to irrigate their land with water from the Ogallala aquifer. The region produces corn and wheat as well as cotton and grain-fed beef—crops with a particularly large water requirement. It is estimated that about 60 percent of the water held in the aquifer has now been removed. The water table has fallen by about 100 feet (30 m), and pumping has become more costly.

Waterlogging Water tables can also rise. The table gives examples of the rate at which this can happen. If a soil is to benefit from irrigation, water should soak through it at a rate of 0.1–3 inches (0.25–7.5 cm) per hour. A soil that is suitable for irrigation should absorb sufficient water over 24 hours to moisten it to a depth of two to three feet (60–90 cm). If the rate of infiltration is slower than this, much of the irrigation water will evaporate before reaching plant roots, and some will drain away horizontally as surface run-off. If water infiltrates faster—through desert gravel or sand, for example—it will descend to the groundwater and

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Rising Water Table Due to Irrigation PROJECT

COUNTRY

RATE OF RISE (feet per year)

Nubariya

Egypt

6.5–10.0

Beni Amir

Morocco

4.9–10.0

Murray-Darling

Australia

1.6–4.9

Amibara

Ethiopia

3.3

Xinjang Farm 29

China

1.0–1.6

Bhatinda

India

2.0

Source: The World Environment 1972–1992, edited by Mostafa K. Tolba and Osama A. El-Kholy. (London and New York: UNEP with Chapman & Hall, 1992.)

be lost that way. Many irrigation systems are so inefficient that less than half the water they deliver reaches the crops for which it is intended. Where this happens, farmers are sometimes tempted to apply more water. What they see is a soil that dries rapidly, and the apparently obvious remedy is to water it more heavily and more frequently. This will have little effect if the reason the soil dries is that its texture is so coarse it cannot retain water. What may happen, however, is that water drains through the soil faster than it can be removed by the flow of groundwater. Water then accumulates belowground, and that is why the water table rises.

Drowning in Soil that Looks Dry At first a rising water table is harmless or even beneficial if the groundwater is replenishing an aquifer that supplies wells elsewhere. Trouble begins when the water table rises to the level of the plant roots. Depending on the crop this may be far enough below the surface that the soil still appears dry to the farmer, but plant growth will slow. The yield will be reduced, and in extreme cases the plants may die. The soil is becoming waterlogged, and the crop is dying for lack of air. Groundwater occupies soil that is saturated. It is water that can sink no lower because there is an underlying layer of rock or other impermeable material it cannot penetrate, so it is held, filling all the spaces between particles. The saturated soil is completely airless. Plant roots must have oxygen for cell respiration, and although some plants of the humid Tropics have evolved adaptations that allow them to grow in waterlogged ground, farm crops have not—and least of all those crops that are grown in or close to deserts. Eventually, the water table may rise all the way to the surface, and pools of water will fill every small depression.

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Irrigation and waterlogging. Provided the irrigation channels are sealed, the water they carry remains in the region of the plant roots. If the channels leak, water soaks and eventually saturates the ground, causing waterlogging that may not be visible at the surface.

Waterlogging is not always caused by excessive irrigation. It often results from inefficient irrigation due to cracks in the pipes and channels carrying irrigation water. Water leaks into the ground before reaching the irrigated field, and by the time the leak is noticed the damage has been done. The diagram illustrates this by showing a cross-section of ground with four irrigation channels carrying water across it. The two channels on the right are sound, and the water table lies some distance below them. The two on the left are leaking. They have saturated the soil beneath them, raising the water table almost, but not quite, to the surface. If the farmer is growing a crop with shallow roots, the damage might not yet be evident, but already a deep-rooting crop would fail. Irrigation channels may also overflow. This, too, may be a problem that develops slowly. River water carries mineral particles. These remain suspended while the water is flowing quickly, but the rate of flow in irrigation channels is slower than that in a river. The amount of material that can remain in suspension is proportional to the energy the water has. As it loses energy by slowing down, the water also loses its capacity for transporting particles. These settle as silt onto the bottom of the channels, gradually raising the channel bed. If the volume of water being fed into the channels remains constant, sooner or later the accumulation of silt will make them overflow, allowing water to drain freely and possibly raising the water table.

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Removing Surplus Water Obviously, the remedy to waterlogging is to keep channels open by dredging out the silt and to repair leaks. Irrigation systems require maintenance, and once the necessary repairs have been made the water table should start to fall. If this does not happen, a more drastic measure is needed. Wells need to be sunk to the desired level of the water table, and water above that level must be pumped out. The pumped water can be poured back onto the land, although spraying through nozzles is better. This sounds paradoxical, but fields are irrigated only where the climate is dry and, therefore, where the evaporation rate is high. As the water flows from the pump some of it evaporates, and if it emerges as a spray of tiny droplets even more will evaporate because the total surface area of water increases. So although water removed from the ground is poured back onto the same ground, a substantial proportion of it is lost. More water is removed than is returned, so little by little the water table falls.

Salination Waterlogging can be cured fairly simply, if expensively, and with a wastage of water, but it is often accompanied by salination, which is a much more serious problem. Farms are not irrigated with drinking water. They use water of poorer quality, which means the water contains concentrations of mineral salts that render it unacceptable for domestic use. Soil also contains mineral salts derived from the parent material (see “The Formation, Development, and Aging of Soils” on pages 55–57). These are highly soluble in water.

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Extent of Soil Degradation on Irrigated Dry Land by Continent (thousands of acres; thousands of hectares in line below) CONTINENT Africa Asia Australia Europe North America South America

SLIGHT OR NONE

MODERATE

SEVERE

VERY SEVERE

TOTAL OF MODERATE, SEVERE, AND VERY SEVERE

21,058

4,396

301

2

4,700

8,522

1,779

122

1

1,902

148,774

60,132

14,302

4,176

786,099

60,208

24,335

5,788

1,690

31,813

4,003

247

321

49

618

1,620

100

130

20

250

24,693

3,311

1,137

259

4,707

9,993

1,340

460

105

1,905

37,082

12,182

1,804

494

14,480

15,007

4,930

730

200

5,860

17,292

2,587

766

148

37,485

6,998

1,047

310

60

1,517

Source: The World Environment 1972–1992, edited by Mostafa K. Tolba and Osama A. El-Kholy. (London and New York: UNEP with Chapman & Hall, 1992.)

Water that is poured onto the land is slightly salty. As it drains downward more salts dissolve into it. Water is also evaporating from the surface. This exerts a pressure drawing water upward by capillarity (see “Except When It Moves Upward, by Capillarity” on pages 68–69) and into the soil around plant roots. Evaporation is the change from the liquid to the gaseous phase, and the molecules escaping into the air are of pure H2O. Any substances that were dissolved in the liquid are left behind. Gradually, the solution from which plant roots draw water and mineral nutrients becomes more concentrated. Salts may even be precipitated as a white crust on the ground surface. At first this has no appreciable effect. The point at which crop plants start to suffer varies according to the species. Barley, tomatoes, and sugar beet are fairly salt-tolerant, cotton, wheat, soybeans, and sorghum are rather less so, and other beans begin to suffer at quite low salt concentrations. Usually the harm is due not to the toxicity of the salts, although in extreme cases plants can be poisoned, but to their osmotic effect.

Osmosis and Its Opposite Where a partially permeable membrane, such as a cell wall, separates two solutions of different concentrations, water will pass from the weaker solution to the stronger solu-

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tion until the concentration is equal in both. The process is called osmosis, and plants absorb water by maintaining a salt concentration inside their root cells that is higher than the concentration in the soil solution. The strength of the force drawing water through cell walls is proportional to the difference in concentration of the solutions on either side of the walls. This difference is like a hill—a concentration gradient. The steeper the gradient, the faster molecules will move down it, and, consequently, the less energy plants need to expend in order to absorb water. Increasing the concentration of the soil solution reduces the concentration gradient, and plants need to work harder to draw in water. Energy they expend doing this cannot be used for other purposes, so their rate of development slows. Seed germination is delayed or even completely inhibited, and the growth of stems and leaves is affected. For most crops—rice is an exception—there is no immediate reduction in the quantity of seeds the plant forms. With increasing salt concentration, however, the problem becomes much more serious as plants struggle to absorb the amount of water they need. Crop yields are then greatly reduced, and it can even happen that osmosis starts working in the opposite direction—called exosmosis. Instead of water entering plant roots, it is drawn from them. This can happen quite suddenly if a pause in irrigation allows the soil to start drying,

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History and the Desert with a resulting abrupt increase in salt concentration. All it takes is a change in the concentration on either side of cell walls. Plants cannot survive these conditions. Salination is widespread, as the table at left shows. These figures describe the condition on dry lands, which are lands that can be farmed only with irrigation, and in the world as a whole about 0.8 percent of all irrigated dry land is degraded. Salination is the principal cause of this degradation.

Removing Salts Once land has been degraded by the accumulation of salts, it may remain barren for a long time, because the only way to remove the salts is by washing them away. Water pumped from wells sunk to below the water table can be used to flush the salt from the ground, but care must be taken to ensure that the salt water leaving the land does not pollute groundwater or nearby rivers. As with so many problems, prevention is much better than cure. It is also a great deal easier, although it increases the initial installation cost of irrigation systems. Seeking to avoid that increased cost is the reason why waterlogging and salination occur. Water must be allowed to flow. It is not enough simply to pour it onto the land; drains must also be provided to carry it away. A combined irrigation and drainage system imitates the way rain penetrates the ground naturally and then flows from it. Farmers may think it paradoxical to pour water through their land only for it to run away again through drains they have also paid for, but this is the only way to guarantee that their fields will become neither waterlogged nor saline. Provided it is planned and installed correctly and subsequently maintained properly, the system will turn a patch of desert into a sustainably productive farm.

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EGYPT

Despite their strong rulers, strict laws, and advanced technology, life was always uncertain for the peoples of the Fertile Crescent. Their rivers, the Euphrates and Tigris, were unreliable and often dangerous, and eventually their irrigation networks began to fail them. Salination became a serious problem. Egyptians, on the other hand, enjoyed much greater security because their river, the Nile, was more dependable. Ancient Egyptians seem to have been happy, lively people, fond of music, dancing, and games. Their country was self-sufficient in everything the people needed, and they did not seek quarrels with other nations. Until the end of the Old Kingdom, in about 2181 b.c.e., the Egyptian permanent army consisted of little more than the king’s bodyguard. Later, when the army was much bigger, the soldiers were paid mercenaries, many of whom

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were foreigners. The Egyptians were not at all warlike and desired nothing more than to live in peace.

Prospering in a Desert Cairo (Al-Qāhirah), the capital city built on the site of the ancient capital of Memphis, has an average annual rainfall of 1.1 inches (28 mm), and all of that falls between the beginning of November and the end of May. It is a desert climate, and it was a desert climate in ancient times. Despite its climate, Cairo is by far the biggest city in Africa or the Near East. Metropolitan Cairo covers 83 square miles (214 km2) and has a population of about 7.6 million. Greater Cairo, which includes all the suburbs, covers more than 33,300 square miles (86,000 km2) and has a population of almost 15 million. Egypt thrives in its arid climate because its farmers do not rely on direct rainfall to water their crops. They farm the land on either side of the Nile, and the river supplies the water (see “Providing Water” on pages 269–274). That water originates as rain, of course, but it is rain that falls far away in Uganda and on the mountains of Ethiopia. Less than 4 percent of the land area of Egypt is habitable. The remainder consists of three deserts: the Western and Southern Desert; the Eastern Desert, which is sometimes considered as part of the Arabian Desert; and the Sinai Desert. Apart from some oases, the deserts are uninhabited. The populated part of Egypt forms two regions that in ancient times were regarded as two countries and that form two administrations in modern Egypt: Upper and Lower Egypt. Upper Egypt extends from the Sudanese border northward to Cairo, a distance of 750 miles (1,200 km). For the first 200 miles (320 km) the Nile flows through a narrow gorge, but before it reaches the city of Aswān the land on either side of it widens into a plain about 100 miles (160 km) across. North of Cairo the river enters its delta, and Lower Egypt. The two kingdoms were united during the First Dynasty (ca. 3100–2890 b.c.e.), and that is when the kings (pharaohs) began to wear the double crown to symbolize their dominion over both.

Early Farming The climate was not always so arid as it is now. When the first farmers began tilling the Egyptian soil, a few centuries before 4000 b.c.e., they did so in savanna grassland grazed by elephants, giraffes, and rhinoceroses. Farms in al-Fayyūm, an area of low-lying land to the southwest of Cairo, grew wheat, barley, cotton, and flax for weaving into linen cloth and raised sheep, goats, and pigs. At about the same time people living farther south at Al-Badari, on the east bank of the Nile near the modern town of Asyūt, were also raising domesticated animals, wrapping dead animals

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in linen, and burying them near their villages. By about 3600 b.c.e. farming had spread to the delta and along most of the length of the Nile in Upper Egypt. In addition to cattle, sheep, goats, and pigs, during the Old Kingdom (2686–2181 b.c.e.) almost every local species of antelope and gazelle was kept in captivity, probably for food. Contemporary pictures show these animals wearing collars and being fed by hand. Hyenas were also kept in captivity, and the illustrations suggest that they, too, were raised for food. There are pictures of them being forcefed. No one knows what part of the animal was eaten. In those days the moister climate made it easier than it is now for people to cross the desert, but conditions were already becoming drier. As they did so the river on which everyone depended was becoming less reliable. The seasonal rains over the Ethiopian mountains were often lighter than they had been previously, and in some years they failed completely. This reduced the level of water in the Nile and the dependability of the annual floods, resulting in the famines that marked the end of both the Old Kingdom in 2181 b.c.e. and of the Middle Kingdom in 1786 b.c.e. Egyptians were country folk, and cities were important to them mainly for their markets serving the rural community. Farming was highly organized. There was a ministry of agriculture, with high officials responsible for such matters as the proper management of the fields and raising of livestock. The land was owned by the pharaoh, by temples, or by aristocrats and farmed by tenants who paid a rent that was fixed by law. Tenant farmers were lent seed, and oxen for plowing were lent or hired to them. Loans and hire charges were repaid from the harvest. People ate simply but usually well. Their diet was based on bread, beans, and onions, but the better off also enjoyed salads of lettuce and papyrus stalks, lentils, peas, and a variety of other vegetables and fruit. They also ate fish, beef, poultry, and game. The Egyptians grew grapes and made wine, were producing olive oil by around 1200 b.c.e., and kept bees. It was they who first used yeast (Saccharomyces cerevisiae) to ferment beer—their everyday drink—and to leaven bread, and they were also the first people to use bread ovens. They were eating leavened bread by 2600 b.c.e. and were very fond of a wide variety of cakes, including some that were sweetened with honey and fried.

The Annual Nile Flood Heavy rains fell in Ethiopia in most years, and water first filled the Blue Nile, and then, when the level of the Blue Nile started to fall, the White Nile filled (see “White Nile” on page 272 and “Blue Nile” on page 272). At Wadi Halfa, where the river enters modern Egypt, the water

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level started to rise in June and reached its maximum in September. During this period the Nile overflowed its banks, flooding the land to either side throughout all of Egypt lying to the north of Aswān. As well as water, the flood brought soil from Ethiopia that had been washed into the river by the torrential rains. When the flood subsided it left the land covered by a layer of fertile silt, called alluvium. The annual delivery of alluvium was equivalent to applying fertilizer, and it made Egyptian farming highly productive. The annual flood was guided to where it was needed. Fields were low-lying and extended from the riverbank to the edge of the desert. Each field was divided from those on either side by high embankments, forming a series of basins. Short canals led from the river to each basin, but water flowed along them only during the flood. Then the muddy floodwater filled the compartments to a depth of several feet, and the fields remained under water for a few weeks during which the mud settled. Salination and waterlogging did not occur because the land drained well, and after soaking the fields the irrigation water joined the groundwater and returned to the river.

The Nilometer To help with predicting the flood, the ancient Egyptians invented an instrument called the nilometer that measured the river level very accurately. Some have survived to the present day. The one at Aswān, on an island known in ancient Egyptian times as Elephantine Island, was restored in 1870. Nilometer records were kept for each year, and one series has survived. It comes from the nilometer at Roda Island in Cairo, and it records the readings for most of the years between 622 c.e. and 1522 c.e. The illustration on page 191 shows how the nilometer works. A horizontal tunnel runs from a little way above the riverbed into a large cistern, the bottom of which is level with the bottom of the river. Water always finds its own level, so the depth of water in the cistern is the same as the depth in the river, but the water is not moving, which makes it much easier to measure. A graduated obelisk mounted in the center of the cistern allows an official to read the depth of water from an observation platform. In some nilometers there were graduations on the side of the cistern rather than an obelisk at the center. When the river level was clearly rising, a message was sent to stations downstream. Important rivers all over the world still use nilometers, invented all those years ago for predicting the Nile flood. They are not called nilometers, of course, but gauging stations. Some measure the river level simply by means of graduations on a pole or on a wall that forms the bank, but

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obelisk

observation platform

tunnel

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Nile

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The nilometer

the most accurate ones use an underground pipe carrying water into a cistern, just like those used by the Egyptians. Engineers can calculate the rate at which water is flowing from changes in the river level.

Irrigation and the Shaduf The Nile was the only source of water, and farmers needed to irrigate their fields from February to July, during the period between floods. It may have been the Egyptians who invented irrigation. They are known to have been practicing it by around 2000 b.c.e. Egyptian irrigation worked by baling water, often using a device known in Egypt as a shaduf, or shadoof, and in India as a denkli, or paecottah. Still used to this day, a shaduf consists of a long, tapering pole mounted on a horizontal crossbeam about 10 feet (3 m) above the ground. A bucket hangs from the thin end of the pole, a big rock is fastened as a counterweight to the thick end, and the crossbeam is positioned much closer to the counterweight than to the bucket. At rest the pole is roughly horizontal. To fill the bucket a worker pulls on a rope to lower the bucket into the river. When the bucket is full he releases the rope, and the counterweight raises the bucket to where he can reach and empty it, pouring the water into a channel leading to the field. Where water has to be raised higher than is

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possible with a single shaduf, two or more can be arranged one above the other. A single shaduf can irrigate about two acres (0.8 ha). The sakia, also spelled sakieh and sāqīyah and known in India as the harat, or Persian wheel, was also used, and, like the shaduf, it is still in daily use. This is a vertically mounted wheel that turns continuously. Buckets are attached close together along a loop of rope passing over the wheel or in some versions of the device to the wheel itself. The buckets fill with water at the bottom of the loop or wheel, and at the top of their travel they automatically empty into an irrigation channel as they cross the top of the wheel. The wheel is connected by a gear to a horizontal wheel that is turned by oxen yoked to an arm from the center of the wheel and walking in a circle.

The Screw of Archimedes Archimedes (ca. 287–212 b.c.e.) is credited with having invented a third device for lifting water. It, too, is still used in Egypt, and some historians suspect that the Egyptians may have been using it long before Archimedes “invented” it. Known as the screw of Archimedes, it is a helix, or screw, that turns inside a watertight cylinder, or simply a helix fixed around a central rod. It is mounted at an angle of about 45°, the lower end in water and the upper end supported. When it is turned, originally by means of a handle at the upper end, water is lifted up the helix and falls from it at the top into a gutter leading to an irrigation channel. The diagram on page 192 shows how it works.

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DESERTS and in the 15th century the Xochimilco were absorbed into the Aztec Empire. The Aztecs then adopted, developed, and extended an agricultural system that had been invented by earlier inhabitants. Archaeologists believe that the system was devised by people who lived in Teotihuacán around 100 c.e., making it ancient even when the Aztecs arrived. Teotihuacán, about 33 miles (50 km) northeast of modern Mexico City, was sacked and burned by the Toltecs some time between 650 and 900. Teotihuacán means “city of the gods” in Nahuatl, the language spoken by the Aztecs.

The Lake of the Moon

© Infobase Publishing

The screw of Archimedes

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CHINAMPAS

Spanish explorers of the 16th century would certainly have heard of the legendary Hanging Gardens of Babylon, but when they entered Mexico in 1519 the conquistadores came across something no less amazing. What is more, what they found was still there and working, unlike the hanging gardens, of which no trace remained. When they arrived in the land of the Aztecs they found not hanging gardens but what were first described as floating gardens. Mexico was ruled then by the Aztecs, a tribe to whom all the other Mexican peoples paid reluctant tribute. Their name was from Aztlán, the white land, which referred to the region, probably in northern Mexico, where they originated. The Aztecs had invaded the region about 200 years earlier after spending centuries leading a seminomadic life as they sought a place where they could settle permanently. After fights with other tribes the Aztecs were confined to two small islands on the western side of Lake Texcoco, which formed part of what they knew as the Lake of the Moon. The Aztec name for this lake was Meztliapán, and the Aztecs were also known as the Mexica—the name that, as Mexico, was later applied to the entire country. The Aztecs founded their capital city in about 1325 and called it Tenochtitlán, after Tenoch, their mythical ancestor. Later, the conquest of the adjoining city of Tlatelolco reduced it to the status of a district of Tenochtitlán, and the city became known as Tenochtitlán-Tlatelolco. In 1352 and 1375 the Aztecs defeated another tribe, the Xochimilco, that had settled farther south in about 1300,

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The Lake of the Moon lay in the Valley of Mexico, a landlocked basin surrounded by mountains and 7,500 feet (2,288 m) above sea level. The lake was about 20 miles (32 km) wide and extended for 40 miles (64 km) from north to south. Rainfall in the basin averages 30 inches (762 mm) a year, of which 27 inches (686 mm) falls between May and October. The valley occupies an area of about 3,000 square miles (7,770 km2), and during the summer rainy season the lake covered one-quarter of it. During the dry winter season evaporation reduced the lake to five separate lakes: Zumpango and Xaltocán in the north, Texcoco in the center, and Xochimilco and Chalco in the south. It was in the southern town of Xochimilco, in the region known then as Chinampan, that the Spaniards first saw the cultivated plots that came to be called chinampas. They were described as floating gardens in a book called Historia natural y moral de las Indias (Natural and Moral History of the Indies), written by a Jesuit missionary named José de Acosta (born in 1539 and died perhaps in 1599, but the date is uncertain) and published in 1590. “Gardens that move on the water have been built by piling earth on sedges and reeds in such a manner that the water does not destroy them, and on these gardens they plant and cultivate, and plants grow and ripen, and they tow these gardens from one place to another.” It was not quite as Father Acosta thought, but his mistake was easily made, because the system did involve towing large rafts of vegetation and the cultivated plots were surrounded by water on all sides. Not much remains today of the Lake of the Moon. Modern Mexico City, on the site of Tenochtitlán, uses so much water that the level has fallen drastically, but in the 16th century the lake formation was complex. The southern part of Lake Xochimilco was swampy because the soil held freshwater that poured in from a large number of springs. The water table there was higher than the open surface of the lake. Canals were dug that allowed the spring water to flow freely into Lake Xochimilco and from there into Lake Texcoco, which was deeper. There were also freshwater springs around Tenochtitlán, from which water entered the

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History and the Desert western side of Lake Texcoco, but the supply from these proved insufficient as the Aztec city grew in size. During the reign of Montezuma I (1440–68) a covered aqueduct was built to bring water to the capital from Chapultepec hill in the west. A second aqueduct was built during the reign of Ahuítzotl (1486–1502), although this transported water so efficiently it caused serious flooding.

Keeping the Water Fresh Freshwater was essential to prevent salination (see “Salination” on pages 187–188). Each summer heavy rain falling on the volcanic hills washed mineral salts into Lake Texcoco, and each winter evaporation concentrated the salts in the deeper, eastern part of the lake. There was no outlet from the Lake of the Moon, so the salts had been accumulating for thousands of years. It was essential to keep them away from the cultivated land. Injecting freshwater maintained the height of the water table, and for a time this prevented the intrusion of saltwater. During the 15th century, however, heavy summer rains caused saltwater to flow westward with potentially disastrous consequences for the Aztec capital. Nezahualcóyotl, a relative of Montezuma I, assembled a workforce of 20,000 men recruited from all the towns in the Valley of Mexico to build a dike of stones and earth enclosed by stockades. The barrier was 10 miles (16 km) long and ran across the Lake of the Moon in a north–south direction from Atzacoalco to Ixtapalapa, separating the western side of Lake Texcoco from the eastern side.

Chinampas—“Floating Gardens” The floating gardens, or chinampas, were rectangular plots of land reclaimed from the shallow lake. Each plot measured between 19.7 and 32.8 feet (6–10 m) wide and between 328 and 656 feet (100–200 m) long and was surrounded by canals. Mud dredged out during the digging of the canals was piled between the canals. This raised the chinampas a little above the water level, and their sides were held in place by fences made from posts interwoven with vines and branches. Willow trees were planted later to replace some of the fences. The people working the plots— known to the Spaniards as chinamperos—reached them in flat-bottomed canoes, but footpaths also intersected chinampa areas. Before a crop was sown the chinampero scooped mud from the bottom of the canal and spread it over the plot. Aquatic plants floated on the surface of the canals, and from time to time a chinampero would cut these plants and tow them, as a raft of vegetation, to be spread on the plot and then covered with mud. Warmed by the hot Mexican sun, this turned the vegetation into a compost pile, and it may

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have been the sight of vegetation being towed that gave the impression of floating gardens. When the level of the plot rose too high the surface was lowered, and the surplus material was often used on another plot. One end of each chinampa was used as a nursery for all the crops except corn, which was sown directly into the chinampa. A layer of vegetation was covered thickly with mud and left to dry. When the mud was firm enough it was cut into rectangular pieces called chapínes. The chinampero would make a hole in each chapín, drop in a seed, and fill the hole with manure. When the chinampa surface had been leveled and tilled, the chapínes were set into the mud. As well as vegetables, the chinamperos caught the carp and other fish that swam in the canals, and axolotls were also eaten. Water birds were netted for food and for their feathers. Flowers were grown mainly for use in the temples. In Nahuatl the name Xochimilco means “place of the flower gardens.”

Who Owned the Land? In the early 16th century, when the Spaniards arrived and the chinampa system was at the peak of its productivity, the population of the Aztec capital and all the towns and villages around the Lake of the Moon was in the region of 400,000. Xochimilco alone had at least 25,000 inhabitants and 100,000 may have lived in Tenochtitlán. Tlatelolco had a market, and on the day of the main market Spanish reports said there were 60,000 people buying and selling. Tenochtitlán was a city of canals, green with plants and busy with canoes carrying people and goods. Under the Aztec administration Tenochtitlán was divided into four wards, called calpullis, probably based on occupation by extended families, and an aristocrat ruled each calpulli. These four calpullis were divided into 12 to 15 smaller calpullis. Tlatelcolco contained 10 to 20 calpullis, and Xochimilco contained 18. Each calpulli had its own name. There were three categories of chinampas. Some belonged to each calpulli. A member of the calpulli could use one of its chinampas to support himself and his family for as long as he did not leave the plot uncultivated for two years in succession. Some chinampas belonged to the administration—the office occupied by an aristocrat but not to any person. Finally, there were privately owned chinampas, which could be bought and sold. The chinampa system was highly productive. It is still practiced in the south, around Xochimilco, by chinamperos who usually harvest two crops of corn and five of other produce every year. They grow beans, chili peppers, tomatoes, onions, and salad vegetables. Plants and flowers—especially dahlias, the national flower of Mexico—from the chinampas of Xochimilco are sold throughout Mexico.

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NOMADIC PEOPLES OF THE SAHARA

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In the seventh century and again in the 11th the warriors of Islam advanced westward from Arabia into North Africa. Many settled, and their descendants still live there. North Africa was not uninhabited at the time of the advance, however, and the descendants of the original, pre-Arab population now live in Morocco, Algeria, Mali, Mauritania, and Niger. They have had several names over the centuries. At one time they were known as the Numidians and allied themselves with the Phoenicians who occupied Carthage, the city located where Tunis is today. Later, after the Romans had destroyed Carthage (in 146 b.c.e., at the end of the Third Punic War) and North Africa was under Roman occupation, the coastal area was called Barbary, from the Greek word barbaros, which means “foreign.” The word barbaros was applied to anyone speaking a foreign language—that is, any language other than Greek. The original meaning of barbarian is “a person who does not speak Greek.” The inhabitants of Barbary were called Berbers. This is the name by which they are most widely known, but they find it alien and prefer to be called the Imazighen, the free men (the singular is Amazigh). In the fourth and fifth centuries the Imazighen had kingdoms of their own. Many Imazighen became Latin-speaking Christians during Roman times, but there were also Jewish people living farther inland. These were traders and metal workers, and some Imazighen converted to Judaism. Many of the North African Jews have now migrated to Israel.

Imazighen—the Berbers Although they resisted the Arab invasion, most of the Imazighen were converted to Islam, but even today they retain religious customs from their own, pre-Islamic past. Many learned the Arabic language and were absorbed into Arab society, but their own Amazigh (Berber) languages are still spoken. About one-quarter of the Imazighen population speaks no Arabic. There are six Amazigh languages, each with several dialects. The languages differ mainly in their pronunciation, the grammar and vocabulary being fairly standard throughout the group. They have their own alphabet, known as tifinagh, and although most of the time these languages are written in the Arabic script, tifinagh is still used for writing the Tuareg, or Tamashek, language. About 9 million Imazighen live in Morocco. They comprise three groups, each speaking their own Amazigh language. The Riffi, or Riffians, live in the Rif Mountains, which is the coastal range, the Tamazight in the Middle Atlas inland from the Rif, and the Shluh in the High Atlas farther inland. Almost 5 million Imazighen live in Algeria,

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about two-thirds of them in the mountains. Following a campaign by Algerian Imazighen for recognition of their culture, in May 1995 the government established a council to promote the Amazigh language. The Imazighen now have two political parties of their own in Algeria. In Morocco news and radio programs for schools are now broadcast in Amazigh. Most Imazighen people are farmers, but some live by farming in the lowlands through the winter and spending the summer in the high meadows with their sheep and goats—this practice is called transhumance—and others live as nomads. The nomads are pastoralists. These are people who live by herding cattle, sheep, and goats, and live in tents made from goat skins. Those Imazighen who spend their summers in the high meadows also live in tents but also build enclosures surrounded by walls made from pounded earth to provide security for themselves and their stores.

Tuareg—the Blue People The Tuareg are the desert people. About 950,000 of them are estimated to live in Niger, about 660,000 in Mali, and significant numbers in the south of Algeria, Libya, and Nigeria. There are also Tuareg living in the Sahel countries of Mauritania, Senegal, Burkina Faso, and Chad. Their ancestors were peasant farmers, living a settled life in North Africa, but they were dispersed by an invasion of Bedouin Arabs (see “Bedouin” on page 197) in the 12th century. Since then they have lived as nomads, dwelling in tents made from animal skins dyed red or sometimes from plastic. They are the Blue People of the desert, a name that refers to the blue color of the traditional robes and turbans worn by the men. At one time men wore a blue veil in the presence of women and strangers, but nowadays the custom is not always observed. Theirs is a matriarchal society, one in which the mother is the head of the family and titles and property are inherited through the female line. Consequently, Tuareg women enjoy much more freedom than Arab women. They are not veiled, their best clothes are colorful, they paint their faces, and they wear much heavy jewelry. Fiercely proud, the Tuareg were once feared. Bands of blue-clad tribesmen would suddenly appear apparently from nowhere to raid travelers, then disappear as mysteriously as they had come. They rode horses and camels and were heavily armed with double-edged swords, daggers, lances, and shields. The desert was theirs. They also traded, forming caravans to transport merchandise, and they collected taxes from other caravans that crossed their desert.

Tuareg Society Tuareg tribes are grouped into several federations, some in the north, where they live in true desert, and others

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Mali and in 1995 in Niger. Since then the Tuareg have lived in peace, but a peace that is fragile. Modernization has also disrupted their way of life. In the 1940s the Tuareg owned and operated some 30,000 caravans. As nomads their mobility made them the natural truckers of the desert, and they largely controlled the movement of goods. Their camels could not compete with real trucks, however, and their business gradually disappeared. Some of the former caravan truckers have become farmers. Others have moved into the cities, but many live in refugee camps near Timbuktu and depend on international relief organizations.

Fulani

A Tuareg man mounted on his camel and wearing the traditional blue gown and turban (Tom Claytor)

in the south, living on savanna grassland just outside the desert. Those in the south breed camels and cattle and sell some of them to the northern Tuareg. Members of the different tribes meet in market towns, such as Agadez at the southern edge of the Aïr Massif in central Niger, and every August nomads meet to trade livestock, hides, and other goods at I-n-Gall, about 68 miles (110 km) west of Agadez. Tuareg society is very hierarchical. Nobles enjoy the highest status, and the priests and scholars, learned in the Qur’an, are just below them. Laborers belong to the lowest class and are descended from the slaves Tuareg once owned. Today their lives are changing. Droughts in the 1970s killed many of their cattle, which are the only form of wealth the Tuareg possess. This forced many of the older people to settle near the cities, and many of the young men joined the army of whichever country they were in. Some Tuareg called for an independent state, and in Mali there were clashes with government troops, followed by retaliation and more fighting. Further conflict that erupted in Niger and Mali in the 1900s was resolved by peace accords agreed to in 1994 in

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Imazighen (Berbers) are not the only nomads. The Fulani, also known as the Peul and Fulbe, are believed to have come originally from Lower Senegal. In the 14th century they expanded eastward, and by the 16th century they were moving into Hausaland, now in northern Nigeria, where many of them adopted a settled way of life and were converted to Islam. In the 19th century a Fulani empire flourished in western Sudan. Their language, called Fulfulde or Fula, is related to the languages spoken in the Niger and Congo basins, but in northern Nigeria they speak the Hausa language. Fulani live today in many parts of West Africa, but especially in Nigeria, Niger, Mali, Guinea, Cameroon, and Senegal. Although many have settled in towns or live by farming, the Fulani are typically nomadic. They own cattle and travel from one area of pasture to another, living in temporary camps or huts. Their animals are their only wealth. They are rarely eaten, but the Fulani trade dairy produce for other foods and goods. They meet the Tuareg at the I-n-Gall August market. Their society is fairly egalitarian, and although most settled Fulani are fervent Muslims, the nomadic people are less strict in their observance, and some are non-Muslim. A group of nomadic Fulani usually consists of a man, his wives, and their unmarried children.

Arab Nomads Nomadic Arab tribes also attend the markets at towns in Morocco and Algeria. There are the Reguibat, who dress in blue as the Tuareg do, the Tekna and Ould Delim tribes, and also the Shluh, who are Imazighen nomads from the Atlas Mountains. Some of these people have settled in towns, but they retain strong links to the nomadic life. Those who farm nowadays raise sheep rather than camels and practice transhumance, spending the summer with their flocks on meadows high in the Atlas Mountains.

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DESERTS SPAIN

AT L A N T I C

Gibraltar

Tangier

OCEAN

Casablanca Madeira Islands

Oran

Rabat

MOROCCO Marrakesh

Agadir GUELMIM TAN-TAN

TINDOUF

SA HA R

A

Canary Islands

WE

ST E

RN

ALGERIA

MAURITANIA

MALI © Infobase Publishing

Nomad market towns in the western Sahara

People meet and trade camels at Guelmim, in southwestern Morocco, which was once an important caravan center, and at Tan-Tan, to the southwest of Guelmim. An annual fair, called a musim, is held in Tan-Tan. Around the oasis town of Tindouf, across the border in Algeria, there is a large nomad population, mainly of Reguibat. The map shows the location of these towns. In the 1980s Tindouf was the most important center for the Polisario Front, most of whose members were Reguibat. They were fighting Morocco, whose troops had annexed what is now called Western Sahara (it was then Spanish Sahara). The Polisario Front sought independence for the territory. With the help of the United Nations, a settlement plan was agreed to August 30, 1998, calling for the war to end and a referendum on independence to be held. In January 1999, no referendum having been held, the United Nations extended its mandate over the territory. In 2004, as the struggle continued, the Moroccan government accused Algeria of supplying military aid to the Polisario. The war left parts of Western Sahara heavily mined. South Africa formally recognized the Saharawi Arab Democratic Republic in September 2004.

PEOPLES OF THE ARABIAN DESERT

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When the civilizations of the Fertile Crescent were at the peak of their prosperity and power (see “The Fertile Crescent” on pages 181–182), there were also farming settlements in Arabia. Probably it was the people who lived in those settlements who domesticated the dromedary (Camelus dromedarius). When this happened is uncertain, but there are historical records of tribes of people who were

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raising camels in the ninth century b.c.e. They were called Aribi, and it seems that in 833 b.c.e. they supplied 1,000 camels to a sheikh named Gindibu, who needed them for a battle fought at Qarqar, Syria, against Shalmanser III, an Assyrian king. Samsi, an Arab queen, lost a battle against another Assyrian king, Tiglat-Pileser III, in 733 b.c.e., and the victor’s booty included 30,000 of her camels. A basrelief from the palace of Ashurbanipal of Nineveh, dated at about 645 b.c.e. and now in the British Museum, shows camels being used in battle. People who lived in the Arabian Peninsula probably domesticated camels some time before 1000 b.c.e. These people were pastoralists, making their living mainly by herding animals.

Domestication of the Camel Camels (dromedaries) have several uses, but when they were first domesticated their main use was for warfare. Arabian warriors mounted on camels raided outposts of the great empires, and the sight of a raiding party could cause panic among the defenders. The Bible describes one such attack by two tribes, the Midianites and Amelektites, that must have taken place prior to about 1050 b.c.e. And they encamped against them, and destroyed the increase of the earth, till thou come unto Gaza, and left no sustenance for Israel, neither sheep, nor ox, nor ass. For they came up with their cattle and their tents, and they came as grasshoppers for multitude; for both they and their camels were without number; and they entered into the land to destroy it. (Judges 6: 5–6)

Generals soon learned the lesson, and Ashurbanipal (r. 669–630 or 626 b.c.e.) was just one of many rulers who used them in battle. Camels were especially effective against horse cavalry, because until they grow accustomed to them, horses will bolt at the smell and sight of camels. There is a record of this in an account by the Greek historian Herodotus of the battle in which Croesus was defeated by Cyrus II the Great. Croesus, renowned to this day for his immense wealth, ruled the kingdom of Lydia, in the west of what is now Turkey, from about 560 to 546 b.c.e. He conquered the Greeks of Ionia, a neighboring country, formed various alliances, and finally embarked on a combined Greek and Lydian challenge to the might of Persia. This failed in 550 b.c.e. at a battle against the Persian king Cyrus the Great. According to Herodotus, when the Lydian horses smelled and saw the camels of the Persian army they turned and fled. In order to secure supplies of camels for military use, the imperial authorities had to enter into alliances with the camel breeders. The Arabian pastoralists acquired status, and many were recruited into the armies. Over the

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History and the Desert course of several centuries Arabs spread throughout the Middle East. From the earliest times, then, Arabs were closely associated with their camels. The name Arab is an Arabic word, ‘arab (singular ‘arabī) meaning “those who speak clearly”— that is, in Arabic. It therefore describes any person whose first language is Arabic. The Arabs were originally from Arabia, and at first many of them led a settled life as farmers. During the fourth century c.e., however, a series of severe droughts affected the entire Mediterranean region, and many farms failed. Southern Arabs, the Yemenites, developed an elaborate irrigation system in the kingdom of Saba (see “The Kingdom of Sheba” on page 183), but after the failure of the Ma’rib dam in the sixth century they abandoned farming and became nomads.

Farmers, Nomads, and Warriors The sixth and seventh centuries were a golden age for nomadic Arabs, and although the deteriorating climate forced many to give up farming, others became nomads from choice, convinced of the superiority of that way of life. This period coincided with the rise of Islam—Mohammed lived from ca. 570 to 632. As Arabs converted to Islam, a state grew up comprising the Muslim community and the lands they occupied, led by a khalifah, or caliph. Muslims were forbidden to fight or steal from each other, so previously warring tribes united and turned their attention to non-Muslims, forming armies composed mainly of Arab nomads. The caliphate expanded widely and did not end until 1258, when the Mongols destroyed Baghdad (see “Failure Due to Changing Climate” on pages 176–177). Many of the nomads settled in the conquered territories. This reduced the number leading a nomadic life. Today the great majority of Arabs live in cities and small villages, but the ancient tradition has not disappeared entirely. In Arabic the nomads are known as desert dwellers, or badw (singular badawi), a name that is written in English as Bedouin. Most Bedouin speak Badawi, which is their own dialect of Arabic, and may not understand other Arabic dialects.

Bedouin The Bedouin live throughout the Near and Middle East and North Africa, making up about 10 percent of the population of the region but occupying a much larger proportion of the land area. The table shows the estimated Bedouin population by country. These figures include Bedouin who have settled on the edge of the desert and are known as fellahin. Starting in the 1920s the nomads were increasingly compelled to obey the laws of the countries through which they traveled. They were forbidden to raid isolated villages or to conduct feuds between tribes. Some enlisted as sol-

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Bedouin Population by Country COUNTRY

NUMBER

Tunisia

2,030,000

Egypt

1,250,000

Syria

1,000,000

Libya

750,000

Saudi Arabia

700,000

Jordan

326,000

United Arab Emirates

180,000

Israel

60,000

Mali

32,000

Burkina Faso

10,000

TOTAL

6,338,000

Smaller numbers of Bedouin live in Iraq and the Sudan

diers or took work in the construction industry, although the Bedouin despise farming and manual labor. Many were forced to settle for political reasons. In the 1950s both Saudi Arabia and Syria took into public ownership what until then had been Bedouin grazing lands, and at about the same time Jordan severely curtailed the grazing of goats. Since that time there have been repeated disputes over land between the fellahin and nomadic pastoralists. Following the return of the Sinai from Israel to Egypt in 1982, the Egyptian authorities encouraged the Bedouin living there to settle as part of a scheme to develop tourism.

Bedouin Life Nomadic Bedouin spend the dry summer months at the edge of the desert and move into the desert during the winter rainy season, when there is pasture for their animals. They are classed according to the animals they own, the most prestigious class being the camel herders, who occupy huge areas in the Sahara and Syrian and Arabian Deserts. Those herding sheep and goats come next in the social structure. They live close to cultivated land, mainly in Jordan, Syria, and Iraq. Cattle herders are found in Sudan and southern Arabia, where they are known as baqqarah. Bedouin are organized into extended family groups, usually called clans. These number several hundred persons, and groups of clans make up tribes. Some tribes are considered noble because they trace their ancestry to either the Qaysi or Yamani tribes of northern or southern Arabia, respectively. Other tribes are considered to be “ancestorless,” and their members live by serving members of a noble tribe,

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for example as entertainers or metalworkers. Tribes tend their livestock within well-defined territories. The head of a Bedouin family is called a sheikh. The title is ancient and describes a distinguished, elderly man, elderly meaning more than 50 years old, so any senior male is likely to be a sheikh. As well as the heads of families, the heads of the clans and tribes are also sheikhs. Those with authority over clans and tribes govern with the help of informal councils of less senior sheikhs. The Bedouin are Muslim, and their religion affects their social structure, although they retain some pre-Islamic beliefs and customs. Bedouin women pray to the new moon, for example, and some people still believe in spirits, called jinni.

Position of Women Men are permitted to have more than one wife, and a family group usually consists of the husband, his wives, and their unmarried children. When Bedouin settle into permanent dwellings, each wife lives in her own house. Women wear the veil in public, and their husbands are permitted to beat them, at least during the first years of their marriage, provided they do not cause bleeding. Women perform most of the chores around a Bedouin camp, including gathering fuel and water as well as pitching, striking, and loading the tents. Traditionally, the women also spun the goat hair and wove it into the cloth from which the tents were made. Although their life is harsh, performing the chores allows women to meet each other with no men present. Herding the animals is a task performed by young men as well as young women and allows them to meet secretly. Marriages used to be arranged, but young people nowadays are often able to choose their partners.

Tents Nomadic Bedouin live in low, rectangular, black tents supported by a line of poles along the center. The length of this line reflects the wealth and importance of the owner of the tent. The sides of the tent can be rolled up to allow air to circulate on calm days, and during bad weather the tent can be sealed firmly. Inside decorative hangings called gata divide the tent into sections. One half of the tent is occupied by the women and children and is also used to store goods and provisions. It has a hearth for cooking. The other half of the tent, also with a hearth, is occupied by the men and is used for entertaining. Hospitality to visitors is very important in Bedouin culture.

Diet The Bedouin diet consists mainly of milk, yogurt, rice, butter, ghee cooking fat, and unleavened bread, with dates and

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other fruits that are obtained during visits to oases. Meat is eaten only on festive occasions. The dairy produce is from camel and goat milk.

Dress Dress, for both men and women, consists of long, flowing robes. These protect against the heat of the day, the cold of the night, and the windblown sand.

NOMADIC PEOPLES OF THE GOBI DESERT

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Mongolia—the official name is Mongol Uls—is a vast Central Asian country. It covers an area of 604,800 square miles (1,566,500 km2), but its population is only 2.356 million, of whom 627,300 live in the capital, Ulaanbaatar (known formerly as Ulan Bator). This means that outside the capital there are on average slightly fewer than three persons for every square mile (1.1/km2). Mongolia is possibly the most sparsely populated country in the world. Saudi Arabia is crowded by comparison, with 22 persons for every square mile (8.5/km2). The average population density of the United States is 83.1 persons per square mile (32.1/km2). The majority of the Mongolian people are Khalkha Mongols, and Khalka Mongolian is the official language. Very little of the land is farmed, and most of the settled farmers are descended from Chinese immigrants. Farming is difficult in the arid climate. Cereals can be grown in some places, but the rainfall is so unreliable that cereals cannot be expected to yield a harvest every year. Mongols have always been pastoral nomads, and most still lead at least a seminomadic life, despite official encouragement for them to settle in the capital or in the second-largest city, Darhan. Nomadism has shaped every aspect of Mongolian culture, and it is a way of life that makes the best possible use of the natural vegetation. Sparse though the population is, no other system of food production could support any greater density. Mongol nomads manage to survive even in the Gobi Desert.

Horses and the “Manly Sports” Mongolians are renowned for their skill in horse riding, and there are keenly fought contests in the three “manly sports” of horse racing, archery, and wrestling. Children from the age of four years take part in the horse races. The main contests take place at the annual Naadam festival, beginning on July 11, the National Day, the most important events being held in Ulaanbaatar.

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History and the Desert Caring for the horses is a task performed only by men, and traditionally Mongolian men considered it demeaning to undertake any work that could not be performed on horseback. In the Mongolian version of chess, the most important piece is not the queen, but the horse. The national musical instrument, called a morin huur, is stringed, played with a bow, and has a horse’s head carved at one end. Legend has it that the first morin huur was made by a rider from the ribs and hair of his favorite horse and used to express his sorrow at the animal’s death. In traditional Mongolian stories the hero is always accompanied by a horse, from which he receives good advice. The national drink, airag, is the fermented milk of mares. It is believed to have special therapeutic and nutritional qualities, and special herds of horses are kept to provide a supply of airag for factory workers and miners. Horses are important, but the principal pack animal is the Bactrian (two-humped) camel (Camelus bactrianus). Of the 367,000 Bactrian camels in Mongolia, two-thirds are in the Gobi. A camel can carry a heavy load, but it can also produce up to 130 gallons (492 l) of milk a year and up to 10 pounds (4.5 kg) of wool that is used to make clothes and blankets. Mongolians consider camels to be very beautiful and even hold competitions to choose the most beautiful animal. In the fall and winter, when conditions are especially harsh, the camels are used for riding rather than for carrying loads.

Surviving Winter Winter is always the hardest time of year. People move with their livestock to winter campsites. At one time the animals had to feed on the hay made in the summer and had nothing more than a stone-walled corral for shelter against the icy wind and blizzards. Severe storms could kill entire herds and flocks—and sometimes did. Since the 1950s the authorities have provided better shelter and fodder. As winter approaches the animals that are unlikely to survive the winter are slaughtered. This reduces the herd to a size that can be fed. The meat from the slaughtered animals is dried or frozen, and people eat it during the winter, when neither horses nor sheep are producing milk. Mongols do not eat horse flesh, but Kazakhs do; about 6 percent of the population is Kazakh. Most Kazakhs live in neighboring Kazakhstan, and their way of life is very similar to that of the Mongols.

Mongol Diet Meat and milk are the staple Mongolian foods. Not only do Mongols dislike vegetables, they positively despise them and have a proverb: “Meat for men, leaves for animals.” Nor do they eat fish, although Mongolian rivers are full of

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them. Horse meat is highly prized by the Kazakhs, but it is a rare treat enjoyed only by the better-off. Summer, when milk is relatively plentiful, is when the airag is made, as well as arul, which is dried curd and cheese. Apart from airag, the everyday drink is tea, made in a large bowl with added salt and milk. Although these are the basic foods and people are able to subsist on nothing but meat, arul, airag, and tea, Mongol nomads also eat a certain amount of flour made from wheat, barley, or millet. They obtain this from farmers in exchange for animal products. Tea from China is also obtained through barter, as are metal tools, cooking pots, and silken cloth. Since everything the nomads possess must be regularly packed into bundles and carried by camels, however, there is a strong incentive to travel light.

Tent Villages The dwelling for both Mongols and Kazakhs is a ger, also called a yurt or yurta. This is a circular tentlike structure made from wooden poles covered with either skins, woven textiles, but most commonly felt. It is waterproof and strong enough to withstand the weather but light enough to be dismantled and moved from site to site fairly easily. Inside there is a hearth near the center, smoke from the fire rising through a chimney hole in the roof, and the furnishings consist of brightly colored rugs. A visitor, even a total stranger, enters a ger simply by walking in and sitting down. There is never just one ger but always a group of at least two and as many as six, forming the herding camp that is the basis of Mongolian society. Members of the households living in the different gers are often related, but not always, and the arrangement is flexible. Membership of the camps is agreed for one year. At the end of the year a household may choose to remain with the others or to leave and join another camp. If the camp becomes so large as to make the grazing difficult to manage, some of the households will move away and start a new camp elsewhere.

Managing the Pasture Like true nomads everywhere, the Mongolian pastoralists have a detailed knowledge of the land in which they graze their animals and a concept of territory as a patchwork of particular places they are allowed to enter and use at particular times. A group of nomads moving across a landscape, their camels laden and their animals scattered over a large area, may seem to be wandering aimlessly, but they are not. Every migration is carefully planned and timed to make the best possible use of the natural resources. As well as camels and horses, the nomads herd sheep, goats, and cattle. Some of the Mongol cattle herds include

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yaks. Sheep are by far the most important of their animals. They provide milk for making butter and arul, as well as meat, wool, hides, and dung. Sheep dung is the fuel used for cooking and heating. A herding camp always has sheep. It has other animals only if there are enough people to tend them and pasture to feed them. Each species is managed separately because their dietary requirements differ, so the families must decide carefully whether the benefit they can expect to derive from a herd of goats, cattle, or horses justifies the full-time work of one or more persons for each species. All the sheep in the camp are managed as a single flock. Every morning they are driven out to the pasture, and every evening they are brought back to the camp. Their grazing is controlled, and they move out from the camp in a spiral, moving a little farther each day. When the fresh pasture is too far away for convenience, the camp moves to a new site. The sheep are brought in at night partly to ensure the supply of dung. Sheep feed all day and defecate in the evening, a behavior that has been exploited in many parts of the world. In Britain, for example, sheep were traditionally pastured on the hillsides by day and in the evening brought onto low ground around the farmstead, where their dung fertilized the fields used for growing crops. Essentially, the method transfers fertility from the hills to the lowlands. Mongolian sheep are also brought to the camp at night for their protection. Wolves and other large predators attack sheep, especially lambs. To help protect the flocks the Mongolian shepherds have dogs that are notoriously fierce and will attack any animal—or strange person—they perceive as a threat to the sheep. The dogs do not herd the sheep, however. Kazakhs are Muslims, but Mongolians are Tibetan Buddhists—that is, their type of Buddhism came to them from Tibet and centers on lamas, who are teachers and spiritual leaders. They also have many beliefs and customs derived from shamanism, the religion that preceded Buddhism. In shamanism a certain individual called the shaman, who may be a man or woman, is believed to be able to communicate directly with the spirits and from that ability to heal the sick and foretell the future.

The Yak Breeds of domestic cattle that are at home on the ranges of North America are able to survive on the arid grasslands of Central Asia, but the native cattle in this part of the world are yaks (Bos grunniens). Domesticated yaks are descended from wild yaks that can still be found in northern Tibet, although nowadays in greatly reduced numbers. They are closely related to domestic cattle and are able to interbreed with them. Yaks produce milk, meat, hides, and long hairs that in some places are used to make fly whisks. They are also used as pack animals, and in the mountains, where the terrain is unsuitable for horses, people ride them.

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Yaks thrive in the climate and on the sparse pasture found 14,000–20,000 feet (4,300–6,000 m) above sea level. As with all cattle, the food they eat ferments in the rumen, one of the four chambers into which the stomach is divided. Fermentation requires a constant temperature of 104°F (40°C), and yaks are able to maintain this, even in the depths of a Himalayan winter. To help them, yaks have a long, very thick coat. Yaks are big animals. A wild bull is up to six feet (1.8 m) tall at the shoulder; cows and domesticated yaks are smaller. They live naturally in large herds and feed on grass. Yaks need a plentiful supply of water, and in winter they are said to eat snow.

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CARAVANS AND THE SILK ROAD

Not only is it difficult to cross a desert, it can be extremely dangerous. People should never attempt it alone. An illness or accident that might be trivial in a city can be fatal in a remote spot in the desert heat or cold. Solitary travelers are also easy targets for robbers. To avoid such dangers as these, people have always crossed deserts as groups. A group of people journeying together is called a caravan, and until modern vehicles superseded them camels were the animals most often used to carry travelers and their baggage across deserts. A caravan was invariably a camel caravan, although it might include mules. Sometimes the camels traveled in single file as a long procession. This is the way they are usually depicted in movies. There were caravans of this type, especially in China, but more commonly the camels moved across open landscapes as strings of up to 40 camels each. The camels were held together by ropes fastened to the saddle and passed through the nose ring of the camel behind. Strings walked three or four abreast. A caravan could contain a large number of camels—often hundreds and sometimes thousands. The actual number depended on how many passengers wished to travel, the volume of goods to be conveyed, the availability of animals, and the dangers to be faced along the route. There was always safety in numbers, so the more hazardous the journey the bigger the caravan. Caravans did not move at all times of the year. Camel drivers who were planning long journeys needed to know there were pasture and water for the camels along the route. A camel can last a long time without food or water (see “The Ship of the Desert” on pages 138–142), but it cannot survive indefinitely and even though it survives, without nourishment it will weaken. It might also be necessary to time a departure in order to arrive at a particular time. Commercial traffic needed to arrive as quickly as possible, but pilgrims also traveled in this way and those undertaking the hajj needed to be in Mecca by the seventh day of the month of

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History and the Desert Dhū al-Hijjah. Consequently, pilgrim caravans would depart for Mecca at particular times from different places. Each camel was usually loaded with about 350 pounds (160 kg), and passengers did not ride them seated behind the camels’ necks. That is the way camel riders ride, but camel passengers were carried in panniers—large bags or baskets— slung one on each side of the animal. In cool weather or over short journeys the camels might carry heavier loads.

The Caravansary Each day the caravan would travel at a steady two to three MPH (3–5 km/h) and stop to rest after eight to 14 hours. In very hot weather caravans traveled by night. Wherever possible the caravan would spend the night at a caravansary, which was the equivalent of a motel but very much larger. Because of its size, the caravansary was usually located outside the walls of a town or village. The caravan entered through the only entrance, which was a wide, high, strong door set in the tall, thick walls. Inside was a rectangular paved courtyard open to the sky that was big enough to contain up to about 400 camels or mules. In one corner of the courtyard there was a hearth for cooking, and water could be obtained from a well in the center. The caravansary was open from dawn until night. Then the door was closed and secured with heavy chains. A covered walkway resembling a cloister surrounded the central court. Rooms opening off the walkway were used for storing merchandise and stabling animals. Wide stone steps led to another walkway running around the upper story. The travelers slept in rooms off the upper walkway. The caravansary was publicly owned and open to all travelers, but it provided only accommodation, not food for people, fodder for animals, or bedding. A porter appointed by the local community supervised the establishment and had assistants to help him. He had a room beside the door, and it was his job to maintain order and also to guard the building, its occupants, and their property. Travelers seeking food and more luxurious lodgings could go into the town or village in search of an inn, called a khan.

Caravan Routes Caravans traveled certain routes, and apart from those used by pilgrims, the routes were developed for trading goods, most often with the Mediterranean civilizations. One route that existed in Old Testament times, for example, was used to convey gold, silver, and frankincense from south Arabia (Yemen) to Gaza or Damascus. Salt was also a valuable commodity, and caravans conveyed it from south Arabia to the Mediterranean. The principal salt routes, however, linked the mines at Taoudenni, in northern Mali, and Bilma, in eastern Niger, to Timbuktu in the south of Mali.

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Where possible, routes called at oases. A caravan route that was in regular use for many centuries ran from Lake Chad to Bilma, from there across the Fezzan region of Libya calling at the oases at Marzūq and Sabhā, and ended at Tripoli on the Mediterranean coast. There are few camel caravans nowadays—although some still cater to pilgrims—but the routes have survived. Trucks, four-wheel drive vehicles, and in some cases buses travel them today, and like the caravans of former times they often travel in convoys for safety.

Emperor Wu-ti and the Silk Road The most famous of all caravan routes was called the Silk Road. It was opened around 100 b.c.e. during the reign of the Chinese emperor Wu-ti, also known as Wudi, of the Han dynasty. His real name was Liu Ch’e. Wu means “martial,” and Wu-ti is the name that was conferred on him after his death. Wu-ti expanded the empire to the south and also to the west, across the Gobi, where his armies tried unsuccessfully to subdue a nomadic people called the Hsiungnu. Autocratic and ruthless, Emperor Wu-ti cared nothing for the dangers and privations faced by his troops, but he made the imperial bureaucracy more efficient, instituted Confucianism as the state religion, and made large areas of Central Asia fairly safe for travelers. Routes were established that encouraged trade, and this trade eventually linked the two great empires of China in the east and Rome in the west by the most famous route of them all. The Chinese had been manufacturing silk for many centuries, but the method of production was a closely guarded secret. Anyone who revealed it to a foreigner was condemned to death by torture. When the first silk cloth reached Europe it was literally worth its weight in gold. It was a fabulous material because no one in the West had the remotest idea how it was made. Spices from the Tropics entered China from the south, and they were also exported to Europe. The Roman Empire exported gold, silver, precious stones, and woolen cloth. Few people journeyed the whole length of the Silk Road, and caravans would travel particular stages, then return. Passengers and merchandise traveling more than one stage would transfer to the next caravan.

Travelers’ Tales It was not only goods that traveled along the Silk Road. Travelers’ tales about China reached the West, and Buddhism from India and Nestorian Christianity entered China. The Nestorians are a Christian sect founded in Asia Minor and Syria in the fifth century whose adherents base their beliefs on the teachings of Nestorius, a fourth-century

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bishop of Constantinople. Today Nestorians live in Iraq, Syria, and Iran. Even so, Western knowledge of China was very rudimentary. Traders could reach India by sea. Europeans had known something of that land since the time of Alexander the Great, but China remained a mystery. Silk was highly prized, but no one knew anything at all about the country that produced it or of the territory it had crossed on its journey to them. Theophactulus Simocattes, also spelled Simocatta, was the first person to obtain any reliable information about the Chinese Empire. He was an Egyptian historian who held the office of prefect and imperial secretary to Heraclius, the emperor who ruled Constantinople from 610 to 641, and he met envoys from somewhere in Central Asia who had arrived at court. It is unlikely that these persons were Chinese, but they had had contact with Chinese officials and could describe them. The rise of Islam made direct contact still more difficult. Islamic countries were hostile to Westerners, effectively placing a barrier across all of the land and sea routes to Asia.

Marco Polo Europeans had to wait until early in the 14th century for a reliable account of the Chinese Empire. That came with the publication of the most famous of all travel books, known in Italian as Il milione (the million) and in English as Travels of Marco Polo. In about 1260 the Venetian brothers Niccolò and Maffeo Polo sailed from Constantinople to the Black Sea port of Sudak. They were merchants seeking better markets for their goods, and another brother, Marco, already had a house in Sudak. Niccolò and Maffeo then traveled on into China. The two Polos made a second journey eastward in 1271, this time accompanied by Marco’s son, also called Marco (1254–1324). The three men set out in a caravan carrying valuable goods they hoped to sell in lands beside the Volga River ruled by Barka Khan, a Tartar lord. Their journey was successful, but war broke out before they could return and they were trapped. They found refuge at Bukhara and spent their time, probably several years, learning the Tartar language and customs. Then they were offered the chance to join a delegation traveling to the overlord of all the Tartar rulers, Kublai Khan. The Polos traveled to the region of northern China that was known then as Cathay, from the Khitai, a tribe that had established an independent state there, and for part of the way they traveled the Silk Road. Eventually, they reached the palace of Kublai Khan at the city then known as Khan-balik and now called Beijing. They spent about 20 years traveling in the service of Kublai Khan, finally returning to Venice in

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1292. Marco wrote his account of their adventures while he was a prisoner of war in Genoa in 1298–99.

The Route of the Silk Road For travelers from China the Silk Road began at the Han capital, Ch’ang-an’ch’eng, “the walled city of Ch’ang-an,” to the northwest of the modern city of Sian, also spelled Xian. That is where smaller trade routes converged from all over eastern China. From Ch’ang-an the road ran northward, meeting and then following the Great Wall past the Nan Shan Mountains. It skirted the southern edge of the Gobi Desert and passed through Tun-huang, a town built around an oasis in the Kansu-Sinkiang Desert. That is where Marco Polo first entered China. It was then called Sha-chou. West of Tun-huang the Silk Road divided into two branches. One passed the northern edge of the lake of Lop Nor and skirted the Tarim Basin on the northern side. The route Marco followed passed the southern edge of Lop Nor and went to the south of the basin. The Tarim Basin occupies the center of the Takla Makan Desert. From the Takla Makan the route led to Soch’e, or Yarkand, then Sufu, or Kashgar—once called Cascar—after which it climbed into the Pamir and Karakorum Mountains on the border between the Sinkiang Uighur Autonomous Region of China and Tadzhikistan. The route then turned south along what is now a paved road, the Karakorum Highway leading to Islamabad, then north again to Samarkand, in Uzbekistan. It crossed to the south of the Caspian Sea through northern Iran skirting the Dashte Kavir Desert, crossed Iraq and Syria, passed through Damascus, and reached the Mediterranean coast at the ports of Alexandria in Egypt and Antioch, the city in southern Turkey that is now called Antakya. Passengers and goods were carried from there by sea to Italy. With the decline of the Roman Empire and the rise of militant Islam, traveling the Silk Road became dangerous, and the route fell into disuse. People still traveled it during periods of political stability, however. When Marco Polo made his journey the Mongol emperor Kublai Khan had established peace in the lands he crossed. There are now plans to restore the Silk Road as the Trans-Asian Highway.

PEOPLES OF THE AMERICAN DESERT

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Despite its arid climate, Arizona has been inhabited for longer than most parts of North America. There is archaeological evidence of human habitation 25,000 years ago, and for the last 2,000 years people have lived in highly

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History and the Desert

Enchanted Mesa, New Mexico, sometimes called Sky City and formerly known as Katzimo, rises 450 feet (137 m) above the valley floor. According to legend it was once the home of the Acoma; archaeologists believe it was occupied around 1150 C.E. (NASA)

advanced societies. It is in parts of Arizona and across the border in New Mexico that Native American peoples built villages. Those peoples are known by the Spanish word for a village as Pueblo Indians. Oraibi, a pueblo in northeastern Arizona, may be the oldest continuously inhabited settlement in the United States. People have been living there since 1150 c.e., and the present population is more than 600. Located on the Third Mesa (see “Mesas, Buttes, and Inselbergs” on page 64), Oraibi is the unofficial capital of the Hopi Reservation. The first Pueblo peoples were the Anasazi (see “The Anasazi” on pages 183–184)—their name is the Navajo word for “ancient enemy”—who lived in the region where Arizona, New Mexico, Colorado, and Utah meet. After 100 c.e. one branch of the Anasazi, known as the Salado people, started expanding peacefully into the part of southern Arizona, along the Gila and Salt Rivers, occupied by the Hohokam people (see “The Hohokam” on page 183).

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Nothing is known of the origins of the Anasazi, but they are the ancestors of all the later Pueblo peoples. The oldest traces of them date from around 100 c.e., at which time they were skilled at weaving baskets—100–500 and 500–700 c.e. are known, respectively, as their Basket Maker and Modified Basket Maker periods. After the arrival of the Salado the Hohokam started making baskets. The Anasazi hunted game and gathered wild plants but augmented these by cultivating corn and pumpkins. No one knows why the Hohokam disappeared, but their name means “those who have disappeared” in the language of the Pima people, who may be descended from them. After their departure, the Pima—the name means “river people” in their language—occupied the central part of what had been Hohokam territory.

Pima The Pima were farmers and lived in the river valleys. Cultivating the land and using river water for irrigation as their predecessors had done, they increased the amount of food available to them, and this allowed them to construct bigger villages than most of their neighbors. Farming conditions were not reliable, however, and in dry years the crops failed. Hunting and gathering augmented the supply

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of cultivated food even in the good years, but in the bad years it was the only means of subsistence. Then people lived mainly on jackrabbits and mesquite beans. Pima tribal organization was strong. Local chiefs were chosen for their abilities and were aided by councils composed of all the adult men living in the village. The councils were responsible for maintaining the irrigation channels, supervising the farming operations, and protecting the village from attack. Apache sometimes raided Pima villages. The tribal chief was also chosen rather than inheriting the position. Today there are about 10,000 Pima. Most live on reservations in their traditional lands. They share the land with the Maricopa, a group of Yuma. The Yuma Desert is an alternative name for the Sonoran and Colorado Deserts.

Yuma The Yuma were also farmers, augmenting their diet by fishing, hunting, and gathering wild fruit and edible seeds. They lived on the flood plains of the Lower Colorado River, downstream from the Pima and at the head of the river delta. Unlike the Hohokam and Pima, the Yuma had no need for irrigation. Instead, their small, low-lying fields were inundated every spring when the winter snow melted in the mountains of Wyoming, Colorado, and Utah, sending a huge surge of water down the Colorado River and making it overflow its banks. As well as water, between them every year the Colorado and Gila Rivers deposit an average of 4.59 billion cubic feet (130 million m3) of silt on the floodplain. The silt fertilized their crops, and the Yuma grew corn, beans, pumpkins, melons, and various grasses. Later they also grew wheat as well as cowpeas from seed they obtained from the Spaniards. Food was stored in baskets. The Yuma lived well. Their village communities consisted of several large families, and a council consisting of the male heads of all the families settled disputes. One of the family heads was the leader of the village. The fields were privately owned and inherited through the male line.

Tohono O’odham Farther upstream the people living to the south of the Pima were the Tohono O’odham, a name that means “desert people.” They are also known as the Papago, and today their population numbers about 8,300, most of whom live on the Tohono O’odham, Gila Bend, and San Xavier Reservations in southern Arizona. As their name implies, water was scarce where the Tohono O’odham lived, and they relied on hunting and gathering much more heavily than did their neighbors. Although they farmed, they lived a partly nomadic life. Unlike the Pima, the Tohono O’odham did not irrigate their fields. Instead, they practiced flash-flood farming. Every year heavy sum-

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mer rains would cause sudden, heavy floods—flash floods. After the first rains had moistened the ground the Tohono O’odham planted their seeds, usually four to six inches (10– 15 cm) deep to prevent them from being washed away, in alluvial fans at the mouths of washes near the limit of the area reached by the floodwater. An alluvial fan is a mass of sediment deposited where the bed of a river suddenly becomes less steep, for example at the foot of a hill. Tohono O’odham men made some ditches and dikes to guide the floods and reservoirs to impound the water. The ground remained moist after the rains for long enough to produce crops of beans, pumpkins, cotton, and fast-growing varieties of corn. While the crops were growing the Tohono O’odham lived in villages near the fields. After the harvest the ground dried, and by the end of the summer both food and water were in short supply. That is when the people moved away from their “field villages” to “well villages” in the hills, where they spent the winter. There was water on the higher ground and game they could hunt. Tohono O’odham villages were small, each village consisting of a number of scattered dwellings housing the various branches of several extended families. A village of this type, with a loose cluster of houses, is known in Spanish as a ranchería. The oldest man who was still active led the village community, and there was a village council to which all the adult men belonged. The council met every evening to discuss matters of common interest. Each village also had its own shaman. A shaman is a person able to communicate directly with the spirits. The shaman led the religious ceremonies. A group of related rancherías was the biggest political unit, and this became important when the Tohono O’odham had to defend themselves against the Apache. Then the villages would move closer together. The councils would collaborate, but once the attacks ceased the bonds uniting them loosened. The Tohono O’odham and Pima are closely related, and both speak dialects of the same language, Piman, which belongs to the Sonoran division of the Uto-Aztecan family of languages. These languages are spoken from the southwestern United States and Mexico to Guatemala. In Chihuahua, northern Mexico, the Tarahumara also speak a Sonoran Uto-Aztecan language. They live on a high plateau divided by deep canyons, and farming is possible only in scattered places where there is good soil. There they grow beans, squash, potatoes, and corn and also raise cattle and goats. The Tarahumara, also known as the Rarámuri, live in rancherías and often move from place to place at different seasons of the year.

Hopi All these desert peoples live in pueblos. For centuries they have farmed the land within the limits imposed by the climate and have led settled, mainly peaceful lives provided

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History and the Desert they were allowed to do so. In 1680 all the Pueblo Indians rose against the Spanish, who had imposed their own political structures, and drove them from their territory, but this was in response to extreme provocation. Pueblo peoples fall into eastern and western groups distinguished mainly by the type of farming they practice and their economic organization. They belong to many linguistic and cultural groups, the Hopi being one of the best known. The Hopi speak a language that belongs to the Shoshonean division of the Uto-Aztecan language family. About 6,000 Hopi survive today. A member of the western Pueblo group, they live in northeastern Arizona on a large reservation surrounded by the Navajo Reservation. Most of their villages are built on mesas high above the surrounding country. Oraibi, on Third Mesa, is about 6,500 feet (1,980 m) above sea level. The Hopi raise sheep and grow corn, beans, squash, melons, and other plant crops. Their culture is similar to that of other Pueblo peoples. Social life centers on the village and is dominated by religious observances. There is an ancestor cult into which boys and girls are inducted soon after their sixth birthdays. Its observances, called kachinas (or katcinas), involve impersonations of gods, spirits, ancestors, and clouds by masked men. There are also secret societies, each owned by a particular clan, that are responsible for fertility and other rituals. Many of the ceremonies are conducted secretly in underground chambers called kivas. Men can join the secret societies, some of which demand a rigorous initiation. There are also secret societies for women. The clans and larger family groupings are matrilineal, with descent through the female line. At one time, although no longer, a bride remained in her mother’s house. Most of the traditions are eroding as the Hopi, like other Pueblo peoples, adopt more and more of the dominant American culture.

PEOPLES OF THE ARCTIC AND ANTARCTIC DESERTS

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On Monday, February 15, 1999, an election was held for the 19 seats that constitute the Legislative Assembly of Nunavut, and on April 1 Nunavut came into being as a newly selfgoverning territory in northern Canada. Between 1999 and 2009 the new government will complete the process started in 1993 by which it takes over the responsibilities previously exercised by the government of the Northwest Territories. All residents of the new territory are entitled to vote, but almost 80 percent of the population are Inuit, so they predominate in the legislative assembly. It is they who have determined the highly decentralized type of administration. It will be the first territory in North America to be governed by an aboriginal people. The name Nunavut means “our land” in

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Inuktitut, one of the two dialects of the Inuit language spoken in the territory (the other is Inuinnaqtun).

Nunavut Nunavut comprises 28 communities living in three regions: Qikiqtaaluk, or Baffin, on Baffin Island; Kivalliq, or Keewatin, to the west of Hudson Bay; and Kitikmeot to the west of Kivalliq. The new territory covers 733,400 square miles (1.9 million km2), which is 55 percent of the area of the Northwest Territories and 19 percent of the total area of Canada. Nunavut is bigger than Alaska and nearly as big as Greenland. It has a population of about 29,400, of whom about 17,500 are Inuit, giving it a population density of barely 0.04 persons per square mile (0.01/km2). For comparison, the overall population density of Canada is nine persons per square mile (3.5/km2). There are only 15.5 miles (20 km) of highway in the whole of Nunavut. The biggest town and the capital is Iqaluit, with a population of 3,600, located on the southern coast of Baffin Island, at latitude 64°N. The most northerly settlement is Grise Fiord, at 77°N, where 130 people live. Grise Fiord is inside the Arctic Circle, and in December the Sun does not rise above the horizon.

Greenland Politics On February 16, the day after the Nunavut elections, the people of Greenland, or Kalaallit Nunaat, also went to the polls in an election contested by 206 candidates. The social democrat Siumut Party won 11 of the 31 seats in the Landsting (parliament), the liberal Attasut Party eight, the Marxist Inuit Ataqatigiit Party seven, Katusseqatigiit, an alliance of populist candidates, four, and one seat went to an independent candidate. The voter turnout was 75 percent, showing that democracy is alive and well in Greenland. Kalaallit (Greenland) Inuit constitute about 79 percent of the 56,676 people living in Greenland (according to a 2003 estimate). The ice sheet covers the interior of the island (see “Greenland or Kalaallit Nunaat” on pages 42–45), but the ice-free coastal strip has a total area roughly equal to that of Italy or Norway. That is where the people live. The country became a Danish possession in 1380, was granted home rule in 1979, and was given full internal self-government in 1981 while remaining an integral part of the Kingdom of Denmark. The Danish government remains responsible for foreign and defense policy.

Inuit Colonization of Greenland The Inuit—the name means “the people”—are the desert people of the far north. They live in the high Arctic and subarctic regions of Siberia, Alaska, Canada, and Greenland. Their

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close relatives, the Aleut, live in the Aleutian Islands and the western part of Alaska, but Aleut numbers have fallen sharply over the past 150 years. There were once about 25,000 Aleut, but today there are no more than about 2,000. Their traditional way of life has almost disappeared. Aleut is a Russian name. The people call themselves the Unganan, or Unganas. Their language has three mutually intelligible dialects and is closely related to the Inuit languages, but Eskimo and Aleut cannot understand each other. They have been separated for far too long and the difference between Inuit and Aleut is about as great as that between English and Russian. It was Inuit from Canada who colonized Greenland. The last immigration took place in the middle of the 19th century, when a shaman called Qillaq—in Greenland he came to be known as Qitdlarssuaq—led a group to northwestern Greenland in search of new lands. His descendants live there still.

Eskimo or Inuit In Canada and Greenland the people prefer to be called Inuit. Those living in Alaska prefer Eskimo, a name of uncertain meaning that is probably derived from aiachkime, a word in Montagnais, a language spoken by one of the Algonquin peoples of eastern Canada. In modern Montagnais assimew means “she laces a snowshoe.” To the Cree the people are the ashkimew. At one time it was thought that the Inuit, or Eskimos, were a group of Amerindians who had moved into the far north and then adapted to the conditions they found there. Amerindians were regarded as forest people and the Inuit and Eskimo as people otherwise similar to the Amerindians but who lived along Arctic coasts and in open country inland. Compared with Amerindians, most Inuit people are shorter, are stockier, and have smaller hands and feet. These differences might have evolved in response to the cold climate, but scientists now know that, in fact, this is not the case. Both the Inuit and Amerindians are descended from Asian people, but that is all they have in common. The two are not closely related. The difference is most apparent in blood types. A substantial proportion of Inuit people have blood type B. This type is common in Asia, especially in northern India, but it is not found at all in Amerindians. Inuit and Aleut languages are also distinctive in not being closely related to any other languages, including any of those spoken by Amerindians. There has been considerable intermarriage between Inuit and European peoples in Labrador and Greenland, and some Greenland Inuit have fair hair and blue eyes.

Anarak, Parka, Kayak, and Husky Anorak and kayak, the two familiar words that Inuit languages have contributed to English, reflect the Inuit way

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of life, although the original anoraks—from the Greenland Inuit word anoraaq—were hooded pullovers made from caribou skins, not synthetic fibers. Parka, the name of the outer coat, is an Aleut word. Kayaks, canoes that carry one person, were made from animal bones covered with skins and used for hunting. Overland the principal means of long-distance transport was the dogsled, drawn by Eskimo dogs. The Eskimo in their name—sometimes in the form of huskemaw—became corrupted to husky, and their feats of strength and endurance became legendary. When sled-dog racing became a popular sport early in the 20th century, one of the trials followed a course of 1,050 miles (1,700 km). It seems likely that the Asian migrants who became the Inuit brought their dogs with them, which means that the ancestors of the modern husky left Asia several thousand years ago. Their owners may have allowed them to breed with wolves—or have been unable to prevent them from doing so. Huskies possess certain wolflike characteristics, and, like wolves, they rarely bark. They are nowadays popular as pets, but these beautiful animals require a great deal of strenuous exercise if they are to remain healthy.

Natural Resources Tundra vegetation consists mainly of mosses and lichens, with scattered sedges, grasses, and dwarf shrubs (see “Plants of Continental and Polar Deserts” on pages 119–122). Some of the shrubs bear berries, and these are about the only edible plant foods to be found in the lands of the Inuit. Farther north, in the true Arctic desert, there are no plants of any kind. Trees grow along the southern edge of the tundra, but elsewhere they are uncommon, so wood is available as a building material only locally. Nowhere can it be used as a fuel. Plants are more abundant on the Aleutian Islands than they are elsewhere in the region, and Aleut women used to weave baskets and other items from grasses. Generally, though, the Inuit and Aleut peoples depend on animals rather than plants as their primary resource, and animal life is abundant.

Aleut Way of Life Traditional Aleut villages were located on the coast of Alaska. The villagers needed a reliable source of freshwater, a beach where they could haul their boats ashore, and a clear view to give warning of attacks from other tribes. Most villages contained a number of related families, and a chief might rule over several villages. About 4,000 years ago the Aleut migrated from Alaska to the Aleutian Islands, but they maintained their way of life. Aleut gathered marine mollusks along the shore, but they obtained most of their food by hunting. They caught birds

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History and the Desert and freshwater fish such as salmon, but they were seafaring people and hunted mainly in coastal waters. They caught seals, sea otters, sea lions, whales, and occasionally walrus. Seals and sea otters were pursued in kayaks and killed with harpoons. Whales were hunted in larger boats called umiaks also using harpoons. People living on the Alaskan mainland also hunted land mammals, especially caribou and bears.

Inuit Hunters Many Inuit people hunted in the same way, and some still do. They also fished through the sea ice, caught seals when they visited their breathing holes, and stalked seals that were resting on the ice. These were mainly winter pursuits. In spring communities that had spent the winter together started to disperse. Some families fished in lakes or rivers, some pursued seals farther afield, and some set off to hunt the bowhead whale (Balaena mysticetus). In summer Inuit families used to move inland, traveling by dogsled to hunt caribou, bears, and other large animals. They used bows and arrows for hunting on land. Animals provided all the materials the Inuit needed. Whereas Amerindian peoples used wood, the Inuit used bone. Bones provided the skeletal framework for their buildings and boats. Skins provided them with tough, weatherproof clothing, tents, and the outer coverings of boats. Blubber, the thick layer of fat that insulates the bodies of whales, walrus, and seals, provided fuel oil as well as food. Whale blubber is 50–80 percent oil by weight, and the Inuit burned it in shallow dishes made from stone or pottery to provide light and heat for cooking.

Eating Only Meat The Inuit diet consists of fish, red meat, and blubber. Meat is cooked and eaten soon after the animal has been killed, which is when the Inuit say it tastes best, but they also eat raw frozen meat, for which the Inuit name is quaq. In the Arctic climate meat freezes naturally and fairly quickly on exposure to the air. The Inuit say the ice crystals that form in the blood and tissues aid chewing. Raw meat is easily digested, and the fat in it is readily converted to energy. This produces a rapid acceleration of the metabolic rate that generates body heat. When meat is cooked it is always boiled and made into a stew or soup. Fish is either boiled or eaten raw and frozen. The Inuit diet seems unbalanced to people brought up to believe it necessary to eat cereals, fruits, and vegetables, but the Inuit thrive on it. Eventually scientists discovered that although the Inuit eat no plant material directly, the animals they hunt do, and the vitamins (including vitamin C) and minerals animals obtain from plants are stored in their

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tissues. Cooking destroys some of them. A diet comprising nothing but cooked meat would be deficient, but the Inuit avoid the problem by eating quaq, which is raw. The first Europeans who had close contact with the Inuit were the employees of whaling companies, and they suffered badly from scurvy, the disease that is caused by a deficiency of vitamin C, presumably because they cooked their meat. Nor do the Inuit ever eat salt. They obtain as much as their bodies require from their food, a high proportion of which comes from the sea.

Weapons Traditionally, animal bones were used to make arrowheads and the heads of harpoons. The harpoon was attached to a long rope made from skin, and three large inflated balloons, also made from seal skin and fastened to the rope, made it more difficult for the wounded animal to dive. Its struggles to do so helped exhaust it. Some Inuit used rawhide nets to catch seals but never used them for fishing. Until the Inuit were able to obtain metal tools, knife blades were made of bone or ivory. Bows were made from wood, bone, or antler and backed with twisted sinew by a method very similar to that used by the Mongols. As it shrinks, the sinew bends the bow into a curve. It is then pulled against the curve, so it bends in the opposite direction, and it is held in this position while being strung. This produces a very strong tension and, therefore, a powerful weapon. Bones and stones were also carved to make articles of religious significance and more recently to make tourist souvenirs.

Inuit Society Inuit society was based on the nuclear family of parents and their children, and the family owned only its dogs—the number of which was limited by the ability of the family to feed them—together with whatever they could make with their own hands or had inherited from others. Kayaks, sleds, bows and arrows, harpoons, tools, tents, and clothing were private possessions, but a dwelling belonged to a family only for as long as they occupied it. Once they moved out, anyone could move in. The important resources, which were hunting territory and fishing waters, belonged to everyone and to no one. Families did help each other. Where life was so precarious, a family would always share its food with others that had none in the expectation that they would be treated in the same way if their hunting or fishing should fail. There was a clear division of labor. Men built the home and provided the food. Women prepared the food, prepared skins, and made the clothes. This arrangement made men and women so dependent on each other that no one could live alone. Most marriages were monogamous, but occasion-

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ally a man might have two or even three wives. Women occasionally found a second husband, although such relationships were usually temporary. If someone died, the partner would remarry as soon as possible, and relatives would always take in those who were too old to remarry. Orphans were found homes, and usually, but not always, they were treated well. Quarrels between families often led to bitter and violent feuds, however. These could continue generation after generation and involve the persecution of the entire family, not only of the individual who caused the original offense. Greenland Inuit tried to control feuding by inventing the “song duel.” This was a public gathering at which two enemies insulted and ridiculed each other in song.

Arrival of Europeans The traditional Inuit way of life began to change when European whalers arrived in the 19th century. The Europeans established whaling stations and traded with the Inuit, who found they could obtain manufactured goods in exchange for skins, furs, and ivory from walrus tusks. This improved the living standard for the Inuit, but contact with the Europeans also exposed them to diseases to which they had no immunity. There were many deaths. Early in the 20th century the Hudson Bay Company moved north in response to increased competition from rivals farther south, and they encouraged the Inuit to trap white foxes. Missionaries accompanied the whalers and traders. They opened missions and schools, and in 1970 the government of the Northwest Territories assumed responsibility for education. The Inuit were encouraged to settle, and many abandoned their nomadic lives. Then, from the 1950s, mines and oil fields began to open. These provided employment for local people, who then settled in the towns built to accommodate them. Today the Inuit have access to schools, medical care, and other modern services, yet life has not improved for everyone. The Inuit have experienced the difficulties faced by other Native Americans in trying to adapt to and be accepted by the majority culture. Unemployment in Nunavut is about 22 percent, about one-third of the people receive welfare, and the resulting demoralization has led to alcohol and other substance abuse.

Modern Greenland Economy Greenlanders have also settled in permanent houses. They live mainly by fishing, especially for shrimp, and sheep farming. More recently mining companies have conducted explorations for minerals. These certainly exist, but the climate and ice sheet make conditions extremely difficult for mining. It will be some years before the industry produces significant quantities or provides much employment. Until

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then the principal industries are fish processing, mainly of shrimps, making handicrafts and preparing furs, and some small shipbuilding. Seal hunting has declined greatly, but Greenlanders are allowed to hunt whales for their own use. Those Inuit who continue to live by hunting and fishing use modern methods. Nowadays animals are shot with rifles rather than harpoons. Dogsleds have been abandoned in favor of much faster, if noisier, snowmobiles. Seal hunters work from boats with outboard motors. Everyone wears clothing made in factories and bought from stores. Inuit life may be less colorful than it once was, but in many ways living conditions have greatly improved. There are schools for the children, medical services are available for everyone, and the necessities of life are more easily obtained.

Antarctica Antarctica has no native population. Vast expanses of stormy seas isolate it from all other continents, and even had hardy voyagers managed to reach it, they would have faced not land but the high cliffs of ice that mark the boundaries of the ice shelves. They would surely have perished. Yet the existence of Antarctica had been suspected long before the first explorers stepped ashore. For centuries people believed there was a large continent in the Southern Hemisphere, but that belief arose from a love of symmetry. Southern lands were needed to balance those in the north. Belief led to rumor and legend, but it was not until the age of the Portuguese explorers that Europeans penetrated deeply into the Southern Hemisphere. In 1488 Bartolomeu Dias (ca. 1450–1500) became the first European to round the Cape of Good Hope, and in 1520 Fernão de Magalhães (ca. 1480–1521), better known as Ferdinand Magellan, discovered the strait named after him. The Magellan Strait separates the South American mainland from the island of Tierra del Fuego. The first European and probably the first person, to cross the Antarctic Circle was the British sailor James Cook (1728–79), on January 17, 1772. Exploration intensified during the 19th century, but it was not until the 1940s that the first permanent stations for scientific research were established. Today there are about 40 stations that are occupied throughout the year (see the map in “Cold Deserts” on page 35), together with a variable number of summer stations. In summer the population is about 4,200, falling to about 1,000 in winter. As well as scientists conducting research, there are support staff to provide medical care, building maintenance, catering, and other services.

Antarctic Tourism Antarctica also attracts tourists. Numbers vary, but a peak was reached in the 1999–2000 summer season, when

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History and the Desert 13,826 tourists landed. The International Association of Antarctica Tour Operators reported that 29,823 visitors landed on the continent during the 2005–06 season. Most of them arrived as passengers on privately organized expeditions sailing on board commercial ships, though others arrived on private yachts. These visitors live on board and spend only a short time visiting sites ashore, most landing on the Antarctic Peninsula. More than one-third (38.9 percent) of the tourists came from the United States. Most of the others were from the United Kingdom (15.4 percent), Germany (10.3 percent), Australia (8.4 percent), Canada (5.6 percent), the Netherlands (3.2 percent), Switzerland (2.4 percent), and Japan (2.1 percent). Because Antarctica has no indigenous population, it has been possible to regulate activities there through the Antarctic Treaty. This allows scientific research but prohibits industrial or commercial development. Activities on Antarctica are managed by the 45 nations that have signed the treaty.

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DESERT BUILDINGS

People build their homes from whatever materials are easily obtainable. If they live surrounded by big rocks, they build stone houses. Elsewhere they use mud or timber. In the far north the most plentiful material is snow, so that is what the Inuit use. Their word for house is igdlu, from which we derive our word igloo. The igloo is a Canadian invention. The Eskimo of Alaska and Inuit of Greenland do not make igloos, but in what is now Nunavut it is the home in which families traditionally spend the winter. Building an igloo takes an experienced man no more than two hours.

How to Build an Igloo First the builder selects a level site close to a deep drift of fine, compacted snow. From this he cuts rectangular blocks using a long-bladed knife resembling a sword. Traditionally the blade was of bone, but today it is usually metal. Each block measures about 48 × 24 × 8 inches (120 × 60 × 20 cm). The first layer of blocks is laid in a circle, and their tops are trimmed so they slope inward. A second layer is laid on top of the first, trimmed in the same way, and as further layers are added the structure curves inward to make a dome. Finally, room remains for one block. This will be the window, and it is made either from a piece of transparent seal intestine or from a block of clear ice. Entry to the igloo is by a semicylindrical passageway with recesses on either side for use as cupboards. A flap of sealskin hangs over the outer end of the entry tunnel, and a low wall made from snow blocks may be built a few feet away. Between them these keep out the

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drafts. Inside there is a low platform covered with twigs and furs on which everyone sleeps. Inuit who do not build in this style make their homes partly below ground, with upper walls and a roof made from stone or from a frame of wood or whalebone covered with turf. In summer, when they might travel widely in pursuit of game, families often live in tents made from animal skins.

Covered Pits At one time semiunderground homes were also built in the deserts farther south. During their Pioneer period, approximately from 300 b.c.e. to 500 c.e., the Hohokam people (see “The Hohokam” on page 183) lived in shallow pits covered with a wooden or brushwood framework sealed with clay. The Anasazi (see “The Anasazi” on pages 183–184) stored food in covered pits of this kind, but they did not live in them. Their homes were either in caves or made from a wooden framework covered with mud that baked hard in the sun—what modern builders would describe as a timber-frame method of construction—and they installed storage pits, often with roofs, in both their caves and outdoor dwellings.

The Houses of Çatal Hüyük Çatal Hüyük in Turkey is a city that was occupied 8,000 years ago. At that time its population was numbered in the thousands, making it the world’s biggest human settlement. Items recovered from the excavations include the oldest known cloth, mirror, and wooden bowl. There were also tools, weapons, and jewelry. The city consisted of square, flat-roofed buildings placed so closely together that although each building had its own walls, the spaces between buildings were too narrow for anyone to pass. There were no streets, and the principal entrances to the buildings—the front doors—were in the roofs. People entered through the ceiling and walked across the roofs from one building to another. There were more conventional doors at ground level, but archaeologists believe these were used only to allow small domestic animals to enter and leave. This style of architecture is still found in parts of eastern Turkey, as is the method of construction, called himis. Each building was supported by a timber frame, and the walls were made from sun-dried bricks. The roof was made from clay mixed with reeds supported by wood. The citizens of Çatal Hüyük probably conducted much of their business and social lives on the roofs, because the interiors of their homes must have been rather dark and poorly ventilated. All the buildings were the same size. Each had a main room with two storage rooms leading off it. There was a stove for heating, an oven, and a raised structure made of

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earth or stone and covered by a woven mat called a killim. This structure served as table, divan, and bed. It is also where the skulls of the dead were buried together with gifts for use in the afterlife. When someone died the body was left in the open until the vultures had reduced it to a skeleton. People then cleaned the skeleton and wrapped it in cloth, but they took the skull into the house, painted it, and buried it. The walls of the main room were decorated with religious paintings, and the occupants changed or renewed the paintings regularly—perhaps annually—by covering the old painting with a thin layer of plaster and repainting. Some of the houses had up to 30 layers of plaster. The paintings were of horned animals, and the houses also contained statues of the heads of bulls. This suggests that as well as dwellings, the houses were places of worship. There is no evidence that animals were sacrificed.

Adobe Blocks of baked mud are still used as a building material, and both the blocks and the material they are made from are called adobe. Adobe is used where timber is hard to find, so buildings cannot be constructed by fitting cladding over a wooden frame. Instead, the walls must be solid and made from either masonry or clay, and of the two, clay is easier and quicker to use. Adobe walls provide excellent insulation against both heat and cold. This makes them especially suitable for desert climates. Adobe was a traditional building material in North Africa, the Middle East, and Arabia by the time Spain came under Moorish rule in the eighth century. The word adobe is the Spanish version of atob, the Arabic name for a sun-dried brick. When Spaniards arrived in North America bringing their knowledge of adobe construction with them, they found that Native Americans were using an identical technique. Adobe soil is a mixture of sand, silt, and clay. When mixed with water it becomes sufficiently plastic to be easily shaped, and it dries into a hard mass. Soil with these properties is common. Work begins with sorting or sieving the soil to remove all the stones bigger than about one inch (2.5 cm) across. Then the soil is mixed with enough water to turn it into a plastic mass of about the same consistency as the clay a potter would use. Once it has been well mixed, it is left to stand for a day or two. This allows time for water to penetrate and soften all the small lumps. If straw or any other fibrous material is available, it is then added. This strengthens the bricks, and straw has been used for thousands of years to strengthen bricks.

Bricks Made with Straw When the captive Israelites, forced to make bricks for the Egyptians, asked Pharaoh for time off so they could go into the desert to make sacrifices, he punished them by refusing to continue supplying them with straw to make bricks, but

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without reducing the quota of bricks they were required to produce. Henceforth they had to find their own straw in their own time. “So the people were scattered abroad throughout all the land of Egypt to gather stubble instead of straw.” (Exodus 5: 12.) The straw is mixed into the clay, and the brick makers trample the whole mass with their bare feet. After that build-

Modern Bricks Bricks are made today in almost exactly the same way they have been made for thousands of years, but on an industrial scale. Clay is the raw material, and modern brick makers have a much wider selection of clays to choose from than did their predecessors. Usually a number of clays with different qualities are blended to give a material the desired characteristics. The clay is ground and screened to remove stones and to ensure that the particles are of uniform size. If the bricks are to be colored, pigments are added at this stage. In the second stage the clay is mixed with water. The older method uses a large amount of water to produce a very soft clay that is poured into steel molds containing sand or water to prevent the clay from sticking to the sides. Clay projecting above the top of the mold is scraped off, and the clay is compressed by applying pressure of up to 1,500 pounds per square inch (10 kPa). This method has been largely replaced by one using much less water to produce a clay with a stiff consistency that is packed into a machine that extrudes it, rather like toothpaste being squeezed from a tube. As it emerges a blade cuts the clay into pieces the size of bricks. The wet bricks are then dried in an oven, in the course of which they shrink to their final size. If the bricks are to be glazed, the glaze is applied to the dry bricks before they enter the kiln for firing. Bricks are fired in a kiln at 1,600–2,000°F (870–1,100°C) depending on the type of clay. Most modern kilns are tunnels that are cool at one end and become progressively hotter farther in. Up to 3,000 bricks are loaded onto a car that carries them through the tunnel so their temperature rises steadily as they advance, then allows them to cool as they approach the exit. As they emerge from the end of the tunnel, machines stack them in blocks of 500 secured with metal straps, ready for transport to wherever they are needed.

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History and the Desert ing can begin. Walls are built on a stone foundation—or nowadays a concrete base. The foundation must be impermeable to prevent water from being drawn into the adobe by capillarity (see “Except When It Moves Upward, by Capillarity” on pages 68–69). Water would make the clay swell, and the lower blocks would crumble, making the wall collapse. North and Central American peoples used to shape the clay by hand into lumps roughly the size and shape of loaves of bread. These were laid one course at a time, and each course was allowed to dry thoroughly before the next was added. The Spanish introduced molds for shaping the adobe into bricks. Nowadays the molds usually make bricks 3–5 × 10–12 × 14–20 inches (8–13 × 25–30 × 35–50 cm). The shaped bricks are dried completely before being used, and they are then used like ordinary house bricks, with wet adobe clay as mortar. Finally, the finished walls are coated with plaster or, nowadays, cement. Clay remains the material from which bricks are made, but nowadays the clay is used without the addition of straw, which would burn during the firing process (see facing sidebar).

Pueblo Apartment Buildings Adobe construction was widely used in the Native American pueblos, but stone was also used. Between 1050 and 1300, known to archaeologists as the Classic Pueblo period, dressed stone came into use. That is stone that is shaped to improve the fit of one stone against another. Walls became thicker and stronger, and buildings were made bigger. The builders constructed apartment buildings up to four stories high and with up to 1,000 rooms. In effect, each block was a village in itself. Rooms on the ground floor often had only one opening, in the roof. The upper rooms had small windows and doors, but each story was set back from the one below. This gave every room except the lowest a terrace on the roof of the room below. The roofs were strengthened to take the weight by laying a layer of adobe six to eight inches (15–20 cm) thick over a layer of rush matting supported on massive timber beams. Pueblos also had underground rooms, called kivas, in which important religious ceremonies were performed (see “Hopi” on pages 204–205). Some of these were 80 feet (25 m) across. Many Hopi pueblos are of this terraced type.

Cliff Dwellings Some pueblos were built on ledges and in sheltered alcoves on the walls of canyons and mesas. These are usually known as cliff dwellings. Mesa Verde National Park, in southwestern Colorado, contains cliff dwellings built in the 13th century. Some are multistory structures built under overhanging cliffs, the largest of which, known as Cliff Palace, contains hundreds of rooms and several kivas.

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Although there is abundant rock suitable for construction, the extreme aridity of the Atacama Desert makes it totally uninhabitable. (Riccardo Giovanelli/Cornell University)

Examples from the peak of pueblo construction can also be seen at Chaco Culture National Historical Park, in San Juan County, northwestern New Mexico. The site covers 53 square miles (137 km2) and contains 13 ruins dating from pre-Columbian times as well as more than 300 smaller archaeological sites. Pueblo Bonito is a multistory settlement built in the 10th century, with about 800 rooms and 32 kivas.

African Homes African houses are usually built as a number of separate buildings standing apart, each of which contains one room. The rooms may be linked to each other—by walls, not passageways—and a fence or wall often encloses the group. All the rooms look identical. One is used for sleeping, one is the kitchen, another is a food store, and the largest is likely to accommodate cattle. Adobe and clay mixed with stones are the commonest building materials. Some houses have more than one story. Those built by the Somolo people of Burkina Faso, for example, have six or seven circular buildings that are merged together, some with two stories and others with three. A single house contains up to 20 rooms. Each wife has her own room, there are rooms for the children, and some rooms are used to store food. The Dogon people of Mali and Burkina Faso live in villages consisting of dwellings clustered on the sides of escarpments. Their houses are made from adobe and roofed with thatch. Each dwelling accommodates a family, and the senior members of the community live in bigger houses. In the Sahara and Middle Eastern deserts most ordinary houses are made from adobe, but stone is often used

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for more important buildings. Most desert buildings are square or rectangular, although in Ethiopia some Tigre farmhouses are circular, made from stone, and are multistoried. When the Tuareg build permanent houses they are usually square and made of stone. Roofs are often flat because there is no need for them to divert rainwater away from the walls or to shed snow, and a flat roof is more economical in its use of materials.

EXPLORERS OF THE POLAR DESERTS

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In 1882, as the Norwegian sealing ship Viking came within sight of Greenland, a young member of the crew caught his

first glimpse of the ice cap. Pondering what he had seen, it struck him that it might be possible to cross the ice cap—to cross Greenland. Over the next few years he developed an audacious plan. The inhabited towns and villages lie along the western coast. His plan was to start from the uninhabited eastern coast, so there was no town to which the party could return and it would be compelled to continue. On August 15, 1888, the party of six explorers arrived at the eastern coast, and their journey began. Their leader, then aged 26, was Fridtjof Nansen (1861–1930; see the sidebar). Nansen was a keen outdoor sportsman and expert skater and skier, and all the members of his team were in peak condition. They needed to be, because the crossing was arduous. On September 5 they reached the highest point, 8,921 feet (2,719 m) above sea level, and on September 26 they reached the coast. It was then too late in the year to

Fridtjof Nansen (1861–1930) Fridtjof Nansen was a Norwegian explorer, scientist, and statesman who won the 1922 Nobel Peace Prize for his humanitarian work with the League of Nations and the International Committee of the Red Cross. Nansen was born on October 10, 1861, at Store-Fröen, on the outskirts of Christiania (now Oslo). He studied zoology at Christiania University. A keen outdoor sportsman, Nansen was an accomplished skater and skier and developed a strong physique and stamina. He first visited the Arctic in 1882 as a member of the crew of the Viking, a sealing ship that sailed close to Greenland. The encounter gave Nansen the idea of crossing the Greenland ice cap. He recruited a party for the trek and set out in August 1888 from the east coast, reaching the west coast almost six weeks later. They spent the winter at Godthåb (now Nuuk), where Nansen spent his time studying the Inuit people. This experience led to his most famous expedition. With financial support from the Norwegian government and private subscriptions, a ship called the Fram (“forward”) was built to Nansen’s design, and on June 24, 1893, it sailed for eastern Siberia with a crew of 13. Nansen’s idea was to allow the ship to be enclosed by sea ice and then to drift with the ice in order to track the direction of the ocean currents. The Fram withstood the pressure of the ice and drifted slowly northward. Nansen left the ship when it reached 84.07°N and headed north by dogsled and kayak accompanied by F. H. Johansen, reaching 86.23°N, the highest latitude

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anyone had then reached, before making for Franz Josef Land. The two men were forced to spend the winter, from August 1895 until May 1896, on Frederick Jackson Land, where they built a hut and hunted polar bears and walrus for food and fuel oil. They finally returned to Norway on August 13, 1896. The Fram also returned safely to Norway, as Nansen had predicted it would. Nansen was also an eminent scientist. In 1896 he was appointed professor of zoology at Kristiania University (the spelling had been changed), but in 1908 this was changed at his request to professor of oceanography. He took part in several scientific cruises, making discoveries about wind-driven ocean currents and the circulation of ocean water. He took an active part in the dissolution of the union between Sweden and Norway that established Norway as an independent nation, and he became the first Norwegian minister in London. During World War I he headed the Norwegian mission in the United States. Nansen headed the Norwegian delegation to the first assembly of the League of Nations, where one of his tasks was to arrange the repatriation from Russia of almost 430,000 German and Austro-Hungarian prisoners of war. He led the efforts by the Red Cross in 1921 to bring relief to Russia, stricken by famine, and he proposed a scheme, adopted internationally in 1922, to issue refugees identification documents. He devoted his Nobel prize money to furthering international relief work. Nansen died at his home at Lysaker, near Oslo, on May 13, 1930.

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History and the Desert return to Norway because of the sea ice, so the explorers spent the winter at Godthåb (now Nuuk). Nansen made use of his time there to study the Inuit and their way of life. They returned home in 1889, and in 1891 Nansen published his book Eskimoliv (Eskimo Life). In 1909 the Belgian explorer Adrien de Gerlache crossed Greenland in the opposite direction, from west to east, at 77°N.

The Fram, Drifting with the Ice Soon after his return from Greenland Nansen was planning his next expedition, which was eventually funded partly by the Norwegian Parliament and partly by private subscriptions, including one from King Oscar II. Nansen’s aim was to study the movement of sea ice, which he calculated would carry a ship from Siberia to the Svalbard archipelago. He designed a ship, the Fram (“forward”), that would be lifted by the ice as the sea froze around it instead of being crushed, and with a crew of 13 the Fram sailed from Kristiania (now Oslo) on June 24, 1893. It reached 78.83°N, 133.62°E, northeast of the Novosibirskye Ostrova (New Siberian Islands) on September 22, where it became locked in the ice. The ship was not damaged, and it drifted with the ice, just as Nansen had predicted. The following March Nansen and one companion, F. H. Johansen, left the Fram to journey north by dogsled and kayak. On April 8 they reached 86.23°N, the highest latitude anyone had reached at that time. The two explorers spent the winter in the far north in a hut of stone covered with a roof of walrus skin and returned to Norway on August 13, 1896. The expedition also studied the seabed. Previously, scientists, including Nansen, had believed the sea around the North Pole was shallow. Soundings from the Fram revealed depths from 11,000 feet to 13,000 feet (3,355–3,965 m). This showed that the North Polar Sea was in fact an ocean—the Arctic Ocean. Nansen’s Fram was the first ship to drift with the sea ice, but it was not the last. In 1937 a Soviet icebreaker, the Georgy Sedov, was trapped by ice in the Laptev Sea and spent 27 months drifting across the Arctic Ocean. Airplanes were able to land on the sea ice, and in 1937 a Soviet airplane based at Zemlya Frantsa-Iosifa (Franz Josef Land) landed four men at the North Pole. There they established a scientific research station, called North Pole 1, on a large ice floe. This drifted for nine months. By the time the ice floe reached the Greenland Sea, it was melting, and the station and its crew were removed.

By Air to the North Pole Other Arctic explorers flew. Richard Evelyn Byrd (1888– 1957) claimed to be the first person to fly across the North Pole on May 9, 1926, as the navigator with the American

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aviator Floyd Bennett (1890–1928) as pilot, although there was doubt over whether the claim was valid. Byrd’s diary of the time, discovered in 1996, suggested that an oil leak from the starboard engine of his three-engined Fokker airplane had caused them to turn back. If that is so, the first flight over the pole was made later in 1926 by an airship, the Norge. That expedition was financed and led by the American explorer and scientist Lincoln Ellsworth (1880– 1951; see sidebar on page 214). The other members of the crew were the Italian aeronautical engineer Umberto Nobile (1885–1978) and Roald Amundsen (discussed later). The first person to cross the Arctic Ocean by air was the Australian-born British explorer Sir George Hubert Wilkins (1888–1958), and he was knighted for the achievement. On April 16, 1928, Wilkins and Carl Ben Eielson (1897–1929) took off from Point Barrow, Alaska, and landed, 20.5 hours later and 2,100 miles (3,400 km) away, at the Svalbard archipelago, north of Norway. Wilkins was not only an aviator. He also pioneered the scientific use of submarines. In 1931 he navigated the U.S. submarine Nautilus beneath the Arctic Ocean as far as latitude 82.25°N. Eielson died in a plane crash in Siberia on November 9, 1929. On another flight later in the year that he flew across the Arctic Ocean, Wilkins also discovered several previously unknown islands off the coast of Antarctica. That flight took him 600 miles (965 km) from Deception Island southward across Graham Land, forming the northern part of the Antarctic Peninsula.

Shackleton His 1929 flight was not the first trip Wilkins had made to Antarctica. He had served as naturalist on the last expedition made by Sir Ernest Henry Shackleton (1874–1922). This was the Shackleton–Rowett Expedition that began in 1921. Shackleton himself never completed it. He died at Grytviken, South Georgia, on January 5, 1922. Shackleton began his Antarctic career in 1901, when he joined the British National Antarctic Expedition as third lieutenant. Sailing in the Discovery and led by Captain Robert Falcon Scott (1868–1912), the team reached the Ross Ice Shelf, and Shackleton, Dr. Edward Wilson (1872– 1912), and Scott traveled south by dogsled to 82.28°S. Shackleton was unable to complete the expedition owing to ill health. In 1908 he returned as leader of the British Antarctic Expedition, sailing in the Nimrod. They failed to reach their intended base and spent the winter on Ross Island, in McMurdo Sound. Shackleton led a party that came within 97 miles (156 km) of the South Pole, for which he was knighted. In 1914 Shackleton led the British Imperial TransAntarctic Expedition. The plan was to cross the continent from the Weddell Sea to McMurdo Sound via the

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South Pole. Their ship, the Endurance, became trapped in ice, drifted for 10 months, and was finally crushed. Six of the explorers spent the following five months drifting on ice floes before escaping to Elephant Island, one of the South Shetland Islands. This involved them in an 800-mile (1,287 km) journey in an open whale boat followed by a crossing of the island to seek help.

Overwintering in Antarctica—the Belgica and Southern Cross By the time of the Scott and Shackleton expeditions the feasibility of overwintering in Antarctica had been established. The first ship to spend the winter there was the Belgica, a

Lincoln Ellsworth (1880–1951) One of the heroes of the early days of aviation, Lincoln Ellsworth was born into a wealthy family in Chicago on May 12, 1880, but his was a lonely childhood. The family home contained a library with many first editions and many valuable paintings, but Lincoln and his sister Clara saw little of their father, who was away on business for much of the time. Their mother died in 1888. Ellsworth was educated at Columbia and Yale Universities, in Canada at McGill University, and at the London School of Mines. In 1902 he took part in the first survey of the transcontinental route for the Canadian Pacific Railroad, and in 1906 he became resident engineer for the company at Prince Rupert, British Columbia. Later he worked in the Rocky Mountain states for a U.S. biological survey. It was during World War I that Lincoln Ellsworth trained to fly as a pilot and combat observer. In 1924 he headed a surveying expedition arranged by Johns Hopkins University of the mountains between the Amazon basin and the Pacific Ocean. His polar explorations began in 1925. On May 21 of that year Ellsworth and Roald Amundsen led four other men on a mission to fly to the North Pole in two Dornier amphibious aircraft. They took off from Spitzbergen but had to make an emergency landing and became trapped by ice. One aircraft was wrecked, and the team spent 30 days carving a runway on the rough pack ice before they were able to fly back to Spitzbergen, heavily overloaded and with only 23 gallons (78 l) of gasoline in the tank. A year later the second polar expedition was in the dirigible Norge. On May 11, 1926, Ellsworth and Amundsen together with the Italian explorer Umberto Nobile left Spitzbergen bound for the North Pole and then Alaska. The journey of 3,393 miles (5,459 km) took them 72 hours, and when they landed 91 miles (146 km) northwest of Nome, the Norge was coated in a thick layer of ice. It was the first crossing of the Arctic basin and was acclaimed across the world. In 1931 Ells-

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worth crossed central Labrador by canoe, a distance of 800 miles (1,287 km), and later the same year he accompanied Hugo Eckener and a party of 15 scientists including Nobile on flights across Franz Josef Land and Novaya Zemlya in the airship Graf Zeppelin. Other explorers had been flying over Antarctica, and Ellsworth determined to make the first transantarctic flight from the Ross Sea to the Weddell Sea and back. After two failed attempts, on November 20, 1935, Ellsworth made the first of several exploratory flights from Dundee Island on the Antarctic Peninsula, flying as observer with Herbert Hollick-Kenyon as pilot. The main attempt began from Dundee Island on November 23 in the Polar Star, an all-metal, low-wing monoplane built by the Northrop Corporation and powered by a Pratt and Whitney Wasp engine. The aviators had to make three landings because of bad weather, waiting sometimes for days for conditions to improve, and one because they ran out of fuel. By then they were about 16 miles (26 km) from the Bay of Whales, which they reached on foot on December 15. They had traveled about 2,200 miles (3,540 km) and were airborne for approximately 20 hours. Ellsworth led a final expedition to the Indian Ocean quadrant of Antarctica in 1938–39. In the course of his expeditions Ellsworth claimed a total of about 300,000 square miles (777,000 km2) for the United States. He named one area James W. Ellsworth Land for his father; it is now called Ellsworth Land. Lincoln Ellsworth died in New York City on May 26, 1951. It was not the first time Ellsworth and Amundsen had flown together. In 1925 they made an attempt at the North Pole. Nor was it Ellsworth’s last emergency. In 1935 with the Canadian pilot Herbert Hollick-Kenyon (1897–1975) he flew from the Antarctic Peninsula to the Little America base, but they ran out of fuel and had to complete the journey on foot. Ellsworth also flew over Zemlya Frantsa-Iosifa (Franz Josef Land) and Novaya Zemlya for the American Geographical Society.

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History and the Desert Belgian vessel commanded by Adrien-Victor Joseph, baron de Gerlache de Gomery (1866–1934). The mate on the Belgica was a Norwegian, Roald Amundsen. De Gerlache explored the region north of the Peninsula then sailed into pack ice, where the Belgica remained trapped through the winter from March 1898 to March 1899. During the same winter an expedition from London led by the Norwegian-Australian explorer Carsten Egeberg Borchgrevink (1864–1934), sailing on the Southern Cross, camped at Cape Adare, on the northern tip of Victoria Land, the region on the western side of the Ross Sea. Borchgrevink also sailed south to the Ross Ice Shelf and found its edge was much farther south than it had been in 1842. This demonstrated that the area of the ice shelves is constantly changing for entirely natural reasons.

Bellingshausen Earlier explorers had approached the Antarctic coast, so those intending to land already had an idea of what to expect. Sir James Cook (1728–79) was the first person to cross the Antarctic Circle in 1772. Nearly half a century passed before anyone else attempted this voyage. In July 1819 the Vostok and Mirnyi sailed from Kronstadt, Russia, on an expedition authorized by Czar Alexander I and led by Fabian Gottlieb von Bellingshausen (1778–1852). They crossed the circle on January 26, 1820, and crossed it several more times before returning to Russia. At one point they came within 20 miles (32 km) of the coast.

Dumont d’Urville and Wilkes In 1837 a French expedition led by Captain Jules-SébastienCésar Dumont d’Urville (1790–1842) sailed from Toulon in two ships, the Astrolabe and Zélée. The crews were promised 100 francs if they crossed latitude 75°S, the farthest point reached by James Weddell (see below), and an additional 20 francs for every degree they traveled farther south. They managed to map the coast of the northern part of the Antarctic Peninsula and chart the waters off shore, but the exploration had to be abandoned when scurvy broke out among the crew of both ships. The explorers returned later and named part of the continent Terre Adélie, after Dumont d’Urville’s wife. The famous Adélie penguins are also named after her, and the principal French research station, on the coast near the magnetic South Pole, is called the Dumont d’Urville Station. One day in January, 1840, the two French ships saw an American ship approaching. This was the Porpoise, commanded by Charles Wilkes (1798–1877), leader of the United States Exploring Expedition that lasted from 1838 until 1842. The two commanders ignored each other, Dumont d’Urville sailing northward, Wilkes maintaining his westerly course, and each believing the other had insulted him. Wilkes sailed

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along the edge of the ice pack for about 1,500 miles (2,400 km), spotting land several times. This was the first proof that an Antarctic continent existed. The part of the continent he saw is now known as Wilkes Land. The Wilkes expedition collected a large amount of scientific observations and specimens.

Ross and Weddell James Clark Ross (1800–62; see the sidebar on page 216) was also in the area at the time. He was a British naval officer leading an expedition with two ships, the Erebus and Terror. Sailing from Hobart, Tasmania, in November 1840, they headed first for the Auckland Islands. There they found two boards bearing notices. One said Charles Wilkes had called there on March 10, 1840. Beside it the other announced that Dumont d’Urville had been there on March 11. Ross found the way south blocked by a smooth, perpendicular cliff of ice 150 to 200 feet (46–61 m) high and flat at the top. He called it the Victoria Barrier. In fact, it was the edge of what was later renamed the Ross Ice Shelf. Ross also named an active Antarctic volcano Mount Erebus after one of his ships and an extinct volcano nearby Mount Terror after the other. Both volcanoes are located on Ross Island, on the western side of the Ross Sea. The U.S. McMurdo base and the British Scott base are both on Ross Island. On his return to England in 1843 Ross was knighted. Ross sailed around the ice of the Weddell Sea. That sea was discovered in 1823 by a British sealer, James Weddell (1787–1834), who named it King George IV’s Sea. In his search for seals Weddell claimed to have sailed about 214 miles (344 km) farther south than James Cook.

Byrd, Ellsworth, and Palmer Other regions of Antarctica are also named for explorers or their wives. Whatever the truth about his Arctic flight, Richard E. Byrd led three highly successful expeditions to Antarctica, in 1928–30, 1933–35, and 1939–41. During the first expedition he established a supply base called Little America on the Ross Ice Shelf close to the Bay of Whales. From there he flew as navigator over a large area of the continent. He discovered the Rockefeller Mountains, which he named after one of his sponsors, and the territory behind them, which he called Marie Byrd Land after his wife. He made his last flight over the South Pole on January 8, 1956. Lincoln Ellsworth, a member of what was possibly the first team to fly over the North Pole, also explored much of Antarctica from the air. The Ellsworth Mountains are named after him, and he named Ellsworth Land for his father. The southern part of the Antarctic Peninsula with its adjacent islands is called Palmer Land. It is named after an American sea captain and explorer, Nathaniel Brown Palmer (1799–1877). He went to sea at 14 and later became a sealer.

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James Clark Ross (1800–1862) James Clark Ross was born in London on April 15, 1800, and entered the Royal Navy at the age of 11. In 1818 he took part in an expedition into Arctic waters searching for the Northwest Passage led by his uncle Captain John Ross (1777–1856; later Rear Admiral Sir John Ross). James accompanied Rear Admiral Sir William Edward Parry (1790–1855) on four Arctic expeditions between 1819 and 1827 and sailed with his uncle from 1829 until 1833 on an expedition that reached the north magnetic pole in 1831. By 1833 Ross had attained the rank of commander. Ross specialized in studying the Earth’s magnetic field and spent most of the years from 1835 to 1838 working on a magnetic survey of Great Britain. He took time out from this task, from December 1835 to August 1836 to search for some missing whaling ships. On April 8, 1839, Ross took command of HMS Erebus, a ship of 414 tons (376 t), with his friend the Irish-born officer Francis Rawdon Moira Crozier (1796–1848) taking command of the 380-ton (346-t) Terror. Both ships were sturdily built, three-masted warships used for transporting mortars, and they were strengthened to equip them for conditions among the sea ice. Ross was highly experienced and knew that his crew would perform best if they ate well. He ensured that the ship carried ample supplies of food to provide a varied and healthy diet. Officials from the admiralty inspected the ships on September 12, 1839, and declared themselves satisfied. All members of the crew were given three months’ pay in advance, and the ships left port on October 5. The main purpose of the expedition was to conduct magnetic observations and to locate the south magnetic pole. On the voyage south they set up magnetic observing stations on St. Helena and at the Cape of Good Hope. The ships spent two months on the Kerguelan Islands in the southern Indian Ocean, where the crew took hourly magnetic readings while Ross made astronomical and

In 1818 he was made captain of the schooner Galina and began his explorations in 1819, mainly as part of a search for seal rookeries.

Scott The two most famous Antarctic expeditions both began in 1910. The British naval officer Robert Falcon Scott

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tidal observations. A storm separated the ships soon after leaving the Kerguelan Islands, but the Terror arrived safely at Hobart, Tasmania (then called Van Diemen’s Land), on August 15, 1840, and the Erebus on the following day. While there Ross supervised the building of a magnetic observatory by convict laborers brought in by the lieutenant governor, Rear Admiral Sir John Franklin (1786–1847), another Arctic explorer. The ships sailed south on November 20, heading for where the German mathematician and physicist Karl (or Carl) Friedrich Gauss (1777–1855) had calculated the magnetic pole should be. They reached the Auckland Islands, where they found two boards announcing visits from Charles Wilkes and Dumont d’Urville. After conducting magnetic surveys there and on Campbell Island they departed on December 17. They encountered their first icebergs on December 27 and crossed the Antarctic Circle on January 1, 1841. From January 5 to 10 the Erebus and Terror penetrated the pack ice into open water and discovered the Ross Sea and then Victoria Land, but the Ross Ice Shelf barred their way farther south. Unable to find anywhere to spend the winter safely, they returned to Hobart, arriving on April 6. On November 23 they sailed southward once more, reaching latitude 70.17°S, but by January they were surrounded by blocks of ice that were being thrown about violently by the waves. Both ships were damaged but continued southward on February 4. They wintered in the Falkland Islands (also called Las Malvinas) then charted part of the coastline, eventually to 71.5°S, before heading for home. Ross was elected a fellow of the Royal Society in 1848. In 1848 and 1849 he led an expedition with two ships, HMS Enterprise and HMS Investigator, in a fruitless search for Sir John Franklin, who had been lost while searching for the Northwest Passage. Rear Admiral Sir James Clark Ross died at Aylesbury on April 3, 1862.

(1868–1912) led one, and the Norwegian explorer Roald Engelbregt Gravning Amundsen (1872–1928) led the other. Scott’s first expedition had lasted from 1901 to 1904. He was the first person to survey the surrounding area from a captive balloon, which he did in 1902. The 1902 expedition used dogsleds to penetrate deep into the interior of the continent. The second, 1910, expedition aimed to study the Ross Sea and reach the South Pole.

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History and the Desert In 1908 Shackleton had used an automobile in his exploration of Ross Island, and he had pioneered a route up the Beardmore Glacier using Manchurian ponies. These achievements persuaded Scott to attempt the journey to the pole using motorized sledges, ponies, and dogs. His party of 12 men set out from Cape Evans, Ross Island, on October 24, 1911. It was not long before the motorized sledges broke down and had to be abandoned. Then the ponies failed and had to be shot. Finally, when they reached 83.5°S, the dogs could not continue and had to be sent back. On December 10 the party commenced the ascent of the Beardmore Glacier by hauling the sledges manually. By the end of the month seven men had dropped out. The remaining five—Scott, E. A. Wilson, H. R. Bowers, L. E. G. Oates, and Edgar Evans—reached the pole on January 18, only to discover that Amundsen had arrived there about a month earlier. On the return journey the weather was very severe, and supplies of food and fuel were low. Evans died on February 17 at Beardmore, and Oates walked away into a blizzard on March 17. Scott, Wilson, and Bowers continued for a further 10 miles (16 km) but were then confined to their tent by a blizzard that continued for nine days. That is where they died from cold and exhaustion, only 11 miles from their depot, where they had left stores that would have saved them.

Amundsen Roald Amundsen, the Norwegian explorer, was born on July 16, 1872, at Borge, to the south of Oslo. He had sailed on the Belgica in 1897, and in 1903 he was the first person to sail through the Northwest Passage, the sea route linking the Atlantic and Pacific Oceans through the maze of islands across the north of Canada. He made this voyage with a crew of six on a tiny ship, the 52-ton (47-t) sloop Gjöa. For his southern expedition he took Nansen’s ship, the Fram. The Fram anchored in the Bay of Whales, on the eastern side of the Ross Sea close to the edge of the ice shelf, 60 miles (96.5 km) farther south than the Scott camp at Cape Evans. Amundsen then led a party to deposit supplies at several points along the first part of the route south. His team of five men, 52 dogs, and four sledges set out on October 19, 1911, and reached the South Pole on December 14. They left the pole on December 17 and reached their base on January 25, 1912. It was after his Antarctic expedition that Amundsen returned to exploring the Arctic with a new ship, the Maud. By then he owned a successful shipping business. During that period he made the airship crossing of the North Pole with Ellsworth and Nobile. He died in June 1928 somewhere in the Arctic Ocean near the Svalbard archipelago while rescuing Umberto Nobile from an airship crash.

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EXPLORERS IN AFRICA, ARABIA, AND ASIA

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The Berbers (see “Imazighen—the Berbers” on page 194) have inhabited North Africa for thousands of years. They, and especially the Tuareg Berbers, know the Sahara well, and in former times they used to maintain trade links between the northern coastal settlements and oasis settlements to the south. The Egyptians, in contrast, did not venture far from their river and oases. The Romans sent several expeditions into the desert, as did various North African, Middle Eastern, and Arab rulers. After the fall of the Roman Empire European interest in the desert revived in the Middle Ages, but it was not until the 15th century that explorers began to travel to Africa. One of the first was a Venetian, Alvise Ca’ da Mosto (or Cadamosto; 1432–88), who sailed on March 22, 1455. He called at Madeira, the Canary Islands, and passed the mouth of the Senegal River. On another voyage in 1456 he reached the Cape Verde Islands and may have been the first European to do so. He sailed some distance up the Gambia River and explored part of the coast. On his return he wrote an account of his adventures, which aroused further interest.

The Romance of Timbuktu At the same time that Ca’ da Mosto was exploring West Africa, so was Diogo Gomes (1440–84), a Portuguese explorer. He also sailed up the Gambia River, and at the town of Kuntaur (then called Cantor) in central Gambia he met men from Timbuktu. Illness among his crew prevented him from traveling farther inland, but the name of Timbuktu became a magnet to later European adventurers. Serious European exploration began in the 19th century. It was part of the colonialist expansion into Africa, and much of it centered on the river systems, which provided routes into the interior of the continent. Timbuktu had not lost its appeal, however, and on August 18, 1826, the Scottish explorer Alexander Gordon Laing (1793–1826) became the first European to reach it.

Laing and Caillé Laing was a soldier, and his first expedition, in Sierra Leone, had been to establish trade in goods and to abolish the slave trade. It was his second expedition that took him to Timbuktu. Starting from Tripoli, Libya, he crossed the desert to Ghudāmis in northern Fezzan, where he entered Tuareg territory and was badly wounded in a fight. He stayed in Timbuktu until September 24, but two days after leaving he was murdered by his guide.

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Two years later the French explorer René-Auguste Caillé (1799–1838) became the first European to visit Timbuktu and return alive. He had made two earlier trips to Senegal and in 1824 began preparing for his journey into the desert by learning Arabic and studying Islam. He left the West African coast in April 1827, posing as an Arab traveling to Egypt. He fell sick on the way, was delayed for five months, but reached Timbuktu on April 20, 1828. After spending two weeks there he headed north for Morocco and then back to France.

Clapperton and Barth Another Scot was the first European to visit and describe northern Nigeria and Lake Chad. In 1822 Hugh Clapperton (1788–1827), a captain in the Royal Navy, joined Major Dixon Denham (1786–1828), an English army officer who had fought at the Battle of Waterloo, and Walter Oudney (1790–1824), a naval surgeon from Edinburgh, to travel south from Tripoli. They reached Lake Chad early in 1823. From Lake Chad the party continued to Kano, Katsina, and Sokoto in what was then the kingdom of Bornu and is now the Nigerian state of Borno. Oudney went on to Senegal, where he died in 1824. Clapperton and Denham returned to England in 1825. Clapperton soon returned to West Africa and died near Sokoto. One of the most comprehensive studies of North and Central Africa was made between 1850 and 1855 by the Prussian geographer Heinrich Barth (1821–65). He spoke Arabic fluently—as well as French, Spanish, Italian, and English—and had already explored the North African coast before embarking on a journey across the Sahara. The expedition left Tripoli in 1850 led by the English explorer James Richardson (1806–51), who had earned his reputation traveling in India, and including the German geologist and astronomer Adolf Overweg (1822–52), who was the first European to travel completely around Lake Chad and sail on its waters. The party traveled in a southwesterly direction into what is now the north of Nigeria. Richardson died there in 1851, and Barth took command. He and Overweg traveled southward and around the southern shore of Lake Chad. Overweg died in September 1852, and although by then his own health was poor, Barth continued alone, traveling westward and eventually reaching Timbuktu. He stayed there for six months before heading back to Tripoli and from there to London. By the time his journey ended Barth had traveled about 10,000 miles (16,000 km). He was able to describe the middle section of the Niger River and had recorded the routes he had followed. For this he used dead reckoning—keeping accurate notes of the compass directions followed, calculating the distances covered, and marking these on a

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map. Between 1857 and 1858 he published an account of his travels together with details of the peoples he met, their histories, and their languages. The resulting four volumes, Reisen und Entdeckungen in Nord- und Central-Afrika in den Jahren 1849 bis 1855 (travels and discoveries in North and Central Africa in the years 1949 to 1855), provide one of the fullest descriptions of those parts of Africa ever written. In 1863 Barth was appointed professor of geography at the University of Berlin.

Duveyrier, Rohlfs, and Nachtigal Henri Duveyrier (1840–92), a French explorer, met Barth and was inspired by him. Like Barth, Duveyrier learned to speak Arabic, and in 1859, when he was 19, he set off on a three-year journey through the northern Sahara. In the course of that and later travels Duveyrier spent a great deal of time living with the Tuareg, studying their dialect and way of life. His book about them, Exploration du Sahara: Les Touâreg du nord (exploration of the Sahara: the northern Tuareg), was published in 1864. Gerhard Friedrich Rohlfs (1831–96) was much more of an adventurer. Born near Bremen in northern Germany, in 1855 he joined the French foreign legion. He learned Arabic, and in 1862, disguised as an Arab, he explored Morocco and the Atlas Mountains and traveled as far south as the Fezzan region of Libya, where he arrived in 1864. In 1865 he embarked on a journey no other European had attempted, from Tripoli across the desert to northeastern Nigeria, then down the Niger River to the Gulf of Guinea, near Lagos. In 1874 he undertook another journey, this time from Tripoli to Egypt. In 1885 Rohlfs was made German consul in Zanzibar, in East Africa. By then the European powers were dividing Africa among themselves, with especially keen competition around Lake Chad, which was mistakenly thought to be of great economic importance. The first to arrive were the French in 1897, followed by the British and Germans in 1902. What is now the République du Tchad came under French occupation and was then made part of the federation of French Equatorial Africa. Gustav Nachtigal (1834–85) also became a consul, in Tunis. He was a doctor, first in the German army and later as physician to the ruler—called the bey—of Tunisia. While in Tunis he made several journeys into the desert, and in 1869 King William I of Prussia sent him to Bornu, the kingdom that today is a state (Borno) in the northeast of Nigeria. Nachtigal crossed the desert and the Tibesti Mountains to enter Borkou (or Borku), a region that was then controlled by the Muslim sultanate of Ouaddaï and inhabited by nomadic and seminomadic tribes. France took control in the early 1900s, but the region remained ungovernable for many years. It is now a prefecture in northern Chad.

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History and the Desert Nachtigal crossed Chad and the Sudan and continued to Cairo, where he arrived in November 1875.

Exploring the Kalahari and Namib Exploration was less intensive in southern Africa. The Scottish explorer and missionary David Livingstone (1813–73; see the sidebar “David Livingstone” on page 20) crossed the Kalahari Desert in 1849. Local people assisted him, but even so the journey from one water hole to the next was arduous. In 1878–79 a party of Boers undertook the Dorsland (“thirst land”) Trek from Transvaal to central Angola. Some 250 people and 9,000 cattle died along the way, most of them from thirst. The Namib Desert remained largely unknown until the 19th century. In the 1890s South West Africa became a German territory. German military expeditions then began detailed surveys and the compilation of maps of the desert. A war of resistance by the Herero people lasted from 1904 until 1907, and in 1914–15, during World War I, South African forces invaded and captured the entire country. After the war the League of Nations mandated South West Africa to the United Kingdom, to be exercised by the South African authorities. Namibia became an independent republic in 1968.

Arabia If exploring the Sahara and Kalahari was difficult for Europeans, traveling in Arabia was even more so. Until the 18th century almost nothing was known in Europe about the interior of the Arabian Peninsula. Many Arabs had visited it, of course, but they had written very little about it. King Frederick V of Denmark was the first European ruler to send a scientific expedition to Arabia, led by a German surveyor, Carsten Niebuhr (1733–1815). The team of five set out in 1762. They visited the Nile, crossed it into Sinai, then sailed southward along the Arabian coast as far as Jidda. From there they traveled overland to Mocha (al-Mukhā) in the far southwest. The philologist (a person who studies the language and literature of a people) traveling with the group died in May 1763, and the naturalist in July. The three survivors, an artist, a surgeon, and Niebuhr, visited Şan‘ā’, in Yemen, then returned to Mocha. Niebuhr wrote two books about the region. Beschreibung von Arabien (description of Arabia) was published in 1772 and Reisebeschreibung nach Arabien und andern umliegenden Ländern (description of travels to Arabia and other surrounding countries) in 1774. A much more important travel book appeared in 1888. Travels in Arabia Deserta was written by the English traveler Charles Montagu Doughty (1843–1926). In 1876 Doughty set out from Damascus, traveling with pilgrims

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bound for Mecca. He did not reach Mecca, but he visited Madā’ in Şālih, near Medina, and on his return journey visited Taymā’, Hā’il, ‘Unayzah, and Aţ Ţa’if, towns in the mountainous region of Jabal Shammar, deep inside Arabia, as well as the port of Jidda.

Philby and Ibn Sa’ūd During World War I Turkey fought on the side of Germany. This made the entire region to the south of Turkey militarily important to the British and French, which meant it was important for the Allies to win friends among the Arab rulers. In 1917 the British government sent Harry Saint John Bridger Philby (1885–1960) to meet ‘Abd al-‘Azīz ibn Sa’ūd. The Sa’ūd tribe had once ruled a large part of Arabia, but in 1880, soon after Ibn Sa’ūd was born, their rivals, the Rashīd clan, drove them out. The Sa’ūd retreated to Kuwait, from where, in 1901, Ibn Sa’ūd led a group to Riyadh, the old Sa’ūd capital. They captured the city, rallied support, and within a couple of years had retaken a large part of central Arabia. The Rashīds sought Turkish help, however, and although the Turks withdrew their troops before the war began, they remained a threat. Ibn Sa’ūd agreed to allow the British to make Arabia a protectorate and to make war on his old Turkish-backed rivals. In return the British paid him £5,000 a month and supplied him with weapons. Despite this, Ibn Sa’ūd did little during the war, and that is why Philby, a British civil servant attached to the Mesopotamian Expeditionary Force, was sent to meet him. After the meeting Philby traveled from Al-‘Uqayr, on the eastern coast just south of the island of Bahrain and opposite Qatar, to Jidda, on the western coast. This journey took him across the northern part of the Rub’ al-Khali, the Empty Quarter. In the 1920s Philby set up his own business in Arabia, and in 1930 he became a convert to Islam. He was a friend and adviser to Ibn Sa’ūd, who had gained control of most of Arabia and who formed the Kingdom of Saudi Arabia in 1932. Philby helped to map Arabia and contributed to the study of its archaeology and the languages of its peoples. He was expelled from the country in 1955 for criticizing the extravagance and inefficiency of the government, and he died five years later in Beirut. Philby’s son Kim (1912–88; his full name was Harold Adrian Russell Philby) was a notorious Soviet agent who worked for British intelligence.

Lawrence of Arabia From 1920 to 1924 Philby represented the British government in Jordan, then called Transjordan. He took over the position from the most famous of all the explorers and adventurers ever to travel in Arabia, T. E. Lawrence— Lawrence of Arabia. No one has done more to popularize and romanticize Arab culture.

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Thomas Edward Lawrence (1888–1935), known later in his life as John Hume Ross and T. E. Shaw, began his career as a student of military architecture. It was this interest that took him to Syria and Palestine and then, from 1911 to 1914, to the banks of the Euphrates as a member of an archaeological expedition. He spent his free time exploring, learning Arabic, and getting to know local people. He helped to map the region between Gaza (Ghazzah in Arabic, ‘Azza in Hebrew) and Al-Aqabah (Aqaba) in southern Jordan. This led to a job preparing a militarily useful map of Sinai in the map department of the War Office in London. He soon returned to the region, however, first to Cairo and then to Arabia. Husayn ibn ‘Alī, the amīr (commander) of the territory around Mecca, was in revolt against the Turks. After consultations with Abdullah, one of his sons, Lawrence went to see another son, Fayşal, who was leading troops near Medina. Lawrence returned to Cairo and persuaded the British authorities to supply money and weapons to the local rulers who were opposing Turkey and to unite them in a resistance that served British purposes. That is how Lawrence came to lead a small guerrilla force operating behind Turkish lines destroying trains and bridges. His other aim was to forge the quarrelsome tribal leaders into a coherent Arab nation using bribes, promises of booty, and his own example of stamina and courage. His force captured Al-Aqabah in July 1917 and reached Damascus in 1918, but in the course of his campaign Lawrence was captured, tortured, and wounded several times. Out of these experiences Lawrence wrote The Seven Pillars of Wisdom. This is an account of the desert campaign, but it also describes the Bedouin with whom Lawrence fought and for whom he had a genuine and deep affection. After the war he argued strongly for Arab independence. Lawrence ended the war with the rank of colonel, but he resigned from the army and refused the honors offered to him—he literally left King George V standing with the box holding a medal in his hand. For a time the Colonial Office employed Lawrence as an adviser, but then he resigned, changed his name to Ross, and enlisted in the Royal Air Force (RAF). The press discovered him and revealed his identity, and the RAF released him. He changed his name again, this time to Shaw, and joined the army as a private. Later he was transferred to the RAF. He completed his RAF service in 1935 and was killed in a motorbike accident a few months later.

Wilfred Thesiger In more recent times the British soldier, traveler, photographer, and travel writer Sir Wilfred Thesiger (1910–2003) kept the romance of the desert alive. Thesiger was born on June 3, 1910, in Addis Ababa, Ethiopia, and he lived there

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until he was nine, so he could claim that northern Africa was his home. Thesiger made several crossings of the Rub’ al-Khali, but his main interest was the Ma’dan, or Marsh Arabs, with whom he lived for some time. They live in southern Iraq in a region of marshland between the Euphrates and Tigris. The marshes form a triangle with the towns of An-Nāşirīyah, Al‘Amārah, and Basra at its corners. Thesiger described them in his book The Marsh Arabs, published in 1964. He died at Croydon, Surrey, on August 24, 2003.

Rediscovering the Silk Road European exploration of the deserts of Central Asia began in the late 19th century. The Silk Road (see “Caravans and the Silk Road” on pages 200–202) had fallen into disuse as sea routes grew safer and more reliable. Its rediscovery began with the work of the Prussian geographer and geologist Ferdinand Paul Wilhelm, freiherr von Richthofen (1833–1905). Von Richthofen traveled throughout China and described the country in his five-volume work China, Ergebnisse eigener Reisen und darauf gegründeter Studien (China, results of my own travels and studies based on them), published between 1877 and 1912. It was von Richthofen who named the travel route the Silk Road. In 1893 the Swedish explorer Sven Anders Hedin (1865– 1952) began a five-year journey across the Ural and Pamir Mountains, past Lop Nor, and to Beijing along the old Silk Road. Later Hedin discovered the way Lop Nor had formed from shifts in the course of the Tamir River. When he was there the area had no inhabitants. The groups of Uighur people who moved into the region later left around 1920 because of a plague that killed many of them. The lake itself, which covered about 770 square miles (2,000 km2) in 1950, ceased to exist in 1970, when dams held back the water of the Tamir that used to feed into it. Between 1899 and 1902 Hedin explored the Gobi Desert, and from 1927 to 1933 he led a Sino-Swedish expedition that discovered evidence of Stone Age cultures in what is now desert.

Sir Aurel Stein and the Cave of the Thousand Buddhas The name most closely associated with the rediscovery of the Silk Road, however, is that of Hungarian-born Mark Aurel Stein (1862–1943). Stein became a British citizen in 1904 and was knighted in 1912. He died in Kabul, Afghanistan. Sir Aurel Stein conducted three expeditions, in 1900, 1906, and 1913, into what are now Xinjiang and the Uighur Autonomous Region of China. These lasted for a total of seven years and followed the old caravan routes between

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History and the Desert China and the West for about 25,000 miles (40,200 km). Stein traced the Silk Road across Lop Nor and found the Jade Gate that had once stood at the border between China and the Western Kingdoms and the walls that had been built to keep nomads from entering China. As he went, Stein excavated ancient sites carefully, discovering long-lost cities and, near the city of Tun-Huang (see “The Route of the Silk Road” on page 202), the Cave of the Thousand Buddhas. The Cave of the Thousand Buddhas is a temple that was carved into a cliff in 366 c.e., at the start of a period lasting to the early 13th century, when the city was a major Buddhist center. Eventually there were nearly 500 temple caves, now known as the Mogao Caves. Many are now open to the public, and in 1987 the area was declared a World Heritage Site. In one of the caves Stein discovered a hoard

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of about 60,000 paper manuscripts and other documents dating from the fifth to 11th centuries that had been walled up in 1015. They included Buddhist, Taoist, Zoroastrian, and Nestorian scriptures, stories, and ballads and were written in Chinese, Sanskrit, Tibetan, Uighur, and other languages. There were also paintings and temple banners. Stein removed many of them, and the French scholar Paul Pelliot (1878–1945) later removed more. These were sent to London and Paris, where they were properly stored, cataloged, and published, but news of their removal reached the Chinese authorities in Beijing. They bought the remaining material and dispatched it to Beijing, but the money to pay for it was stolen before it reached Tun-huang. Most of the material was stolen before it reached Beijing, and much of it has now been lost.

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6 Economics of the Desert Deserts are harsh, inhospitable places where it is difficult to scrape together even a meager living. It seems paradoxical to talk of the economics of lands that are barely habitable, yet a harsh climate does not imply a lack of resources that do not depend on rain and mild temperatures. The deserts of the Middle East are rich in oil, and their oil has made several Middle Eastern nations extremely wealthy. This chapter examines the history of the oil industries and their likely future, and it describes living conditions in the oil-rich countries. Although the world possesses very large reserves of oil, these will not last forever. Long before the last drop has been squeezed from the last well, the industrial world will have moved to alternative fuels. The Sun is an obvious source of energy, and the desert climate provides the clear skies needed to exploit it. The chapter describes some of the technologies that may be used to make sunshine perform useful work. Not every desert country possesses oil reserves, of course, but other countries, listed here, have metals and other minerals they can exploit. Mining for minerals or oil is not the activity for which desert peoples are most famous. Their place names—Persia, Afghanistan, Bokhara, Turkey, and many others—conjure images of textiles and especially of carpets. These have been exported for centuries, and they are still an important source of export earnings. Carpets contribute to the romance of desert lands. Visitors travel from afar to see and experience the places where they are made, but deserts have much more to offer. These are regions of the world where civilizations rose and fell and where several major religions were born. Tourism is now one of the most valuable industries in many countries.

Titusville, Pennsylvania. The story goes that the reply they gave him was: “Drill for oil? You mean drill into the ground to try and find oil? You’re crazy!” Perhaps their attitude is not so surprising. Drake was not a colonel at all but an impoverished former streetcar conductor who wore a top hat spattered with mud. In any case, oil seeped from the ground not far from Titusville. There was no need to go looking for it. All the same, “Colonel” Drake succeeded in persuading enough workers to join him, and they sank a well to a depth of 70 feet (21 m). Oil gushed from the ground, and for the people of Titusville prosperity followed. The Titusville well was said to be the first oil well in the world. In fact, though, the Chinese struck oil while drilling for salt—the two often occur together—in the third century c.e., and the Greek historian Herodotus, writing in the fifth century b.c.e., described how salt, oil, and bitumen, a more viscous kind of petroleum, were obtained.

The Birth of the Oil Industry People have been using oil for at least 5,000 years, but the uses were limited. The oil was obtained from places where it seeped from the ground, and the most liquid part of it was used for lighting. In both Arabic and Persian, liquid oil was called naft, from which we derive our word naphtha. Bitumen was used as mortar, for setting jewels, as a cement for laying mosaics, and as an adhesive for attaching the handles of tools and weapons to their blades. Lighter oil was used as a solvent for cleaning fabrics as well as for lighting. Various medicines and ointments contained petroleum.

Asphalt Roads

OIL AND THE ECONOMIES OF MODERN DESERT COUNTRIES

■

In 1859 Colonel Edwin L. Drake (1819–80) was trying to recruit workers to drill for oil just outside the city of

As long ago as the fifth century b.c.e. asphalt was being used to surface roads. Herodotus, the Greek historian who lived at that time and who was a great traveler, described walking the length of one of those asphalt-surfaced roads, known as the Royal Road. The Royal Road ran between Susa, which was the capital of a country called Elam in

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Economics of the Desert what is now southwestern Iran, and the Aegean coast in what is now Turkey. The road was 1,500 miles (2,413 km) long, and Herodotus claimed to have walked the entire length of it. Good roads make walking more comfortable, but the real need for them became evident when knowledge of horseback riding reached the Middle East from the region of what is now Ukraine, where it began. Domesticated horses were used as draft animals as well as for riding, and in addition to drawing carts to transport goods to and from markets, horses drew two- and four-wheeled war chariots. These were rather heavy, clumsy vehicles that were probably used to carry soldiers into battle rather than as weapons in their own right. In modern terms they were personnel carriers rather than armored cars. The design was improved over the years. Originally they had solid wooden wheels. These were later replaced by much lighter wheels with spokes, and the new fast chariots were usually drawn by four horses running abreast. Chariots were used in warfare, for hunting, and for racing. Civilian models were the fast sports cars of their day, but, just like modern sports cars, they were not designed to be driven across rough ground or through mud or loose sand. A poor surface slowed them down, and a rough surface could damage them. That is why the Assyrians and Persians covered their roads with asphalt to make a smooth, hard surface. One ancient inscription included the claim by a king that he had found his kingdom in mud and left it laced with roads glistening with asphalt. Asphalt is still used in road building. Asphalt is a sticky black or brown substance that is a viscous liquid when hot but sets hard when it cools. It was known in India as earth butter, and its earliest use may have been so seal the brick walls of a reservoir in Mohenjo-Daro some time soon after 4000 b.c.e.

Rock Oil and Rockefeller Oil was not “invented” at Titusville, but the sinking of the Titusville well did mark an important development because for the first time petroleum—at first known as rock oil— was available in large amounts all in one place. That made it an industrial commodity ripe for exploitation. It could be processed—refined—and new uses for it could be found. One of the first businessmen to appreciate the significance of the increasing output in Pennsylvania was John Davison Rockefeller (1839–1937). He built an oil refinery in 1863 near Cleveland, Ohio, and used oil as the fuel in its furnaces. Rockefeller founded Standard Oil. Eventually, the trust he established was declared illegal and was disbanded, but the successors of Standard Oil include such familiar names as Exxon, Mobil, Amoco, and Chevron. By 1890 petroleum was also being obtained from wells and refined in Russia.

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Still, the market was fairly small, and it was likely to shrink as gas lighting, using gas made from coal, replaced oil lamps. Its expansion began with the mass production of automobiles, which demanded highly refined gasoline. In the 1930s the rapid expansion of air travel and aircraft manufacture increased the demand still more, and for fuel of even higher quality. When jet engines were introduced, demand increased again. Jet engines burn less refined oil than piston engines, but they use a great deal more of it. With the increasing demand for fuel oils there also went a demand for lubricating oils. More oil was needed, and oil companies began searching for it outside America. The use of oil for heating buildings began in the 1920s.

Oil from the Middle East In 1901 the Iranian government granted an oil-field concession to an English investor, William Knox D’Arcy (1849–1917). The concession covered more than 500,000 square miles (1,295,000 km2). D’Arcy made his first strike in May 1908 in southwestern Iran, and in 1909 he formed the Anglo-Persian Oil Company. Wells were drilled, crude oil was piped to a refinery at the port of Ābādān on the Persian Gulf, and the first oil was exported from there in March 1912. More wells were drilled and more refineries built, and by the late 1930s Iran was a major oil producer. By 1914 the British government had become a major stockholder in the Anglo-Persian Oil Company. Eventually, it was the single biggest stockholder, and in 1955 the name was changed. The company became British Petroleum (BP). The government sold its stock in the late 1970s and 1980s, BP became Britoil, and Britoil acquired Standard Oil (Ohio). Farther south, Ibn Sa’ūd—or to give him his full name ‘Abd Al-Azīz Ibn ‘Abd Ar-Rahmān Ibn Fayşal Ibn Turkī ‘Abd Allāh Ibn Muhammad Āl Sa’ūd—ruled Saudi Arabia, but both he and his country were very poor. In May 1933 he granted a concession to an American oil company, but five years passed before they struck oil. That was in the Dammām field, south of the port of Ad-Dammām on the Persian Gulf and close to the modern town of Az Zahrān (or Dhahran). Then World War II began. Operations were halted, so the king was no better off. The oil company resumed work after the war, and it was then that the king started to receive substantial payments. By 1953 these amounted to $2.5 million a week. Ibn Sa’ūd had no idea what to do with all his money. Such wealth offended his austere religious convictions, and swindlers and criminals of all kinds were moving into his country. He hated all of it. In the 1950s oil was discovered in Libya. Until then that country had had few natural resources and relied on foreign aid to pay for the import of many of the commodi-

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ties it needed. Once the oil began flowing the economy of the country was transformed, and Libya became one of the wealthiest countries in Africa. It used its wealth to develop agriculture and manufacturing industries, and it established health and educational services for its people. In 1956 oil was discovered in Syria, but production did not commence until 1968. Petroleum and products made from it account for more than half Syria’s exports, but Syrian oil fields are much smaller than those of the major Middle Eastern producers. Oil fields discovered in Algeria in the 1960s proved to be among the biggest in the world. They are also rich in natural gas, of which Algeria is now one of the world’s most important producers. The Al-Burmah (or El-Borma) oil field in southern Tunisia opened in 1964. Tunisian reserves are smaller than those of Libya and most Middle Eastern oil-producing countries, but they are economically important nevertheless. Oil production also began in Oman in 1964. Nigeria became an oil-producing country in the late 1960s. In the late 1980s oil was discovered in Yemen and now accounts for more than 90 percent of that country’s exports.

Oil and National Wealth Deserts are hard places to live, and in material terms their populations are poor. The discovery and exploitation of oil clearly makes an important difference to them, although this difference is not so obvious as it might seem. The table compares the per capita gross domestic product (GDP) in most of the desert countries of the world. Where possible it is measured as purchasing power parity (PPP), and in countries for which the PPP value is not obtainable, it is measured as nominal GDP. The gross domestic product is the combined value of all the goods and services produced within a country in a year, but not including income from overseas investments and not allowing for depreciation or the use of capital in the process of production. The per capita GDP is the GDP divided by the size of the population. It does not represent the amount individual citizens are actually paid. All amounts are converted to U.S. dollars, but the conversion can introduce distortions, because particular currencies may be overvalued or undervalued with respect to dollars. PPP corrects for such distortions by measuring what a given amount of each currency will purchase. This is now the preferred way to compare economies. For comparison, the per capita GDP in the United States in 1999 (the latest year for which complete figures are available) was $31,910, which was also the PPP because the United States is the standard against which other economies are measured. The table indicates those countries that produce oil. Those labeled OPEC are members of the Organization of Petroleum Exporting Countries. As the table shows, the smallest oil-producing countries are also the richest. This is

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Per Capita GDP in Desert Countries, 1999 (US $) Algeria

4,840

OPEC

Bahrain*

7,640

oil producer

Burkina Faso

960

Chad

840

Djibouti* Egypt Ethiopia Iran

790 3,460 620 5,520

OPEC

Iraq*

600

OPEC

Israel

18,070

Jordan

3,880

Kuwait*

22,110

Lebanon*

3,700

Libya*†

6,700

Mali

OPEC OPEC

740

Mauritania

1,550

Mexico

8,070

Mongolia

1,610

Morocco

3,320

Namibia

5,580

oil producer

Niger

740

Nigeria

770

Oman*

5,950

Qatar*†

11,600

OPEC

Saudi Arabia

11,050

OPEC

Somalia*

110

Sudan*

330

Syria Tunisia United Arab Emirates* Yemen

3,450

OPEC oil producer

oil producer

5,700 17,870 730

OPEC oil producer

OPEC means member of the Organization of Petroleum Exporting Countries. Oil producer means a non-OPEC country that exports petroleum. *Expressed as nominal GDP. †1998 figure. Source: 2006 Book of the Year. Chicago: Encyclopaedia Britannica, 2006.

because their smaller populations mean there are fewer people among whom the GDP is shared, and it demonstrates that oil wealth is based on the exploitation of a natural

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Economics of the Desert resource rather than on manufacturing. In economic terms oil is a primary product, and although a country may process the petroleum into oil products, thus adding value to it, the economic activity remains wholly dependent on the oil itself, which is the primary product.

OPEC The Organization of Petroleum Exporting Countries (OPEC) was established in September 1960, and since 1965 its headquarters have been in Vienna, Austria. Its original members were Iran, Iraq, Kuwait, Saudi Arabia, and Venezuela. Since then Qatar, Indonesia, Libya, United Arab Emirates, Algeria, and Nigeria have joined. The aim of OPEC is to regulate the oil market by coordinating production and export among its members. If world prices fall, production is reduced, and if prices rise, production increases, but without individual OPEC members taking unfair advantage of each other. The effectiveness of OPEC was first demonstrated at its meeting in September and October 1973, when it increased world oil prices by 70 percent, and in December 1973, when it raised them a further 130 percent. There were additional price rises in subsequent years. Today OPEC is less influential. This is partly because of production outside the OPEC area, in Alaska, the Gulf of Mexico, and the North Sea, for example, and partly because oil consumers have turned to alternative sources of energy, mainly nuclear power and coal, and have increased the efficiency with which energy is used.

Are We Running Out of Oil? For years there have been fears that a day will come when the oil fields have been drained and the world will run out of petroleum. Oil is measured in barrels: one barrel = 42 gallons (158.97 l). It is true that the world has only a limited supply of petroleum, and each gallon that is used is lost—the resource is finite. Oil experts have estimated that originally the world contained between 2,050 billion and 2,390 billion barrels of crude oil (86,100–100,380 billion gallons; 325,888–379,938 l). So far the world has used between 45 percent and 70 percent of this, amounting to some 922–1,670 billion barrels (38,724–70,140 billion gallons; 146,570–265,480 billion l). Between 1965 and 2005 the world used about 918 billion barrels (38,556 billion gallons; 145,934 billion l). The U.S. Geological Survey (USGS) estimates that the remaining proven reserves amount to approximately 1,300 billion barrels (54,600 billion gallons; 20,666 billion l). At this rate the known oil reserves are likely to be exhausted by about 2050. There are also reserves that remain undiscovered but that are calculated to exist on the basis of studies of rock

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formations. These are estimated at 275–1,469 billion barrels (11,550–61,698 billion gallons; 43,717–233,527 billion l). When the remaining known reserves, the reserves that cannot be exploited at current prices and with present technology, and the undiscovered reserves are added together, the USGS estimates that the total comes to about 3,000 billion barrels (126,000 billion gallons; 476,910 billion l). The table lists the amount of oil in the 20 countries with the largest proven reserves. The figures appear alarming, and there are large margins for error in all of them. Some countries have incentives to exaggerate or minimize the size of their reserves. For example, OPEC sets limits to the amount of oil each member is permitted to produce, calculating the limit as a percentage of the proven reserves in that country. Clearly, if a country increases the estimate of its reserves, its production quota will also increase and it will gain more income. Kuwait did precisely this in 1985, and other OPEC coun-

Proven Oil Reserves of the Top 20 Countries, 2006 COUNTRY

RESERVES (BILLIONS OF BARRELS)

Saudi Arabia

264.3

Canada

178.8

Iran

132.5

Iraq

115.0

Kuwait

101.5

United Arab Emirates

97.8

Venezuela

79.7

Russia

60.0

Libya

39.1

Nigeria

35.9

United States

21.4

China

18.3

Qatar

15.2

Mexico

12.9

Algeria

11.4

Brazil

11.2

Kazakhstan

9.0

Norway

7.7

Azerbaijan

7.0

India

5.8

WORLD TOTAL

1,292.5

Source: Oil and Gas Journal 103, 47 (December 19, 2005). Washington, D.C.: U.S. Energy Information Administration.

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tries quickly followed. There are other countries that may not wish to draw attention to the size of their reserves for fear of outside political or even military interference, so the published figures should not be taken too literally. It is unlikely that the world will ever run out of oil in a literal sense, but there is a very real possibility that as the reserves are depleted prices will rise steeply, eventually making oil too expensive to be used as a fuel. As demand increases, so does the intensity of exploration, which reveals new sources, but it would be unwise to rely on the hope of reserves that no one can be sure exist. There are vast reserves of tar sands and shale oils that could be exploited— the Canadian oil reserves listed in the table are mainly of this type. Obtaining oil from these sources is much more expensive than taking it from conventional oil fields, and it might be difficult to exploit them in ways that are environmentally acceptable, but they exist. Natural gas is even more abundant, locked in marine sediments in compounds called natural gas (or methane) hydrate. Estimates suggest that the reserves of methane hydrate are approximately double those of all other fossil fuels (coal, oil, and natural gas) combined.

Methane Hydrate When water freezes, its molecules lock together in an open structure. That is why water expands as it freezes and why ice is less dense than liquid water. Under certain conditions of temperature and pressure, a molecule of a different compound can become trapped inside the ice crystal, as though held in a cage. A compound of this type is called a clathrate. Methane hydrate is a clathrate compound consisting of a molecule of methane (CH4) held inside a cage of ice. It forms in shallow sediments in permafrost, where the temperature is very low, and in seabed sediments where the water is more than about 1,640 feet (500 m) deep, where the pressure is very high. Often the crystals form at temperatures higher than the ordinary freezing temperature of water. Lumps of methane hydrate look like ice, but when held in the hand they feel more like Styrofoam. The existence of methane hydrate deposits has been known for more than a century. French scientists studied them in the 1890s, and in the 1930s, when engineers were extending natural gas pipelines in northern Russia, methane hydrate was a nuisance because it continually formed inside the pipes, blocking them. Then, in 1964 a drilling team working in the Messoyakha gas field in northern Siberia discovered a deposit of methane hydrate that had formed naturally. Methane is the principal ingredient of natural gas, so methane hydrate deposits were possibly of great value. The next development occurred in 1981, when the research ship Glomar Challenger, designed to drill into the seabed, bored into a methane hydrate deposit off the coast of Guatemala.

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Unexpected finds such as this pose a risk to oil companies. Occasionally an oil drill penetrates the cap of rock covering a conventional oil well. When this happens friction from the drill bit may melt the clathrate, releasing methane that explodes without warning. An oil well standing on top of an undetected layer of clathrate may be at risk of collapsing because the methane hydrate may shift under the weight of the platform. There is a further risk that decomposing clathrate might trigger a submarine landslide that could release highly flammable gas and generate a shock wave big enough to cause a tsunami. Deposits have since been located all around the North American coast, off the Central American coast, and in smaller amounts off the coasts of South America, Africa, and Eurasia. The smaller amounts identified in these places almost certainly reflect the much greater effort devoted to the search around North America. The U.S. Geological Survey estimates that in the world as a whole, the amount of carbon in methane hydrate reserves is equal to at least twice the amount in all other fossil fuels—oil, natural gas, and coal—combined. Demand for natural gas has been growing steadily throughout the industrialized world, because it is a cleaner and more efficient fuel than oil or coal. Once a way has been found to extract and process them, methane hydrates will supply the world with natural gas for several centuries and possibly for much longer.

The Process of Economic Development A challenge nevertheless remains for the oil-rich desert countries. This is to use their wealth to develop economies that will continue to sustain them when oil production declines. It is what most of them are doing. Commodities such as agricultural produce, timber, coal, and petroleum are said to be primary. This means that the commodities exist in the form in which they were harvested or extracted from the ground. When commodities are processed into some other form, they become secondary. Wheat grain is a primary product, for example, and bread is a secondary product. Processing a primary commodity into a secondary one adds value to it because of the labor that must be expended in the processing, and, obviously, the processing provides employment. If the processed commodity can be exported, the national economy benefits. This transition, from an economy based principally on primary production to one based principally on secondary production, occurred first in the countries of western Europe and the United States. In these countries part of the wealth produced by the primary sector of the economy was saved. Some was invested in building factories to process the primary produce, and some was used to improve public services, especially education. Attempts to extend industrialization to other parts of the world have proved difficult.

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Economics of the Desert There is no single, simple reason for this. The difficulties arose from the combined effects of shortages of capital for investment, defining industrialization in terms only of manufacturing using expensive modern technologies, and lack of an adequately educated and trained labor force. Today the economies of the less-industrialized countries are growing, but the desert countries still lag behind. In Thailand, for example, manufacturing accounts for about 35 percent of the GDP and provides almost 15 percent of all employment. In Zambia the corresponding figures are 10 percent and less than 2 percent. As the table shows, however, manufacturing contributes no more to the economies of the oil-producing countries than it does to those without oil. Manufacturing is the process by which primary commodities are converted into secondary goods. As economies develop, the first primary commodities to be exploited are those that are available locally. In the case of the oilproducing countries the primary product is petroleum— called “crude” because it is in the state in which it emerges from the well. Crude petroleum is chemically very complex. When it is heated, its constituents separate. One by one they vaporize as the temperature reaches their boiling points. Each vapor in turn is then collected and cooled to condense it. In this way petroleum is “cracked” to break it into its “fractions,” and the overall operation is called refining. Other constituents are removed by using solvents. The lighter fractions are used as fuel, but all the chemical compounds obtained from petroleum have uses as the raw material for the chemical industries based on petroleum—the petrochemical industries. Plastics, paints, synthetic fibers, and adhesives are just a few of the groups of substances obtained from petrochemicals. Many of the plastics have familiar names. PVC is polyvinyl chloride, foamed polystyrene is called Styrofoam, polymethyl methacrylate is Plexiglas and Perspex, polyamide is nylon, and polytetrafluoroethylene is Teflon. These are the materials from which goods are made, and they are the secondary commodities an oil-producing country would be expected to make as it sought to build an industrial economic base. In fact, very few do so. Of the total value of all Saudi Arabian exports, refined petroleum contributes 16 percent and petrochemicals 5.2 percent. Refined petroleum contributes nearly 12.3 percent of Algerian export earnings. Other oil-producing countries produce some petrochemicals, but not sufficiently for them to be of major economic importance.

Births, Deaths, and Life Expectancy There is another way the level of economic development is reflected in national statistics. Vital statistics include those that describe the birth, death, and infant mortality rates and life expectancy at birth. The birth rate is the number of babies born each year for every 1,000 of the population,

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Manufacturing as a Percentage of GDP and Employment COUNTRY

% OF TOTAL VALUE

% OF LABOR FORCE

Algeria

5.8

5.4

Bahrain

11.2

16.2

Burkina Faso

12.6*

1.4

Chad

8.6

1.5

Djibouti

3.4

11.0†

30.0*

10.5

6.7

1.6

Iran

11.2

15.9

Iraq

2.1

n.a.

Israel

14.8*

14.4*

Jordan

13.6

10.6

Kuwait

7.2

6.0

Lebanon

9.7

n.a.

Libya

2.7

10.5

Mali

6.9

n.a.

Mauritania

8.0

4.6

18.7

14.5

Mongolia

5.3

5.8

Morocco

16.6

11.9**

Namibia

10.9

3.5

Niger

6.6

2.8

Nigeria

3.9

8.1

Oman

8.2

8.6

Qatar

5.6

12.7

10.2

8.7

Somalia

3.0

12.0**

Sudan

8.2

n.a.

Syria

3.5

12.1**

Tunisia

17.8

33.9**

United Arab Emirates

13.7

13.6

5.6

3.7

Egypt Ethiopia

Mexico

Saudi Arabia

Yemen

* includes mining ** includes mining and public utilities † includes mining and construction n.a. = not available Source: 2006 Book of the Year. Chicago: Encyclopaedia Britannica, 2006.

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Vital Statistics for Desert Countries COUNTRY

BIRTH RATE PER 1,000

DEATH RATE PER 1,000

Algeria

17.8

9.0

Bahrain

18.5

Burkina Faso Chad

INFANT MORTALITY PER 1,000 LIVE BIRTHS

LIFE EXPECTANCY AT BIRTH (YEARS) MALE

FEMALE

32.2

71.2

74.3

4.0

17.9

71.5

76.5

46.3

16.2

94.4

46.6

49.5

46.6

17.0

94.7

45.3

48.6

Djibouti

40.4

19.4

105.5

41.8

44.4

Egypt

23.8

5.3

33.9

68.2

73.3

Ethiopia

39.3

15.3

94.2

47.5

49.9

Iran

17.1

5.6

42.9

68.3

71.1

Iraq

33.1

5.7

52.7

67.1

69.5

Israel

21.7

5.7

11.0

77.5

81.5

Jordan

28.1

3.2

20.9

70.6

72.4

Kuwait

21.9

2.4

10.3

75.9

77.9

Lebanon

19.3

6.3

25.5

69.9

74.9

Libya

27.2

3.5

25.7

74.1

78.6

Mali

50.3

17.6

111.2

46.3

50.3

Mauritania

41.8

12.7

72.4

50.2

54.6

Mexico

18.8

4.5

12.6

72.7

77.6

Mongolia

18.1

6.5

22.2

61.6

67.8

Morocco

20.1

5.5

43.3

67.5

72.1

Namibia

26.3

17.6

49.8

45.2

44.4

Niger

51.4

21.4

121.0

43.3

43.2

Nigeria

40.9

17.4

100.4

46.0

47.0

Qatar

15.0

4.5

19.3

70.9

76.0

Saudi Arabia

29.7

2.7

13.7

73.3

77.7

Somalia

46.0

17.3

118.5

46.0

49.5

Sudan

35.8

9.4

64.1

57.0

59.4

Syria

28.9

5.0

30.6

68.5

71.0

Tunisia

15.7

5.1

25.8

73.0

76.4

United Arab Emirates

18.7

4.1

15.1

72.5

77.6

Yemen

43.2

8.8

63.3

59.5

63.3

United States

14.1

8.3

7.3

74.6

80.4

Source: 2006 Book of the Year. Chicago: Encyclopaedia Britannica, 2006.

and the death rate is the number of deaths each year for every 1,000 of the population. The infant mortality rate records the number of babies that die within their first year. The average life expectancy at birth is the average number of years a newborn child can expect to live, and it is calculated separately for males and females. A high birth rate is characteristic of a less-developed economy and a comparatively low standard of living. It indi-

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cates a traditional society in which children are valued for their labor in family enterprises, usually subsistence farming, and for the economic support they provide for their parents in later years. It also indicates a society in which many infants die, so a large number of babies are born to ensure that enough will survive. A high death rate indicates a society in which people die young, usually from infectious diseases and illnesses linked to poor nutrition. As health

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Economics of the Desert care improves the death rate falls, and so a higher percentage of the babies survive. This is what causes a population to increase in size. The facing table gives the birth rate, death rate, infant mortality rate, and life expectancy at birth for a number of desert countries, with those for the United States included for comparison. The world average birth and death rates are 21.1 and 9.0, respectively. Taken together, these data provide a measure of the level of economic development in each country. They show that the people of oil-producing countries do not all enjoy a high standard of living and that those living in desert countries that have no oil are extremely poor.

Quality of Life Nevertheless, standards of living are improving in ways that can be measured. The rate of infant mortality, shown in the facing table, is a good indicator of the quality and extent of health care provision. This is also reflected in life expectancy, but that indicator also reflects the adequacy of the national diet. Access to health care can also be measured by the number of doctors and hospital beds in relation to the size of the population. In the United States, for example, there is one doctor for every 346 members of the population and one hospital bed for every 295 persons. This is typical for a developed country, although some do better. France has one doctor for every 306 persons and one hospital bed for every 126 persons, and Germany has one doctor for every 271 persons and one hospital bed for every 152 persons. Kuwait, on the other hand, has one doctor for every 625 persons and one hospital bed for every 455 persons. United Arab Emirates, with a per capita GDP similar to that of Kuwait, (US$17,870 and US$22,110, respectively) has one doctor for every 599 persons and one hospital bed for every 394 persons. These figures are lower than those for developed countries, but without the revenue and economic opportunities provided by the possession of large oil reserves they might be very much worse. Mauritania is a poor desert country with a per capita GDP of US$1,550, although it is not the poorest. It has one doctor for every 10,000 of its people and one hospital bed for every 1,250. That said, health care provision is to some extent a matter of government policy. In Cuba, for example, there is one doctor for every 167 persons and one hospital bed for every 161 persons. The literacy rate and figures for school attendance are also useful indicators of well-being. In the United States all children receive primary education, and the adult literacy rate is virtually 100 percent. In Kuwait only 8.6 percent of children receive primary education, and 44.8 percent have no formal education of any kind. The adult literacy rate is 82.9 percent, and somewhat higher for males (84.7 percent) than for females (81.0 percent). Kuwait is the richest of the

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desert countries. The second-richest is Qatar, a tiny state of only 4,412 square miles (11,427 km2) bordering the Persian Gulf, with a population of 744,000 and a per capita GDP of US$11,600. In Qatar there is one doctor for every 399 persons and one hospital bed for every 447 persons. At 81.7 percent, the adult literacy rate is similar to that in Kuwait, and rather more females (83.7 percent) than males (80.8 percent) are able to read and write. But 34.8 percent of the population have no formal education, and only 13.0 percent receive primary schooling. Mauritania fares even worse, with an adult literacy rate of only 52.5 percent and a marked difference between males (60.1 percent literate) and females (45.3 percent). These figures clearly illustrate that there is more to economic and social development than can be read from the per capita GDP.

■

SOLAR ENERGY

One day the world’s oil reserves will be depleted to such an extent that oil will become too scarce and too expensive to use as a fuel. No one knows when that day will come, but by the time it arrives, those countries that depend on oil exports for their income will need to have found some other resource to exploit. At the same time, the world will need to have found alternative sources of energy. Various alternatives to oil have been suggested. Several of them involve harnessing the wind, the heat of the Sun, or sunlight, and deserts have an abundance of both wind and sunshine. Provided ways can be developed to transport energy from the places where it is generated to the places thousands of miles away where it is needed, desert economies may continue to flourish. What is more, those economies may be based on a genuinely inexhaustible resource. If they are to succeed, the new industries will need to generate energy on a very large scale, and a number of technologies are being explored. Some of them have already been installed on an experimental or pilot basis. Possibly the most dramatic of these is the solar chimney. In effect, this is a device that produces a tornado and extracts energy from it.

The Solar Chimney Picture a chimney, like a very tall factory chimney, that stands by itself on a flat desert plateau. Around its base the desert surface is covered by a glass roof supported about 6.5 feet (2 m) above the ground. Close to the chimney the roof is double-glazed. Because of its size the structure is visible from afar. The glass roof covers an approximately circular area about 4.25 miles (6.8 km) in diameter, and the chimney at its center rises to a height of about 4,900 feet (1,500 m). At present the tallest structure in the world

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is the CN Tower in Toronto, with a height of 1,815 feet (553 m). This mighty chimney is nearly three times taller. Its purpose is to harness solar power for power generation—it is a solar chimney—and it produces about one-fifth the output of a big, modern nuclear or coal-fired power plant. So far the solar chimney exists only as a plan, but a smaller experimental prototype was built in 1983 at Manzanares, 93 miles (150 km) south of Madrid, in Spain. The prototype was 640 feet (195 m) tall, it had a glass roof 787 feet (240 m) across, and it generated 50 kilowatts of power. The chimney ran for seven years and proved that the idea works. Two more demonstration plants were then built in Sri Lanka. Preparations to build a much bigger one in the Thar Desert, in India, reached an advanced stage before they had to be abandoned for political reasons in 1998. That would have had a solar chimney 2,000–3,000 feet (600–950 m) tall with a glass-covered area of 38.6 square miles (100 km2.). Jörg Schlaich, the German civil engineer and professor of engineering at the University of Stuttgart who first thought of the idea, built the Spanish one, and planned the Indian one, wants to build one this size as a demonstration model to attract investors. The 4,900-feet (1.5-km) chimney is planned for the southern Kalahari Desert, in South Africa. It has been designed by a team led by a German physicist, Wolf-Walter Stinnes, as part of a study for the government of Northern Cape Province. Chimneys are also planned in other countries, and a 25,000-acre (10,118-ha) Australian site for a chimney has already been purchased at Mildura, Victoria. Construction is planned to commence in 2006. The Australian chimney will be 3,280 feet (1,000 m) tall, and the covered area will be approximately 37,000 feet (11,285 m) across.

How the Solar Chimney Works The illustration shows how such a chimney works. Sunshine passes through the glass and heats the ground beneath. Air in contact with the ground is heated but cannot rise because it is trapped below the glass. The double-glazing near the center reduces the amount of heat lost by conduction through the glass. Hot air rises up the chimney, drawing cooler air beneath the glass cover and creating a constant flow, and as the air converges to the base of the chimney it will rotate, its rotational speed increasing as its radius of rotation decreases. In other words, it will behave like air drawn into the base of a tornado. In the design for the Australian chimney the air accelerates to about 35 MPH (56 km/h). As it rises the air turns one or more turbines set inside the chimney and linked to the generating plant. The illustration shows one turbine, but the Australian chimney will have 32. The amount of power that can be generated is proportional to the difference in air pressure at the bottom and top of the

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collecting area

turbine

© Infobase Publishing

Solar chimney. Air beneath the transparent cover is heated, rises upward and accelerates toward the base of the chimney, then rises up the chimney, turning one or more turbines.

chimney. Because air pressure decreases with height, the taller the chimney the lower the pressure will be at its top, so increasing the height of the chimney also increases its power output. At the same time the air beneath the glass will try to expand as its temperature rises, but because it is trapped its pressure will increase. Efficiency can be increased both by building higher to reduce pressure at the top and by increasing the covered area to increase pressure at the bottom. The glass cover is the most expensive part of the structure to build, so designers opt for taller chimneys and smaller collecting areas.

Too Costly? Deserts lack many resources, but they possess in abundance those that a scheme of this kind needs. They have large areas of land that cannot be farmed or used for housing, and they have ample sunshine. The plant itself would be built mainly from concrete and glass, both of which can be made from desert rocks and sand. Nevertheless, it would be expensive to build. The fullscale South African plant would cost about $400 million, and the cost of the Australian chimney is estimated to be between $500 million and $750 million. That is a great deal of money. The electricity the plant generated would cost

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Economics of the Desert much more than electricity from a conventional plant, especially in South Africa, where coal is cheap and plentiful, but it would also last longer. The chimney should go on working for 100 years, and the turbines would also last well because they would spin in an airflow moving at constant speed. It is stopping, starting, and changing speed that cause most wear on moving parts. Its long life means the cost of construction can be spread over a much longer period than is the case for a conventional power plant, maintenance costs would be low, and the fuel itself costs nothing. Taken together, the designers hope that solar chimneys may produce electricity as cheaply as a coal-fired plant. A solar chimney also has a major advantage over all other solar devices—it goes on working 24 hours a day. Experiments have found that the airflow continues even at night. During the day sunshine warms the ground. The flow of air carries heat away, but the rate of heating exceeds the rate of cooling, so the ground grows continually hotter until the hottest part of the day. At night the warm ground continues to heat the air above it and maintain the airflow. It might be possible to increase this effect, for example, by a system of pipes that carried water heated by day into an insulated reservoir and returned it at night to warm the air.

Solar Furnace In 1946 French scientists and engineers supervised the construction of the world’s first large-scale solar furnace at Odeillo, in the eastern Pyrenees 5,000 feet (1,500 m) above sea level. It is a place that enjoys clear skies and bright sunshine most of the time. The present solar furnace entered service in 1970. It focuses sunshine to generate temperatures between 1,470°F (800°C) and 4,500°F (2,500°C). That is equivalent to 1 MW of thermal energy. Modern furnaces are much bigger. Solar One, designed by the U.S. Department of Energy, was an experimental furnace completed in 1981 near Daggett, in the Mojave Desert about 10 miles (16 km) from Barstow, California. It generated 10 MW of power and operated from 1982 until 1986. Nevada Solar One, located near Boulder, Nevada, is expected to become operational in 2007. It will generate 64 MW. The High-Flux Solar Furnace, built on top of a mesa near Golden, Colorado, by the National Renewable Energy Laboratory, has been in operation since 1998. It generates 10 MW by concentrating sunlight to produce the power of up to 50,000 Suns. A 1 MW plant is being built in Arizona, two 50 MW plants in Spain, and a 500 MW plant in Israel. The diagram shows how a modern solar furnace works. Sunshine falls onto an array of heliostats. A heliostat is a plane mirror mounted on a mechanism that makes it track the Sun across the sky. The Odeillo solar furnace uses 63 helio-

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p focus

solar rays

parabolic reflector heliostat © Infobase Publishing

Solar furnace. The heliostat tracks the Sun across the sky and reflects solar rays onto the parabolic reflector. The parabolic reflector focuses the rays onto the focus, producing very high temperatures.

stats, but other designs use only one. The High-Flux Solar Furnace has just one heliostat with an area of 344 square feet (32 m2). The heliostats reflect sunlight onto a parabolic reflector, called a primary concentrator, made from curved mirrors. The primary concentrator at the High-Flux Solar Furnace has 25 mirrors, each with an area of about 5 square feet (0.5 m2). The primary concentrator reflects the sunlight onto a secondary concentrator, or focus. The diagram shows this projecting from the surface of the parabolic reflector, but it is usually located inside a building. Solar furnaces generate heat that can be used industrially. More commonly the heat is used to vaporize water to drive steam turbines for power generation. It is a tried and tested technology that has great potential for desert countries.

Wind Power Solar furnaces have been built. They are known to work, but there are other ways of harnessing solar energy. Wind generators are the most familiar and count as solar energy devices because the weather systems that produce the wind are driven by solar energy. They can be erected for much less than a solar chimney, but they suffer from a major disadvantage. Although a desert is a very windy place, wind energy is very diffuse and intermittent even there. A modern wind generator consists of an aerodynamically efficient rotor, very much like an airplane propeller. Depending on the design, it has two or three blades, and it

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is mounted on a tower high enough to expose the rotor to winds that are not slowed by friction with the ground. To achieve this the rotor must be at least 33 feet (10 m) taller than the top of any nearby tree, building, or other obstruction. Gearing connects the rotor to a generator. The power the rotor produces is proportional to the square of the diameter of the circle swept by its rotor and the cube of the wind speed. This means the structure must be as big as possible. It is possible to install a small wind generator on the roof of a house to supply that property, and a single generator erected outdoors can supply an isolated community. Where wind generators are to feed useful amounts of electricity into a grid, they are built as groups forming a wind farm, and wind farms use large machines. As the technology has developed, the size of wind turbines has grown. A large wind generator has a rotor diameter of about 165–260 feet (50–80 m) mounted on a tower of the same size up to the base of the rotor—a 165-foot (50-m) rotor would need a tower about 246 feet (75 m) tall. The air slows down as it passes through the rotor, and as it slows it spreads to the sides. This sets a limit to the proportion of the energy of the wind that any rotor is capable of extracting. The theoretical maximum for a propeller-type rotor is about 30 percent, but in practice most wind generators extract 10–20 percent of the wind’s energy. That figure is used in calculating the specified output from the generator. The wind does not blow all the time, however, so the actual annual power output of a wind generator will always be lower than the figure given in its specification multiplied by the number of hours in a year. The ratio of the amount of power actually produced to the figure stated in the specification is called the capacity factor of the generator. In fact, the most efficient wind generator on the best possible site will generate no more than about 35 percent of its stated output. In other words, a 1 MW wind generator with a capacity factor of 35 will actually generate no more than 350 kW. This compares with an average efficiency (actual output compared with stated capacity) of about 90 percent for a nuclear plant, 70 percent for a coal-fired plant, and 30 percent for an oil-fired plant. Wind generators are not new. Some were designed in the 1930s, although they were not built. The first very large one to be built, with a rotor diameter of 87 feet (26.5 m) and a tower 117 feet (36 m) tall, was erected in 1940 on a hill in Vermont called Grandpa’s Knob. Its generating capacity was rated at 1.25 MW. After running for several years metal fatigue weakened one of its blades and destroyed it. Wind power is expanding rapidly. Although it still accounts for less than 1 percent of the total amount of electricity generated throughout the world, its output quadrupled between 1999 and 2005, and in some countries it makes a significant contribution to the power supply. Wind generators provide 23 percent of electricity in Denmark, about 8 percent in Spain, and more than 4 percent in Germany. In

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the United States wind power is the fastest growing method of electricity generation. There is one possible long-term disadvantage. Any device that extracts energy from moving air removes that energy from a weather system. At present the scale of wind power generation is far too small for this to be of any importance, but wind power might affect the climate if it were ever to supply electricity on anything approaching the scale of coal, gas, oil, or nuclear power today.

Solar Cells Photoelectric cells convert light into electricity. Arrays of cells are used to power spacecraft and satellites. All they need is sunlight, they have no moving parts, and the technology on which they are based is well understood, mainly because of its development by the space industry. The cells exploit the fact that an electric field exists at the junction where two layers of different materials are in close contact. A photoelectric, or solar, cell has an upper layer of dark material that absorbs sunlight. Beneath that there is a top-junction layer comprising a metal grid. This forms one of the two electrical contacts that are needed for a current to flow. Light passes through the grid to the absorber layer, where two dissimilar materials in close contact produce an electric field. Below that there is a metal layer covering the entire base of the cell. This is the back-junction layer, from which the current flows out. The absorption of light falling on the device passes energy to electrons in the three layers. The electrons move in the direction imposed by the electric field at the absorber layer and are carried away by a wire connected to the back-junction layer. To complete the circuit, the external wiring must rejoin the cell. Flowing electrons—the electric current—lose the energy imparted by the absorbed sunlight by doing useful work and return to the cell to absorb more. Although photoelectric cells generate power reliably, they do not generate it cheaply. They are made from fairly costly materials, and, because their efficiency is fairly low, a large number is needed if a useful amount of power is to be generated. Under standard testing conditions, of light falling at 93 watts per square foot (1,000 W/m2) and a temperature of 77°F (25°C), most solar cells convert no more than about 15 percent of the light into electricity. Scientists working on more advanced cells are aiming to achieve efficiencies of up to 40 percent.

Solar Ponds Solar ponds exploit the fact that salt water is denser than freshwater, so a layer of freshwater can be used to insulate a layer of salt water. A pond, usually several feet deep and with a large surface area, is lined with black plastic to

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Economics of the Desert absorb radiant heat. A layer of water saturated with salt covers the lower part of the pond, and freshwater is carefully poured into the pond above it. Freshwater and salt water do not mix readily because of their different densities, so the freshwater forms a layer above the denser salt water. Solar heat passes through the freshwater and is absorbed by the black lining. The lining heats up and warms the salt water in contact with it. Convection currents warm the rest of the saltwater layer, but they do not affect the overlying freshwater. The salt water grows hotter, but the freshwater remains at the same temperature as the air—any heat it does absorb is lost to the air in contact with the surface. The salt water can reach a temperature approaching 212°F (100°C). It is piped away to a heat exchanger, where pipes containing hot salt water pass through a tank containing cool freshwater, much like a domestic hot water tank, and back to the solar pond. From time to time more freshwater must be added to the upper layer to replace the water that evaporates, although evaporation losses can be eliminated by covering the entire pond with transparent plastic.

A Place for Solar Energy Those desert countries with reserves of oil, gas, or coal have the fuel they need to provide energy for their industries and for commercial and domestic use. For countries without fuel reserves of their own, however, the cost of energy is a major constraint on development. In most of the poor desert countries the power supply is erratic and does not serve every community. In most modern factories and offices using computers, printers, fax machines, and other electronic devices as well as machines powered by electricity, a power failure means work has to stop. Solar power provides a means of exploiting space and sunshine, the two resources that are available in every desert. Although the devices are costly to install and cannot compete with other generating plants, they gain a competitive edge where fuel is expensive. Technologies that may be inappropriate in an industrial country, especially one in a temperate region, may have much to offer in an industrializing country in a low latitude.

MINERALS, METALS, AND TEXTILES

■

Not all desert landscapes conceal reserves of oil or natural gas, but in some places they contain other riches that can be exploited. The landscapes consist of vast areas of bare rock and sand exposed by the lack of vegetation. These are used in the construction industries, of course, but they may have other uses as well. Sand is the raw material for making

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glass, for example, although not all sand is of sufficiently high quality, and the silicon crystals from which computer chips are made are also obtained from sand. Rock is made from minerals, which are inorganic compounds each with its own distinct and often complicated chemical composition. Granite, for example, contains at least 20 percent quartz by weight, together with alkali feldspar, mica, and a variety of other minor ingredients. Quartz is the mineral silicon dioxide (SiO2) and the substance from which most sand is made. Alkali feldspars are also minerals. They vary in composition, but all contain sodium (Na) or potassium (K) with aluminum (Al), silicon (Si), and oxygen (O). Sodium feldspar, or albite, is NaAlSi3O8, and potassium feldspar, also known as sanidine, orthoclase, or microcline, is KAlSi3O8. Every rock contains a suite of minerals, and some minerals are economically valuable. Quartz is familiar as sand, but it also occurs as much larger pieces of attractive rock crystal and is sometimes colored by the presence of chemical impurities. Colors resulting from these impurities change its name. Pink quartz becomes rose crystal, purple quartz is amethyst, dark brown quartz is cairngorm, and light brown quartz is topaz. Onyx and agate are also varieties of quartz. Mix silicon dioxide with a little beryllium (Be) and aluminum, and it becomes beryl (Be3Al2Si6O18 ). Transparent green varieties of beryl are known as emeralds, and bluishgreen ones are aquamarines.

Industrial Minerals Jewels and semiprecious stones are obviously valuable because they are pretty and also rare. Other minerals have industrial uses. Almost all the metals in everyday use are obtained from minerals extracted from rocks. A mineral that contains a high enough concentration of a useful metal for it to be extracted economically is called an ore mineral. Certain rocks can be processed for the chemicals they contain. Phosphate rock is composed of minerals, all of which consist mainly of calcium phosphate (Ca3(PO4)2). The rock can be crushed and processed to obtain phosphorus, which has several industrial uses, or to make phosphate fertilizer. The “phosphate” in phosphate fertilizer is phosphorus pentoxide (P2O5). Gypsum is hydrated calcium sulfate (CaSO4.2H2O). It is precipitated when seawater evaporates, and it has many industrial uses. It is used as a fluxing agent to help the process of smelting and alloying metals and as a filler in textiles and paper. It is also used in wall plaster because it spreads easily when wet but sets hard as it dries. A particularly finegrained variety of gypsum that is mined near Paris is called plaster of paris. Gypsum can also occur as a solid rock called satin spar that is used to make ornaments and jewelry. One kind of satin spar is called alabaster, which can be polished and is used for sculpture. It is very expensive.

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Rock lies beneath every landscape, of course, but in deserts the rock is exposed. No agricultural land or wild vegetation has to be sacrificed to gain access to it. Desert countries that lack oil ought to be able to mine enough minerals to compensate.

Mining Unfortunately, both the geology and the economics are a little more complicated. Commercially useful minerals are not distributed evenly. They are abundant in the Sahara, for example, but not equally so everywhere in the Sahara. Consequently, some desert countries have large reserves, and others have none at all. Even where reserves are known to exist, it may not be possible to exploit them. Exploitation requires capital investment in machinery and the means to transport the recovered minerals to factories for processing or to seaports for shipping overseas. Poor countries may be unable to finance such a costly operation, and the remoteness of the prospective mines, difficulties in recruiting local labor, political uncertainty, or any one of dozens of other factors may make it impossible for the government to attract foreign investors. Bahrain, Burkina Faso, Chad, Djibouti, Egypt, Ethiopia, Kuwait, Lebanon, Oman, Qatar, Somalia, Sudan, and United Arab Emirates are all desert countries with no valuable mineral reserves. In some cases the countries are very small, of course. Bahrain covers only 268 square miles (694.2 km2) including its offshore islands. Lebanon occupies 4,016 square miles (10,400 km2), Qatar 4,412 square miles (11,427 km2), and Djibouti 8,950 square miles (23,200 km2). Perhaps it is not surprising that they have no important mineral resources: Where resources are distributed randomly, these small areas are missed. Sudan, on the other hand, is the largest country in Africa, with an area of 966,757 square miles (2,503,890 km2)—more than 3.5 times the size of Texas.

North Africa North African countries other than Egypt do possess minerals. Libya, to the west of Egypt, has reserves of manganese, gypsum, iron ore, and lignite (also called brown coal), which is a poor-quality coal that produces less heat than better coals and much more smoke. Libya’s mineral reserves are of little importance compared with the country’s huge oil reserves. Tunisia, Libya’s neighbor to the northwest, has some of the largest reserves of phosphate rock in Africa. The reserves are mined, and most of the rock is used domestically to manufacture industrial chemicals and fertilizers, which are exported. Phosphate products account for about 4.9 percent of Tunisia’s export earnings.

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Algeria has high-grade iron ore and phosphate rock. It also produces sand, gravel, and crushed stone used in the construction industries. Like Libya, its minerals are of little economic importance compared with its petroleum and natural gas. Morocco does exploit its mineral resources. Phosphoric acid (H3PO4) provides 7.6 percent of the value of Moroccan exports, and phosphates provide 4.6 percent. Phosphoric acid is the most commercially important compound derived from phosphate rock. Some, made by a process that leaves it containing some impurities, is used to make fertilizer, but the purest phosphoric acid is used in the foodstuffs and detergent industries. The country also has iron ore, zinc, lead, manganese, and copper as well as rock salt and coal.

Southern Sahara and Sahel Mauritania exports substantial amounts of iron ore, accounting for about 56 percent of its export earnings. The country also has reserves of copper, titanium, gypsum, and phosphate rock. Its neighbor Mali also has reserves of iron ore as well as nickel, copper, manganese, and bauxite, the ore from which aluminum is obtained. These remain largely undeveloped. Gold is the one metal that is exported, and it accounts for 50 percent of the country’s export earnings. Gold is used to make jewelry, of course, but nowadays it has more important uses in electronics. Although gold has only 71 percent of the electrical conductivity of copper, unlike copper it is chemically inert. This makes it an ideal metal for printed circuits and for plating terminals and other contacts. Its softness and ductility mean gold can also be made into a very thin film, and this is also a useful attribute. One ounce (28 g) of gold can be made into a sheet of gold leaf with an area of 187 square feet (17.4 m2), and an even thinner film will reflect 98 percent of the light falling on it yet allow someone on the dark side to see through it clearly. Gold film is used to cover the face visors on space suits and on satellites to prevent them from overheating, and it is also used on the windows of some large buildings. It adds to the beauty of the building while at the same time reducing bills for air conditioning. Niger is one of the world’s most important producers of uranium. This accounts for almost one-third of the country’s export earnings. It also possesses very high-grade iron ore—the ore is about 50 percent iron—as well as phosphate rock and coal. Nigeria has iron ore and tin as well as limestone that can be used for building.

Arabian Peninsula and the Middle East There is also high-grade iron ore in Saudi Arabia as well as gold, bauxite, ores of copper and lead, gypsum, salt, lime-

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Economics of the Desert stone, and marble. In the south of the Arabian Peninsula, although Oman has no significant mineral reserves, Yemen has iron ore. This is mined and smelted at Mount Nuqum, north of Şan ‘ā’. There are more reserves elsewhere, although these are of only minor economic importance since the discovery of oil and natural gas. Yemen also produces salt. Syria produces significant amounts of phosphate rock and also has reserves of iron ore. Iran exports small amounts of iron and steel. The country also has reserves of copper and coal, although they are largely undeveloped. Iraq has deposits of sulfur, phosphate rock, gypsum, and salt, but these are of little economic importance. Jordan exports phosphate and potassium (potash) fertilizer made from its own deposits, and between them they amount to about 11 percent of the value of total exports. Jordan also has reserves of uranium, copper, gypsum, kaolin—the source of china clay, used to make fine ceramics and as an inert filler in many products, including paper— and sand of glass-making quality. Israel is one of the world’s principal producers of potash fertilizer—mainly potassium chloride (KCl) and potassium sulfate (K2SO4)—and it also has deposits of phosphate rock, gypsum, and glass-quality sand. Other minerals, including compounds of bromine and magnesium, are extracted from the water of the Dead Sea. Diamonds are its principal export, however. Cut diamonds provide 28.2 percent of the country’s export earnings, and rough diamonds provide 6.5 percent.

Namibia and Mongolia In southern Africa, Namibia is richly endowed with minerals. There are reserves of uranium, tin, tungsten, and copper as well as smaller amounts of lead, zinc, cadmium, lithium, and silver. Gold and diamonds are mined. Diamonds provide 41 percent of Namibian export earnings, and other minerals, principally gold, zinc, copper, lead, and silver, provide almost 15 percent. Together, therefore, minerals account for more than half of Namibian exports. Mongolia is also rich in minerals. It has ores of iron, tin, copper, zinc, molybdenum, and tungsten as well as gold, a variety of semiprecious stones, a phosphate rock called phosphorite, and fluorite. Fluorite, also called fluorspar, is calcium fluoride (CaF2) and is used in the ceramics and chemical industries and in the smelting of iron. Copper is Mongolia’s most important export, accounting for 28 percent of export earnings. Gold provides 14 percent of export earnings, and fluorspar 4 percent.

Mexico Mexico has the richest mineral reserves of all the desert countries and is the world’s biggest producer of bismuth

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and the second-biggest producer of silver, fluorite, and celestite. Celestite (SrSO4) is the most important ore mineral for strontium (Sr). Bismuth (Bi) is a metal used in alloys as solders for special purposes and in safety devices, such as the heads on automatic sprinklers and release mechanisms for fire doors. Bismuth compounds are also used in the manufacture of plastics and paints. In addition to these, Mexico is also a major producer of cadmium, lead, gypsum, zinc, sulfur, copper, iron ore, and gold. It also has deposits of arsenic, antimony, mercury, and graphite. Its minerals are not exported directly but in the form of manufactured goods. Mining contributes only 1.2 percent of the Mexican gross domestic product and provides 0.3 percent of the employment, but manufacturing contributes more than 17 percent and provides 18.6 percent of the jobs. Metal goods are economically the most important manufactures, followed by chemicals.

Adding Value Metallic ores, other minerals, and even oil have no economic value while they lie undisturbed in the ground. They acquire value only when capital is invested and labor is recruited to extract them. Even then their value is fairly low, because they cannot be used in the forms in which they are taken from the ground. Even diamonds and other jewels and semiprecious stones have to be cleaned and shaped before they can be sold to their final users. Gold and a few other elements such as copper, sulfur, and carbon (graphite as well as diamond) occur in their pure form—they are known as native elements—but most have to be separated out of chemical compounds. Each of the operations by which a raw mineral is converted into a product that can be retailed adds value to it. It is therefore in the interest of the producing country to perform as many of those operations as possible within its own borders. This is what Mexico has achieved. Not all countries are able to add value in this way. Just as some countries are unable to raise or attract from overseas the capital needed to mine their mineral resources, others that do mine them are unable to refine or process them for the same reason. Instead, they export them in their unprocessed state to be processed by factories in wealthier countries.

Cashmere Minerals are not the only resources capable of economic exploitation, however. Although desert countries support little agriculture, their peoples do raise sheep and goats, which produce wool or hair that can be spun, dyed, and woven. Hair from the Kashmir breed of goats is soft and fine, with fibers one to 3.5 inches (2.5–9 cm) long. Like wild goats and sheep and some other domesticated breeds, the coat of a

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Kashmir goat has two layers. The outer layer consists of long, coarse, water-repellant hairs, and the inner layer is of soft hairs that provide thermal insulation. In spring the animals molt their thick winter coats, and it is then that the cashmere wool is obtained by plucking or combing it from the goats, although in Iran the goats are sheared like sheep. Some wool is also obtained from goats that have been slaughtered. This is called pulled wool, and it is of inferior quality because the fibers deteriorate after death. The fleece is then cleaned, and the fine and coarse hairs are separated. The coarse hair is used to make ropes, bags, and tent curtains. Cashmere is the finest of all types of wool and was first used to make shawls. Today it is also made into sweaters, suits, dresses, and overcoats. A goat yields from a few ounces to about one pound (0.5 kg) of cashmere each year. It takes the wool from four to six goats to make a sweater, and an overcoat requires wool from 30 to 40 animals. European factories started making imitation Cashmere shawls early in the last century, but these are a poor substitute for the real thing. Only a small amount of cashmere wool is produced, the demand for it is very large, and consequently it is expensive. It comes mainly from China, Mongolia, and Iran, with smaller amounts from Afghanistan and Turkey.

Carpets and Rugs Nomadic peoples have always woven woolen carpets. These decorate their tents and can be rolled up easily when it is time to move. They are laid on the ground inside the tent, hung as flaps over the entrance, used as blankets and saddle covers, and made into bags. People carry their small prayer rugs with them everywhere. More durable than pottery and lighter than metal articles, a decorated carpet is a form of ornamentation especially suited to the nomadic way of life. Carpets are also the only medium of artistic expression available to their makers. The designs are traditional but very elaborate, and the carpets and rugs have always been made to very high standards. Although at first made for tents, they were soon ornamenting the interiors of mosques and palaces. The export of carpets and rugs to Europe from the desert countries of the Middle East and Central Asia began in the 16th century. From the start they were much too expensive to be laid on the floor. Their wealthy owners hung them on walls or over balconies or laid them over chests and tables. They were displayed where they could be seen and admired without being harmed. Carpet making reached a peak in Persia (Iran) in the 15th and 16th centuries. At that time the finest carpets and rugs were made in royal workshops attached to palaces. Sheep were bred specially to produce suitable wool, gardens were sown with dye plants and carefully tended, and skilled spinners, dyers, and weavers were well rewarded. Most of

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the carpets were made from wool, but the very finest were made from silk. Persian carpets became famous throughout Europe and have remained so ever since. Two techniques are used. Flat carpets, called kilim, are woven. The weft (cross-threads) is made from dyed wool or silk and the warp (longitudinal threads) from cotton, linen, or hemp. These carpets resemble woven tapestries. Pile carpets are made by individually knotting colored threads around the warp threads. Several types of knot are used, and some Indian carpets have about 2,500 knots to every square inch (388/cm2).

Spread of Persian Techniques Persian carpet making influenced the designs and techniques used in neighboring countries. Egyptian rulers of the Mamlūk dynasty (1250–1517) sponsored carpet manufacture in their country. Mamlūk carpets were knotted using a knot that originated in Persia, and many were exported. Persian techniques also spread into the Caucasus, where carpets were made for local aristocrats. Carpets are still being made by Kazakh, Sarūq, and other nomadic peoples living near the Caspian Sea and on the borders of Iran and Iraq. Early Turkish carpets also followed Persian designs and used Persian techniques. They were made mainly in Anatolia, in central Turkey, by nomadic peoples, including the Kurds. By the 18th century Turkish carpets were so popular abroad that they were being made for export in the western port of Izmir (formerly Smyrna). Carpets are still among the goods exported through Izmir.

Bukhara Bukhara, or Boxoro in the Uzbek language, is the principal city in the province of Buxoro, in Uzbekistan. Set at the center of the Bukhara oasis, its history goes back a long way. The Arabs captured it in 709, Genghis Khan captured it in 1220, Timur, also called Tamurlane, captured it in 1370, and in 1506 it was taken by Uzbek people and became the capital of the khanate of Bukhara. Today it is the fifth-largest city in Uzbekistan, with a population of about 238,000, and it has grown substantially since natural gas was discovered nearby in the 1950s. Its name is associated with Bukhara carpets, but these are made not in Bukhara, but over a large area of western Turkistan. Turkistan is not a single country, but the name that used to describe an area of more than 1 million square miles (2.6 million km2) in central Asia bounded by Siberia, Tibet, Afghanistan, India, and Iran and inhabited by nomadic Turkic peoples. Bukhara carpets are made by members of nomadic Turkmen tribes, mainly the Tekke, Yomut, Afghan,

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Economics of the Desert Sarūk, Ersar, Beshir, and Baluchi, most of whom live in Turkmenistan and adjacent parts of Uzbekistan and Tajikistan. Turkmen carpets are made entirely from wool and are knotted. Not all are meant as floor coverings. Nearly square rugs that are laid on floors are made mainly for trading. Others are used as flaps to cover the entrances to tents or to be hung inside as decoration, and some are made into saddlebags. Textile exports earn desert countries useful amounts of foreign exchange, but they are much more important indirectly. Their fame attracts visitors seeking to buy them close to where they are made and in some cases to see them being made. These visitors are tourists, and usually they are wealthy.

■

TOURISM

Today there are new travelers on the Silk Road. Most visitors do no more than visit a few of the towns along the route, but there are some who journey along parts of one of the old roads. The more adventurous arrange their own itineraries using local transport services or even bicycle for part of the way. For the less confident, there are tour operators who will make all the necessary bookings and advise about the visas foreigners require. Many visitors travel in organized parties by train, bus, or hired car. Tourism is now of major economic importance to most countries, not least to those occupying desert lands.

Legendary Cities Visits to the more remote parts of the world are becoming easier and less expensive. For less than $600 a week, for example, it is possible to spend five or six weeks traveling along the Silk Road with a guide, crossing mountains and deserts. The romance of the place names along the route exerts an obvious attraction. Samarkand has existed since the fourth century b.c.e. Then it was known as Maracanda and was the capital of a kingdom called Sogdiana. Alexander the Great captured it in 329 b.c.e. In the centuries that followed the city was occupied by the Turks, Arabs, and Persians. It was destroyed by Genghis Khan in 1220 c.e. and was rebuilt, and its citizens revolted against their Mongol rulers in 1365 (see “Rise of the Mongol Empire” on page 177). After that Samarkand became the capital of the empire established by Timur (1336–1405), also known as Tamerlane or Tamburlane, who was born not far from the city. He was the last of the great Mongol conquerors and probably the most violent. His empire extended from Delhi, which he destroyed, to Moscow. His troops burned many cities and killed all their inhabitants. Timur lived as a nomad and never

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had a permanent home, but he tried to make Samarkand the finest of all cities, although he never spent more than a few days there. He was buried in the ornate Gūr-e Amīr mausoleum. This still stands and is one of the many places visited by tourists. Strangely, in the 18th century Samarkand was completely abandoned for 50 years.

Mongolia Samarkand, Tashkent, Bukhara, and the Silk Road are names that resonate through the cultures of Europe and Asia, but cities, buildings, and ancient monuments are not the only attractions that draw visitors. So do lifestyles that are radically different from those of the West. Mongolia also welcomes tourists, who come to meet the descendants of the ancient Mongol warriors, explore their capital, Ulaanbaatar, join in their festivities, and visit their homes, the traditional ger. The name Ulaanbaatar means Red Hero and dates from 1924, when Mongolia became a people’s republic. Before that it was known as Niislel Khureheh, Capital of Mongolia, a name it was given in 1911 when what was then Outer Mongolia declared its independence. Before that it was called Da Khure, which is the name of the Tibetan Buddhist monastery built in 1639 that provided the focus for a permanent settlement.

Greenland Thousands of miles away, Greenland has become a tourist destination where people can visit what claims to be the biggest national park in the world, established in 1974 and covering 289,500 square miles (750,000 km2) in the northeast of the country. Greenland offers splendid scenery, with glaciers and the ice sheet, and a thriving culture in the coastal towns and villages. The number of tourists allowed into the country is controlled but has risen over recent years, and the Greenlandic government is keen to promote tourism. Approximately 5,000 overseas visitors traveled to Greenland in 1993. By 2002 the figure had risen to about 32,000.

North Africa and the Middle East Figures are not available for the income Greenland and Somalia derive from tourism, but, as the table on page 238 shows, for many desert countries it is large enough to be economically important. North African beaches attract European tourists, Tunisia and Morocco being the countries that have developed and promoted this resource most vigorously. Tunisia earns more than $1.5 billion a year from tourism and Morocco more than $2 billion, and not only from visitors who wish only to soak up the sun. The site of the ancient

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Tourism in Desert Countries (Millions of US$) COUNTRY

INCOME FROM VISITORS

EXPENDITURE BY NATIONALS WHO TRAVEL ABROAD

Algeria

161

248

Bahrain

740

327

Burkina Faso

39

22

Chad

25

80

4

4

4,584

1,321

114

50

1,777

4,190

Djibouti Egypt Ethiopia Iran Iraq Israel

14.5

30.6

2,039

2,550

Jordan

815

377

Kuwait

117

3,349

Lebanon

1,016

n.a.

Libya

79

548

Mali

104

36

28

55

9,457

6,253

Mauritania Mexico Mongolia

143

108

Morocco

2,856

485

Namibia

333

74

34

21

Nigeria

263

950

Oman

219

557

3,418

4,165

n.a.

n.a.

Niger

Saudi Arabia Somalia Sudan

118

119

Syria

1,147

760

Tunisia

1,838.6

300

Yemen

139

77

(n.a. not available) Source: 2006 Book of the Year. Chicago: Encyclopaedia Britannica, 2006

city of Carthage is now a suburb of Tunis, the Tunisian capital, and the Carthaginians established a series of trading ports along the Moroccan coast. The city of Larache was founded in the 12th century b.c.e. by Phoenicians from Tyre. Syria also earns more than $1 billion a year, though

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not mainly from its beaches, since Syria has only a short stretch of coastline. Egypt earns $4.584 billion a year from tourism. Visitors to Egypt are there primarily to see the pyramids, the Sphinx, and the ancient temples. It is also history that draws people to the other countries of North Africa and the Middle East. These are regions containing archaeological remains dating from Roman times and, in the lands bordering the Euphrates and Tigris Rivers, from the oldest of all Western civilizations. Three of the world’s major religions also arose in the Near and Middle East, and some of the visitors to the region are pilgrims. They are especially important in Saudi Arabia and Israel.

Opportunities for Greater Tourism For political or religious reasons not all desert countries encourage tourism, but most benefit to some extent. For many years the Libyan government thought tourism unimportant, but in 1997 it changed its mind. In keeping with the national ethos, it offers the desert itself as a destination and experience. Libya was politically isolated for some time and is difficult to reach, but when its isolation ended its annual tourist income rose from $7 million in 1997 to $79 million in 2003. There is a possibility for economic expansion in many desert countries. Oman, for example, earned only $92 million in 1997 but increased this to $219 in 2003. Oman has been inhabited for at least 10,000 years, and in ancient times its people exported frankincense to markets throughout the Mediterranean and Middle East. An Omani empire once extended through part of East Africa, and Zanzibar was the capital of Oman. Yemen also produced frankincense, and in the 4th century b.c.e. the Ma’īn kingdom was exporting it to Egypt as well as spices and other goods. Saba was the kingdom of Sheba (see “The Kingdom of Sheba” on page 183). Al-Baydā’, a city of about 500,000 people, was the capital of a sultanate that ruled a large area of southern Arabia from the seventh to 16th centuries. In 2003 Yemen earned $139 million from tourism. Fossils of Australopithecus species—ancestors of humans—up to 4 million years old have been found in Ethiopia, making that part of Africa the oldest of all in terms of human habitation. In the second millenium b.c.e. the kingdom of Da’amat was established and grew wealthy through trade. Soon after 300 b.c.e. it was superseded by the kingdom of Aksum, ruled, according to legend, by descendants of Solomon and the Queen of Sheba. During the fourth century c.e. the kings became Christian and linked to the Coptic Christians in Egypt. Their power also spread into Arabia and in the sixth century extended as far as Yemen. Ethiopia has much to offer the visitor, and despite its remoteness it may be able to increase its income from tourism from the $114 million it earned in 2003.

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Economics of the Desert

Need for Investment and Peace There is no shortage of history, tradition, and ancient culture that might fascinate tourists, but there are difficulties to be overcome before a thriving tourist industry can emerge. Extremely poor countries, such as Ethiopia, Chad, Somalia, and Sudan, lack the infrastructure and facilities that tour operators demand. Before vacationers start to arrive and enjoy the country, airports, good roads, reliable buses and trains, restaurants, and comfortable hotels must be provided. Of course, building, maintaining, and operating such facilities provide employment, but adequate capital must be available for investment before work can begin. Even then, tourists must feel safe as well as comfortable, and political instability in some desert countries makes this difficult. Egypt, Yemen, and Turkey are among the countries where foreigners have been attacked, kidnapped, or threatened, and war, the threat of war, and civil disturbance have placed Iraq, Lebanon, Somalia, Sudan, and certain other countries off-limits to Westerners. Just as tourists can visit a country, so citizens of that country can travel abroad, becoming tourists themselves. Tourists bring foreign currency into the lands they visit, but it is currency they have removed from their own countries. Foreign currency is valuable, but the economics of tourism need to balance. The table shows the amount spent by nationals traveling abroad against the amount brought in by foreigners. In some countries, such as Algeria, Kuwait, Iran, Libya, and Saudi Arabia, the two are distinctly unbalanced. Proximity to a source of wealthy tourists also helps. Namibia does well by attracting South Africans. The most successful country of all, however, is Mexico, where Americans spend more than $9 billion a year.

Is Tourism Destructive? Although tourism brings in needed foreign currency, the industry also brings dangers. Visitors require accommodations, transport facilities, and a wide range of amenities. These generate employment for local people, but it is mainly unskilled or semiskilled work. Jobs are poorly paid and often seasonal. Many hotels, restaurants, and shops close down at the end of the tourist season, and their employees are out of work until preparations begin for the next season. The temporary nature of the employment and the limited skills it requires often attract migrant workers and students needing vacation jobs. At the end of the season the workers disperse. Companies based outside the tourist

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area often own many of the tourist facilities, especially the large hotels, which means that a substantial proportion of the profits of tourism leave the area. Providing tourist facilities can also damage the environment, including the very features visitors wish to experience. Skyscraper hotels lining the coast may destroy the tranquillity of the natural scenery. When wealthy outsiders buy vacation homes in attractive villages, property values often rise to levels local people cannot afford. Unless checked, villages are transformed into resorts occupied for only part of the year by people whose main homes are elsewhere. The social and economic activities that made the villages vibrant, colorful places to visit die out as the original villagers are driven out of the area. Villages that are taken over by tourists are victims of their own success in attracting visitors. The countryside can also suffer in the same way. If too many people visit an area of scenic beauty, the trampling of the ground can cause erosion. Many birds and mammals are highly sensitive to disturbance. Simply the proximity of too many visitors will drive them away. Tourism may also cause pollution. Visitors consume food and drink and buy souvenirs and gifts for family and friends, all of which must be procured and transported to where it will be consumed. Hotels and restaurants need to launder linen. Tourists wish to travel, so they need to hire cars, bus services, railroads, and ferries in addition to the long-distance transport that brings them to their destination and takes them home when the vacation ends. All of these activities generate wastes, and unless tourist resorts dispose of the wastes safely, the pollution they are capable of causing will damage the environment and may even result in visitors falling sick. Tourism is attractive. It brings visitors to a country to admire its scenery, culture, and history and to meet the people. This is flattering and, since the visitors are prepared to spend generously, it is also highly profitable. Small wonder, then, that most countries seek to promote tourism and take advantage of the ease and affordability of long-distance travel. Tourism has grown into a vast industry, and in years to come its growth is likely to continue. Yet it is an industry capable of doing harm as well as bringing benefits. It tends to provide low-paid, unskilled, seasonal employment. Many of the profits leave the tourist area. Local economies can be seriously distorted. The pressure of too many visitors may harm wildlife, and providing the facilities tourists require may destroy the natural scenery and cause environmental pollution. The dangers can be averted, but only with careful and sensitive planning.

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7 Health of the Deserts Deserts are inhospitable places, and they have a timeless quality. They seem to have existed since the beginning of time, to be relics of some imagined original, barren landscapes that life had to struggle to conquer and, once conquered, that were transformed into the world’s grasslands and forests. How can so harsh and uncompromising an environment be described as healthy or unhealthy? Of course, the impression is wholly mistaken. Although deserts do not teem with life, they are very far from being lifeless, and all living communities are vulnerable to major environmental changes. Deserts occupy lands that were not always deserts, and there are grasslands and forests growing today on what were once desert soils. While the conditions that maintain them prevail, the deserts can be deemed healthy. Should those conditions change, the health of the deserts may deteriorate. This chapter considers the two changes that may affect deserts: climate change and poor management. Climates change constantly, and at present the world is gradually growing warmer. The chapter begins by examining particular climatic events in the recent past and the natural cycles that govern many natural changes. It considers the present concerns over global warming and the greenhouse effect and their likely consequences for desert climates. The second part of the chapter begins by describing ways in which the weather might be modified. Already, for example, there are wind machines that protect crops against frost and techniques for squeezing rain from clouds that might otherwise have failed to release it. It then discusses ways of improving land management in order to make deserts more productive and to prevent their spread onto adjacent land. It describes some of the plants that may become the new agricultural crops that would achieve this. Deserts are defined by their aridity, and the chapter ends by considering safe, sustainable ways to provide irrigation for farm crops.

CLIMATE CHANGE AND THE FUTURE FOR DESERTS

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In the summer of 1934 a dust cloud three miles (4.8 km) tall covered an area of 1.35 million square miles (3.5 million km2) from Canada to Texas and from Montana to Ohio. Birds choked by the dust fell dead from the sky, and as fast as the dust was cleared from the desk of the president in the White House more settled. Dust fell on ships in the Atlantic 300 miles (480 km) from the American coast. This was the worst of the dust bowl years, when drought reduced exposed soil to a fine powder, and the wind carried it away from farms in Kansas, Colorado, Oklahoma, and Texas. A number of factors combined to make that drought unusually severe, but it was neither the first nor the last. Drought ruined crops again in the 1950s, 1970s, and 1990s. Severe droughts occur at intervals of 22 or 23 years. Drought afflicting countries along the southern border of the Sahara, in the Sahel region, caught the world’s attention in the 1970s. That drought began in the 1960s and lasted until the 1980s, but it was in the 1970s that its tragic effects became evident. It was not the first time the Sahel had experienced severe and prolonged drought. Several occurred in the 17th century and caused severe famines. Those earlier droughts coincided with the coldest part of the Little Ice Age (see “The Little Ice Age” on page 11). Between 1690 and 1699 the average temperature in England was 2.7°F (1.5°C) lower than the temperature between 1920 and 1960. Still farther back in time, the Middle Ages were a warm period, and beyond them, from about 7,000 to 5,000 years ago in the period known as the postglacial climatic optimum, temperatures in Antarctica and Europe were 3.6°F to 5.4°F (2°C–3°C) warmer than they are today. About 10,000 years ago the most recent full-scale ice age came to an end.

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Constantly Changing Climate Until the 1950s most scientists assumed that the climates of the world changed little from one century to another. The ice ages were known, of course, but it was thought that during them the climate remained steadfastly cold all the time, and during interglacials, such as the one in which we are living now, conditions were warmer and equally unchanging. There were relatively warm years and cool years, of course, as well as periods of dry weather that caused droughts and excessive rains that caused flooding, but over time these extremes canceled each other. Plot average temperatures and rainfall for a particular place over several centuries on a graph, and the lines would rise and fall as a series of spikes. Fit a trend to them, and the resulting line would be level. No one believes that now. Climates are changing constantly and have always been changing, but at rates that are slow compared to a human lifetime. They are also erratic, with periods when the climate seems stable interrupted by episodes of rapid change. This makes significant changes difficult to detect against the natural variation of warm, cool, wet, and dry years. For example, the global climate grew warmer from about 1880 to 1940. This was most pronounced in the Arctic and in Siberia, but in the Southern Hemisphere south of about 30°S there may have been a cooling. From about 1940 to some time between 1975 and 1985 temperatures fell. Then, from the late 1980s they began to rise again, and they are still rising. Detecting this rise calls for powerful statistical methods to interpret data recorded by extremely sensitive instruments. Scientists now agree that the global climate is warming by an average 0.02°F to 0.03°F (0.012°C–0.019°C) every year. This is less than the ordinary variation from year to year. Sea level is also believed to be rising, but at no more than about 0.08 inch (2 mm) a year, and then not everywhere. Some coastal areas are rising as they continue to rebound from the removal of ice sheets, the weight of which once depressed them. Since the end of the last ice age, 8,000 to 10,000 years ago, eastern Canada and Scandinavia, where the weight of ice was greatest, have risen by 820 to 1,320 feet (250–400 m). That is about half the amount by which they were depressed, and eventually they will complete their rise—unless a new ice age generates ice sheets thick enough to depress them again. In these places the sea level is falling because sea level is measured against coastlines.

Sahel Drought and the Intertropical Convergence The Sahel drought was caused by a change in the seasonal migration of the Intertropical Convergence Zone. This

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is the belt along which the trade winds of the two hemispheres meet. Their convergence produces a region of low surface pressure and rain. The position of the low-pressure zone changes with the seasons. During the Southern Hemisphere summer its northern boundary is at about latitude 5°N, but most of the zone lies south of the equator. In the Northern Hemisphere summer the zone shifts northward. In August its northern boundary lies along the southern border of the Sahara, runs through Yemen and Oman, and crosses the Indian subcontinent along the southern foothills of the Himalaya in latitudes between about 15°N and 25°N. Its northward migration brings summer rains to the Sahel. During the drought years the zone remained well to the south of its usual location, and the rains failed to arrive.

Was Air Pollution to Blame? The seasonal migration of the Intertropical Convergence Zone is an entirely natural phenomenon, and any change in its migration pattern would also appear to be natural. If the convergence moves a little earlier or later, or a little farther or not so far, that, too, seems to be a natural occurrence. It now appears that this may not be entirely true. Clouds develop when water vapor condenses onto minute particles called cloud condensation nuclei (CCN), forming droplets of liquid water. Various substances make suitable CCN. One is sulfate (SO4). Sulfur dioxide (SO2) enters the atmosphere from volcanoes and through the oxidation of sulfur compounds emitted by living organisms. It also results from the burning of oil and coal containing sulfur, from the smelting of sulfate metal ores, and from the processing of wood pulp in pulp and paper mills. In the air some of the SO2 forms sulfate particles. These are extremely small—less than 0.00004 inch (1 μm) across—and water vapor condensing onto them forms droplets that are too small to fall. The clouds containing them are brighter than other clouds and survive for longer because they release less rain. The bright clouds reflect sunlight, shading and cooling the surface below (see the sidebar “Global Dimming and Changing Clouds” on page 242). Computer models suggest that the surface cooling associated with these bright clouds weakens the rain belt that accompanies the Intertropical Convergence Zone, and the weakening suppresses its movement. Most atmospheric sulfur dioxide enters the atmosphere in the Northern Hemisphere because that is where the industries releasing it are located. Consequently, it is the northward migration of the convergence that is suppressed. The convergence does not move so far north. The end of the Sahel drought coincided with steps that were taken by the industrialized nations to reduce sulfur dioxide emissions.

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Global Dimming and Changing Clouds In the 1960s atmospheric scientists noticed a reduction in the intensity of sunlight reaching the Earth’s surface. The extent of the reduction varied from place to place, but between 1960 and 1990 it averaged about 5 percent. It was later calculated to be reducing the sunshine reaching the surface by a global average of 2–3 percent every decade. Reducing the brightness of sunshine cools the surface. In 2001 Gerry Stanhill coined the term global dimming to describe it (Stanhill, G., and S. Cohen. “Global Dimming: A Review of the Evidence for a Widespread and Significant Reduction in Global Radiation with Discussion of its Probable Causes and Possible Agricultural Consequences.” Agricultural and Forest Meteorology 107 (2001) 255–278.) Scientists believe global dimming is caused by certain very small particles, especially sulfate particles, acting as cloud condensation nuclei. Water vapor condenses onto sulfate particles to form droplets that are so small and light they do not fall. Because they do not fall, they have few opportunities to collide with other cloud droplets, which would allow them to merge and form larger droplets. Clouds containing these small droplets are very bright, and they endure for much longer than other clouds because they do not release their moisture. Since about 1990 the trend has reversed. Global dimming no longer occurs, probably because the governments of industrialized nations have legislated to reduce emissions of the sulfur dioxide that is the source of the sulfate particles.

Trade Winds and El Niño Weather extremes are also associated with the El Niño phenomenon or, to give it its proper name, an El Niño– Southern Oscillation (ENSO) event. These occur at intervals of two to seven years and are sometimes especially severe. ENSO events have been occurring for thousands of years. In this century they were especially severe in 1925–26, 1941–42, 1957–58, 1965–66, 1972–73, 1976–77, 1982–83, 1986–87, 1991–94 (this event was continuous over several years), and 1997–98. Ordinarily, the trade winds just south of the equator blow from the southeast and drive a current of warm water,

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the South Equatorial Current, across the South Pacific in a westerly direction, from South America toward Indonesia. The surface water is strongly warmed by the Sun, and as water is carried across the Pacific a deep pool of warm water accumulates near Indonesia. The westward movement means that the layer of warm surface water near South America is fairly shallow. Friction and the Coriolis effect (see “The Coriolis Effect” on page 79) combine to push the Peru Current, flowing northward from the Southern Ocean parallel to the South American coast, away from the coast. This allows deep water to rise to the surface as a series of upwellings. The Peru Current carries cool water, but the upwellings make the surface water even cooler. Air crossing the cold water is chilled, making it very stable and contributing to the aridity of the Atacama Desert. During an El Niño event the normal pattern reverses. The trade winds weaken, causing the South Equatorial Current to slacken or even to reverse direction. The warm pool around Indonesia becomes shallower, causing drought in southern Asia, and the accumulation of warm water off the South American coast suppresses the upwellings. Air flows toward South America, crossing the warm water where it gathers moisture, and deposits its moisture as heavy rain over the normally dry lands. This usually occurs in the middle of summer—late December in the Southern Hemisphere—and because it brings rain when the crops need it most, it is regarded as a gift from God and called El Niño.

Walker Circulation and the Southern Oscillation Air rises over Indonesia and flows from west to east at a high altitude. This return flow of air is called the Walker circulation, after Sir Gilbert Walker (1868–1958), the British mathematician and meteorologist who first described it in 1904. It produces a region of low surface pressure around Indonesia, where air is rising, and one of high pressure off South America, where air is sinking. From time to time this pattern reverses. This reversal was first observed in 1897 as a change in pressure distribution, and a number of scientists studied it. The phenomenon was described fully by Walker in his presidential address to the Royal Meteorological Society in 1928. During a reversal pressure is low off South America and high near Indonesia. Walker called this a southern oscillation, because it occurs only in the Southern Hemisphere. A southern oscillation causes the upper air to flow from east to west and the surface winds either to weaken or reverse direction. The wind-driven surface current also reverses direction, causing a thinning of the warm water pool near Indonesia and a deepening of the warm water off South America. It is then called an El Niño.

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La Niña At other times the usual pattern of trade winds and ocean currents intensifies. Pressure rises in the already highpressure region in the east, pressure falls in the low-pressure region in the west, and the southeasterly trade winds blow more strongly. This accelerates the east-to-west surface current, deepening the Asian warm water pool, and produces a very thin layer of warm surface water off South America. This intensification, the opposite of El Niño, is known as La Niña. Together, a complete cycle of a Southern Oscillation, El Niño, and La Niña make up an ENSO event. ENSO events affect the weather over a large area. In particular, El Niño brings heavy rain to the usually arid belt along the western coast of South America and drought to Australia. La Niña makes the aridity even more extreme in South America but brings heavy rain to Australia.

Climate Cycles over the North Pacific ENSO events vary in strength, however, and scientists have now discovered that their strength is influenced by another cycle, called the interdecadal ENSO. This comprises a slow increase, then decrease in the surface temperature of the eastern Pacific that has been traced back as far as 1860. A similar cyclical temperature change—so far without a name—affects part of the central Pacific, and there are two temperature cycles in the North Pacific. These cycles have periods, the time that elapses between one peak or trough and the next, measured in decades, and when they coincide with the temperature changes produced by an ENSO event, they intensify them. The event is then a severe one. When the cycles are out of phase with ENSO, so that an El Niño warming coincides with cool water in the eastern Pacific, the ENSO event is weak.

Cycles over the North Atlantic There is also a climatic cycle in the North Atlantic, called the North Atlantic Oscillation (NAO). This consists of a change in the distribution of surface pressure, but between north and south rather than east and west, as in the southern oscillation. An area of low pressure is located more or less permanently near Iceland, and there is an area of high pressure near the Azores. These are called the Iceland low and Azores high, respectively, although the high-pressure region sometimes drifts to the opposite side of the Atlantic and is then known as the Bermuda high. Air circulates counterclockwise around the Iceland low and clockwise around the Azores (or Bermuda) high, so between them the air flows from west to east. The difference in pressure between the two centers drives weather systems across the Atlantic, but the speed and intensity of the weather systems

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vary with changes in relative pressure. The pattern is measured according to an NAO index. When pressure is lower than usual in the Iceland low, it is usually higher than usual in the Azores (Bermuda) high. This produces a high NAO index. When pressure is higher in the Iceland low, it is lower in the Azores high, producing a low NAO index. When the NAO index is high, European winters are mild, winters in northeastern North America are cold, and the Mediterranean region, including North Africa, is dry. This was the situation from around 1975 until 1995. Then the contrast sharply decreased. A low NAO index brings mild winters to eastern North America, cold winters to Europe, and wet weather to the Mediterranean region. The NAO may be part of the Arctic Oscillation (AO), which involves the distribution of pressure between the North Pole and approximately latitude 55°N. When pressure is low over the pole, it is high farther south, and the AO is said to be positive. When pressure is high over the pole and low farther south, the AO is negative. A positive AO produces storms in high latitudes, warm weather across Eurasia, and dry weather in California and around the Mediterranean. When the AO is negative, California and Mediterranean lands are wet, and Eurasia experiences cold weather. Climate cycles such as ENSO, the NAO, and the AO are linked in the sense that they affect each other.

The Greenhouse Effect Today the change that worries many climate scientists arises from the possibility of an enhanced greenhouse effect. Nitrogen, oxygen, and the other gases of the atmosphere are transparent to solar radiation. It passes through them unimpeded and warms the surface of land and sea. Once warmed, these surfaces then begin radiating heat at wavelengths inversely proportional to the temperature to which they have been raised—the hotter the body, the shorter the wavelength at which it radiates. Consequently, the land and sea emit radiation at much longer wavelengths than those of the radiation they receive from the Sun. Certain atmospheric gases are partially opaque to these wavelengths. Their molecules absorb the radiation. This warms the air, but there are some long wavelengths that are not absorbed. These are called windows, and radiation escaping through them allows the overall balance to be maintained. By day the amount of incoming solar energy exceeds the amount of outgoing radiation, and the air warms. At night there is no incoming radiation, and the air cools, although its temperature never falls so low as it would if there were no gases absorbing radiation. In that case the average temperature at the surface of the Earth would be -0.4°F (-18°C) rather than 59°F (15°C), which is the actual average. This is the so-called greenhouse effect, although in a real greenhouse the warming is due mainly to the inability

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of warm air to escape and mix with the air outside. The greenhouse effect is entirely natural, and without it life on Earth would be extremely uncomfortable. The gases that cause it, the greenhouse gases, are principally water vapor, carbon dioxide, methane, nitrous oxide, ozone, and also chlorofluorocarbons (CFCs), which do not occur naturally. The capacity of gases, and especially carbon dioxide, to absorb radiation was first noted in the 19th century. The Swedish physical chemist Svante Arrhenius was the first scientist to calculate in detail the climatic consequences of

altering the concentration of carbon dioxide in the atmosphere (see the sidebar).

Global Warming Millions of years ago and over millions of years, carbon dioxide was removed from the air by photosynthesis (see“Photosynthesis” on pages 96–101) and then stored in the form of organic material that failed to decompose completely, which would have oxidized its carbon, returning it

Svante Arrhenius (1859–1927) Svante August Arrhenius was a physical chemist who received the 1903 Nobel Prize in chemistry for his work on electrolytes. He was born on February 19, 1859, on the estate of Vik, near Uppsala, Sweden, that was owned by the University of Uppsala, and in 1860 the family moved to Uppsala. Svante began his education at the cathedral school in Uppsala and was accepted as a student at Uppsala University when he was only 17. He graduated in 1878 in chemistry, physics, and mathematics and was awarded his doctorate in 1884. His thesis, Recherches sur la conductibilité galvanique des électrolytes (investigations on the galvanic conductivity of electrolytes), was awarded only the lowest grade of doctorate, but it was the work he described in it that later won him his Nobel Prize. Toward the end of 1884 he was offered a post at Uppsala and later a traveling fellowship that allowed him to meet other scientists working in the same field. In 1891 Arrhenius accepted a lectureship at the Stockholms Högskola (high school), where in 1895 he became professor of physics. From 1897 until 1905 Arrhenius was also rector. The high school was equivalent to the science faculty of a university, but it was not empowered to award degrees or to accept doctoral theses. It became the University of Stockholm in 1960. Arrhenius retired from the professorship in 1905. In that year the Swedish Academy of Sciences decided to establish a Nobel Institute for Physical Chemistry, and Arrhenius was appointed director. He remained in this position until shortly before his death. Arrhenius was a man of wide interests. He applied his knowledge of chemical reactions to the effects on the body of toxins and antitoxins, describing this in a series of lectures he delivered in 1904 at the University of California. He was interested in immunology and in the origin of life on Earth. He wrote about astronomy, especially comets and the possibility of life on Mars.

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He also studied what is now called the greenhouse effect. In 1896 he published a paper, “On the Influence of Carbonic Acid in the Air upon the Temperature of the Ground.” He was not the first scientist to consider the absorption of energy by carbon dioxide, and it was the French mathematical physicist Jean-Baptiste-Joseph Fourier (1768–1830) who suggested in 1827 that the atmosphere acts like the glass of a greenhouse, allowing light in but preventing heat from leaving. Arrhenius was not content with speculation. He calculated the effect carbon dioxide would have if the atmospheric concentration of it were altered. He worked out the resulting change in mean temperature for 13 belts of latitude, for each of 10 degrees from 70°N to 60°S, for the four seasons of the year, and for the mean for the year. For each of these belts of latitude and seasons and for the whole year he worked out what the temperature would be if the carbon dioxide concentration were 67 percent, 150 percent, 200 percent, 250 percent, and 300 percent of the concentration that actually existed in the late 19th century. He calculated that a doubling of atmospheric carbon dioxide would increase the mean annual temperature by 8.91°F (4.95°C) at the equator and by 10.89°F (6.05°C) at 60°N. The task involved thousands upon thousands of calculations, all of which he performed by paper and pencil. Arrhenius married twice, first in 1894 to Sofia Rudbeck, by whom he had a son, and in 1905 to Maria Johansson, by whom he had one son and two daughters. He was a happy, contented, genial man who made many friends and delighted in meeting them. He was also a popular lecturer and author. In his later years he was in constant demand and traveled widely to attend meetings and deliver lectures. His incessant hard work may have weakened his health, because he was only 68 when he died in Stockholm on October 2, 1927. He is buried in Uppsala.

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Health of the Deserts to the air. This organic material is now coal, oil, and natural gas. The possibility of enhancing the greenhouse effect arises because burning these fuels completes the oxidation of their carbon and returns the carbon dioxide to the air. There is then more of it to absorb radiation. Clearing forests and burning unwanted vegetation also release carbon dioxide, the digestive systems of cattle and sheep and bacteria in flooded rice fields release methane, and cars and factories release nitrous oxide and unburned fuel that react to produce ozone. Calculations by computer models show that if these gases continue to accumulate in the atmosphere, they are likely to cause a general rise in temperature. This would be the enhanced greenhouse effect, and its extent is usually given as the temperature rise that would be caused by increasing the concentration of greenhouse gases by an amount equivalent to doubling the amount of carbon dioxide. The resulting increase is between 2.7 and 8.1°F (1.5–4.5°C), and it is calculated to occur at a rate of rather less than 0.5°F (0.3°C) every 10 years. The warming would be most marked in the Arctic and Antarctic but somewhat less so in temperate latitudes, and there would be least change in the Tropics. So far this does not quite correspond with the changes scientists observe. Measured by surface weather stations, weather balloons, and orbiting satellites, the average global temperature is increasing steadily by 2.2°F to 3.4°F (1.2°C–1.9°C) a century, which is equivalent to a rise of 0.2°F to 0.3°F (0.1°C–0.2°C) every 10 years. The rise in temperature is occurring mainly in the Northern Hemisphere north of 30°N, and it is strongest in the Arctic basin. Two-thirds of the temperature rise takes place in winter and most of it at night.

Wetter or Drier? Any rise in temperature will increase the rate at which water evaporates. Water vapor then condenses, and precipitation increases. A worldwide period of warm weather is usually a period of wet weather, and the computer models agree that precipitation will increase in a world that is growing warmer. Water also evaporates from the ground, however, and when this factor is taken into consideration the consequences of higher temperatures become more complicated. If the increase in precipitation is smaller than the increase in the rate at which water evaporates from the ground, the ground itself will become drier. In the reverse situation, whereby the increase in precipitation exceeds the increase in the rate of evaporation, the ground will become moister. It seems likely that the threshold between the two effects occurs with an increase in temperature of approximately 3.6°F (2°C). If the temperature rise is smaller than 3.6°F (2°C), the ground will become moister, and if it is greater the ground will become drier.

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At present the rate at which the temperature is rising indicates that the ground will become moister. Computer models suggest that precipitation will increase, easing the problems of water shortage in those parts of the world with an arid or semiarid climate. The peoples of the western half of the United States, North Africa, and the Near and Middle East will have more water. This will not necessarily solve problems of water shortage, but it will help.

Would the Deserts Bloom? The most obvious effect of a general rise in temperature would be to shift the climate belts away from the equator. The humid Tropics would extend farther north and south, and rainfall would increase in the tropical and subtropical deserts. At the same time, however, climates might become somewhat drier in what are now temperate regions in central Europe and the central United States, although drier conditions in these areas would be offset by the overall increase in precipitation. Unless the present rate of temperature rise increases substantially, it is likely that today’s deserts will receive more rain. If so, they will shrink in area as it becomes possible to cultivate the more hospitable areas.

Uncertainties The output from the computer models is very far from being a prediction of what is likely to occur, because there are many uncertainties. The most obvious of these is the fact that observations do not match the calculations in every respect. At present the average temperature is rising, but at the lower end of the rate produced by the models, and not all of this increase is linked to the release of greenhouse gases. Some studies have found there is a longterm climate cycle, with a period of approximately 1,300 years of alternating warm and cool periods. There was a warm period during the time of the Roman Empire, a cool period during the Dark Ages, a medieval warm period, and the Little Ice Age (see “The Warm Middle Ages” and “The Little Ice Age” on page 11). It may be, therefore, that the present rise in temperature heralds what some climate scientists are calling the modern warm period. There is no doubt that the accumulation of greenhouse gases contributes to the warming, but the extent of that contribution is uncertain. Some scientists believe most of the warming is natural and related more to changes in solar output (see “Sunspot Cycles” on pages 246–249) than to releases of greenhouse gases. In some places the warming in Alaska, northern Canada, and Siberia is already beginning to melt the permafrost, the ground that lies below the surface and is permanently frozen. If the warming continues, melting the permafrost would

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release water, so much of the tundra would become marsh interspersed with lakes. Permafrost contains large quantities of methane, and it lies above soil containing even more. If the permafrost were to melt, some or all of this methane might be released to add to the amount of this greenhouse gas that is already present in the air. This release has been detected in northern Mongolia. On the other hand, some researchers have found that when permafrost melts, increasing biological activity means that the ground absorbs carbon faster than the methane discharge releases it.

Weakening the Great Conveyor There is another fear. As the last ice age was drawing to its close, the Laurentide ice sheet covering much of North America began to retreat, releasing icebergs into the North Atlantic. Together with meltwater from the retreating ice, these formed a layer of freshwater that lay on top of the ocean. The freshwater suppressed the formation of North Atlantic Deep Water, and, because of that, it shut down the Great Conveyor (see “The Great Conveyor” on pages 88–89) and the North Atlantic Drift, the ocean current that brings warm water into the northern Atlantic. This triggered the cooling, almost back to ice age climates, of the Older and Younger Dryas (see “Varying Climates” on pages 179–180). This time there is no huge North American ice sheet to melt, but some calculations suggest that a substantial increase in rainfall would also deposit a layer of freshwater over the North Atlantic and might have a similar effect on the Great Conveyor. If that happened, global warming might, paradoxically, produce cooler conditions over much of northwestern Europe.

Melting Ice Sheets and Sea Levels There is no large North American ice sheet, but Greenland still lies beneath ice, and so does the much bigger area of Antarctica. If these ice sheets were to melt, they would release water into the oceans that at present is lying on land. The volume of the oceans would increase, and sea levels would rise accordingly. The complete melting of the Greenland ice sheet would raise sea levels by 20 to 23 feet (6–7 m), and if the entire Antarctic ice sheet melted sea levels would rise by 164 to 330 feet (50–100 m). This would inundate many low-lying islands and coastal regions. Much of the eastern United States would disappear under water. At present sea levels are rising, probably by 0.04 to 0.08 inch (1–2 mm) a year. This is caused almost entirely by the expansion of seawater as its temperature increases. Mountain glaciers are melting throughout the world except for a few in Scandinavia, but these contain only a small proportion of the world’s freshwater. Outlet glaciers from the

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Greenland and west Antarctic ice sheets have also accelerated in recent years. Studies of the southern, most vulnerable, part of the Greenland ice sheet have found it to be thinning, especially in the east, in places by as much as three feet (1 m) a year. The thinning is occurring at elevations up to 5,000 feet (1,500 m) above sea level. At higher levels the ice sheet is growing thicker, in some places by up to 10 inches (25 cm) a year. Overall, the southern ice sheet is thinning, but not greatly, and the central and northern ice sheet is thickening. Overall, the Greenland ice sheet is losing more ice than it is gaining, but so far the difference is small. Two ice sheets cover Antarctica, separated by the Transantarctic Mountains. The east Antarctic ice sheet is about double the size of the west Antarctic ice sheet. The east Antarctic ice sheet covers a high plateau, and the west Antarctic ice sheet covers an archipelago of mountainous islands. The east Antarctic ice sheet is growing thicker, and the average temperature over most of Antarctica has been falling since the 1980s. During the last 20 years the area of ice surface that melts in summer has decreased from 11.4 percent to 7.2 percent of the total area of the continent. The area covered by sea ice is also increasing. The west Antarctic ice sheet is less securely grounded, but at present it, too, is growing thicker. The Antarctic Peninsula, on the other hand, is warming faster than anywhere else on Earth, probably due to changes in wind patterns and ocean currents. Its warming has continued for several decades, and it has been losing ice (see the sidebar “The Larsen Ice Shelf ” on page 41). The west Antarctic ice sheet grounding line, beyond which the base of the ice is no longer in contact with the solid surface, has been retreating for the last 7,500 years due to complex movements of the ice. Overall, the west Antarctic ice sheet seems to be gaining ice faster than it is losing it. All the evidence suggests that the melting of the polar ice sheets will make only a very small contribution to rising sea levels. If the ice sheets gain ice faster than they lose it, their contribution may be to reduce the rate at which the sea levels rise due to thermal expansion.

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SUNSPOT CYCLES

People have always been fascinated by the stars and planets, and for as long as astronomers have had telescopes with which to observe them, part of that fascination has centered on sunspots. These are dark patches—spots—that appear on the face of the otherwise featureless Sun, and what is interesting about them is that they come and go. To early scientists the sunspots looked like imperfections in the Sun. It is important to remember that you must never look directly at the Sun unless you have equipment that is specifically designed to allow you to do so safely. Ordinary sun-

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Health of the Deserts glasses are not sufficient protection. Looking directly at the Sun can cause serious and permanent damage to the eyes. Asian astronomers were counting and recording sunspots many centuries before European observers began studying them. In about 1611, when the first telescopes were becoming available, Galileo (1564–1642) and several other scientists became interested in them. It was in 1843 that the German astronomer Samuel Heinrich Schwabe (1789–1875) discovered the sunspot cycle. Heinrich Schwabe was trained as a pharmacist, and at the age of 17 he took over his mother’s business in Berlin, preparing and dispensing medicines. Astronomy was his passion, however, and he devoted many hours to the search for a planet inside the orbit of Mercury. To do this he projected the image from his two-inch (5-cm) telescope onto a screen, hoping to see the silhouette of the supposed planet as it crossed the Sun. What he found, after watching the Sun closely since 1826, was that the number of sunspots increased and decreased over a cycle of 10 years. The Swiss astronomer Rudolf Wolf (1816–93) confirmed this and refined the cycle to 11.2 years. Wolf also confirmed that the sunspot cycle had been running at least since 1700, which he believed to be the limit of reliable records.

What Are Sunspots? Sunspots are dark because they are regions where the temperature is markedly lower than that of the visible surface of the Sun, called the photosphere, surrounding them. They consist of a dark center, called the umbra, surrounded by what look like dark filaments, forming the penumbra. Some appear by themselves, but most occur in pairs, the first to appear being called the leader and the second the follower. Each cycle begins from a sunspot minimum, when there are at most only a few sunspots near the solar equator. New spots appear farther from the equator, and their number increases to reach a maximum after two or three years. After some years the number starts decreasing, returning to a minimum about 11 years from the time the cycle began. The spots are magnetic phenomena. Each sunspot is a magnetic pole, which is why they occur in pairs—one spot of each pair is a north pole and the other a south pole. A sunspot that appears isolated does have a partner, but the two may be a very long way apart. Pairs of sunspots tend to form groups. Groups also occur as pairs, and lines of magnetic flux link both pairs and groups. During a very active maximum there might be 10 groups with up to 200 spots each, as well as more than 100 pairs that are not in groups.

The Solar Wind and Cosmic Radiation As well as radiation, the Sun emits a stream of electrically charged particles. These constitute what is known as the

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solar wind. They are deflected by the Earth’s magnetic field, but some become trapped in it, flowing along the flux lines between the magnetic poles. Where the magnetic lines descend at the poles, the particles lose energy. This energy is emitted in the form of light, and it produces the Aurora Borealis and Aurora Australis. The strength of the solar wind—and the frequency and brightness of the auroras— are related to the number of sunspots. The more sunspots there are, the stronger the solar wind blows. The Earth is also bathed in radiation from every other part of the sky. This is cosmic radiation, and it does not come from the Sun. Cosmic radiation consists of a stream of particles carrying electromagnetic charge and traveling at very high speeds as well as gamma rays. As it passes through the atmosphere the radiation collides with nitrogen and oxygen atoms, producing showers of secondary particles and more gamma radiation. These collisions ionize the nitrogen and oxygen atoms, and the ions react to form particles that act as cloud condensation nuclei. Cosmic radiation is deflected by the solar wind, so the stronger the solar wind the less intense is the cosmic radiation experienced on Earth—or, more to the point, in the Earth’s atmosphere. The more intense the cosmic radiation, the more clouds that form in the atmosphere and, therefore, the more incoming solar radiation that is reflected back into space. Intense cosmic radiation produces cool temperatures, and when cosmic radiation is weak, with fewer clouds, temperatures rise.

The Spörer Minimum and the Maunder Minimum In 1889 the German solar astronomer Gustav Spörer (1822–95) published an article describing his discovery of a sunspot minimum in the late 16th century that lasted for an unusually long time. The article was largely ignored, but it caught the attention of Edward Walter Maunder (1851–1928; see the sidebar on page 248), a photographic and spectrographic assistant at the Royal Greenwich Observatory, in England. Searching through old records at the observatory, Maunder discovered another, even longer period of minimum sunspot activity lasting about 70 years from 1645 until 1715. During that time hardly any sunspots had been observed. It included several periods of 10 years during which no sunspots at all were reported and one period of 32 years during which no spots appeared north of the solar equator. Maunder knew his discovery was important for two reasons. The first was that it challenged the prevailing idea that the Sun never changed. Clearly it did change, and quite dramatically. The second arose from a paper published in the Philosophical Transactions of the Royal Society in 1801. It was by the eminent German-British astronomer

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Edward Walter Maunder (1851–1928) Edward Maunder was the scientist who first identified the period from 1645 to 1715, now known as the Maunder minimum, during which the recorded number of sunspots and auroras was extremely low. Maunder was born in London on April 12, 1851, the youngest son of a Methodist minister. He was educated at King’s College, London (since 1900 a part of the University of London, but then an independent institution). Following his graduation he went to work at a bank in London. In 1873 a vacancy occurred at the Royal Observatory, Greenwich, London, for a photographic and spectroscopic assistant. This was a position within the British Civil Service, for which there was an entry examination. Maunder passed the examination and was appointed to the position, although he had no formal qualifications as an astronomer. Maunder was given the job of photographing sunspots and measuring their areas and positions. As he did so, he discovered that the solar latitudes in which sunspots appear vary in a regular fashion during the course of the 11-year sunspot cycle. While engaged in photographing and measuring sunspots, his attention was drawn to the work of the German astronomer Gustav Spörer (1822– 95), who had identified a period from 1400 to 1510 when very few sunspots were seen. This period is now known as the Spörer minimum. Maunder began searching through old records at the Royal Observatory to see whether Spörer was correct and whether there were any other such periods. It was this search that led to his discovery of the Maunder minimum. His discovery of the sunspot minimum was overlooked. Other scientists thought he placed too much reliance on old records that, in their view, were likely to be incomplete or inaccurate. The Maunder minimum is now known to be a real phenomenon. Edward Maunder was made a fellow of the Royal Astronomical Society in 1873. He died at Greenwich on March 21, 1928.

Sir Frederick William Herschel (1738–1822), who had noted a link between sunspot activity and wheat prices. These fluctuate according to the size of the harvest, which

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in turn is determined by the weather. It suggested a connection between sunspots and climate. Maunder wrote an article on the subject in 1890. In 1894 he wrote another, called “A Prolonged Sunspot Minimum,” and in 1922 he published a third, longer article with the same title. No one took his idea seriously, but it was revisited half a century later by the American solar astronomer John A. Eddy at the High Altitude Observatory in Boulder, Colorado. Eddy checked Maunder’s findings against records from other parts of the world and found them correct. He also checked the climatological implication, working in collaboration with the British climatologist and historian of climate Hubert H. Lamb (1913–97).

Cosmic Radiation and Carbon-14 It was possible to correlate changes in cosmic radiation with climatic changes because cosmic radiation produces carbon 14 (14C), a radioactive isotope of carbon. As cosmic particles—mainly protons and alpha particles (comprising two protons and two neutrons) traveling at 186–435 miles per second (300–700 km/s)—strike atoms in the atmosphere, they liberate free neutrons. When a free neutron (n) strikes a nitrogen atom, it is absorbed, and a proton (p) is released. A nitrogen atom has a total of 14 protons and neutrons and is written as 14N. Absorbing one neutron and losing one proton leaves the number of particles unchanged, but the loss of one proton alters the chemical characteristics of the atom, changing it from nitrogen to carbon: 14N + n → 14C + p. Carbon 14 is unstable. It decays with a half-life of 5,715 years, but it behaves chemically just like the most common carbon isotope, 12C. It oxidizes to 14CO2 and is absorbed by photosynthesizing plants. When the plants die the 14C continues decaying and no more is absorbed, so the proportion of 14C in the plant tissues decreases at a steady rate. This provides the basis of radiocarbon dating, but variations in the intensity of cosmic radiation create large anomalies in the proportion of 14C in organic material. Instead of decreasing steadily over time, there are sudden increases or decreases that can be measured. This ability to detect variations in the intensity of cosmic radiation in wood that had been independently dated by counting tree rings allowed Eddy to place dates on the changes in cosmic radiation and, from that, on the sunspot maxima and minima.

Sunspots and the Little Ice Age The period with so few sunspots that Maunder had found is now known as the Maunder minimum, and the earlier one is known as the Spörer minimum. They both coincide with especially cold parts of the Little Ice Age. This was a period lasting altogether from about 1420 to the latter half

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Health of the Deserts of the 19th century, during which the climate all over the world was generally cooler than it is now or was earlier. There were relatively warmer and cooler episodes within the overall period, but every warmer period was followed by a return to cold, wet years. During the Spörer minimum, between 1560 and 1599, mean winter temperatures in central Europe were about 2.3°F (1.3°C) lower, and in Denmark from 1582 to 1597 they were 2.7°F (1.5°C) lower than the 1880–1930 average. It was during the Maunder minimum that temperatures reached their lowest. Eddy found that over the last 5,000 years there were at least 12 sunspot minima as big as the Maunder minimum. Each of them coincided with a period of cold weather and glacial advance. More recently, scientists have used tree rings to identify 19 warm periods over the last 10,000 years. Of these, 17 coincided with sunspot maxima. Sunspots come and go over an 11-year cycle, but at present there are more during the peak of the sunspot cycle than there have been for about 300 years. In other words, the late 20th and early 21st centuries are a time of a sunspot maximum. Some scientists believe that this increased solar activity and the consequent reduction in the intensity of cosmic radiation is responsible for some, or perhaps most, of the present rise in atmospheric temperature. When the sunspot maximum ends, the warm period will also end, to be followed by a new sunspot minimum and much colder weather.

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MILANKOVITCH CYCLES

Edward Maunder was not the only scientist who investigated astronomical causes for climatic changes on Earth. Maunder concentrated on variations in the amount of energy Earth receives from the Sun and discovered the sunspot minimum associated with the coldest part of the Little Ice Age. In Belgrade, meanwhile, a Serbian mathematician and physicist was also looking for astronomical influences, but in this case influences that could be traced over a much longer period. Their existence had first been suggested in 1864 by the Scottish climatologist James Croll (1821–90). Milutin Milankovitch (1879–1958) was educated in Vienna but moved to the University of Belgrade in 1904 and remained there for the rest of his life as professor of applied mathematics. He spent 30 years studying the relationship between the Earth and Sun and its climatic consequences over the last 650,000 years and published his findings in 1930. He was interested not in variations in the amount of energy radiated by the Sun, but in variations in the rotation of the Earth on its axis and in its orbit about the Sun. These affect the amount of radiation the Earth receives. Sunspots are generated inside the Sun by processes that are still not fully understood and that so far remain unpredictable. Milankovitch studied the movements of astro-

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nomical bodies, which are well known. Not only can they be predicted with great accuracy, their past behavior can also be calculated precisely. Milankovitch discovered three variations that affect climate. Two relate to the Earth’s rotation and the third to its orbit.

Axial Tilt The illustration on page 250 shows three diagrams. Drawing A, labeled “axial tilt,” shows that the Earth’s axis of rotation is not vertical—it is said to be oblique. That is to say, the rotational axis is not at right angles to the plane of the ecliptic. The plane of the ecliptic is the imaginary disk enclosed by the path of the Earth’s orbit around the Sun. Instead of being at 90° to the plane of the ecliptic, the Earth’s rotational axis is at 66.5°, a difference of 23.5°. This is the angle of tilt at present, but it varies between a maximum of 24.4° and a minimum of 21.8° over a period of about 41,000 years. Diagram C shows that it is the tilt, or obliquity, of the Earth’s axis that produces our seasons. If the axis were perpendicular to the plane of the ecliptic, the Sun would be directly overhead at the equator at noon on every day of the year, day and night would always be of equal length everywhere, and there would be no summer and winter. In the drawing one hemisphere is tilted toward the Sun and is enjoying summer, while the other is tilted away from the Sun and is experiencing winter. The bigger the angle of tilt, the more extreme the seasons are, because the more each hemisphere in turn is tilted toward and then away from the Sun. This is one of the cycles Milankovitch found to have a climatic influence.

Precession of the Equinoxes The Earth is a spinning object, with its mass concentrated around the rim. In other words, it is a gyroscope, and in one important respect it behaves as one: It is subject to precession. If a force is applied to the rotational axis of a gyroscope, precession causes the gyroscope to move at 90° to the force in the direction of rotation. This makes precession sound complicated, but there is a familiar example of it in action. Remember what happens when a child’s spinning top turns. The axis around which it spins is hardly ever vertical. Because it is tilted, gravity pulls at it on the side that is leaning. Instead of falling over, however, the spinning top wobbles. The top of its axis describes a circle around the vertical. That is because precession causes the force to act at 90° in the direction of rotation. As the top slows down, it ceases to be a gyroscope and so eventually falls. The Earth behaves in the same way, as shown in diagram B, labeled “axial wobble.” It takes the rotational axis about 21,000 years to complete one turn. This wobble alters the dates of the spring and autumn equinoxes and

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A

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axial wobble

summer

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aphelion July 4

perihelion January 3

orbital stretch © Infobase Publishing

Milankovitch cycles. There are three cycles: axial tilt; axial wobble; and orbital stretch.

spheres will have exchanged the months in which each season occurs.

Earth’s Eccentric Orbit summer and winter solstices—the dates at which the Sun is directly overhead at the equator at noon (the equinoxes), and the dates when the noonday Sun is overhead at the Tropics (the solstices). This gradual change is called the precession of the equinoxes. At present in the Northern Hemisphere the spring, or vernal, equinox is on March 20–21, the summer solstice on June 21–22, the autumnal equinox on September 22–23, and the winter solstice on December 22–23. About 11,000 years from now spring will be in September, summer in December, autumn in March, and midwinter in June. In other words, the hemi-

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The precession of the equinoxes would make no difference to the climate at all if the Earth moved in a perfectly circular orbit, but it does not. The orbit is elliptical. An ellipse has a center and two foci. In the case of the Earth’s orbit, the Sun is at one focus and the other is unoccupied. This means that although the Earth orbits the Sun, the Sun is not at the center of its orbit. The orbit is eccentric, and the extent of its eccentricity can be calculated. The diagram shows the orbital ellipse, with the Sun at one focus, F1, the other focus F2 unoccupied, and the center, C. The distance between F1 and C is the linear eccentricity, le, and the dis-

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are not. The difference, with much more land in the north and much more ocean in the south, actually reverses the effect, because land warms and cools quickly and water warms and cools slowly, with the consequence that continental climates experience much more extreme temperatures than do maritime climates. In fact, therefore, northern winters are the cooler than those in the Southern Hemisphere, and northern summers are warmer. Eventually, though, the precession of the equinoxes will bring the Earth to perihelion in the middle of the northern summer and to aphelion in the middle of the northern winter. This will reverse the effect. Winters will then be much colder in the more continental climates of the Northern Hemisphere, and summers will be much warmer, although the effect will be less marked in more maritime climates of the Southern Hemisphere.

tance between C and the orbital path is a. The eccentricity (e) is then calculated as: e = le/a.

Perihelion and Aphelion Clearly, at one time in the year the Earth is closer to the Sun than it is at other times. When it is at its closest approach, the Earth is said to be at perihelion, and when it is furthest away, it is at aphelion. At present the Earth is at perihelion on January 3 and at aphelion on July 4. This means that Earth is closest to the Sun in the middle of the Northern Hemisphere winter, and its proximity makes the winter somewhat milder than it would otherwise be. Earth receives about 7 percent more radiation at perihelion than it does at aphelion. That would make northern winters warmer than southern winters and southern summers warmer than northern summers if land and sea were distributed equally in both hemispheres, but they

Stretching the Earth’s Orbit The orbit is elliptical, but the shape of the ellipse changes. Sometimes it is almost circular, and at other times it is stretched. Diagram C in the illustration of the Milankovitch cycles shows this, though not to scale. At present the orbital eccentricity, e, is 0.017, but it varies from e = 0.001,

Orbital eccentricity. The Earth’s orbit about the Sun describes an ellipse with the Sun at one focus (F1). The extent of its eccentricity is equal to le/a.

le

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C F2

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which is almost circular (e = 0), to e = 0.054. Like the other changes, the orbital stretch increases and decreases in a cycle, in this case with a period of about 95,000 years. At maximum stretch the Earth is farther from the Sun at both perihelion and aphelion than it is at other times. This alters the amount of solar radiation reaching the Earth, and it also affects the dates of the solstices and equinoxes. By itself, each of these three cycles has only a small but nevertheless significant effect. Sometimes they coincide, however. When this happens the effects of each cycle are added together. They account for the fact that from 12,000 to 9,000 years ago the Northern Hemisphere received 8 percent more solar radiation in summer than it does now. That was enough to trigger the melting of the ice sheets that brought the last ice age to an end. Increased warmth in summer was accompanied by a reduction in solar radiation in winter, but this affected the Southern Hemisphere subtropics, so it did not halt the glacial melting. For a time all the heat was absorbed as latent heat in the melting of the ice, so climates did not begin to grow warmer until most of the ice had gone. Much earlier, the Milankovitch cycles coincided to produce the reduction in solar radiation that caused the ice sheets to start advancing. Milankovitch suggested that the cycles coincided with nine ice ages. For many years climatologists doubted that the Milankovitch cycles could affect the climate strongly enough to alter it. As more has been learned about the way feedback mechanisms can enhance climatic influences, for example when pale colors of sand or snow reflect sunlight or dark colors of forests absorb it, and about past climates, the evidence has tended to support the Milankovitch theory. Most scientists now believe the astronomical cycles he described exert a strong influence on the initiation of ice ages and interglacials.

Snowblitz Their influence is strong, but the astronomical variations Milankovitch described are not enough in themselves to trigger the onset of an ice age or the ending of one. An additional factor is needed, and this is provided by changes in the area covered by ice and snow. Snow- and ice-covered surfaces reflect up to 90 percent of the sunshine that falls on them. Sunshine comprises heat as well as light, and, obviously, the radiation that is reflected cannot be absorbed. Consequently, air in contact with ground that is covered by snow or ice remains cold. In its most extreme form this influence has been called the snowblitz theory. It was an idea that became popular in the 1970s, when evidence suggested that the global climate was growing cooler. There were fears of a return to the cold conditions of the Little Ice Age or even of the onset of a fullscale ice age. According to the snowblitz theory, the climate

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can change in a few decades from the mild conditions of an interglacial to a full ice age. The change begins when there is a series of cool summers in high latitudes. It is the summer temperatures that matter, not winter temperatures. Cooler summers mean that each year the winter snows melt a little later, until in one of those cool summers not all the winter snow melts. It remains until the first snowfall of the following winter joins it. Throughout the summer the snow, although dirty by this time, continues to reflect much of the sunshine falling on it, so the air adjacent to the snow remains cold. Year by year the area covered by snow expands, and the period grows shorter when summer temperatures rise above freezing. Snow accumulates, and as its weight increases the lower layers are compressed into ice. Ice sheets begin to form in northern Europe and Canada. The ice sheets expand southward a little each year. An ice age has then begun, and according to the snowblitz theory the entire process could happen very rapidly.

Snowball Earth The snowblitz scenario is based on the change that occurred as the most recent ice age was coming to an end, when suddenly the Northern Hemisphere was plunged back into the ice age conditions of the Younger Dryas (see “Varying Climates” on pages 179–180). Evidence from Greenland ice cores shows that the change took no more than about 10 years, and the Younger Dryas ended no less abruptly. The snowblitz theory explains how this may have happened. Few climate scientists believe that could happen today, because there is no source for the large amount of freshwater that would be needed. The cooling of the Younger Dryas was triggered by the release into the North Atlantic of vast amounts of water due to the melting of the Laurentide ice sheet covering much of eastern North America. There is evidence to suggest that there have been times in the past when ice covered the entire Earth except for the tops of the highest mountains. Totally ice-covered, the planet became Snowball Earth. This condition may have occurred four times between 750 million and 580 million years ago. Mean temperatures were about -58°F (-50°C), and all the oceans were frozen to a depth of more than 0.6 mile (1 km). Heat from the rocks below the seabed prevented the oceans from freezing completely. It all began because a large supercontinent had recently broken apart, and the continents were small and low-lying. Rainfall increased over the land. The rain carried dissolved carbon dioxide that reacted with minerals in the rocks and was carried into the sea, and the atmospheric concentration of carbon dioxide decreased. Then temperatures began to fall, and large ice packs formed on the oceans in high latitudes. The ice packs reflected sunshine, triggering a snow-

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Health of the Deserts blitz cooling. Once the oceans were sealed by ice, no liquid water was exposed to the air, and precipitation ceased. Volcanoes continued to erupt, however, releasing carbon dioxide into the atmosphere, and with no precipitation to remove it the carbon dioxide accumulated. After about 10 million years the atmosphere contained enough carbon dioxide to trigger a strong greenhouse effect. Temperatures rose. Within a few centuries the ice had melted, but the temperature continued to rise, eventually reaching more than 120°F (50°C). The rate of evaporation increased, rainfall intensified, and gradually the rain washed the excess carbon dioxide from the air, allowing the temperature to fall and the climate to stabilize. There is an alternative, and perhaps more plausible, explanation of these dramatic events. Researchers have found clear evidence that the “Snowball Earth” episodes coincided with periods of intensive and rapid star formation, called “starbursts,” in part of the Milky Way galaxy not far from the solar system. Starbursts are invarably accompanied by the collapse and death of many young stars and as they collapse the stars emit radiation that reached Earth as a large increase in cosmic radiation. The cosmic radiation would have increased greatly the amount of low-level cloud shielding the surface, causing temperatures to drop sharply. Calculations show this would have been sufficient to trigger a worldwide ice age. When, at last, the starburst episode ended, the level of cosmic radiation fell, the clouds cleared, and as more sunshine penetrated to the surface, the temperature began to rise.

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RAINMAKING

Think just how many of the world’s problems might vanish if only the weather could be brought under control. There would be no more droughts, like those that in the last few decades have devastated Ethiopia and the countries along the southern border of the Sahara and that earlier last century caused the dust bowl tragedy in the Great Plains. Hurricanes would no longer be allowed to wreck homes and destroy crops, or tornadoes to bring destruction, death, and terror. And deserts would disappear from the face of the Earth. One day all these things may become possible, but not by using brute force to contrive the weather people would prefer. Society lacks the power to achieve that. An average summer thunderstorm releases energy equivalent to burning 7,000 tons (6,356 t) of coal. An average hurricane releases double that amount, and an average tornado, lasting no more than a few minutes, releases about as much energy as is used to keep all the streetlights in New York City shining for one night. Even if there were countries that could spare so much power and could afford to generate it, there can be little doubt that producing and releasing energy on

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this scale would cause more environmental problems than it would solve. If people are to alter the weather, a subtler approach is needed. Fortunately, a few exist. They do not provide control over every aspect of the weather, but they permit it to be manipulated just a little under certain conditions. Sometimes this can help.

Wind Machines to Protect Against Frost Fruit orchards are often planted in places sheltered from winds, which can dry and chill the growing tips of branches and damage blossom. The trees thrive in such places, but they face another danger: frosts. These can occur where cold air sinks into valleys and hollows and accumulates there. In the last century Florida citrus growers found a way to protect their trees. They used wind machines. These are huge propellers mounted on tall columns and driven by motors. As the propellers turn they generate a wind, and this mixes the air, preventing the cold air from settling into a layer close to ground level. It is simple but ingenious, and the technique manipulates the weather on a very local scale. Sometimes the wind machines were not enough. Then the growers used what they called smudge pots. These are no longer used because of the pollution they cause. They were heaters that burned oil set out on the ground at a density of up to 70 to an acre (173 per hectare). The protection they afforded came not from the heat they produced, but from the smoke. If the pots were lit in the early evening when a frost was predicted, the smoke would form a layer that prevented the cold air from accumulating. Wind machines and smudge pots were often used together.

Fog Dispersal Kerosene burners were used in the 1940s to disperse fog from airfields. The method went under the unwieldy name of Fog Investigation Dispersal Operations but the happier acronym of FIDO. The burners were positioned along both sides of a runway, and the heat they produced decreased the relative humidity of the air, causing the fog droplets to evaporate. FIDO was effective but very costly in terms of the amount of fuel it used.

The Bergeron–Findeisen Process Research into weather manipulation on a much bigger scale also began in 1946 and is continuing. It is based on the Bergeron–Findeisen process. This is an explanation of the way certain clouds form that was proposed in 1935 by the Swedish meteorologist Tor Bergeron and first demonstrated in the laboratory in 1938 by the German meteorologist Walter Findeisen. The Bergeron–Findeisen process occurs only in mixed clouds that contain both ice crystals and water droplets. The

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air temperature in a cloud of this type is between 32°F (0°C) and -40°F (-40°C). Warmer clouds contain only water droplets, and colder ones only ice crystals. Water molecules are constantly escaping into the air from the surface of water or ice. In the layer of air next to the surface, water molecules are pressing against the surface and condensing into it. If the number of molecules condensing onto the surface is equal to the number escaping from it, the vapor next to the surface is saturated. The pressure exerted on the surface by molecules in a layer of saturated water vapor is known as the saturation vapor pressure. Molecules can escape from an ice surface without the ice melting first, but with more difficulty, because they are bound together more tightly in the solid than in the liquid. Consequently, the saturation vapor pressure over an ice surface is lower than that over a liquid water surface. Inside a cloud ice crystals and water droplets are constantly moving. If the air next to the liquid droplets is saturated, then it will be supersaturated with respect to ice crystals because of the lower saturation vapor pressure over an ice surface. Molecules will then attach themselves to ice crystals, which will grow bigger. The molecules that are removed from the air and added to the ice crystals bring the air to below saturation with respect to the liquid droplets. Water evaporates from them, making them smaller. The overall effect is that the ice crystals grow rapidly at the expense of the water droplets. When ice crystals collide they stick to one another and form snowflakes. The type and shape of the snowflakes depend on the temperatures they encounter as they move about in the cloud. As they move, now and then they come into contact with liquid droplets. These are supercooled— their temperature is below freezing—and they freeze instantly on contact with any solid surface. In clouds where the temperature is below freezing at high level but above freezing at low level, droplets repeatedly freeze onto crystals, fall to a lower level where their surfaces melt, and are then carried aloft where they freeze again and gain more ice by collisions with supercooled droplets. That is how hailstones form.

Cloud Seeding The research that began in 1946 aimed to exploit the Bergeron–Findeisen process. If crystals of a suitable size and shape are fed into a mixed cloud, they will stimulate the formation of ice crystals. In early attempts aircraft dropped sodium chloride (common salt) and calcium chloride crystals and sprayed fresh or salt water into clouds. Powdered salt was also blown upward from the ground. These had little effect, but silver iodide and dry ice— solid carbon dioxide—proved more successful (see the sidebar “Vincent Schaefer and Bernard Vonnegut” on page 255). Silver iodide crystals are very small and similar

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in shape to ice crystals, so water condenses onto them as it would onto ice. The usual way to apply silver iodide is to dissolve it in acetone to make a solution that is burned at about 2,000°F (1,100°C) in small devices suspended beneath aircraft. The Russians have also tried using artillery to fire projectiles that explode inside clouds, each projectile releasing 3.5 to seven ounces (100–200 grams) of lead iodide or silver iodide. Rockets have also been used. Silver iodide can be generated at ground level. The crystals are carried upward in the rising hot air and then swept into the cloud by air currents. Dry ice has a temperature of about -108°F (-78°C) and chills the air. That saturates the air and causes water vapor to condense. The dry ice is usually made into crushed pellets and dropped into a cloud from aircraft. Carbon dioxide is little used nowadays because it soon vaporizes and loses its effectiveness. These methods are called cloud seeding, and the aim is to stimulate the production of ice crystals by an amount that is sufficient to cause precipitation to fall from a cloud that might otherwise have yielded none. It is hard to tell whether a cloud would have delivered precipitation if left undisturbed, but experiments suggest that cloud seeding works on about 10–20 percent of occasions.

Taming Storms Causing precipitation is only one of the potential uses of cloud seeding. If seeding material is injected into a cumulonimbus (storm) cloud at the right moment, it can cause the formation of large numbers of small hailstones that fall from the cloud, rather than a smaller number of much bigger hailstones that could damage crops. Some years ago attempts were also made to use cloud seeding to reduce the intensity and alter the track of hurricanes. In Project Stormfury, carried out between 1962 and 1983, clouds forming the eye wall of a hurricane were seeded with silver iodide. The idea was that seeding would increase production of ice crystals. Their formation would release latent heat, and this would intensify convection in the cloud. The original eye wall would weaken, and a new one would form outside it, at a larger radius. This would reduce the force of the winds around the eye because their intensity is due to the conservation of angular momentum. As the rotational radius of the storm increases, its wind speed decreases. At first the experiments seemed promising. Wind speeds apparently decreased by 10–30 percent in four of the eight attempts made. Later, however, scientists began to question the results. Today it is thought the wind reduction was entirely natural and that cloud seeding is unlikely to alter hurricane behavior. Seeding can have other effects. If more than a certain amount of material is used, a cloud is overseeded.

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Vincent Schaefer (1906–1993) and Bernard Vonnegut (1914–1997) Vincent Joseph Schaefer was born at Schenectady, New York, on July 4, 1906. After leaving school he worked for a time in the machine shop at the General Electric Corporation (G.E.C.) in Schenectady. Then, thinking outdoor work would suit him better, he attended classes at Union College, New York, and enrolled at the Davey Institute of Tree Surgery, from which he graduated in 1928 and became a tree surgeon. He was a keen skier and loved the snow, but unable to earn an adequate salary at tree surgery, he had to abandon this profession. In 1933 Schaefer returned to G.E.C. as a research assistant to Irving Langmuir (1881–1957). Schaefer became a research associate in 1938, and he remained at G.E.C. until 1954. During World War II Langmuir and Schaefer studied the problem of icing on the wings and other external surfaces of aircraft. This was extremely dangerous and caused many aircraft to crash, but before remedies could be found the scientists had to discover what was causing ice to form. Working with his colleague Bernard Vonnegut, Schaefer studied the formation of ice and snow using a refrigerated box with an inside temperature that remained at a constant -9°F (-23°C). He hoped to be able to induce water vapor to be deposited as ice around dust particles. There was a spell of unusually hot weather in July 1946, and to maintain the temperature inside the box, on July 13 Schaefer dropped some dry ice (solid carbon dioxide) into it to chill the air. The result was dramatic. The moment the dry ice entered the air in the box, water vapor turned into ice crystals and there was a miniature snowstorm. This suggested a way to make precipitation fall from a cloud that otherwise would not have released it. By November 13 Schaefer was ready for a full-scale trial. An airplane flew him above a cloud

Overseeding rapidly produces a very large number of very small ice crystals. Their formation removes water vapor from the air, reducing it to well below saturation, and the crystals evaporate into it, dispersing the cloud. Large-scale weather modification seems beyond the capability of modern technology. Cloud seeding has some effect, but there is no evidence that severe storms can be dissipated in this way. Research continues, however, and in years to come it may prove possible to intervene at least to reduce the ferocity of storms.

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at Pittsfield, Massachusetts, about 50 miles (30 km) southeast of Schenectady. Schaefer dropped about six pounds (2.7 kg) of dry ice into the cloud and started the first artificially induced snowstorm in history. This discovery led to the development of other techniques for cloud seeding. Bernard Vonnegut was born at Indianapolis, Indiana, on August 29, 1914. He was educated at the Massachusetts Institute of Technology (M.I.T.), graduating in 1936. He obtained his Ph.D. from M.I.T. in 1939 for research into the conditions that produce icing on aircraft. From 1939 until 1941 he worked for the Hartford Empire Company, and from 1941 until 1945 Vonnegut was a research associate at M.I.T. In 1945 Bernard Vonnegut moved to the G.E.C. laboratories, where he continued his research into icing in collaboration with Vincent Schaefer. Following the discovery that dry ice was effective at cloud seeding, Vonnegut began searching for other materials that might perform the same task. Deciding that crystals of silver iodide were the right size and shape to act as freezing nuclei, he experimented with them and was proved correct. Silver iodide largely replaced dry ice as a seeding medium. Unlike dry ice, it can be stored indefinitely at room temperature, and it does not have to be released from an airplane flying above the target cloud. Silver iodide can be released at ground level and will be carried into the cloud by vertical air currents. Vonnegut moved to the Arthur D. Little Corporation in 1952, and in 1967 he was appointed distinguished research professor of the State University of New York, a position he held until his death at Albany, New York, on April 25,1997. Schaefer died at Schenectady on July 25, 1993.

HALTING THE SPREAD OF DESERTS AND NEW CROPS FOR DRY CLIMATES

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In the early 1970s, as the continuing drought in the Sahel region drove starving people southward in search of pasture for their animals and food for themselves, there were fears that the desert itself was spreading. More than

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100,000 people died as a result of that drought, as well as up to 4 million head of livestock. In 1973 the drought in Ethiopia is believed to have claimed up to 250,000 lives, and drought returned again in 1984–85. People became familiar with the word desertification through newspaper and magazine articles and television programs. In 1977 the United Nations held a conference on desertification in Nairobi. The United Nations claimed to have coined the term desertification, but in fact it was first used in 1949, admittedly in French, in a book called Climats, fôrets et desertification de l’Afrique tropicale, by A. Aubreville. The word was not new, and neither was knowledge of the process it described. Fears of the southward encroachment of the Sahara were being expressed as long ago as the 1930s.

The subject was also debated at the Rio Summit (the United Nations Conference on Environment and Development) held in June 1992 in Rio de Janeiro, where it was agreed that a treaty should be prepared as a guide to nations in halting land degradation. This led to the UN Convention to Combat Desertification (CCD). Unlike many international agreements, the CCD aims to involve local people and make use of their special knowledge and expertise. Information about schemes that are devised and succeed in one place will be passed to groups in similar situations in other parts of the world. The program to achieve the aims of the CCD is based on national schemes that are coordinated internationally through what has been called the Global Mechanism (GM). The GM is administered by the International Fund for Agricultural Development working in collaboration with the UN Development Program and the World Bank. The industrialized countries are funding the CCD, and individual countries are working closely with particular projects. The Netherlands, for example, is working with Burkina Faso, Spain with Latin American countries, and Germany with Mali. A desert studies program has been instituted at the University of Kuwait, and Kuwait has also started collecting data on the movement of sand dunes. Senegal has opened a center to monitor the environment. Argentina, Bolivia, and Paraguay are collaborating in the development of methods to manage dry lands.

wind, carrying the topsoil, blows over adjacent land where plants are still growing. Friction with the plants slows the wind, causing it to lose energy. As the wind loses energy the soil settles. It buries the plants, clogging their stomata and shielding them from the sunlight, so they are unable to photosynthesize (see “Photosynthesis” on pages 96–101) and die. Their roots then decay and cease to bind the soil, which also blows away. In this way the desert spreads into the semiarid land along its borders. People living in these semiarid lands see their crop yields fall and harvests fail. There is less pasture for their livestock. They have to walk farther to find firewood because the nearby trees have gone. Eventually, their lives may become unsustainable, forcing them to move far away from the area. Some or all of a number of physical changes accompany this process. The water table falls, so wells dry up, and plants wilt because their roots can no longer reach moist soil. Where there is water it may be brackish or salty, making it undrinkable. Soils may be poisoned by excessive amounts of salts (see “Aquifer Depletion, Waterlogging, and Salination” on pages 184–189). Soil erosion may increase. Despite the dramatic description and the familiarity of the word, there is no agreed upon definition of desertification, and it is by no means certain that the Sahara is spreading. The UN Environment Program defines desertification as “land degradation in arid, semi-arid, and dry sub-humid areas resulting mainly from adverse human impacts.” Most scientists would qualify this. The deterioration must affect both vegetation and soil, it must be caused at least partly (but not necessarily mainly) by human management, and, most important, it must be sustained for at least 10 years. Deterioration for a shorter period than this may be part of a cycle from which the land will recover naturally (see “Desert Advances and Retreats” on pages 264–266). Although the word desertification conjures a picture of land turning into a desert resembling the Sahara, this is the most severe condition. Most deteriorating land is in a better state than this, and not all of it is located along the edges of deserts. Deterioration can occur anywhere on land with an annual rainfall of less than about 31.5 inches (800 mm). As the map shows, although Africa and Asia between them have almost two-thirds of the world’s 23.5 million square miles (61 million km2) of vulnerable arid lands, every continent has some.

How Deserts Expand

Improving Land Management

Desertification results from the effect of drought on bare, exposed soil. Removing trees for wood or fuel and allowing grass and herbs to be overgrazed (see “Overgrazing and Desertification” on pages 268–269) combine to destroy the root systems that bind soil particles together. As the surface soil dries to a powder, it is blown away by the wind. The

At present there is no way to modify the climate on a large scale (see “Rainmaking” on pages 253–255), but it should be possible to improve land management. The first aim of any improved land use must be to maintain a vegetation cover at all times. This serves two purposes. Plants increase the roughness of the ground surface, which increases fric-

UN Convention to Combat Desertification

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North America 12%

Europe 5%

South America 8%

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Asia 32%

Africa 32%

Australia 11%

© Infobase Publishing

Distribution of arid lands. These are the parts of the world most at risk from desertification.

tion with moving air and slows the wind, and plant roots bind soil particles. Together, these greatly reduce soil erosion, and they can often prevent it almost entirely. In the 1970s there were schemes to halt what was then believed to be the southward spread of the Sahara by planting a belt of trees across Africa from the Atlantic to the Red Sea. This was overambitious. The tree belt was never completed, but the CCD is starting to take effect at a more modest level—and it begins by acknowledging that desertification can be combated successfully.

Making People Feel Secure Making people feel secure in their access to the resources that sustain them is an important first step. Traditionally, many of the inhabitants of deserts and the lands bordering them have lived as nomads. They did not own or even rent the land, but their way of life was based on a detailed

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knowledge of the seasonal distribution of pasture and water. Each group had certain places it visited and to which it enjoyed unchallenged rights. That way of life was badly disrupted in the course of the 20th century, and many of the former nomads now live in settled communities. Wars and the famines that often accompany them generate waves of refugees seeking food and sanctuary. Unable to cope with the scale of the need, countries sometimes close their borders. This also affects the nomads, who can no longer be sure they will be able to lead their herds and flocks to the traditional pastures on the other side of what to them is an arbitrary line defended by soldiers. Settled farmers also need security of tenure. Providing this is an essential first step in recruiting local people to schemes for reducing and preventing erosion. This is because they are being asked to do extra work in order to improve the land. Unless they can be confident that they and their children will enjoy the benefit of that improvement, it may be impossible to persuade them to invest the necessary labor. If the land is improved, it may well be the landowner who benefits and not the farmer. That is because improving the land will increase crop yields, making the land more

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valuable, and the landowner may evict a poor farming family in order to install a more prosperous one that can pay a higher rent. Without that assurance of security of tenure, the farmers are being entirely rational when they refuse to undertake extra work. Work consumes energy that must be provided by food. Where food is scarce and the work guarantees no long-term benefit, the price is not worth paying. Once farmers and nomadic pastoralists feel secure, they can be helped to improve their agricultural methods. This is not a matter of sending teams of foreign experts to dictate to them, and far less does it mean trying to impose American or European techniques in parts of the world where the climate and soils make these inappropriate. It means discussing with the farmers and nomads the types of modifications that might reduce erosion and then making sure they have access to the finances and materials needed to make those modifications. Given the materials and equipment, some simple changes are very effective. For example, spraying sand dunes with a mixture of oil and synthetic rubber stabilizes the surface by binding sand grains. Tree seedlings, especially of eucalyptus and acacia species that can grow with only six inches (150 mm) of rain a year, are then planted through the surface coating. Heavy rain is the principal cause of erosion on hillsides. Rain washes away the topsoil. Building low, dry-stone walls from the abundant rocks that litter the desert surface will prevent this type of erosion. Where gullies have already formed, walls across each of the gullies will slow the water, so soil it is carrying is precipitated. Farming and nomadic communities living on the borders of the desert are invariably composed of the very poorest people—those who can afford to do so move to where the living is easier. The very poor have no means of raising the money to buy materials, and banks are reluctant to give them credit. Alternative sources of employment in smallscale light industry or tourism, for example, can provide income. Those who leave farming for other employment will spend part of their earnings on buying food from the farmers, so some of that income may be invested.

Corridor Farming Trees can be planted to shelter cultivated land, not on the grandiose scale of a tree belt the width of a continent, but locally. Crops can then be grown between rows of trees. This is called corridor farming or alley cropping and has been practiced successfully in several semiarid regions using acacia trees. Acacias are legumes, with bacteria attached to their roots that fix atmospheric nitrogen, so they need very little fertilizer. Apple-ring acacia, also called winter thorn and camel thorn (Acacia albida), has the unusual habit of bearing leaves through the dry season and shedding them at the start of the wet season. This means the tree provides food that live-

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stock can eat at a time of year when there is little other food available. Animals that feed on its shoots and leaves remain healthy, and its branches can be cut and stored without losing their nutritional value. The trees are planted as seedlings at the same time as the farm crop. The farm crop is harvested as soon as it is ripe, and the trees are left to continue growing, which they will do through the dry season. In the following year a new crop is planted, and the trees are cut down almost to ground level. Their leaves are fed to livestock, and the wood is used as fuel. This is similar to coppicing, a European management system that produces a regular crop of small poles and firewood. Far from killing the tree, cutting it in this way usually extends its life. The tree becomes smaller and bushier, but it will continue to produce fodder and fuel wood for many years.

Dry Farming Dry farming methods were devised in North America after the dust bowl drought of the 1930s to allow cultivation in semiarid climates. One method is to grow a crop, harvest it, and then leave the land fallow for three years. During those years plants are allowed to grow naturally, but they are controlled by light plowing at regular intervals. The plants accumulate moisture in their tissues, and the plowing prevents them from losing too much of it by transpiration (see “Transpiration and Why Plants Need Water” on pages 101–103). After three years the buried and partly decomposed plants will have moistened and fertilized the soil sufficiently for the next crop to be grown. On a farm using this method, one-quarter of the area is under cultivation at any one time, and the cropped area changes each year according to a regular four-year rotation.

New Crop Plants Farming has developed over thousands of years by exploiting a very small proportion of the plants that exist on Earth. There are 10,000 species of grasses, for example, of which humans have domesticated just seven for human consumption: wheat, oats, barley, rye, rice, maize (corn), and sorghum. Of the 18,000 species of legumes, farmers grow just six: peas, beans, soybeans, peanuts, alfalfa, and clover. These crops may not be the best choices for all soil and climatic conditions. Increasingly, scientists are looking to entirely new crops or to traditional crops that have been genetically modified in their search for ways to halt erosion in dry climates, and the search is proving very successful.

Vetiver A few years ago staff employed by the World Bank in India came across an obscure grass called vetiver (Chrysopogon

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Montezuma’s Well, in Arizona, is a sinkhole—a hollow made by the collapse of a subterranean limestone cave—that has filled with water. Despite its name, Montezuma never visited it. It illustrates the fact that there is water in some places even in deserts. (Nathan Lee Ingersoll)

[or Vetivera] zizanioides). Looking rather like pampas grass, vetiver grows in clumps and is up to 6.5 feet (2 m) tall, with roots that penetrate to a depth of up to 10 feet (3 m). A perennial that can live up to 50 years, it occurs naturally in rain forest, in the Himalaya Mountains, and in the deserts of Rajasthan. It thrives in shade, on sun-drenched sand, in snow, and even for up to 40 days underwater. Despite its toughness, it is unlikely to become a weed because it propagates itself mainly vegetatively and very slowly, rather than by seed. Its roots grow almost vertically downward to a considerable depth. Rows of vetiver can be planted on hillsides, parallel to the contours, where the plants merge to form a screen that is strong enough to hold back soil and other material from moving down the slope. This prevents erosion on hillsides that were once forested. Vetiver is widely grown in Indonesia, the West Indies, Africa, and Polynesia. Apart from its value in preventing soil erosion, the plant is also used for thatching and for making baskets and mats. Its fragrant roots are woven into screens, and a thick, amber oil obtained from the roots by steam distillation is used in perfumery and in aromatherapy.

Grasses and Amaranths Vetiver is not edible, but another grass, Echinochloa turnerana, produces a highly nutritious grain. It grows wild in Australia and grows well with just one watering. Amaranths (Amaranthus species) also produce edible grain. They are natives of Central and South America, where they were cultivated for their grain until they were displaced by corn

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and cereals introduced by European colonists. A. hypochondriacus has edible leaves as well as seeds, and the seeds contain about 15 percent protein and 63 percent starch. The Spanish church forbade the growing of A. hypochondriacus because it was used in Aztec ceremonies the Christians were determined to suppress. A. caudatus is the garden plant love-lies-bleeding and is also known as Inca wheat. The value of amaranths is that they are rich in the amino acid lysine. This is an essential ingredient in the human diet, but very few plants contain it (soybean is one that does). People obtain most of their needs from animal products, which do.

Quinoa Quinoa (Chenopodium quinoa) is another South American plant. Little known outside the Andes of Bolivia, Chile, Ecuador, and Peru, where it grows naturally, quinoa is one of the best sources of protein of all plants. The outer layer of its seeds contains saponins and other substances that make them unpleasantly bitter. This may deter insect and bird pests, making the plant easier to grow, and the bitterness can be removed by washing the seeds in cold water.

Buffalo Gourd From the deserts and wastelands of Mexico and the southwestern United States, buffalo gourd (Cucurbita foetidissima)—also known as chilicote and mock orange— produces abundant fruits containing seeds that are rich

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in oil and protein. One plant, growing in very dry conditions, can produce an average of 60 fruits with 2.5 pounds (1.15 kg) of seeds. One acre sown to buffalo gourds could produce more than one ton (2.3 t/ha) of seeds that are 34 percent polyunsaturated oil and 30–35 percent protein. The fruits can be harvested mechanically, and they dry out so completely that the seeds can be threshed from them. The dried pulp is fed to cattle. Belowground the buffalo gourd grows a tuber in which it stores water. After two growing seasons the tuber weighs around 70 pounds (32 kg), of which 70 percent is water, and it is filled with starch. One buffalo gourd grown in very dry conditions produces as much starch as 20 potato plants grown in moist, well-drained soil.

Jojoba Jojoba (Simmondsia chinensis) is one plant from the arid regions of Central America and the southwestern United States that has been domesticated. It yields a liquid wax with many industrial uses. Jojoba wax has largely replaced oil from sperm whales as a high-quality lubricant. The wax can also be hydrogenated to make a solid, white wax that can be used for polishing. About 40,000 acres (16,200 ha) of farmland in the southwestern United States are now growing jojoba. Most of the oil goes to the cosmetics industry, but jojoba oil might be suitable as a substitute for gasoline. This would be a less lucrative market than cosmetics, but it could absorb much greater quantities. Jojoba might prove a highly profitable crop for farmers in desert countries.

Apple-ring Acacia As well as shoots and leaves for feeding livestock, especially cattle, the apple-ring acacia (Acacia albida) produces seeds carried in pods like peas and beans that contain up to 27 percent protein. A full-grown tree can produce up to 220 pounds (100 kg) of pods each year. The seeds can be eaten by humans but are more commonly fed to cattle. The seeds and bark are also used in folk medicine. The acacias can be grown beside groundnuts. Both crops yield approximately similar amounts of protein, the acacia producing its crop at the start of the wet season and the groundnuts producing theirs at the start of the dry season. The wood has several uses, and the ash from burned wood is used in making soap, as a depilatory, and in tanning. The tree has fierce thorns, and its branches are used to make fences. Apple-ring acacia, also known as the ana tree or winter thorn, grows to a height of about 65 feet (20 m) with a trunk that is more than 6.5 feet (2 m) in diameter. Native to the dry regions of Africa, it is well known for increasing the yield of crops grown beside it. Unlike most trees, apple-ring

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acacia produces leaves in the dry season and sheds them during the rainy season. The fallen leaves improve the soil around the tree, and shedding them prevents the tree from shading the crops growing around its base.

Ramón Ramón (Brosimum alicastrum), also called breadnut, Maya breadnut, capomo Maya nut, ojoche, ojite, ojushte, ujushte, ujuxte, capomo, mojo, ox, masica, and snakewood, bears leaves throughout the year. It occurs naturally in much of Central America as far north as southern Mexico and, less commonly, in Jamaica and Cuba. The tree grows in humid tropical forests. Its roots descend so deeply it can always find water, and so it is very tolerant of drought. Cattle relish its leaves and shoots, pigs enjoy its fruits, and people can eat its seeds raw, boiled, or roasted—they taste rather like potatoes. The dried seeds can also be ground into a meal and mixed with maize meal to make tortillas. This was a staple food for the people of the Mayan civilization, but nowadays it is usually regarded as a famine food. The tree itself can be tapped for its sap, which can be drunk like milk. A close relative from Venezuela, B. utile, is known as the cow-tree.

Genetic Engineering Today scientists are also working to make conventional plants more tolerant of drought and to grow in soils contaminated with salt. Altering plants in this way has always been possible, but slow. It was achieved by identifying varieties of crop plants, or species related to them, that possessed desirable qualities of drought resistance or salt tolerance. These were then cross-bred with varieties that produce dependable high yields. The most promising individuals from the resulting progeny were selected and bred with each other, and after many generations a new variety emerged with the desired characteristics. This was traditional plant breeding. Until recently it was the method of genetic manipulation by which all the varieties of cultivated plants were developed. It is now possible to achieve the same result much more quickly. Plants with the desired characteristics are examined genetically to locate and identify the genes that confer those characteristics. The relevant genes are isolated and transported into the cell nuclei of the target crop plant. They become incorporated into the genetic structure of the crop, which then acquires the characteristics they confer. This is genetic engineering. It is difficult in practice and not always successful, but the techniques involved are advancing very rapidly. Already and increasingly in years to come dry and semiarid marginal lands will be reclaimed and become productive through the use of crops genetically engineered for the purpose.

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IMPROVED IRRIGATION

Deserts are barren for lack of water. Where water is available a wide variety of crops can often be grown quite easily. A supply of water is all that is needed to make the desert productive. Obviously, that is what has been tried repeatedly for thousands of years. Unfortunately, it is not so simple as it seems, even if water for irrigation can be found. A desert climate is defined as one in which more water can evaporate in a year than falls as rain or snow. In other words, the air is very dry. Consequently, liquid water will evaporate rapidly if it is exposed to the air, and pouring water onto the ground is extremely wasteful because most of the water will evaporate and never reach the crop roots. Compensate for this by adding more water, and as the cost rises so may the water table until the ground becomes waterlogged (see “Aquifer Depletion, Waterlogging, and Salination” on pages 184–189). Then evaporation from the surface may leave a deposit of salts that accumulates and eventually poisons the soil. If irrigation is to prove effective and safe, it must be applied with care. First, though, a source of water must be found. This may be a well, but there is always a risk of depleting the groundwater and lowering the water table until the well runs dry. Water can be taken directly from rivers, but that can also cause problems (see “Cotton from Central Asia and the Aral Sea” on pages 275–279). The third alternative is to find some means of holding water that would otherwise be lost.

Capturing the Monsoon Rains In the monsoon climate of the Indian subcontinent the average annual rainfall is often more than would qualify the area as desert, although some places on the border between India and Pakistan receive barely 10 inches (250 mm) of rain a year. The trouble is that even where it is abundant measured over a year, all the rain falls in the space of about one week during the summer monsoon. The rain then is so intense there is no time for it to soak into the ground. Instead, the huge, heavy raindrops batter the soil surface into an impermeable covering, called a cap, and the water flows across the ground and is lost. Indeed, it flows so fast it carries much of the topsoil with it. Local people have found a way to capture and hold water from the monsoon rains. On the lower ground they have used earth to build dams across the valleys. Each dam has a sluice gate that can be opened if necessary to allow excess water to escape, but mostly it remains shut. The rains flow off the hillsides and into the valleys as countless torrential streams. There their progress is checked, and the water accumulates. For about a week after the rains have ended there is an artificial lake behind each dam, but its level falls quickly as the water soaks into the ground. Then, when no

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more water lies on the surface, the farmers move in to plant their crops of wheat and chickpeas in the damp soil. Even after the water has apparently vanished, it has not been lost. It drains downward and becomes groundwater that can be reached by wells. So once the fields have dried irrigation is possible to extend the growing season. Their method does more than conserve water. It also captures the topsoil that has eroded from the hillsides. This is deposited on the fields as a rich, fertile silt. This system for trapping and holding water long enough for it to soak into the ground is very similar—and the principle is identical— to that devised long ago in the Negev Desert (see “Farming the Negev” on page 183).

Drip Irrigation Drip, or trickle, irrigation greatly reduces waste. Water is carried to the crop along plastic pipes, 0.5–1.0 inch (1.3–2.5 cm) in diameter, that have small openings, called emitters, at intervals along them. The pipes are usually buried between 2.75 inches and 12 inches (7–30 cm) below the surface, and the spacing of the emitters can be varied to meet the requirements of particular crops. A fruit tree might have up to eight emitters spaced around it, for example, or there might be one or two to supply water to a grapevine. Water trickles from the emitters very slowly. No more than one gallon (3.7 liters) of water may flow from each emitter in an hour. The water moves through the soil mainly by capillarity (see “Except When It Moves Upward, by Capillarity” on pages 68–69), and only the soil near the plant is moistened. Drip irrigation works especially well in hot, dry climates, and it has been used extensively in Israel, where it was introduced during the 1960s. There, the pipes are buried belowground so the water is delivered directly to the roots of the crop plants and the ground surface remains dry. This almost eliminates losses by evaporation. Drip irrigation allowed Israeli agricultural production to increase by a factor of 16 over 50 years, during which time the population increased by a factor of seven. Drip irrigation makes it possible to cultivate steep slopes, where water would rapidly run off the surface. Because water is applied directly to the root area, the ground surface is usually dry. Drip irrigation also helps control weeds by supplying water to the crop but not to weed seeds lying near the surface. This irrigation method is simple to install and requires little maintenance, although installation is costly and the emitters easily become clogged and infiltrated by roots. The irrigation water must be filtered carefully before it enters the system to avoid clogging, and chemicals are sometimes used to discourage roots from entering the pipes. Crops watered in this way produce shallow roots concentrated close to the emitters. If a pipe fails or a few emitters clog, the plants they serve are likely to show signs of stress very soon and may

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be lost unless repairs are carried out quickly. Soil cultivation and crop harvesting methods may need to be modified because of the presence of the irrigation pipes.

Water and Fertilizer Together There are disadvantages to drip irrigation, but the advantages greatly outweigh them. Because the rate of flow is so low, fertilizer can be added to the irrigation water. This nourishes the growing plants very efficiently. Fertilizer amounts can be precisely controlled and adjusted when necessary, and none of the fertilizer drains out of the soil and is lost. Water used for irrigation is not pure enough for domestic use. It is not fit for drinking, but there is no need to purify it to such a high standard. In fact, irrigation water can be quite salty. The salts it contains are deposited in the soil but do not harm the plants. The soil around the plant roots is kept permanently moist, so the salts accumulate around the outer edges of the wet area. Impure water can harm the health of workers who are exposed to it frequently over a long period, but drip irrigation minimizes exposure by feeding the water into the pipes directly from a storage facility.

Salt-Tolerant Plants Most of the water on Earth is salty. Land-dwelling animals cannot drink it, and it will kill most plants—but not all species. In mangrove forests and salt marshes there are plants that tolerate high salt levels, for example. If the physiological features that allow them to grow in a saline environment could be transferred to other, intolerant species, they could also be grown in salt water. This is an aim of scientists working to engineer genetic changes in crop plants. So far the success has been limited, because although some ordinary crop plants are more tolerant of salt than others, none is very tolerant. Transferring relevant genes from slightly salt-tolerant to even less salt-tolerant species represents only a small gain. The date palm (Phoenix dactylifera; see “Date Palms” on page 112) is one of the most tolerant, but it can grow in soils containing no more than five parts per thousand, or per mile (‰), of salt. Seawater contains an average of 35 parts per thousand of salt, and in places where an arm of the sea is almost landlocked and the evaporation rate is high, the salinity rises to 40 parts per thousand. It reaches this level in the northern part of the Gulf of California, for example, between Baja California and Sonora, and in parts of the Persian Gulf.

How Some Plants Thrive in Salty Environments The alternative to engineering salt tolerance into conventional crops is to exploit the salt tolerance of halo-

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phytes—plants adapted to saline conditions found along coasts. At present it is impossible to transfer genes conferring salt tolerance from halophytes to halophobes (salt haters), however, and it may always remain impossible. That is because the salt tolerance of a halophyte is based not upon one or even a small set of characteristics, but on the entire physiology of the plant. It can include roots with an outer layer of cells through which salt cannot pass. Layers of waxy material may separate cells inside the plant, forcing water to pass through the cells, where salt is removed and excreted. There may be specialized saltstorage organs, or salt bladders, on the leaves. These accumulate salt and burst when they are full, releasing it. All these features are determined genetically, but transferring them to a halophobic species would involve completely restructuring the plant. The task seems daunting, but in March 2006 at a meeting of growers hosted by the University of Western Australia, Tim Flowers announced that a salt-tolerant variety of wheat might soon be ready for testing. Flowers, of the University of Sussex, England, and a visiting professor in the School of Plant Biology at the University of Western Australia, had been working with a team led by Tim Colmer. They had identified the genes that make sea barley grass (Hordeum marinum) salt tolerant and planned to cross that plant with wheat (Triticum aestivum). Sea barley is a close relative of the barley already grown as a farm crop, and wheat varieties vary widely in their salt tolerance. The scientists hoped that the result of the cross would be a salt-tolerant wheat suitable for feeding to livestock. The wheat would also tolerate waterlogging.

Domesticating Salt-Loving Plants Another approach is possible, however. Some natural halophytes can be domesticated. The domestication process begins with the collection of hundreds of halophytic plants. These are screened to determine the extent of their salt tolerance, their edibility, and for those that are edible, their nutritional value. Having made a selection, the second step involves learning how to grow, harvest, and use them. Several species show promise. Palmer’s grass (Distichlis palmeri) yields seeds that were once eaten by some Native Americans. Livestock can eat glassworts (Salicornia species). Glasswort seeds are edible by people, with a nutty flavor, and they contain an oil that can be used for cooking. Grown experimentally in Mexico, the United Arab Emirates, Saudi Arabia, and India, glassworts have yielded an average 7.5 tons per acre (17 t/ha) of the whole plant, from which 1,780 pounds per acre (2 t/ha) of oil was obtained. Saltbushes (Atriplex species) are also edible by farm animals. Care must be taken to make sure salt does not accumulate in the soil while the crop is being grown. Since salt water is being used

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Health of the Deserts for irrigation, the only way to prevent its accumulation is to irrigate copiously and almost continuously, in some cases by flooding the fields. The plants still require domestication. One important feature of domesticating any plant grown for grain is to prevent it scattering its seeds before harvest, which is what wild plants do. Early farmers had to solve the same problem with wheat and barley, and they did so by growing crops from seed they saved only from those plants that retained their seed when ripe. This suggests that the problem is soluble. There is also a problem that may not be soluble. Halophytes contain salt in their tissues. The salt has no nutritional value, but it occupies space, making the plant relatively bulky in relation to its nutrient content. The bulk limits the amount an animal can consume at a time, but volume for volume the food is less nutritious than that from halophobic plants.

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Seawater Is Abundant Despite the difficulties, halophytic crops could be very useful. Seawater is abundant. In some places the groundwater is too salty for agricultural use, and there are large areas suffering from salination caused by poor irrigation in the past. Halophytes might be grown in such places and certain plants might also be able to tolerate and absorb levels of elements that would poison other species. Most of the salt water is in the sea, of course, and can be used only on land near the coast. Even so, this limitation still leaves large areas of coastal deserts that might be cultivated. It remains to be seen whether farms growing halophytes and irrigated by salt water will prove both biologically and economically feasible. Research into their viability began as long ago as 1949, and there are still no commercial halophyte farms. If one day they do start to appear, land that is now barren will be brought to life.

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8 Management of the Desert Wherever they live, people actively manage the land that sustains them. They may not be aware that they are managing the land, but they are. Pastoralists, who guide their herds and flocks from one area of pasture to another, allow the animals to graze only until they have eaten the most nutritious plants. Then they move on, allowing the pasture time to recover. They are ensuring that the pasture will continue to feed them in the future. Even those who live by fishing, hunting game, and gathering the edible parts of wild plants are careful not to overexploit the resources on which they depend. It is a form of management. If desert peoples are to thrive in years to come, managing the deserts will remain important. Management methods will change as new technologies make possible innovations, such as the desalination of seawater on an industrial scale, that would have been beyond the capabilities of any previous society. This chapter discusses the management of the world’s deserts, and it begins by describing some of the traditional ways of desert life in Australia and Africa and how these have changed in modern times. It describes pastoralism in and near the Sahara, and it outlines the way certain developments have caused serious problems. The provision of water is essential, and the chapter continues by describing approaches to irrigation. These include the building of large dams and the diversion of rivers, but they also include building artificial oases and harvesting water belowground. The scarcity of water has been a source of conflict between nations in the past and will be again unless countries can agree to share their resources. Desalination is an old technology that today is becoming more affordable than it used to be and that could be used to improve supplies. Finally, the chapter turns to the frozen deserts of the polar regions. There, the conflicts center on fishing and whaling.

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DESERT ADVANCES AND RETREATS

Deserts seem eternal. Vast expanses of barren wilderness, it is hard to see how they might ever change, yet they do

change. They change with the arrival of the rains, with droughts that can last for years, and with longer-term climatic changes. People who live in or on the edge of a desert must adapt to those changes. This is never easy, but where people depend on desert resources the difficulty increases in proportion to their degree of dependence. There are mining towns in many deserts, for example, populated by miners and other workers and their families. Their necessities are supplied by shops stocked with food and other goods imported from outside the desert. The people are not at all dependent on the desert. Whether the rains arrive or fail is of no concern to them. Farmers, growing crops or husbanding livestock, are much more vulnerable. Adequate rain means good harvests and luxuriant pasture, with adequate food supplies to last until the next season. Drought means hardship, but hardship that can be postponed at least for a time, because there are food stores to fall back on and livestock can be killed for food before they starve. Most vulnerable of all, though, are the people who rely on the desert itself for all their needs. These are the hunters and gatherers, the people whose food consists of wild plants, game, small animals, honey, eggs, and insects and whose few possessions are made from the materials that occur naturally in their environment. They have no reserves, and their only response to hard times is to move elsewhere. Some people romanticize the apparently simple way of life such people follow, but in truth it is harsh in the extreme.

Australian Aborigines When the first humans arrived in Australia, perhaps as long as 60,000 years ago, they found a land that was very different from the forests and lush pastures of southern Asia, the region from which they had come. There was tropical forest and grassland in the northern part of their new homeland, but much of the interior was desert. They were not farmers—they arrived in Australia long before farming was invented. In any case, Australia had no animals that could

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Management of the Desert be organized into herds. Kangaroos, the biggest mammals, cannot be corralled and driven like cattle or sheep. They are a source of meat, however, and the new colonists lived by gathering wild plants and hunting game. As their numbers increased, the local groups would divide. These groups occupied a particular area and shared a common language. The subgroups would then depart in search of a new territory that would supply their needs, and in the Australian deserts those needs were defined in terms of water. Eventually, there were about 500 languagebased groups. Each group comprised a number of smaller groups, all of them living permanently in an area centered on a source of water around which their ancestors had originally settled and where their spirits were believed to have remained. By the time European settlers arrived in the 18th century, some estimates gave the size of the existing population at about 300,000. Others said there were more than 1 million. The Europeans called the people aborigines or aboriginals, a name that simply means “the people who were already there when we arrived.”

Aboriginal Society All the members of a group were related to each other, but relationships were not confined to men and women. Various nonhuman species were also classed as relatives. The components of their natural surroundings and natural phenomena, such as rain, were part of their social order, and there were rituals for communicating with them. Aboriginal people learned to exploit the meager resources available to them, but their life was always hard and precarious. They built no permanent dwellings, had very few material possessions, and did not wear clothes. They would construct a simple windbreak for shelter, but a small campfire and the dogs that slept beside them provided their only warmth on cold nights, when the temperature can fall to 50°F (10°C). Their life was primitive. Much of their time was spent searching for water and food, but the conditions were made endurable by their rich mental life. They developed very complex social arrangements and religious rituals, with a culture based on the concept of the Dreamtime, through which past and present merge and spatial distances vanish. Aborigines are now Australian citizens. They are emerging from a long period of oppression by white Australians that has left them impoverished and dispossessed. There are few groups that still live by hunting and gathering. Most live in towns and have jobs.

Dwellers in the Kalahari It is not possible for people to live a settled life if the desert must provide all their food, drink, and raw materials. The

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resources are too thinly spread and undependable. When it rains plants flourish, animals arrive to feed on them, and times are good (see “When the Desert Blooms” on pages 106–108). Between rains, when the land is parched, food is hard to find. The interval between rains is unpredictable, but it is sometimes measured in years. In the Kalahari Desert of southern Africa there are people who live by hunting and gathering, but their traditional way of life is not so wholly dependent on the desert as that of the Australian Aborigines. The San, also known in some places as the Basarwa, are a group of people some of whom were once farmers. They were driven into the desert long ago by incoming Bantu-speaking peoples and had to adapt to foraging for plants and hunting game. Many San traded with the Bantu peoples, and for a time they lived quite well by trading. At first they traded cattle, then ostrich feathers, which were fashionable dress ornaments strongly in demand in the cities of Europe and America. They also traded ivory. These goods were exchanged for agricultural produce and the guns with which they shot elephants. Eventually, the elephants disappeared, the cattle died from a serious disease called rinderpest, and the market for ostrich feathers collapsed when the fashion changed, as fashion does. The San people were driven into increasingly abject poverty until hunting and gathering was the only means of subsistence available to them.

Myths of Abundance Among the !Kung There are several San groups, of which the most closely studied are the !Kung, !xong, and G/wi—“!” and “/” represent a click sound. A famous study of the !Kung made in the early 1970s found them living well. They were healthy and needed to devote only two or three hours each day to the search for food, and their diet was rich, nutritious, and varied. Based on this report, some Europeans and Americans came to see the !Kung as the protectors of a peaceful, leisured way of life in tune with their natural surroundings, from which they effortlessly obtained everything they required. It was learned later, however, that this study had lasted only three weeks and by chance had coincided with a brief period of abundance following heavy rains. Usually, their lives are harsh. Their life expectancy at birth is 30 years, many babies and infants die, and when food is scarce the adults lose so much weight they are close to starvation.

Life of the G/wi Among the G/wi, who live a similar life to the !Kung, in most years there is enough food between November and about July for a band to live together as a single community. Water

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holes are full for up to eight weeks during that period. The group cannot remain in one place, of course. Every three or four weeks, when the edible plants have been depleted in the area around the camp, the people move to a new location. In early winter, when frosts damage the plants, reducing the food supply, the community divides into its constituent households. There are up to about 16 households in a band, and between them they forage in an area of 300 to 400 square miles (780–1,040 km2). The G/wi diet is based on about eight species of plants. These provide the staple food, different plants being available at different seasons—they are not all eaten at the same time of year. Meat is provided by hunting grazing mammals and by collecting reptiles, birds, and insects. Eggs are also eaten. Antelope skins provide clothing, blankets, and bags for carrying small items.

Changing Times In 1971 a diamond mine opened at Orapa, in Botswana. Other mines appeared in succeeding years in various parts of the Kalahari. The mines provided jobs and attracted people from outside the desert. Cattle ranches were established on the edge of the desert. Local crafts became popular among visitors. A tourist industry began. Little by little the San came to work for the ranchers or found other employment. The Botswana government persuaded many to leave their traditional lands in the Central Kalahari Game Reserve and resettled them in villages outside the reserve. Few still practice the old hunting and gathering way of life. No doubt there are those who regret the change and feel disoriented by it. In time, though, an improved diet and health care will lead to improvements in health and longevity. Children can now attend schools, so their lives will be enriched by education. Most of all, the people have at last escaped from their dependence on the ever-changing, never dependable desert.

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PASTORALISM

Along the southern edge of the Sahara, in the Sahel region, climate cycles bring successive years when the rains make farming possible, but from time to time they also bring prolonged drought. Some years ago SOS Sahel, a Britishbased volunteer agency, sponsored the Sahel Oral History Project, in which individuals living in Mauritania, Senegal, Mali, Burkina Faso, Niger, Chad, Sudan, and Ethiopia were interviewed in order to obtain firsthand accounts of the traditional way of life in their countries. Farming is widely practiced across the Sahel, but in the drier regions, closer to the desert itself, live the pastoralists.

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These are people who grow no plant crops but live by tending their livestock. Theirs is a nomadic way of life, but rather different, perhaps, from the image the word nomad conjures.

Fatimetou’s Story The parents of Fatimetou Mint Mohamed el Mokhtar, a 70-year-old Mauritanian woman, owned cattle, camels, goats, and donkeys and spent their time traveling in search of pasture. The family knew the location of all the wells. Where possible, the livestock and humans used separate wells to prevent contamination of the water the people drank and used for cooking. “While we lived as nomads,” Fatimetou said, “I never worked. We didn’t farm and I didn’t even sew, because I was lazy and had many slaves to do all the menial work.” The slaves tended the animals, and Fatimetou cooked the meal of rice with meat, peanuts, butter, and milk. They drank camels’ milk. The animals supplied all their food and the material for making their tents, which were erected over a frame made of wood gathered from the acacia trees. “Our men brought us anything else we needed.” Slaves were not paid, but the head of the family was responsible for feeding and clothing them. There are no slaves nowadays, and everyone who works is paid, in theory at least. Indeed, some of the men complain that modern women are marrying former slaves now that they are no longer expected to marry within their tribe. The drought of the 1970s was not the first that Fatimetou had endured. There was an earlier one when she was in her 20s and recently married, possibly linked to the 1925–26 ENSO event (see “Trade Winds and El Niño” on page 242). Their animals began dying, and they made a long, difficult journey to a place where they heard there had been rain. Hardly had they arrived before the locusts joined them, eating all the vegetation and even starting to eat the tents. Eventually, they were reduced to killing and eating their camels, one at a time. After the 1970s drought the land was very slow to recover. Fatimetou and her family moved from place to place, but eventually they were forced to admit defeat. They sold what remained of their livestock and moved to the Mauritanian capital, Nouakchott, a city of about 600,000 inhabitants not far from the coast. There her husband set up a small shop. It prospered, and after a time he was able to move into the market. Fatimetou was one of the formerly nomadic women allocated some land by SONADER, a government agency. The women were taught how to cultivate vegetables and were provided with free seed and water for the first three years. They grow cucumbers, tomatoes, carrots, turnips, potatoes, beetroot, aubergines, salads, and mint. The children go to school and help with light work. Because the government

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Management of the Desert gave her the land with the documentation to confirm her ownership of it, her children will inherit it.

Surviving the Sahel Drought Not all the pastoralists settled in the city after the 1970s drought. One man described a life spent wandering from place to place, living in tents that the women made from wool. Often they dug wells to find water for the animals, sometimes spending all night digging. Many of the animals died during the drought, but when the rains returned people started rebuilding their herds. “The bush is our home,” this man told the interviewer, “we feel lost in the city. As we sit here, we live in the hope that the vegetation will recover enough for us all to resume our nomadic life.”

Life Among the Tuareg Fauré Maussa is a Tuareg. She was 90 years old when she was interviewed in Niger, and she gave fascinating insights into traditional nomadic life before it began to disappear as a result of modern changes. Her father owned one of the biggest herds in the area, she said. He had between 200 and 400 cattle, goats, sheep, and camels. The family also kept chickens and guinea fowl. All the animals were marked so they could be identified if they wandered away or were stolen, and the species were managed separately, each tended by its own specialist. Ordinarily, the entire family accompanied the father as he traveled from one pasture to the next, but when he embarked on very long journeys some of the family stayed behind. Despite being nomads, they had a base where they spent much of their time. Among the Tuareg the animals belong to the head of the family. When he dies his wife inherits them, and she decides how to distribute them among her children. If the man had more than one wife, a teacher and prayer leader, called a marabout, decides how the cattle should be apportioned between them. Each child is entitled to inherit something, but males receive twice as much as females. Many years ago, when Fauré was young, the desert was even more dangerous than it is today. Journeys were made on foot, and bandits laid in wait to rob travelers. As protection against them, the marabouts accompanying the party used their magical powers to render everyone invisible, so the bandits passed without seeing them. According to Fauré, this worked! Journeys also took a long time. She and her husband made the pilgrimage to Mecca. This is a long way from Niger, and after they had returned she calculated they had been away for seven years. While she was in Mecca Fauré gave birth to four children. She told the interviewer that her eldest son was still living there. He was married with a family of his own and

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refused to return to Niger but had said he would come looking for his parents and take them back to Saudi Arabia to live with him. Fauré was waiting for him and seemed content to retire from the nomadic life.

Difficulty Settling There are strong pressures to persuade nomads to settle, but it is difficult for the older people to make such a radical change. Sheikh Ahmed el Sigaydi, then 70 years old, was living in El Meseiktab, a settlement near Shendi, to the north of Khartoum, in Sudan, but he longed to resume his nomadic life. He was head of his tribe and had spent most of his life moving around the desert with his herds of cattle, sheep, and camels. He lost many of his animals during the drought of 1983–85 and had been forced to sell some of the survivors to pay taxes and buy essential supplies. He had tried to keep as many female animals as possible to give him a chance of rebuilding his herds, though his son doubted whether this would ever be possible. The old man could see the advantages of a settled life, starting with education for the children. “The younger generation sees a future in the new settlement, with chances for education and a better life.” But he had reservations: “. . . the children are not so healthy as before. Perhaps this is because they no longer tend animals, which gave them fresh air and plenty of milk.” Meanwhile, life was hard. The 57 households of the settlement—a total of about 450 individuals—had no means of saving, and their only way to earn enough money to feed themselves was by doing small, unskilled jobs for the villagers. “We are nomads, and I fear we do not know enough to start up settled farms. We used to practice rain-fed farming in such valleys as Hawada in the Butana plains, and those with pack animals still go there in the rainy season. Last year, my sons and I couldn’t go, because we no longer had such animals. We have problems with our valley wells becoming covered with sand and earth and we have no animals to help dig new ones. The last drought affected the water table, so that wells are now too deep to dig.”

A Way of Life Slowly Ending Drought and famine are themes that recur in the accounts of life in and near the desert. Newspapers and television reported the suffering this brought in the 1970s, so the world paid attention and grew alarmed at what some maintained was a consequence of climate change brought about by emissions of so-called greenhouse gases from factories, trucks, and cars (see “The Greenhouse Effect” on pages 243–244). This was one drought among many, however. Elderly men and women who have spent their entire lives in the region have experienced up to five severe droughts,

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and they have endured—and survived—the famines associated with them. Until now nomadic pastoralists had few choices. Tending animals was the only way of life open to them. Unskilled and poorly educated, they were not equipped to take up other occupations, even if jobs had existed. So after each drought and famine their suffering continued for as long as it took to rebuild their herds and flocks. The most recent droughts, of the 1970s and 1980s, are different in having befallen the region at a time when alternative ways of life were being offered to the nomads. For the first time people are able to choose whether to resume their former lives. The choice is hard for the older members of the communities but much simpler for the young. Most of them opt for education, lifestyle choices, and the settled life these imply. Gradually, nomadism is dying.

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Cattle prefer grass that is long enough for them to seize a clump of it. They bite off the clump and chew it. Sheep are different. They nibble the grass and prefer it to be short. Goats are famous for being able to eat almost anything. They will eat leaves from shrubs and the low branches of trees, and they will climb trees in search of tender leaves when food is scarce. Camels will eat grass that is so dry and brown other animals refuse it, and they will also nibble the twigs and tough leaves of even the thorniest shrubs. Donkeys can also survive on very poor vegetation. Each species feeds differently, so between them they use pasture very efficiently without competing. Traditionally, a party of nomads would arrive at an area of pasture, set up their camp, and stay there for several weeks. Their animals fed on the natural vegetation, but in doing so they trampled the ground and urinated and defecated on it, until such plants as remained were unfit to eat. When the pasture was becoming seriously degraded, one or more of the men would ride out in search of a new site. Once this had been identified, the group packed up their tents and moved. That is how the pastoral way of life functioned. The pasture they left behind was useless. Every last bit of nutritional value had been wrested from it. At the same time the feces and urine deposited by the livestock fertilized it. Left undisturbed, the plants soon recovered, and the pasture was restored to its original condition.

Veterinary Care The pastoralist way of life had developed over the centuries into a highly sensible, sustainable way to manage the

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sparse vegetation of the deserts and their semiarid margins, but animals were often sick. Their owners did their best to cure them and had considerable understanding both of their stock and of the uses of medicinal herbs, but most livestock illnesses were incurable. The animals died. Today that has changed. There are centers to which pastoralists can take sick animals to be examined and treated by trained veterinarians. Modern veterinary medicines and techniques are available. Consequently, animals that fall ill have a better chance of surviving than they did even a few decades ago. A result of this is that livestock numbers have increased.

Rising Demand for Meat Demand for livestock has also grown. Traditionally, the nomads would sell no more animals than was necessary to give them the money to buy materials they could not produce themselves. They needed to buy grain, for example, and metal tools and implements. They might also sell sick animals they knew would not recover. Occasionally, they would kill an animal for their own use, most commonly to celebrate some important event in their lives, but not often. They were very attached to their livestock and did not like to eat them. Economic development has led to the emergence of a new class of city dwellers who are much wealthier than most people were in the past. Traders, entrepreneurs, administrators, and members of professions, they enjoy a relatively high standard of living. The visible evidence of a high living standard is a diet that includes meat as a regular item. There is now a market for meat, and the meat can be obtained fairly cheaply from the pastoralists. That gives the pastoralists an even greater incentive to increase the size of their herds and also to change the composition of the herds. Camels are rather less in demand now that motor vehicles are used to transport goods and people. Cattle, sheep, and goats are in demand for their meat, but many herds have come to consist mainly of cattle.

Cowboys versus Farmers Although the pastoralists make good use of the land, they have always faced competition for it from farmers. This is a conflict as ancient as the invention of farming itself. It is the theme underlying the biblical story of Cain and Abel and of many stories about fights between cowboys and farmers in the “Wild West.” In Africa the conflict continues still. The conflict grew worse during the droughts of the second half of the 20th century. These affected everyone, but in different ways. The pastoralists kept moving for as long as they could, and when the desert defeated them they migrated to cities or villages. Farmers were not so mobile. They had to

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Management of the Desert watch as shifting sand dunes the size of houses moved with the wind. They saw houses buried and had to try to dig them out with no earth-moving machinery to help. People planted trees to hold back the sand, but the trees were also buried and died. They struggled on, growing more trees from seed and doing everything possible to reclaim their lands, but in the end they also moved. In some areas they started farming land that was formerly pasture. The pasture may have been unoccupied and apparently free for the taking, but the nomadic pastoralists depended on it.

Land Hunger This competition for marginal land is not unique. There have been many occasions in European history when prolonged bad weather meant the land was unable to provide as much food as people needed or when the failure of alternative employment—in mines or factories, for example— left people with the choice between farming or starving. Then they started cultivating any land they could find that was not already being farmed. It was called land hunger, and in Europe it led to the plowing of upland moors and other poor land that in better times would not have been thought worth farming. As in Africa, until it was enclosed and plowed that poor land was used for grazing animals. In Africa land hunger drives the farmers onto the pasture used by pastoralists. In Niger, for example, nomads have been forced to move because their traditional grazing lands were enclosed by fences and turned into fields. The farmers allowed the pastoralists to graze their animals in the fields after the harvest, but they charged them for this. Unable to afford to pay the farmers and in any case resentful at being charged for the use of land to which they formerly had free and unrestricted access, the nomads had no choice but to move on. In one case a group of them found another area where the pasture was adequate and free and where there was also a water pump they could use. All was well for a time, but then farmers newly arriving in the area became interested in that land, too. They started enclosing fields and growing vegetables. These required much watering, and the authorities introduced charges for water use that both the horticulturists and pastoralists had to pay.

Squeezing the Nomads Several factors were now at work. Improved veterinary care and the opening of markets for meat had led to increases in livestock numbers. The change in the composition of the herds altered the way the pasture was being used—it came to be grazed less efficiently. Drought had reduced the area and quality of the pasture, but it had also destroyed farmland. That caused land hunger. Farmers expanded into what had formerly been pasture.

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At a time when their herds were increasing in size, therefore, the pastoralists were being squeezed into ever smaller areas. As their animals began to die from hunger, many took those that remained and, with encouragement from the authorities, settled in permanent villages. There they had even closer access to veterinary care for their stock as well as health care for themselves and their families, education for their children, and the possibility of paid employment.

The Tragedy of Overgrazing As their crowding increased, the livestock became increasingly desperate. On the remaining pastures and most of all around the new settlements, the vegetation came under attack. Sheep that used to nibble the grass until it was short and then move on nibbled it a little lower. Such close nibbling destroyed the point, almost at ground level, from where grass plants grow. Eventually it killed the grass. Goats ate what they could find on the ground, then took to climbing into the shrubs and trees in search of food. Stripped bare of leaves, the trees and bushes died. This is the tragedy of overgrazing, and it extends even further. Once the pasture has been destroyed the land is left exposed to the wind. Where the degraded land lies close to a desert the desert is likely to claim it. Overgrazing can and often does contribute to desertification. Some people blame the pastoralists. They accuse them of keeping too many animals out of greed, because they can sell meat to traders who supply city markets. In particular, they point to the harm caused by goats, which eat anything and everything until not a vestige of vegetation remains. They accuse them of failing to comprehend the damage their way of life can cause. They blame them for the spread of deserts. It is a clear example of the type of catastrophe known as the tragedy of the commons (see the sidebar). Blaming the pastoralists is quite wrong, however. They are not the perpetrators of damage, but its victims. Drought, due to entirely natural causes, has injured everyone living in and near the deserts, and the nomads have suffered most of all. As their animals die or are sold, often it is only their goats that can survive. The goats did not destroy the pasture. They are the only survivors of the disaster. Their owners, meanwhile, have lost the way of life on which their culture was based. There are compensations and the promise of a better life in the future, but this loss is no small matter.

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PROVIDING WATER

Most deserts are far from any major source of water, but there are a few exceptions. West coast deserts (see “West Coasts and Rain Shadows” on pages 5–6) are close to the

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Tragedy of the Commons In 1833 William Forster Lloyd (1794–1852), an economics professor at the University of Oxford and fellow of the Royal Society, published a pamphlet with the title The Tragedy of the Commons. It attracted little attention at the time, but in 1968 it was revived and updated by Garrett James Hardin (1915–2003), professor of human ecology at the University of California at Santa Barbara from 1963 until his notional retirement in 1978. Using Lloyd’s title, Hardin published his article in the journal Science. Hardin invited his readers to imagine the commons of the title as a pasture that is open to everyone. Each livestock owner seeks to maximize his own gain from the pasture, and the most obvious way to achieve this is to increase the size of his herd. Consciously or unconsciously, the herdsman must evaluate the consequences of doing this, and when he does so he finds there is a positive component arising from adding one more animal to the herd and a negative component. The positive component is the profit he will make from the eventual sale of the additional animal that has grown fat on the free food supplied by the pasture. That is a big gain. The negative component arises from the fact that adding one more animal depletes and beyond a certain point degrades the pasture. The cost of the deterioration of the pasture is shared among all those using it, however. When the herdsman adds one animal to his herd, he enjoys all the gain but pays only a share of the cost. It is clearly to his advantage to put the additional animal out to graze. All the livestock owners are performing the same calculation, and each of them reaches the same conclusion. Each herdsman stands to gain by adding to his herd and by doing so repeatedly. Hardin maintained that the inevitable outcome is the destruction of the pasture and the consequent failure of all the herds. This happens because each herdsman

sea, and some deserts lie beside a major river. Modern Iraq used to be called Mesopotamia, a name that means “between the rivers”—the Tigris and Euphrates. But the Nile is by far the most famous river to flow through a desert. Water from the Nile has sustained farming in Egypt for thousands of years. Rivers are not always reliable, however. There are dry years when the flow is insufficient to irrigate the fields and wet years when the irrigation channels are overwhelmed and floods destroy the crops. One solution to this problem

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seeks to maximize his gain, and no one takes proper account of the cost. Hardin applied his argument to every aspect of the natural environment. The air, soil, forests, rivers, oceans, and every other component of the global environment is free for everyone to use and is owned by no one. Consequently, degradation of the environment is inevitable unless someone takes responsibility for it. His proposed solution was to make the environment into private property that its owners would defend by rationing its use. Hardin’s critics pointed out that his central assumption was mistaken. A real commons is not free for everyone to use. It is owned by people called commoners, and people who are not commoners have only very limited rights of access to it. Commons are managed sustainably and have remained in constant use for centuries. Also, arguments based on estimates of what would happen if everyone were to behave in the same way are always dubious. People do not all behave in the same way, and in the case of Hardin’s commons the availability of pasture is only one consideration the herdsman must balance. He might choose not to increase the size of his herd because there would be too many animals for him to manage. Perhaps he had insufficient accommodation for them in his yard. Maybe he could not afford the extra veterinary bills. Or he might think that the most obvious result of a general increase in livestock numbers would be a fall in prices. Nor should components of the natural environment be thought of as commons in the sense Hardin means. Politicians do respond to the concerns of people who feel that the environment is theirs. Nevertheless, Hardin’s essay attracted much attention and stimulated an important and valuable debate.

is to build one or more dams to hold back the water and release it at a measured rate, as it is needed. Egypt suffered many severe floods prior to the damming of the Nile, but these floods were minor events compared to those China has suffered throughout its history. In order to regulate the flow of its mightiest river, the Yangtze, and at the same time to generate electricity, the Chinese have now completed the construction of the world’s largest hydroelectric dam. As well as being the world’s biggest dam, the Three Gorges Dam is also one of the most controversial.

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The Three Gorges Dam The Three Gorges Dam is more than five times the size of the Hoover Dam. It stands 607 feet (185 m) high, and it is approximately 1.43 miles (2.31 km) long. The reservoir behind the dam is ordinarily 574 feet (175 m) deep, 373 miles (600 km) long, and holds 1,388 billion cubic feet (39.3 billion m3) of water. Construction began in 1993, and the reservoir began filling in 2003. The building work was completed in 2006, but it will be 2009 before all the 26 generators are installed and working. When they are, the dam will have a total generating capacity of 18.2 GW. It will produce almost 4 percent of China’s electricity, and Yangtze floods will no longer destroy lives and property (see the sidebar “China’s History of Flooding.”) The idea to dam the Yangtze was not new. Sun Yat-sen first proposed it in 1919. It was reviewed several times over the years, but the cost and China’s troubled history meant it was not until 1979 that the government authorized it. It was immediately controversial, and the opposition to it was so intense that in March 1989 the state council ordered a five-year delay for further consideration. Planning resumed in 1992, however, and physical preparations began in 1994. Once work began there were corruption scandals, allegations of bribery, and work so shoddy that in 1999 part of the structure had to be demolished and rebuilt. The principal criticisms centered on the area the reservoir would cover. Millions of people had to be relocated

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from good arable land, and not all of them were properly compensated. Prior to their eviction these people were not safe, however, because they lived on land subject to periodic flooding. The flooded area also contained approximately 1,300 archaeological sites. These included the burial sites— in caves on the cliff sides—of the Ba, a people who settled in the area more than 4,000 years ago. The reservoir also submerged the gorges themselves, and the Three Gorges were internationally renowned for their scenic beauty. Construction projects on this scale always create problems and arouse fierce opposition. With the dam built and working, eventually the controversy will die down. Floods will cost no more lives and cause no more famines by destroying crops, and China will generate at least some of its electricity without burning fossil fuels. Only then will it be possible to calculate whether the benefits the dam has brought outweigh the disruption and damage its construction caused.

The Nile and the Aswān Dams Regardless of the advances and retreats of the Sahara, Egyptian farmers have always had access to water for irrigation. That water has allowed them to feed and clothe a civilization that has existed for thousands of years. Theirs is the most sustainable agricultural system in the world, for it has maintained the fertility of the land throughout this long period. It is the River Nile that delivers water to them.

China’s History of Flooding China’s two great rivers, the Yellow (Huang Ho) and Yangtze, are among the longest in the world. The Yellow River is 3,395 miles (5,464 km) long, and the Yangtze, known in China as Ch’ang Chiang (“long river”) extends for 3,915 miles (6,300 km); it is the world’s third-longest river. Both rivers have flooded repeatedly throughout Chinese history. The Yellow River is sometimes called China’s Sorrow. When the rivers flood the damage and loss of life occurs on a truly appalling scale. In 1887, for example, up to 2.5 million people died when the Yellow River burst its banks. A flood in 1931 inundated about 34,000 square miles (8.8 million km2) of farmland. An estimated 1 million people died from drowning and in the famine and epidemics that followed, and 80 million were made homeless. The Yellow River has even been used as a weapon of war. In 1642 about 900,000 people died when the author-

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ities putting down a peasant revolt opened dikes in order to flood the city of Kaifeng. Kuomintang forces opened the dikes in 1938 during the Sino-Japanese war to halt a Japanese advance on Chengchou, but on that occasion the river flowed out of control, submerging more than 9,000 square miles (233,100 km2). The flood caused 500,000 deaths, and 6 million people lost their homes. The Yangtze is also prone to overflowing its banks. There were more than 1,030 major floods between 206 B.C.E. and 1960 C.E., and there has been a catastrophic flood every 50 to 55 years. More than 3.7 million people died when the Yangtze flooded in 1887, mainly because of the famine that followed the destruction of crops. The river flooded again in 1954, causing a famine in which 30,000 people lost their lives, and it overflowed in 1981 and 1983. When it burst its banks in 1998, the floods lasted from June to August. That flood affected an estimated 230 million people and cost 3,656 lives.

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Although Egypt and Sudan have a desert climate, the lands to the east and south, through which the Nile flows, have abundant rainfall. There have been years when the Nile failed the Egyptian farmers, however. Whenever that happened many people went hungry. In order to avert famine, engineers set out to regulate the flow of the river, and they did so by constructing dams.

White Nile There are two Nile rivers, the White Nile and the Blue Nile. The White Nile rises on the East African plateau, where two river systems drain water from the region around Lakes Victoria and Albert. Most of the plateau is more than 4,000 feet (1,220 m) above sea level, and the equator crosses it. The rainfall is very high, but so is the rate of evaporation. In Lake Victoria the two almost balance. From Victoria the water flows to Lake Kyoga and from there to Lake Albert. Lake Albert feeds the Al-Jabal River, which flows northward, collecting more water from its tributaries and flowing through swamps and lakes known as As-Sudd. When the Al-Jabal water reaches the town of Malakāl, in southern Sudan, the river becomes known as the White Nile. The flow from the Al-Jabal supplies about half the water that flows into the White Nile. The other half comes from the highlands of southwestern Ethiopia. The Ethiopian water is carried mainly by the Rivers Baro and Pibor, which unite to form the River Sobat. For about 200 miles (320 km) the Sobat crosses a level plain, joining the Al-Jabal at Malakāl. Summer rains over the East African plateau and the Ethiopian highlands supply all the water entering the White Nile, but the lands that the rivers cross act like a sponge, absorbing water and releasing it later. The Al-Jabal flows through the As-Sudd and feeds water into the White Nile at a fairly constant rate. The heavy rains feeding the Sobat begin in April, and from July to October the river floods the Ethiopian plain. After the rains have ceased water draining from the plain keeps the river level high until December. This seasonal variation in the level of the Sobat is the main cause of the changing level in the White Nile, but the White Nile flows strongly throughout the year. When the level in the main Nile is at its lowest, about 80 percent of its water is derived from the White Nile.

Blue Nile The Blue Nile differs from the White Nile in that its waters are not delayed by swamps or flooding. This also makes seasonal variations in the river level more extreme. During the Nile flood about 70 percent of the water comes from the Blue Nile.

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South of Lake Tana, in northern Ethiopia, water flowing from a spring near Mount Amedamit becomes the Little Abbay River. Abbay is the Ethiopian name for the Blue Nile and on November 4, 1770, the Scottish explorer James Bruce (1730–94) identified the spring feeding the Little Abbay as the source of the Blue Nile. It flows northward for about 85 miles (135 km), and it is the biggest river flowing into Lake Tana. Its reputation as the source of the river has endured, although the lake itself is the reservoir from which water flows to the Blue Nile. Lake Tana is the largest of the many lakes in Ethiopia and is 6,000 feet (1,800 m) above sea level. Its surface area is 1,418 square miles (3,673 km2), but it is shallow. Nowhere is the water more than 45 feet (14 m) deep. The lake drains an area of 4,500 square miles (11,650 km2). The Abbay (its Arabic name is Al-Bahr al-Azraq—al-bahr means “the river”) flows out of the lake through a series of rapids and then drops into a gorge that in some places is 4,000 feet (1,220 m) deep. The river flows through a deep canyon and then heads northwestward through Sudan. Two tributaries, the Ar-Rahad and Ad-Dindar Rivers, join the Blue Nile in Sudan. These also rise in Ethiopia, and between them they contribute much more of the water carried by the Blue Nile than does Lake Tana, although they cease to flow during the dry season. Other important tributaries join the river in Sudan. At Khartoum the Blue and White Niles merge. They are then simply the Nile, or in Arabic El Bahr en Nil.

Atbara The Blue Nile contributes about four-sevenths of the water in the Nile, the White Nile accounts for about three-sevenths, and the remaining one-seventh comes from the River Atbara. Like the Blue Nile, the third major component of the Nile rises in Ethiopia, to the north of Lake Tana. It flows for about 500 miles (805 km), receiving water from two important tributaries, the Angareb and Satīt Rivers, and joins the Nile 200 miles (322 km) north of Khartoum at the town of Atbara. It floods at the same time as the Blue Nile, but during the dry season the Atbara is reduced to a chain of pools. There are farms in some places along a narrow strip to either side of the Nile from Khartoum to the north of Aswān, which is in Egypt. The Nile is navigable from the Mediterranean coast as far south as Aswān and from Aswān almost to Wadi Halfa, in the far north of Sudan, close to the Egyptian border—and in the ancient kingdom of Nubia. Wadi Halfa is six miles (10 km) south of a waterfall. Between Aswān and Khartoum the Nile is interrupted by a series of six such waterfalls, known as the cataracts, and Wadi Halfa is close to the second cataract (the first is at Aswān). This stretch of the river is navigable only between the cataracts.

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Flood Irrigation The farms bordering the river are irrigated by Nile water. North of Aswān the irrigation system is more elaborate, and the cultivated area widens, eventually to a maximum width of 12 miles (19 km). Until about the beginning of the 20th century, all irrigation water was supplied by the Nile flood, as it had been since the days of the pharaohs. The cultivated land lay below the level that the river surface reached during the flood. Earth embankments built at right angles to the river and extending as far as the edge of the desert divided the land into basins ranging in size from 2,000 to more than 50,000 acres (800–20,000 hectares). Short canals, sealed by dikes for most of the year, led from the river to the basins. During the flood the dikes were opened, and the canals carried water that inundated the land and deposited some of the mud the river was carrying. The Blue Nile and Atbara had carried the mud, and it settled as a layer of richly fertile silt that grew thicker each year. As the flood subsided the water drained back to the river below ground, and the seeds were sown into the wet soil. This system of flood irrigation was still being used as late as the 1960s on about 700,000 acres (283,290 hectares) of land. The method works especially well in Egypt because the land slopes from south to north at a gradient of about five inches every mile (8 cm/km), and it also slopes away from the river banks, so water overflowing the bank moves away from the river, not back toward it.

Predicting the Flood In May the river level in the Blue Nile begins to rise markedly at Khartoum. That is when water from the heavy Ethiopian rains reaches the junction of the two Niles, and the level continues to rise until the end of August or the first week in September. The total rise averages 20 feet (6 m). After that the level in the Blue Nile starts falling again. While the Blue Nile is in flood, however, it blocks the flow from the White Nile. The White Nile overflows its banks and floods land to the south of Khartoum, where the Jabal al-Awliyā’ Dam adds to the ponding effect. When the Blue Nile flood abates, water from the White Nile maintains a high water level for some time longer. Riverside communities must know when to have their land ready and prepared to receive the flood, and they need to know when it is time to open the dikes. This is not always obvious, because the river level fluctuates, the flood level varies from year to year, and the flood does not arrive on a precise date. What appears to be the start of the flood may not be. If the dikes were opened early, water would flood the land and drain away, and seeds might be planted only to be destroyed a week or two later when they were buried beneath a deep layer of mud.

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Historically, therefore, predicting the size and timing of the flood was of great importance. The two are related, because the higher the flood level, the faster the flood water travels downstream. The flood takes between 10 and 35 days to cover the 1,540 miles (2,478 km) from Roseires, south of Khartoum on the Blue Nile, to Aswān, for example, depending on the river level, and until modern times no one understood the cause of the annual floods. To help them predict the flood the ancient Egyptians invented an instrument called the nilometer that measured the river level very accurately (see “The Nilometer” on pages 190–191). Some nilometers have survived to the present day. The nilometer at Aswān, on an island known in ancient Egyptian times as Elephantine, was restored in 1870. The Egyptians kept the nilometer records, and one series has survived covering the years from 622 c.e. to 1522 c.e. This record is from the nilometer at Roda Island, in Cairo.

When the Flood Failed to Arrive Predicting the flood did not control it, of course. The nilometer record from Roda Island shows that in some years the flood level was high, in other years it was low, and sometimes the flood failed to arrive. High or low flood levels often continued for several years in succession. High floods might overflow the basins and damage buildings and stores. Low floods might not fill the basins completely, so the harvest would be poor. When the flood failed entirely famine was probable. It was all very unsatisfactory, and dependence on flood irrigation had a further disadvantage: It allowed only one crop a year to be grown. As the Egyptian population increased this was no longer sufficient, and from about the middle of the 19th century flood irrigation began to be replaced by perennial irrigation.

Damming the Nile Perennial irrigation had always been practiced in those places where water could be lifted from the river by means of a shaduf or similar device (see “Irrigation and the Shaduf ” on page 191), but its use needed to be extended. To achieve this it was decided in 1843 to build a series of dams across the river about 12 miles (19 km) downstream from Cairo, at the head of the delta. By holding water back, the dams would permanently raise the river level upstream, allowing river water to be fed into irrigation channels whenever it was needed. This would allow farms to grow two or even three crops each year. The scheme was completed in 1861. Another dam was added in the delta in 1901, and in 1902 the Asyūt Barrage— barrage is another word for dam—was built near the town of Asyūt, about halfway between Cairo and Aswān. Later two

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more dams were built upstream of Asyūt. The dam at Isnā was built in 1909, and the one at Naj’ Hammādī in 1930. A dam was completed in 1925 at Sannār, on the Blue Nile in Sudan. This made perennial irrigation possible on the land south of Khartoum between the Blue and White Niles.

The Aswān Dam In 1902 a much bigger dam was completed at Aswān. The first Aswān dam has four locks to allow ships to pass, and it was enlarged twice, between 1908 and 1911 and again between 1929 and 1934. After the second enlargement the dam had a granite wall 1.5 miles (2.4 km) long with 180 sluices to allow surplus water to escape. At the time it was one of the biggest dams in the world. The Aswān Dam also houses a hydroelectric plant with an installed generating capacity of 345 MW that came into operation in 1960. The lake behind the dam extended upstream for 150 miles (240 km). As the water level rose toward the flood peak the sluices were opened, allowing water to flow through the dam, taking its silt with it. When the flood peak passed the sluices were closed, and water in the lake was used to irrigate crops through the dry season.

The Aswān High Dam Work on a much bigger dam began in 1960 four miles (6.4 km) upstream of the earlier Aswān Dam. This was the Aswān High Dam. Designed by West German and Soviet engineers, its purpose was to allow perennial irrigation throughout the whole of Egypt. The project cost $1 billion, it was completed in 1970, and the dam was inaugurated by President Sadat at a ceremony held on January 15, 1971. While the dam was being built the region behind it had to be cleared in preparation for being flooded. About 90,000 Nubians living there were resettled. Those living in Egypt were relocated about 28 miles (45 km) away, but those living in Sudan were moved up to 370 miles (600 km). The Nile Valley behind the dam contained archaeological sites of major international importance. Artifacts from those sites and 19 of the most important monuments had to be moved to safety. The biggest and most difficult to move were the two temples of Abu Simbel, also called Abū Sunbul, built to honor Ramses II, who reigned from 1279 to 1213 b.c.e. The temples were carved out of the sandstone cliff on the western bank of the river, and in front of them there were four statues of Ramses in a seated position, two on either side of the entrance to the Great Temple. These statues are 67 feet (20 m) high, and there were smaller figures around their bases representing Ramses’s queens, including Nefertari and their children. The temple itself comprised three halls extending 185 feet (56 m) into the cliff. The smaller temple had two statues, each 35 feet (10.5 m) high, of the king

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and queen. Between 1963 and 1967 scientists and engineers from many countries supervised a team of workers who cut away the cliff, dismantled the temples, and moved the statues. The rescue operation was sponsored by the Egyptian government and the United Nations Educational, Scientific, and Cultural Organization (UNESCO) and paid for by 50 countries. The temples and statues were reassembled nearby on ground 200 feet (60 m) above the riverbed. The dam wall is rock-filled, and its volume is 17 times that of the Great Pyramid at Giza. The dam is 364 feet (111 m) high, is 3,280 feet (1,000 m) thick at its base, and at its crest is 12,566 feet (3,830 m) long. Inside the dam there is a hydroelectric plant with an installed capacity of 2,100 MW that supplies nearly half of Egypt’s electricity. Behind the dam lies Lake Nasser, or, to give it its Arabic name, Buheiret Nāsir, named after Gamal Abdel Nasser (1918–70), who was president of Egypt when the work began. Lake Nasser extends upstream for 310 miles (499 km)—125 miles (201 km) into Sudan—and averages six miles (9.6 km) in width. In 1959 Egypt and Sudan agreed on the maximum amount of irrigation water each country is allowed to draw from the lake each year, Egypt receiving three times more than Sudan. About one-quarter of the total capacity of the lake is held in reserve in anticipation of the highest flood likely to occur in a century. The lake is designed to hold enough water to provide for the needs of both Egypt and Sudan during the most severe drought likely in a century.

Advantages and Disadvantages Since the Aswān High Dam was completed and Lake Nasser filled, Egypt has been fully protected against damaging floods, has enjoyed a reliable water supply for all purposes, even during prolonged drought, and has had a reliable power supply. These advantages greatly outweigh the disadvantages, but there have been disadvantages. The project was controversial from the moment it was proposed. The lake has raised the water table beside the river. In some areas this has caused problems of waterlogging and salination (see “Aquifer Depletion, Waterlogging, and Salination” on pages 184–189). The reduced river flow downstream of the dam has led to the incursion of salt water from the Mediterranean into the delta, resulting in the salination of some delta soils. Water downstream of the dam carries no silt. Egyptian farmers now have to buy fertilizers to replace the nutrients formerly delivered by the flood. Previously, silt deposited in the delta protected the coastal region from erosion. Its loss has led to increased coastal erosion. Changes in the water discharged into the Mediterranean have increased the salinity there. This, combined with the loss of nutrientrich silt entering the sea, has reduced the fish population, with adverse consequences for the fishing fleets working in the eastern Mediterranean.

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Management of the Desert ■

DIVERTING RIVERS

It is the Nile that made the Egyptian civilization possible, just as the Tigris and Euphrates Rivers made possible the civilizations of the Fertile Crescent (see “The Fertile Crescent” on pages 181–182) and the Indus sustained the cities of Mohenjo-Daro and Harappa (see “The Indus Valley” on page 175). By providing a fairly reliable supply of water for irrigation these rivers made the land fertile. Large urban populations could be fed, and it is the concentration in cities of large numbers of people, most of whom are not engaged in food production, that triggers the emergence of civilization. Rivers can make deserts habitable, and nowadays it is sometimes possible to remedy a lack of rivers. If a river flows close to a desert, engineers are able to redirect its course, diverting it to where its water is needed. Unfortunately, the consequences of doing so can be catastrophic.

Ob and Irtysh In September 1949 the Soviet Union detonated its first atomic bomb. Soon after that the chief Soviet delegate to the United Nations made a speech to the General Assembly in which he said that in order to further the development of his country’s economy, atomic power would be used to move mountains and change the courses of rivers. He was referring to an article by V. O. Obruchev that had appeared recently in a geological journal published by the Academy of Sciences. In it Obruchev suggested diverting rivers that flow into the Arctic Ocean so they could provide irrigation water to the deserts of Soviet Central Asia. To the west of the River Yenisei and to the north of a ridge running east and west across Kazakhstan, approximately along a line passing to the north of Karaganda and south of Magnitogorsk and Moscow, lies one of the flattest plains on Earth. It is called the Western Siberian Lowland, and it slopes very slightly from south to north, falling no more than about two inches every mile (2.4 cm/km). Nearly 2,000 rivers drain the plain, the most important being those of the Ob and Irtysh systems. These drain northward into the Kara Sea, which is part of the Arctic Ocean. The map shows their location. As winter commences, water at the mouth of the Ob freezes. Farther south the rivers are not yet frozen, so the water piles up behind an ice dam. Then the rivers freeze upstream. In spring the thaw sends vast torrents of water into the two rivers. This water is still held back by the ice dam, and it floods a large area of the flat, low-lying land near the coast. Finally, the ice dam breaks, and all this freshwater flows into the Arctic Ocean. To the south of the Irtysh and Ob, near where the River Tobol rises, there is a gap in the ridge that separates

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the Kirgiz Steppe from the Western Siberian Lowland. The Russian plan was to dam the Irtysh and Ob. That would flood an area of lowland to produce a lake, and a canal 1,500 miles (2,400 km) long would be constructed to carry water through the gap and into the River Volga.

Arctic Water to Irrigate the Desert The scheme would achieve two aims. The canal would bring water to irrigate the arid steppe and desert land to the north of the Caspian Sea, and the increased flow in the Volga would replenish the Caspian Sea, the level of which had been falling for years. That fall was attributed to the excessive use of water taken from the Volga to irrigate farmland. Those were heroic days in the Soviet Union, when children were taught in school that they must grow up to discover and conquer their country. It would become truly theirs, the textbooks told them, when columns of tractors drawing plows advanced to break soil that had never been cultivated. Diverting two mighty rivers was a project on an appropriate scale that would help to increase Soviet agricultural output. Work was started on the diversion, and although it was scaled down in 1953, the operation was still being defended as late as 1985, when Nikolai Basilyev, the minister for land reclamation and water resources, said the work was essential and would continue. It was not until 1991 that the scheme finally died with the collapse of the Soviet Union. The plan had always been controversial. No one doubted that the diversion would bring water to the arid lands to the south, but there were fears of what might happen in the north. The Ob-Irtysh River system carries freshwater from the south into the Arctic Ocean. Without this flow the water in the partly enclosed Kara Sea would be cooler than it is. The ice that covers it in winter might persist for longer into the spring, its white surface reflecting incoming sunlight and delaying the spring warming over a much wider area. There might be a marked effect on the climate throughout the Arctic. Soviet planners maintained that this had been taken into account. Their calculations showed the effect would be too small to be of any importance, but doubts remained. There were also concerns about the ecological consequences of diverting so much water. The Kara Sea would become saltier, which would affect marine organisms, and freshwater plants and animals would disappear from the rivers to the north of the dam.

Cotton from Central Asia and the Aral Sea In the end water from the Ob and Irtysh never did reach the cotton fields in the parched lands of the south. Cotton had been grown in that part of Central Asia for a long time, but

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DESERTS

B A R E N T S S E A Murmansk

K A R A

L.Tajmyr

S E A

Cheta

Divina

Pech ora

Archangelsk

PECHORSKIY

Noril'sk

BASSEYN

CENTRAL SIBERIAN P L AT E A U Ob

Pur

Suchona

Nizn'aja

Yenisei

Taz

Pod

R U S S I A

kam

Kirov

ennaya

Kazan

Kama

Nizhniy Tagil

Perm

Ir t y sh

Vakh

WESTERN SIBERIAN

Yekaterinburg

Ufa

Ket

L OW L A N D

Kurgan

ra Anga Tomsk

Chelyabinsk

Ural

Tob ol

Magnitogorsk Orenburg

Krasnoyarsk Petropavlovsk

Omsk

Om' Ob Novosibirsk

Kemerovo

Rudnyy Barnaul

Irty Aqmola

Karaganda

© Infobase Publishing

i

Semipalatinsk

K A Z A K H S T A N Aral Sea

Jenise

KIRGIZ STEPPE

sh

Ishim

Abakan

Novokuznetsk

Ust'-Kamenogorsk L. Zajsan

CHINA

The Ob and Irtysh basin in western Siberia

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Management of the Desert in the 1920s the Soviet authorities decided that the country should become one of the world’s leading cotton exporters. The farms expanded, and the plan succeeded. Within a few years the Soviet Union was one of the top three cotton traders, along with the United States and China. After about 30 years the irrigation system that had been installed to serve the cotton fields was running short of water. None was available from the northern rivers, so instead, in the 1960s the cotton crops began to be irrigated with water taken from the Rivers Syr Dar’ya in Kazakhstan and Amu Dar’ya farther south in Uzbekistan. The map shows their location. Mile after mile of canals led from these rivers into the fields—and the canals were unlined, so much of the water soaked away. So far as the farmers were concerned it was lost. Until they were diverted to supply the farms, these two rivers fed water into the Aral Sea. Their diversion has produced one of the greatest environmental catastrophes ever recorded, with appalling damage to communities and to the health of local populations. The Aral Sea derives its name from Aral-denghiz, also spelled Aral Tengizi, which means “sea of islands” in the Kyrgyz language. A thousand islands more than 2.5 acres (1 ha) in extent once dotted its surface. In 1960, before the river diversion began, the Aral Sea was the fourth-largest lake in the world, with a surface area of 26,300 square miles (68,000 km2). For comparison, the surface area of Lake Huron is 23,000 square miles (59,570 km2). The sea measured a maximum of nearly 270 miles (435 km) from north to south and a little more than 180 miles (290 km) from east to west. It is shallow. In 1960 the average depth was 53 feet (16 m), although it was 226 feet (69 m) deep in some places on the western side. Several large bays formed indentations along the northern shore. On the northern part of the eastern side the Syr Dar’ya formed a huge delta where it met the sea, and the Amu Dar’ya formed a delta of similar size on the southern side. To the west the sea faced the high Plato Ustyurt (Ustyurt Plateau). The Aral Sea is 175 feet (53 m) above sea level, and the climate is that of a continental desert. In summer the water temperature averages 73°F–77°F (23°C–25°C). In winter the surface freezes. The amount of water evaporating from the surface each year was approximately equal to the amount entering from the two rivers. All rivers carry some salt. This entered the sea and was left behind when water evaporated. Consequently, salt accumulated in the sea over millions of years, raising the salinity to about 10 parts per thousand ‰. The water was brackish but much less salty than seawater, which has an average salinity of 35 parts per thousand ‰. Once water from the rivers was diverted into irrigation channels, the supply to the sea was reduced. Evaporation continued as formerly, of course, and so inevitably the sea began to shrink. Because it was so shallow over most of

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its area, reducing the volume of water also reduced the area the water covered. What was once a single sea is now two, the Large Aral Sea in the south and the Small Aral Sea in the north. Since 1960 the sea has lost 80 percent of its volume. Scientists have calculated that if it continues to shrink at the same rate, by about 2010 the Aral Sea will be reduced to three small “Aralet” lakes. This is not the first time that the Aral Sea has been reduced to two small seas, the North and South Aral Seas, and the waters of the Amu Dar’ya and Syr Dar’ya have traditionally been used for irrigation. Previous changes in the area of the sea were due to climatic and environmental changes, and in 300 c.e. the total area of the two small seas was approximately the same as it is today. The towns of Aral in the north and Mŭynoq in the south were once ports. Mŭynoq used to be a busy town with a population of 25,000. As well as a fishing fleet, it had a fish cannery processing 25,000 tons (22,700 t) of fish a year and a tourist industry. Visitors came for the beaches and the swimming and to take advantage of the therapeutic properties of the salt waters. Today the seashore is about 43 miles (69 km) to the north of Mŭynoq, and it is still retreating. Fishing boats lie rusting in the sand. The cannery is still there, but now it processes no more than about 1,600 tons (1,450 t) of fish a year. There is very high unemployment. Up to 100,000 people have left the area. As the Aral Sea shrank, so its salts became more concentrated, and it is now much saltier than the ocean. Most of its fish have been killed. Salt from the dried-up bed has been blown by the wind and now covers a large area. The soil, too, has turned salty. Cotton growers were allowed as much water as they wanted. They used it wastefully, and this, combined with seepage from the unlined canals, raised the water table. Water could no longer drain freely, and with the high surface evaporation salt accumulated. The soil now contains about 280 tons of salt on every acre of land (628 t/ha), and salt contaminates the drinking water. Cotton is still grown, but yields are falling. They are now less than one-third of those in Israel, a country with a very similar climate. There are frequent dust storms, but these are not ordinary desert storms. Mixed with the dust there is salt from the sea and also fertilizer and pesticides that were spread and sprayed onto fields, then dried to a powder. The combination makes the dust poisonous. In the part of Uzbekistan close to the sea there were 167 deaths from respiratory illnesses for every 100,000 of the population in 1993. This is one of the highest levels in the world. Tuberculosis is common. In some towns there are 400 cases for every 100,000 people. Many people are anemic, including most pregnant women, and infant mortality rates are high.

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DESERTS

Yrghyz Dzhezkazgan

K

A

Z

A

K

H

S

T

A

N

CCAAS SP PI AI ANN L LOOWWL LAANNDD Syr Darya Kzyl-Orda Aral Sea

P L AT O U S T Y U R T

Muynoq Altynkul’ Chimbay

K

Y

Z

Y

L

K

U

M

Nukus Dashhowuz

U Z B E K I S T A N Urgench

Amu Dar

Nawoiy

ya

Turkmenbashi

Darvaza

Bukhara Nebitdag

K A R A K U M

D E S E R T Chardzhou

Gyzylarbat

T U R K M E N I S T A N

Turkmenskiy Turkmenskiy Bay © Infobase Publishing

Atrak

Ashgabat

The Aral Sea

It was because of the plight of the Aral Sea that Nikolai Basilyev believed it essential to divert the Ob and Irtysh. Their water was needed to flush the salt from the soil and replenish the sea. That plan was abandoned, and if the sea is to recover the process will now be a very long one.

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Karshi

Karakum Canal

Kerki

Preventing or even reducing leaks in the irrigation system would mean less water was needed from the rivers and at the same time would allow the water table to fall. Then—but it would probably take decades—salts could be flushed from the land. Scientists and engineers from many countries are working on solutions. The Global Environment Facility (GEF) helps with the task of restoring the Aral Sea and the surrounding land. Established in 1990, the GEF is managed by the World Bank,

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✦

Management of the Desert which controls two-thirds of its funds. The United Nations Development Program (UNDP) controls the remaining one-third of GEF funds. In 1994 the heads of state of five Central Asian countries agreed to a plan to save the sea, and in 1998 the GEF helped set up the Water and Environmental Management in the Aral Sea Basin Project. The 4.5-year project is managed by an agency working under the auspices of the Republic of Uzbekistan. Its aim is to reduce the amount of water abstracted from the Amu Dar’ya and Syr Dar’ya Rivers by 2.4–3.6 cubic miles (10–15 km3) a year, allowing the sea to accumulate water and restoring the wetlands in the Syr Dar’ya delta.

Las Vegas Paradise

Bullhead City

Palmdale

San Fernando

C A L I F O R N I A Lake Havasu City Parker

San Bernardino Riverside Palm Springs

Los Angeles

Salton Sea

a C O L O R A D O do Brawley D E S E R T

R iver

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Blythe

Color

America has its own example of a river diversion scheme that failed. In 1905–06 engineers attempted to divert part of the Colorado River into irrigation canals. They lost control, and the entire river changed course. For 16 months water poured into the Salton Trough, a salt-covered depression that is part of the bed of a prehistoric lake. Finally, the engineers managed to divert the river back into its original course. Levees were built in 1907 to prevent any more water from flowing in, at which time the lake was about 40 miles (60 km) long, was 13 miles (21 km) wide, and had a surface

ARIZONA

M O J AV E D E S E R T

Diverting the Yangtze

The Salton Sea

Henderson

N E VA D A

SIERRA N E VA D A

Oceanside

The Chinese also have plans to divert a river, the Yangtze. The Yangtze rises in Tibet and flows in an approximately northeasterly direction across southern China to Shanghai. It is China’s longest river and carries a substantial proportion of the water draining from southern and central China. More than half of all the cultivated land in China is in the north, however, where water drains from the land into China’s second-longest river, the Yellow River (Huang Ho). This river also rises in Tibet and flows through Tsinghai Province in northwestern China in a big loop around the Ordos Desert and enters the sea at the Po Hai, or Gulf of Chihli. Lakes in Tsinghai feed rivers that flow into the Huang Ho, but in the 1990s drought dried out many of them, reducing by about 20 percent the amount of water entering the river. In some years the lower reaches of the Huang Ho run dry. The proposed diversion would involve building a canal on the Tibetan Plateau to link the two rivers. At the point where the canal would leave the Yangtze, that river is already large. The other end of the canal would enter the headwaters of the Huang Ho. The effect would be to send water from the Yangtze into the Huang Ho, so less water flowed into southern China and more into northern China. The Chinese diversion has not yet been built, so it is too early to tell whether it proves successful.

279

San Diego

Mexicali

Tijuana

Yuma

SONORAN D E S E RT

M E X I C O Ensenada

© Infobase Publishing

Salton Sea, California

area of about 400 square miles (1,000 km2). Since then the abstraction of irrigation water from rivers flowing into the lake has reduced its size. It now covers about 380 square miles (984 km2), but since the 1960s the volume of inflowing water has exceeded the amount lost by evaporation. The water level has risen. Nowhere is the lake more than 50 feet (15 m) deep. The lake was called the Salton Sea, and it lies in the Colorado Desert about 93 miles (150 km) east of San Diego, California. The map shows its location. It is a lake but is called a “sea” because the salinity of its water is about the same as that of seawater.

Dying Birds and Fish Like the Aral Sea, but on a much smaller scale and for entirely different reasons, the Salton Sea has turned into an ecological disaster. Vacationers used to visit the beaches to enjoy swimming, boating, and camping. Millions of birds spend the winter beside the sea or call there to rest on their migratory journeys, so bird watchers were attracted from miles around. There were edible fish in the sea, providing sport for anglers. Then, in the late 1980s visitors were warned not to eat fish from the sea. These were found to contain unacceptably high levels of selenium, which in large doses can cause liver damage. The water was also found to be contaminated with

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bacteria from sewage entering from a polluted stream flowing northward from Mexico. After that matters went from bad to worse. As different algal species bloom the water changes color alarmingly. Birds began to die. Since 1992 more than 200,000 have been killed by avian cholera and botulism and from causes no one has been able to diagnose. One of the first species to succumb was the eared grebe, also known as the black-necked grebe (Podiceps nigricollis). In 1992 150,000 of them died. No one knows why. A few years later botulism, bacterial food poisoning caused by Clostridium botulinum, killed thousands of American white pelicans (Pelecanus erythrorhynchos) and more than 1,000 brown pelicans (P. occidentalis). Double-crested cormorants (Phalacrocorax auritus) have died from Newcastle disease, caused by a virus. Fish are killed by ammonia and hydrogen sulfide released from rotting plants and animals and by asphyxiation because the decomposition of organic matter removes dissolved oxygen from the water. Fish also suffer because the sea has become saltier—it is now saltier than the oceans, with a salinity approaching 44 parts per thousand.

OASIS FARMING AND ARTIFICIAL OASES

■

Egypt depends heavily on the Nile, but not every Egyptian farmer uses water from the river to irrigate his fields. Far to the west of Aswān farming thrives around Al-Khārijah, a town with more than 40,000 citizens. The town is in the northern part of the Al-Khārijah oasis, and it has existed at least since about 700 b.c.e. Even earlier than that, around 1000 b.c.e., the oasis was a place to which political dissidents were exiled. Since the 1970s more land has been brought into cultivation around the oasis, and the Egyptian government has encouraged farmers from the Nile Valley to settle there. As well as the farms, there are quarries and phosphate mines in the oasis. Controlling rivers is one way to provide water for agriculture, but many desert farms, like those of Al-Khārijah, produce food far from the nearest river. They are located in oases, and that is where about two-thirds of the population of the Sahara live settled lives in permanent towns.

Healing the Salton Sea

How Big Is an Oasis?

Scientists are debating how best to improve the condition of the Salton Sea. The efforts to achieve this are managed by the Salton Sea Authority, but the possible remedies create further problems. It might help if the amount of plant nutrients flowing into the sea could be reduced, but this would be difficult, because it comes mainly from fertilizer used on nearby farms and wastewater from cities. It might be possible to seal off part of the sea. Evaporation would then maintain a constant level in the enclosed area, but the salinity of the water would increase. Pumping water from the open area into the enclosed area would shift salt from one area to the other, steadily reducing the salinity in the open area. Then one channel could be constructed to bring in freshwater and another to allow the salt water to flow into the Gulf of California. There are fears, however, that releasing the salt water might harm a biosphere reserve in Mexico. Diverting rivers to provide water for irrigation seems straightforward, but as other examples have shown, this can lead to serious ecological damage and, in the case of the Aral Sea, direct harm to the health of people living over a wide area. Once the damage has been done, remedying it is difficult, slow, and extremely expensive. That is not to say that river diversions must invariably lead to disaster, but only that they must be planned with great care. Years ago scientists knew too little about the way natural ecosystems function to be able to foresee harmful consequences. Today they understand much more. It may still be impossible to predict in detail what side effects a major scheme will cause, but it is possible to recognize risks that are so serious it would be better not to take them.

Oases vary greatly in size. They are often depicted as no more than a few palm trees grouped around a pool, but this is misleading. It is true that some oases are no more than about 2.5 acres (1 ha) in extent, but others are very much bigger. About 40 miles (65 km) from the coast of the Persian Gulf in Saudi Arabia, for example, the Al-Hasa, or Al-Ahsā’, oasis covers about 30,000 acres (12,000 ha). Siwa oasis, or Wāhat Sīwah, is between four and five miles (6–8 km) wide and about six miles (10 km) long. It is in the far west of Egypt, and there was once a temple there containing an oracle that Alexander the Great consulted.

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Oasis Farms The ancient Egyptians knew the Siwa oasis as Sekht-am, which means “palm land,” because, like most oases, date palms grow there (see “Date Palms” on page 112). Dates are important for food, of course, but that is only one reason for growing them. They also provide shade, which is almost as important. Beneath the date palms the oasis farmers grow olives, figs, peaches, apricots, pomegranates, and citrus fruits as well as cereal crops, especially millet, barley, and wheat. As well as supplying food, the leaves of the date palm are used as fuel, fibers from the leaves are twisted to make ropes or woven into a coarse cloth, and the timber is used for building. Oases also provide food for domesticated animals. There is pasture for cattle, sheep, goats, and camels, and chickens are raised. At the center of an oasis there is usually a pool, and the cultivated land is worked by very labor-intensive methods,

12/6/07 10:25:26 AM

Management of the Desert almost like a series of gardens. In Judaism, Christianity, and Islam, religions that developed in desert lands, paradise is conceived as a garden. The idea of the garden of paradise is based on the gardens around the center of an oasis. These gardens represent islands of tranquility, shade, fruit, and clean water in the midst of the harsh, hostile, unforgiving desert. Where water is plentiful, however, salination often causes difficulties (see “Aquifer Depletion, Waterlogging, and Salination” on pages 184–189). Some oases are surrounded by land contaminated by salt.

Oases and Caravan Routes Caravan routes across large stretches of desert take travelers from one oasis—one garden—to another (see “Caravans and the Silk Road” on pages 200–202). Consequently, in the days before modern rail and air transport many oases were commercially important, prosperous places. Sakākah oasis, for example, is on a caravan route leading from the Mediterranean coast to the interior of Arabia. It lies between the Syrian Desert to the north and the AnNafūd Desert to the south. There is another oasis, Al-Jawf, some distance to the southwest of Sakākah. Atar, in central Mauritania, built around an oasis on a route leading to the capital, Nouakchott, is an important town with a population of more than 20,000 people. It is the capital of Adrar, which is a region of the country, and it has an airstrip. Apart from agriculture Atar is an important center for rug weaving and has a school for rug weavers. As old routes across the Sahara fall from use, new ones are developed. Nafţah, in western Tunisia, lies on one of the routes that are growing in importance. It is built around an oasis located near the shore of a saline lake, with a bigger oasis about 15 miles (24 km) away.

Artificial Oases Oases are not confined to the Sahara. All deserts have them, apart from those deserts in the Arctic and Antarctic. Kattakurgan, in Uzbekistan, is an oasis town with a population of around 60,000. It was founded in the 18th century as a trade and handicraft center. Nearby a modern reservoir holds water from the Zeravshan River. This provides irrigation water and also recreational facilities. Water from the reservoir increases the agricultural output, and so, in a sense, it creates an artificial oasis in the desert. This is not unique. People in the Middle East learned how to make artificial oases long ago, and at one time there were some regions where all the most important towns and villages received their water in this way. The method was probably invented in Iran around 4500 b.c.e., and it spread to desert countries from Egypt eastward to Afghanistan. The system of water management on which the oases are

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based goes by various names. In Arabic it is called qanāt or kanat, in Persian karez, and in the Berber dialect of Arabic foggara.

Qanāt Irrigation Artificial oases are fed by a system of underground channels—qanāts—that convey water from the mountains to lower ground in the desert. No one is quite sure how these channels came to be made in the first place, but work probably began after an experienced individual had identified surface vegetation that suggested the presence of water belowground. This site would be in the mountains and on an alluvial fan, a fan-shaped deposit of gravel and other sediment that forms where the gradient of a stream abruptly decreases. Alluvial fans often occur where a mountain river emerges onto more gently sloping ground, and the water table is often higher there than it is elsewhere. When a promising site had been identified, a test well would be dug in the form of a vertical hole. The workers would dig downward, placing the soil they removed into a basket to be hauled to the surface by a windlass, until they reached the water table. Then they would measure the rate at which water was flowing. If the flow was strong enough, work would proceed to the next stage. The expert would calculate the path for the underground channel, linking the well with the point where the water was needed. Digging would commence at the lower (outlet) end. Tunneling proceeded in a straight line achieved by using lamps placed at intervals for sighting and at a very gentle uphill slope. Typically, the gradient in a qanāt was between 1:500 and 1:1,500. As the tunnel advanced, its depth below the ground surface increased, because the gradient in the tunnel was lower than that of the surface above. The sides and roofs of tunnels often needed support, just like the galleries of a mine. Stone and clay were used for strengthening. Additional vertical shafts were dug at intervals along the route. These provided a means for checking the depth of the qanāt, and they marked the line the tunnel followed. They also allowed air to reach the workers belowground. Eventually, the tunnelers reached and penetrated the water table, and water began to flow back along the channel they had made, which might by then be several miles long. Once one channel had been made, others might be made not far from the outlet, branching from it to distribute water over a wider area. Digging was always done during the dry season, when the flow was at a minimum. This was a safety measure. Heavy rain during the wet season could flood the tunnel and drown the diggers working in it. At the outlets water flowed from the qanāt as a stream of mountain water. It would enter distribution ditches and begin to irrigate the fields. Crops would appear, and an

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oasis would have been made where there had been no oasis in the past.

Qanāts That Are Still in Use No one digs qanāt systems today. They are too expensive, both to install and to maintain, and there are cheaper ways to obtain water. Nevertheless, the fact that several thousand of them are still in use in Iran and Afghanistan shows their reliability. As a means of supplying water the method is distinctly attractive. The water is carried belowground all the way from its source to its outlet. That means there is no loss by evaporation and no opportunity for the water to become polluted. Nor is any power needed for pumping, because the water flows by gravity. Finally, a qanāt delivers a very reliable water supply, and in some cases, if it is properly maintained, it continues to do so for centuries.

CONFLICTS OVER WATER RESOURCES

■

Isaac was a very successful farmer. He was so successful that the Philistines envied him, and he and his followers had to move. For all the wells which his father’s servants had digged in the days of Abraham his father, the Philistines had stopped them, and filled them with earth. And Abimelech said unto Isaac, Go from us; for thou art much mightier than we. And Isaac departed thence, and pitched his tent in the valley of Gerar, and dwelt there. And Isaac digged again the wells of water, which they had digged in the days of Abraham his father; for the Philistines had stopped them after the death of Abraham . . . And Isaac’s servants digged in the valley, and found there a well of springing water. (Genesis 26: 15–19)

Finding water was clearly only the start for people living in desert countries. Once they found water they had to keep it. The dispute over the ownership of wells between Abraham, then his son Isaac, and the Philistine king Abimelech arose when the envious Philistines denied Isaac and his people access to their wells. It was resolved after the king had asked Isaac to move with his people from the coastal plain of Gaza inland to somewhere near Beersheba, in the southern part of the Negev Desert in what is now southern Israel. Isaac agreed, they all moved, and there they found a buried riverbed in which they were able to dig wells.

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Lagash, Umma, and the Jordan River Isaac and his followers were fortunate. Had they failed to find freshwater they might have been compelled to fight the Philistines. On that occasion conflict was avoided, but in about 2500 b.c.e. two city states, Lagash and Umma, located between the Tigris and Euphrates Rivers, went to war over access to water. Disputes continue. Water in the Jordan River basin is shared among Lebanon, Syria, Israel, and Jordan. During the 1967 Six-Day War Israeli forces attacked a dam built by Jordan and Syria on the Yarmūk River. This is a tributary of the Jordan, joining it a little way south of the Sea of Galilee, and it forms the border between Syria and Jordan. The dam had previously supplied hydroelectric power to Jordan as well as water for irrigation. Although the Gawr irrigation canal, completed in 1966, carries water from the Yarmūk to part of northern Jordan, Jordan is chronically and severely short of water despite having one of the lowest water consumption rates per person of any Middle Eastern country: 1.3 gallons (5 l) per day. The war left Israel in control of most of the basin and headwaters of the river, and Israel now takes so much that Jordan does not receive enough for its needs. Jordan has not gone to war specifically over water, but in 1975 Syria and Iraq came close to it. Syria and Turkey had built two new dams on the Euphrates. As the reservoirs behind them filled, the water level in the river dropped, to the alarm of the Iraqi authorities. Later during the 1991 Gulf War, Iraqi troops were ordered to dismantle desalination plants in Kuwait. Oil spilling from damaged refineries polluted the gulf, and that also damaged desalination plants in Saudi Arabia. In 1985, when he was Egyptian foreign minister, the former UN secretary general Boutros Boutros-Ghali warned that the next Middle Eastern war would be fought over water, not politics. The United Nations has calculated that each individual needs at least 2,640 gallons (10,000 l) of water a year. Each Israeli citizen has only 660 gallons (2,500 l)—one-quarter of the internationally recognized minimum. Others are even worse off. People in the Gaza Strip, for example, have only 137 gallons (520 l) per person. The following table lists desert countries in order of the amount of water available per person, with the 48 coterminous states of the United States included for comparison.

Water That Crosses Frontiers Where water is scarce, a person who uses it denies its use to others. If an aircraft successfully seeds a cloud to make rain fall (see “Cloud Seeding” on page 254), that rain cannot fall somewhere else, which otherwise it might have done. Abstract surface water or groundwater for irrigation, and there will be less for the people living downstream—and

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Management of the Desert

Water Availability per Person per Year COUNTRY

GALLONS

LITERS

Mongolia

36,295

137,390

United States*

28,628

108,370

Namibia

26,975

102,110

Mali

23,274

88,100

Kazakhstan

17,906

67,780

Mexico

12,215

46,240

Mauritania

11,301

42,780

Iraq

8,683

32,870

Niger

8,208

31,070

Afghanistan

7,888

29,860

Nigeria

6,641

25,140

Uzbekistan

5,352

20,260

Iran

5,165

19,550

Ethiopia

4,620

17,490

Eritrea

4,549

17,220

Syria

4,285

16,220

Somalia

4,063

15,380

Lebanon

3,331

12,610

Burkina Faso

2,864

10,840

Morocco

2,565

9,710

Egypt

2,269

8,590

Tunisia

1,273

4,820

Algeria

1,263

4,780

Djibouti

1,255

4,750

Oman

1,025

3,880

Israel

729

2,760

Yemen

589

2,230

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most of the aquifers that supply the rivers and wells straddle the borders of two or more countries. In the dry lands of Central Asia many of the international frontiers are new. The Aral Sea, for example, once lay inside the Soviet Union, so its problems could be addressed internally by a single government. Now the sea lies between Kazakhstan and Uzbekistan, two independent nations. Theirs is only one of the new borders in the region that share water resources. The republics of Kyrgyzstan, Tajikistan, and Turkmenistan could also find themselves drawn into disputes over water.

The Nile and Tigris Egypt depends on the Nile, but it is not the only country to do so. The White Nile flows through Sudan and the Blue Nile through Ethiopia before both rivers enter Egypt as the River Nile, and those countries also use its water for irrigation, potentially reducing the volume downstream. All three countries are already short of water, and farther west so are Tunisia, Algeria, and Morocco. There is competition for the waters of the Tigris, which rises in Turkey and flows through Iraq. Control of the river is based on diverting its water into storage reservoirs to protect against destructive flooding during the rainy season from March to May and to provide water during the dry season, when no rain falls. This requires agreement between the two countries over the amount to store and how and when the store should be used. Some years ago there were plans for huge irrigation schemes fed by water from Lake Titicaca, which lies on the border between Peru and Bolivia. These could have led to conflict, because there was not enough water to maintain a reliable supply for both countries.

Bahrain

478

1,810

“Virtual Water”

Jordan

473

1,790

Saudi Arabia

312

1,180

Libya

298

1,130

Qatar

248

940

United Arab Emirates

153

580

Gaza Strip

137

520

26

100

Despite the risks, competition for water has not been the primary cause of any war for thousands of years. This is remarkable because water is an essential resource for the manufacturing industry and domestic use as well as for agriculture. If there is insufficient water, economies falter and public health deteriorates through malnutrition and an inability to maintain standards of hygiene. Water is a resource for which nations might be expected to fight, yet they do not, even though many have access to less water than their industries and people need. Part of the reason is that during the second half of the 20th century governments, even those of poor countries, have been able to import food they needed but were unable to grow on their own land. A country that feeds its people with imported food reduces the pressure on its own farmers to maintain a high output, which they may be unable

Kuwait *48 coterminous states

Source: Food and Agriculture Organization (FAO). Aquastat 2002.

those people may live across an international frontier in a different country, so that a dispute between neighbors becomes a dispute between nations. In the Middle East

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to do without more water. Water that the farmers might have used can then be supplied to homes and factories. The imported food represents “virtual water”—the water that was used to grow it in another part of the world. In effect, countries in the deserts and semiarid lands are importing water. Americans and Europeans, for example, are sending water to the Middle East. This succeeded because food—and in terms of international trade as well as diet food means cereal grains—was often being sold for less than it cost to produce, and governments were making up the difference by subsidizing agriculture. This generated huge grain surpluses and intense competition for markets among the producing countries. The competition reduced the price so much that in the 1980s grain was being bought and sold at half its production cost. By 1996 grain prices had risen to above production cost. They then dropped back again, reaching an average for all grains of $129 per ton ($117/t) in 1999–2000, after which prices recovered. The FAO expects them to reach $173 per ton ($157/t) in 2009–10. Governments are likely to find the import of “virtual water” in the form of food more expensive in years to come. Producing countries are struggling to reduce their surpluses, which are costly in terms of agricultural support as well as storage. In the European Union (EU) the Common Agricultural Policy is a system of agricultural support and management that generates surpluses. It is by far the biggest item of EU expenditure and is in the process of being reformed. The aim of the reforms is to reduce the cost, which means trying to eliminate overproduction, and that will tend to keep world grain prices above the cost of production.

Agreeing to Share Water It was not until the second half of the 20th century that very large quantities of food began to be traded internationally, so the import of virtual water cannot explain the peaceful resolution of disputes over water prior to that. The answer is probably that such disputes were familiar, easy to understand, and potentially catastrophic. There are many places where it would not be difficult for one country to divert or dam the river on which a neighboring country depended. This would have a devastating effect, and it could happen only if the country seizing the water were so much more powerful than its victim that it could impose its will with impunity. If the two countries were more evenly matched militarily, war would be inevitable. Almost all such conflicts had to be resolved peacefully, and so, by and large, they were. There was fierce argument, ill feeling, and much hurling of insults, but over about the last 4,500 years there have been more than 3,500 international treaties regulating the sharing of water—not far short of one treaty every year. Today such treaties are negotiated through the auspices of the United Nations, especially the UN Environment

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Program (UNEP), which is the UN body charged with coordinating intergovernmental monitoring and protection of the environment. UNEP works by identifying potential conflicts in advance and approaching the governments of the affected countries. It can then call on the appropriate experts to assess the situation and propose ways to resolve difficulties. Despite the historic success in avoiding war, the need has never been more urgent. Water for sustained human use is taken from the world’s renewable freshwater resources (RFWR) or from water entering the atmosphere by evapotranspiration from nonirrigated cropland that condenses and falls back to the surface as precipitation. At present humans withdraw less than 10 percent of the total RFWR and about 30 percent of the water entering the air through evapotranspiration. This suggests that the world has ample freshwater, but the impression is misleading, because the water is not distributed evenly. Some regions of the world have abundant water and others very little, and in many parts of the world the availability of water changes with the seasons. Experts have estimated that in 2006 there were 2.3 billion people—40 percent of the world population—living in areas that were chronically short of water, and they expected this figure to rise to 3.5 billion by 2025. Across North Africa and the Middle East there are 45 million people without an adequate supply of drinking water. Of the 25 countries with the highest proportion of their populations lacking access to safe drinking water, 19 are in Africa. Water can be transported by tanker or pipeline from places where it is abundant to places where it is scarce, but this is costly. Crop irrigation accounts for 90 percent of water use, and most of the remaining water is used by industry and for domestic washing and cleaning; very little is required for drinking and cooking. Water for agriculture and industry must be inexpensive. Pipeline transport is feasible only where the water flows through the pipe by gravity; pumping costs more than users can afford. Tanker transport costs even more. In some places purifying seawater—desalination—is the most practicable alternative.

DESALINATION AND MINING ICEBERGS

■

Near Cape Comorin, in the state of Tamil Nādu, at the southernmost tip of India, is a town called Kanniyākumāri. It is a tourist resort, one of the most popular in all of India. It is also a center for pilgrims wishing to visit the temple to Shiva and to celebrate the fact that according to legend Kanyā Kumāri, a goddess, killed a demon there. It is a popular place and often crowded, with many hotels and rest houses to accommodate the thousands of visitors who arrive every day to swell the resident population of about 25,000.

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Management of the Desert Visitors to Kanniyākumāri use water. They can buy it by the bucket or bottle in shops, to which it is delivered every day by fleets of tankers. In the countryside round about, countless wells have been drilled into the groundwater, and pipelines carry the water to the hotels and rest houses in the town. The land is fertile, but farmers have learned that water earns them more money than growing rice or vegetables. Their wells are lowering the water table, and in some places seawater is infiltrating the aquifers. There are no mountains to trap rain in the southern part of Tamil Nādu, and village women have to walk miles to obtain dirty and often muddy water from small rivers and ponds. They filter it through cloth, but it is all they have for drinking, cooking, and washing.

How Distillation Purifies Water When a chemical substance dissolves in water (H2O), its atoms attach themselves to either the positive hydrogen (H+) or negative hydroxyl (OH-) parts of the water molecules. Common salt, for example, is sodium chloride (NaCl). In water the salt molecule separates into Na+ and Cl- ions, which attach themselves to hydroxyl and hydrogen ions, respectively, and the water becomes a salt solution. Evaporation and freezing involve changes in the structure of the water. Water evaporates when its molecules gain sufficient energy to break the hydrogen bonds that link them together in small groups. The energetic molecules then escape into the air, but it is only the water molecules that escape. Any substance dissolved in the water is left behind. In the case of salt, the Na+ and Cl- rejoin, forming NaCl. Then, if there are free H+ and OH-, the salt dissolves once more. Water evaporates as pure H2O, and the remaining solution becomes increasingly concentrated. Freezing has a similar effect. As the temperature falls the water molecules link more tightly together with hydrogen bonds, forming a crystalline structure, ice, that leaves no room for the dissolved substance. Ice is pure water, even when it is frozen seawater. Distillation is a process that evaporates water from a solution, then passes the water vapor through a condenser. This is a chamber that is chilled so that the temperature of the water vapor passing through it falls low enough to cause condensation. The condensed water is then removed. The condensed water may not be entirely pure after a single distillation, and converting seawater to water that is drinkable usually requires several distillations.

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Kanniyākumāri lies at the tip of a triangle with water on two sides, but it is seawater. Seawater can be rendered drinkable, and some years ago the Indian government built 14 desalination plants to purify seawater about 250 miles (402 km) to the north. These were not properly maintained, however, and only one of them is still working. People in Tamil Nādu are working to install filters to clean local water sources and reservoirs to collect rainwater—nearly 80 percent of the 38 inches (965 mm) the state receives each year flows directly back into the sea. One day it is likely they will have to repair their desalination plants, but for the time being it is much cheaper to capture rainwater. Desalination is expensive, so it is used only where no cheaper alternative exists, but the price is falling. Distillation has been known for thousands of years (see the sidebar). Both Aristotle and the physician Hippocrates described the process by which sailors in ancient Greece were using distillation to obtain drinking water from seawater in the fourth century b.c.e., and in the eighth century c.e. an Arab writer published a more detailed account of the process. In 1869 the British built the first modern distillation plant at Aden, in Yemen. It supplied freshwater to ships calling at the port.

Desalination Plants Today there are approximately 15,000 desalination plants in more than 125 countries, and some plants are huge. The one at Al-Jubayl, in Saudi Arabia, produces 1.2 billion gallons (4.7 billion liters) of water a day—more than half the world total. About 75 percent of all desalted water is produced in the Middle East, about 10 percent in the United States, about 5 percent in Africa, and about 5 percent in Europe. The following table lists the amounts of desalted water produced each year. Not all plants are large or located in arid regions. There is a desalination plant on the Isles of Scilly, off the coast of southwest England. It is needed because although the islands receive abundant rain, they are too small to have reservoirs to store enough water for their residents and summer vacationers and are too far from the mainland to have water piped to them. Desalination is especially appropriate on offshore islands where it is impossible to capture enough rainwater in reservoirs. It has also been proposed for parts of mainland England, where demand for water threatens to exceed supply and there are land-use or environmental objections to building large reservoirs.

Multistage Flash Evaporation There are several ways to separate water from the salt dissolved in it. The most widely used method is multistage flash evaporation, illustrated in the diagram. The Al-Jubayl

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Annual Production of Desalted Water, 1992–2000 COUNTRY

MILLION GALLONS

MILLION LITERS

Bahrain

263,380

997,000

Djibouti Egypt Iran Israel* Kazakhstan

0.26 66 7.7

1 250 29

832

3,150

3,508

13,280

Kuwait

610

2,310

Libya

185

700

Malta

83

314

Mauritania

4.5

17

Morocco

9

34

Oman

90

340

Qatar

260

986

1,886

7,140

Saudi Arabia

the next chamber, where the pressure is a little lower than it was in the previous one. Again, the water flashes, and vapor condenses. The process is repeated several times, with the pressure in each chamber lower than that in the preceding one so the water flashes in each chamber. Fuel has to be burned to heat the incoming seawater, but as the pipe carrying the incoming water passes through the flash chambers the water inside it is warmed. This preheating reduces the amount of fuel that is needed. There are several alternatives to multistage flash evaporation, the most widely used of which is called the longtube vertical distillation process. Again, the plant contains a series of chambers. Steam is used to heat and vaporize salt water contained in long, vertical tubes. The vapor condenses and is carried away to a storage tank, and the steam is used to heat salt water in the next chamber. Flash evaporation uses a great deal of energy. Typically, a flash evaporation plant uses six to 10 kWh of electricity to produce one gallon of freshwater (1.5–2.5 kWh/l). This makes it the most costly desalination technique, but it remains the most popular in the Middle East because it is better than any alternative process at dealing with the salt water of the Persian Gulf.

Somalia

0.26

1

Sudan

1

4

Tunisia

22

83

Distillation by Freezing

5

Freezing also separates water from substances dissolved in it. Distillation plants based on this principle also rely on the fact that latent heat is released or absorbed when water vaporizes, condenses, and freezes. In one type of plant seawater chilled almost to freezing is sprayed into a chamber where the pressure is very low. Some of the water vaporizes instantly because of the low pressure, absorbing the latent heat to do so from the water around it and thereby lowering the temperature of that water enough to freeze it. A mixture of salt water and ice falls to the bottom and is piped to a second chamber, where the salt water is drained off, leaving the ice. The vapor from the vacuum chamber is compressed and fed into the second chamber. Compression causes it to condense. It is freshwater and is used to wash the remaining brine from the ice and to melt it. Alternatively, a refrigerant such as propane or butane is used. Cold seawater enters the first chamber, and the refrigerant liquid is mixed with it. The refrigerant vaporizes, absorbing latent heat from the water and causing ice to form. The ice and remaining brine are separated. The ice is washed in a second chamber, then carried into a third chamber. The refrigerant vapor is piped from the top of the chamber through a compressor, which raises its boiling point, and into the third chamber. There, still contained in its pipe, the refrigerant condenses, giving up its latent heat and melting the ice. From there the refrigerant is returned to the first chamber to vaporize again in a new batch of seawater.

Turkey United Arab Emirates Yemen

1.3 1,017

3,850

26

100

Source: Earth Trends. World Resources Institute; *Includes output from a plant due to commence operation in 2007; *Israel Water Commission, 2005, estimated production for 2010.

plant is of this type. The seawater enters along a pipe (from the right in the diagram) that passes through a series of chambers. This water is cold, so vapor will condense onto the pipes carrying it. After it leaves the final chamber the pipe passes through a heater. There it is heated under pressure to prevent it from starting to boil. In most multistage flash evaporation plants the temperature is raised to about 195°F (90°C). From the heater the water is sprayed into the first of the series of chambers. The pressure in this chamber is lower than that in the pipe, so the water boils instantly—the technical term for instant boiling is flashing, hence the name of the process. Vapor released by flashing condenses on the cold pipe carrying incoming seawater and drips from there into a collector. The condensed water is fresh, and it is piped away to a storage tank. The salt water, which is now slightly saltier because of the freshwater that has been removed from it, is carried to

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p

seawater

brine

freshwater

© Infobase Publishing

Multistage flash evaporation. Seawater enters at the top right and flows through a heat exchanger in each chamber before entering a final heat exchanger where the water is heated. Hot water flows back through the chambers. Water evaporates, and water vapor condenses on the heat exchangers carrying cold seawater. The freshwater collects and is piped away. The residue, a concentrated brine, is removed for safe disposal.

The disadvantage of this method of freezing is that although the ice itself contains only freshwater, salt water becomes trapped between ice crystals. This must be washed out, and washing uses and contaminates an unacceptably large amount of freshwater.

Reverse Osmosis There is also an entirely different method to desalinate water, known as reverse osmosis (RO). Today most new desalination plants use RO. The technique is based on the fact that some very thin membranes have pores that allow certain molecules to pass but not others. A semipermeable membrane allows water molecules to pass but not molecules of substances dissolved in the water. If a semipermeable membrane separates two solutions of different strengths, water will pass through the membrane from the weaker to the stronger solution until the strengths of both solutions are equal. This process is called osmosis, and it is an osmotic pressure that drives water molecules through the membrane.

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Osmosis occurs naturally in living cells, but it can be made to work in the opposite direction if the stronger solution is placed under sufficient pressure. Water will then flow from the stronger to the weaker solution. That is reverse osmosis. Salt water is brought under pressure against a semipermeable membrane. Freshwater flows through the membrane, and the salt remains on the other side. It takes a great deal of pressure to force osmosis to work “the wrong way.” Even so, older RO plants use only about 10 percent of the amount of energy that distillation plants consume, and new plants require only about 2.5 percent. Consequently, RO plants produce water much more cheaply. The Ashkelon Desalination Plant in Israel, which is the world’s largest RO plant, produces water for about $0.20 a gallon ($0.05/l). When it becomes fully operational in 2010 the Ashkelon plant will produce 285 million gallons (1,080 million l) of freshwater a year. Electrodialysis is an alternative method. It uses two semipermeable membranes. Ordinary salt is sodium chloride (NaCl) in which the sodium is positive (Na+) and the chlorine negative (Cl-). With electrodialysis an electric current flowing through the salt water drives positive ions, such as the Na+, through one membrane and negative ions, such as Cl-, through the other. Freshwater is left between the membranes.

Vapor Compression Distillation Clearly, all these processes use a great deal of energy, and the high cost of desalination is due to the fuel that is needed.

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This cost has been greatly reduced for reverse osmosis, and it will fall further as membranes made from new materials are introduced. Sometimes, however, the energy cost can be reduced still more. The vapor compression distillation process uses the condensation of water vapor to boil more seawater. Incoming seawater is heated to about 200°F (93°C), then piped into a chamber where it vaporizes and rises to the top. At the top of the chamber some of the water vapor condenses on the inside of vertical tubes, from where the freshwater is carried away, but at the top of the chamber there is a steam separator. This directs some of the water vapor into a pipe leading to a compressor. Compression heats the vapor, and it is then fed back into the chamber, where it causes more salt water to vaporize.

Solar Power to Distill Water Warm sunshine is one resource that is in plentiful supply in desert countries, and it can be used to power the simplest of all distillation plants. A transparent cover is placed over a tank of salt water with a large surface area. Solar heat passes through the transparent cover, it heats and vaporizes water, and the vapor condenses as freshwater on the underside of the cover. If the cover is slightly inclined the water droplets will run down it to one end, like raindrops running down a window pane, where the water can be collected in a trough. Solar energy can also be used to heat water in a rather more concentrated way by means of a solar collector of the kind sometimes fixed to houses to provide hot water or space heating. The collector is a long tube that passes back and forth between two sheets of material. The lower sheet insulates the tube to reduce heat loss, and the upper sheet is dull black to absorb heat. Water flowing through the tube is heated and can be used to preheat seawater entering a desalination plant.

Membrane Distillation Under ideal conditions solar power alone can supply enough energy to operate a small desalination plant. One design combines distillation with the properties of a microporous membrane. It is known as the membrane distillation (MD) process, and an MD plant running continuously will deliver up to about five gallons (20 liters) of freshwater a day. German engineers invented the process, and an MD plant has been running since 1993 at Ibiza, Spain. The membrane is no more than about 0.001 inch (30 μm) thick, with pores averaging about 0.0004 inch (1 μm) in diameter, and pores cover about 80 percent of the surface. Water vapor is able to pass through the membrane but not liquid water, and the process works at ordinary atmospheric

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pressure—there is no need to compress vapor or create a partial vacuum—and at a temperature of only 176°F (80°C). It is because of the low operating temperature that solar heat or waste heat from a factory or generating plant is all that it needs. The membrane is arranged in a spiral inside a chamber. Hot seawater is in contact with one side of the membrane, and water condenses on the other, cold side. Energy is used to heat the water, but some of the energy is recovered because the latent heat of condensation is used to preheat the incoming water. Desalination is not free from environmental risks. The plants produce drinkable freshwater, but their by-product is a brine that is much saltier than seawater. The brine contains minerals, some of which may be worth recovering, but finally the brine must be disposed of carefully if it is not to cause pollution. As seawater is heated, minerals are precipitated from it as scale on pipes and other surfaces that come into contact with the water. Plant designers seek to minimize scaling, because it threatens to clog pipes and means that the operation must be shut down from time to time while the scale is removed.

Bottled Icebergs Freeze seawater in a desalination plant and salt water remains trapped between the ice crystals. At sea, however, there is ice that is not contaminated in this way. It is sold as bottled water in Newfoundland and is advertised as being “pure as the driven snow.” The claim is not surprising, because the ice is made from driven snow. The Newfoundland ice comes mainly from western Greenland and drifts across the sea in the form of icebergs. Individual Arctic icebergs vary greatly in size, but large ones can be about 600 feet (183 m) in length and 150 feet (46 m) in height, and they float with only one-seventh of their mass visible above the surface. Greenland icebergs form by breaking away from glaciers where these flow into the sea (see “Icebergs” on pages 41–42). Most come from the Illulissat (Jakobshavn) Glacier. Antarctic icebergs form by breaking away from the edges of ice shelves. They are commonly five miles (8 km) long and are usually flat-topped. About 67 cubic miles (280 km3) of ice enter Arctic waters every year as icebergs, and about 430 cubic miles (1,800 km3) break away from Antarctica. This is a vast amount of very pure freshwater and, not surprisingly, people sometimes imagine that icebergs might be towed by ship into lower latitudes, where water from them could be supplied to desert communities. So far no one has attempted this.

Can Icebergs Be Towed? Towing icebergs might be feasible, but the water would be expensive. Icebergs big enough to be worth towing weigh

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Management of the Desert between 100,000 and 200,000 tons (91,000–182,000 t). Only the most powerful ships could attempt to tow them, and once the icebergs were moving it would be extremely difficult to control or stop them. This is not to say it would be impossible, but it would consume a very large amount of fuel. They would have to be towed fairly fast, because icebergs start melting as soon as they enter warm water. Although they can last for years as long as they remain in the Arctic Ocean or Southern Ocean, in water at 32 to 39°F (0–4°C) their height decreases by about 6.5 feet (2 m) a day. An iceberg 394 feet (120 m) long that once drifted into tropical waters, where the temperature was about 80°F (27°C), lasted for only 36 hours before it disappeared completely. All but about 3 percent of the freshwater in the world is contained in the polar ice sheets. If just a little of that water could be relocated, there would be no more talk of water shortages. Unfortunately, it is unlikely to prove practicable. Even if it were, both moored icebergs being mined for their water and seawater desalination plants suffer from one obvious geographic constraint: They depend on proximity to the sea. They may provide water for coastal regions and might even produce enough water for some to be piped inland, but they have little to offer the deserts of Central Asia or the central Sahara, thousands of miles from the nearest coast.

FOOD FROM THE ARCTIC AND ANTARCTIC

■

There is no shortage of water in the polar deserts, most parts of which are covered by sheets of ice thousands of feet thick. Far from making the polar lands more hospitable to plants and animals, however, the ice sheets make conditions worse. Plants can utilize water only in its liquid form—ice and snow are useless to them—and the thick cover of ice makes it impossible for plant roots to find the mineral nutrients that lie in the soil beneath. So, despite all that water, far fewer plants grow in polar deserts than in hot deserts, where plants have easy access to such water as there is. People living in the tundra region to the south of the Arctic desert in Europe and Asia are able to herd reindeer (caribou, Rangifer tarandus), and there is some sheep farming in Greenland (see “Greenland or Kalaallit Nunaat” on pages 42–45). The wide expanses of the tundra resemble the expanses of other deserts and make it possible to manage large herds of animals. Reindeer migrate between summer pastures on the tundra and the edge of the taiga to the south, where they spend the winter, and their herders move with them. Most of the reindeer are kept for their meat, fat, and skins, but some are trained to pull sledges, making

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them the equivalent of pack animals such as the asses and camels used in other deserts. This Arctic way of life is very similar to that of the nomadic peoples of the Sahara, Middle East, and Central Asia. Like them, the reindeer herders live in tents, in their case conical tents covered with reindeer hides. The settled life of the Greenland sheep farmers also resembles that of many desert peoples dwelling around oases far to the south.

Whaling Sheep farming and nomadic pastoralism are not how most Arctic peoples live. In the High Arctic, beyond the tundra, there is no food even for reindeer, but food is nevertheless plentiful in the sea—for those who know how to catch it. There are whales, seals, walrus, birds, and, most of all, there are fish. It is not only the Inuit who hunt in the northern oceans, however. So do fleets of vessels from many other countries. At one time they hunted whales. Some countries still do so, and it is likely that hunting for minke whales (Balaenoptera acutorostrata) will increase somewhat in years to come. Numbers of this species are now quite large. Catching a strictly regulated number will not cause them to decline, although the number of catches that might be permitted is still being calculated. The International Whaling Commission (see the sidebar) estimates there are 120,000 to 182,000 minkes in the North Atlantic, 12,800 to 48,600 in the Northwest Pacific and Okhotsk Sea, and 510,000 to 1,140,000 in the Southern Hemisphere. Minke whales are caught commercially by Norwegian whaling ships and for scientific research by Japanese and Icelandic ships. Until the size of whale populations is known more precisely and scientists understand better the breeding rate of these animals, the International Whaling Commission (IWC) has set a quota of zero for all commercial whaling. Norway formally objected to some aspects of the way the quota was calculated, and under IWC rules it is not bound to obey the effective ban. Norwegian ships are therefore catching whales legally.

Subsistence Whaling In 1986 the IWC suspended all commercial whaling to allow endangered stocks to recover and to allow time for scientists to assess the size and age structure of whale populations. The moratorium allowed Inuit and other Arctic peoples whose way of life depends partly on the use of whale products to catch some whales. This is called aboriginal subsistence whaling, and quotas are set by the IWC. The Alaska Eskimo Whaling Commission supervises whaling in Alaska, where the hunt takes approximately 50 bowhead whales (also known as Greenland right whales, Balaena mysticetus) a year from a population of about

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The International Whaling Commission The International Whaling Commission (IWC) is the world body charged with overseeing the conservation of whale stocks and the orderly development of the whaling industry. It was established on December 2, 1946, in Washington, D.C., under the International Convention for the Regulation of Whaling. The headquarters of the IWC are at Cambridge, England. That is where the 17 members of the permanent staff live and work. The convention sets down measures that govern the conduct of whaling. These include the full protection of certain species, making it illegal to hunt them, and the designation of sanctuaries inside which whaling is prohibited. The convention forbids the killing of calves that have not yet been weaned and of females accompanied by calves, sets limits on the number and size of whales that may be killed, and defines open and closed seasons and sea areas for whaling operations. The convention also requires the operators of whaling fleets to maintain reports of all catches and to collect other biological and statistical data. The IWC exists to ensure compliance with the conditions of the convention. It also encourages and funds research on

8,000. People in the Russian Far East are allowed to take up to 140 gray whales (Eschrichtius robustus) in the eastern North Pacific, where there are 21,900 to 32,400 of them. Greenlanders are allowed to catch around 10 fin whales (Balaenoptera physalus) a year, of which there are estimated to be between 27,700 and 82,000 in the North Atlantic, as well as 150 minkes each year off western Greenland and 10 off eastern Greenland. Canadian Inuit catch a small number of whales, but Canada left the IWC in 1982 and is not bound by the IWC suspension of whaling. There are no data on the number of whales caught. In addition to the whales caught in Arctic waters, a small amount of subsistence whaling is also permitted in the Caribbean and off Indonesia.

Harvesting Krill Such large animals are able to thrive in Arctic and Antarctic waters because of the abundance of smaller animals on which they feed. The most important of these are krill (see “Krill” on page 171). There are several species of krill, the most abundant being the Antarctic krill (Euphausia superba). Its numbers increase and decrease over a cycle, as do populations of lemmings and other small, land-dwelling animals in the Arctic, but for a different reason. Along with many other small aquatic animals, krill feed on the

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whales, publishes research results, and promotes studies of such matters as the humaneness of the methods used to kill whales. Its interests cover all cetaceans—dolphins and porpoises as well as the toothed and baleen whales. Membership of the IWC is open to any country that signs the convention, regardless of whether its citizens engage in whaling or plan to do so. The IWC currently has 77 members. At meetings of the IWC each member state is represented by a commissioner, assisted by expert advisors. The commissioners elect the IWC chair and vice chair. The IWC holds an annual meeting, usually in May or June, attended by all the commissioners. The IWC has four committees: the scientific, conservation, technical, and finance and administration committees. The conservation committee is a recent addition, formed in 2004. The scientific committee consists of up to 200 of the world’s leading whale biologists, many of them appointed by their governments. There are also subcommittees and working groups to deal with particular matters, such as infringement of regulations (called infractions) and subsistence whaling.

algae that grow on the underside of sea ice. While they are there they are also sheltered from predators. Consequently, krill numbers follow changes in the area of sea ice. The extent of the ice varies according to changes in surface air pressure and temperature that move around Antarctica as the Antarctic Circumpolar Wave (ACW), taking about seven years to complete one circuit around the continent. It is the ACW that causes krill numbers to reach a maximum every seven years, then decline to a minimum over three to four years, after which they increase again. Female krill lay several batches of eggs each year with up to 10,000 eggs in each batch, so in good survival years the population builds rapidly. Antarctic krill are caught for human consumption, although the Commission for the Conservation of Antarctic Marine Living Resources now restricts catches to a maximum of 1.1 million tons (1 million t) in order to leave food for the other species that depend on them. Most of the krill that is caught is processed into food for farmed fish, bait for anglers, and fish products for human consumption. People do not eat krill directly, like prawns. Krill fishing began in the 19th century or possibly earlier, but it expanded in the 1960s and 1970s. Japan, Russia, and Ukraine are the principal nations that catch krill. There was a peak catch in 1983 of more than 581,000 tons (528,000 t), of which

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Management of the Desert the then Soviet Union caught 90 percent, but the annual catch has since stabilized at approximately 110,000 tons (100,000 t) from Antarctic waters. There is also a krill fishery around Japan, catching about 77,000 tons (70,000 t) a year, and small quantities of other krill species are caught in Canadian waters.

Antarctic Cod and Icefish Fish are the really important food people obtain from the polar seas. Antarctic cod (Notothenia coriiceps) was once found in huge shoals, but it was fished almost to extinction. About 24 inches (60 cm) long when adult, the Antarctic cod is related to the perches (order Perciformes) rather than the cods (order Gadiformes), which occur only in the Northern Hemisphere. Icefish (Chaenocephalus aceratus) have also been fished very heavily. A member of the same order as the Antarctic cod and about the same size, the name icefish refers to the fact that its blood is almost transparent because it lacks the red pigment hemoglobin. Without hemoglobin the fish transports oxygen in its blood plasma. This is not very efficient, and as a result icefish are very sluggish.

Cod, Capelin, and Their Relatives It is in the Northern Hemisphere that fishing has been most intense and where competition among national fleets is keenest. Some countries, most notably Iceland and Greenland, are economically heavily dependent on their fisheries. In Iceland fishing accounts for one-sixth of the gross domestic product and provides one-ninth of all jobs. Icelandic fishing is based mainly on cod (Gadus morhua) and capelin (Mallotus villosus). Between them these two species account for more than two-thirds of all the fish caught. Cod has been an important food fish for centuries, and for many years about 400 million of them were being caught every year in the North Atlantic. The cod is a big fish, up to four feet (1.2 m) long, that often swims in shoals. Its numbers have been greatly reduced by overfishing, but it is so prolific—a female can lay 60 million eggs a year—that now

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that the fishing effort has been reduced it should recover. A related species, the North Pacific cod (G. macrocephalus) occurs in the North Pacific. The capelin is related to the smelts (order Salmoniformes, family Osmeridae). It is quite small, growing to no more than eight inches (20 cm). Many smelts spawn in freshwater, as do salmon, but the capelin is different. It spawns close to shore, sometimes laying its eggs on gravel beaches. Most of the codlike fishes occur throughout the northern North Atlantic, from the latitude of northern Spain to the Arctic, and fishing fleets pursue most of them. They include the haddock (Melanogrammus aeglefinus) and saithe (Pollachius vi rens), both of which grow to about 30 inches (76 cm), whiting (Merlangius merlangus), which averages 14 inches (36 cm), and ling (Molva molva), a slim, almost eellike fish up to six feet (1.8 m) long.

Regulating Fisheries Modern fishing boats are equipped with instruments to locate fish and have engines powerful enough to allow them to use nets that are so large they can catch entire shoals. Over recent years the level of fishing effort has been so high that stocks of most of the commercially important species have been seriously reduced. Efforts are now being made to regulate fishing, but this is difficult and often unpopular, because fishing communities are understandably reluctant to forego opportunities to earn income—and even more reluctant to decommission boats and reduce the size of their fleets. Through regulation, voluntary agreement, or bankruptcy due to the collapse of stocks, the capacity of modern fishing fleets to catch fish will gradually be brought into balance with the capacity of fish stocks to sustain a regular harvest. Once that balance is achieved, fishing can continue indefinitely. Meanwhile, it is likely that an increasing number of sea fishes will be raised in captivity. At present sea fish, the principle food of the Arctic and Antarctic, are the only animals still caught by hunting. If people are to continue to enjoy eating them, sooner or later hunting will have to give way to some kind of farming.

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Conclusion Deserts cover a substantial proportion of the Earth’s land surface. They are found in the subtropics, where subsiding air produces the hot, arid conditions of what most people think of as typical deserts, with vast seas of sand dunes and endless tracts of bare rock and gravel. Given that image of a typical desert, it must seem paradoxical that some of the driest deserts are located along the coasts of South America and southern Africa, yet that is the case. This book began by locating deserts. As it described those parts of the world where deserts occur, it became evident that the “typical” desert, in fact, is but one of several distinct types of desert. The world’s driest deserts are found in the interior of Antarctica and over the Greenland ice cap, places very different from the Sahara. A desert occurs wherever certain climatic conditions are met, regardless of latitude. Having established that there are different types of desert, the book described each of these types. It also explained that the desert climate of the frozen north has several times expanded southward to grip much of North America and Europe. The book explained how desert landforms develop, how oases form, and how the world’s climate systems produce deserts. Deserts appear barren, devoid of life—deserted. It is obviously true that they are inhospitable, but a surprising variety of plants and animals have adapted to the conditions they offer. There are plants that spring to life, flower, and produce seeds in the brief interval between a heavy fall of rain and the drying out of the soil. The camel can

tolerate the desert heat and famously endures long periods without food or drink without coming to any harm. The sandgrouse uses its breast feathers as a sponge to carry water to its young. Desert life is remarkable and its adaptations often surprising. Yet deserts support people as well as wildlife. They are home to some of the earliest civilizations and to ways of life that changed little for centuries, until the modern world caught up with them. Today those ways of life are under threat, and many have already disappeared. This is not necessarily a bad thing. Modernity brings schools, hospitals, new opportunities, and a broader vision of the world as well as the confusion and distress that come with the abandonment of traditional ways. In a world that is changing rapidly, the book finally considered the economics of desert nations and the possible future for deserts. It described what has been done already to make life more secure for those who must earn a living from desert soils and some of the future developments that may bring prosperity. Deserts will not disappear, but in a changing global climate they may migrate. It is likely in years to come that some of today’s deserts will shrink in size, but it is also possible that new deserts may form elsewhere. Deserts are harsh places that challenge the skill, strength, and stamina of those who cross them and the adaptability and ingenuity of those who inhabit them permanently. Those challenges will continue.

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Appendix A

AREAS OF THE MAJOR WORLD DESERTS, BY CONTINENT CONTINENT

DESERT

AREA (square miles)

(square kilometers)

Africa Kalahari

275,000

712,250

Namib

97,000

251,000

Sahara

3,513,530

9,100,000

5,500,000

14,200,000

Gobi

374,420

970,000

Iranian

150,000

390,000

Takla Makan

115,000

297,850

Thar

231,600

600,000

Turkestan

750,000

1,900,000

Australian

979,700

2,538,000

Chihuahuan

200,000

518,000

1,600,000

3,000,000

Syrian

200,000

518,000

Colorado

165,000

427,350

Antarctica Asia

Australia Central America Middle East Arabian North America Mojave

15,000

38,850

Sonoran

120,000

310,000

Greenland ice sheet

700,000

1,813,000

Atacama

140,155

363,000

Patagonian

300,000

777,000

South America

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Appendix B

CLIMATIC AVERAGES AND EXTREMES FOR MAJOR DESERTS DESERT

PLACE

Antarctic

interior coast Vostok Riyadh overall Antofagasta

Arabian Atacama

AVERAGE -84 (-64)

82 (28) 65 (18)

TEMPERATURE °F (°C) MAXIMUM

MINIMUM

117 (47)

-126.9 (-88.3) 49 (9)

85 (29)

41 (5)

RAINFALL INCHES (MM.) 2 (50) 15 (380) 2.4 (61) 0.4 (10)

Chihuahuan

Colorado

Durango El Paso overall Death Valley

67.5 (20) 64 (18) 90 (32.2) 76 (24)

106 (41) 115 (46.1) 134 (57)

-6 (-21) 32 (0)

Ulaanbaatar

61 (16)

92 (33)

34 (1)

Francistown

69 (20)

107 (42)

Las Vegas

65 (18)

115 (46)

8 (-13)

Walvis Bay

62 (17)

104 (40)

25 (-4)

Sarmiento

52 (11)

Gobi Kalahari

21 (536) 9 (229) 4 (102) 1.5 (38) 1.5 (38) 17.5 (445)

Mojave Namib

4 (100) 4.4 (112) 2 (51) 0.8 (20)

Patagonian 5 (127)

Sahara

Sonoran

Al ’Aziziyah In Salah overall Phoenix

136 (58) 122 (50)

78 (26) 55 (13) 70 (21)

26 (-3)

118 (48)

16 (-9)

Deir ez Zor overall Kashgar

68 (20) 41 (5) 55 (13)

114 (46)

6.2 (157)

106 (41)

16 (-9) 1 (25) -15 (-26)

Jacobabad

81 (27)

127 (53)

32 (0)

3.5 (89)

0.6 (15) 5 (127) 7.5 (190)

Syrian Takla Makan

3.3 (84)

Thar Note: Average temperatures are calculated from average daytime and average nighttime temperatures for each month. Temperature maxima and minima occur during the day and night, respectively.

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Appendix C

SI UNITS AND CONVERSIONS Scientists usually use SI (Système International d’Unités) units of measurement. These comprise seven base units plus two supplementary units, the radian (rad) and steradian (sr), as well as a number of derived units that are defined in terms of the base units. The base units, with their abbreviations, are as follows:

means per square foot. This is the way scientific units are usually written. Prefixes are used to increase or decrease the value of units by factors of 10. These are as follows: FACTOR 1012 109 106 103 102 10 10-1 10-2 10-3 10-6 10-9 10-12 10-15 10-18

Length: meter (m) Mass: kilogram (kg) Time: second (s) Electric current: ampere (A) Thermodynamic temperature: kelvin (K) Luminous intensity: candela (cd) Amount of substance: mole (mol) The most commonly used derived units include the following: Area: square meter (m2) Volume: cubic meter (m3) Frequency: hertz (Hz) = 1 cycle per second Density: kilogram per cubic meter (kg m-3 or kg/m3) Velocity: meter per second (m s-1 or m/s) Angular velocity: radian per second (rad s-1 or rad/s) Angular acceleration: radian per second per second (rad s-2 or rad/s2) Force: newton (N) = m kg s-2 Pressure: pascal (Pa) = N m-2 Energy, work, or quantity of heat: joule (J) = N m Power: watt (W) = J s-1 (J/s) Electromotive force: volt (V) = W A-1 (W/A) Electric resistance: ohm (Ω) = V A-1 (V/A) 2

Superscripted numbers (e.g. ) raise units to the power of the number indicated. A superscripted 2 means squared (multiplied by itself), a superscripted 3 means cubed (multiplied by itself twice). The minus sign attached to a superscripted number means per, so s-1 means per second and ft-2

NAME teragigamegakilohectodecadecicentimillimicronanopicofemtoatto-

SYMBOL T G M k h da d c m μ n p f a

Factors for converting between SI and everyday U.S. units are given below.

LENGTH 1 inch = 2.54 cm; 1 centimeter = 0.39 inch 1 foot = 30.48 cm; 1 centimeter = 0.033 foot 1 yard = 0.914 m; 1 meter = 1.094 yard 1 mile = 1.609 km; 1 kilometer = 0.6214 mile 1 nautical mile = 1.852 km; 1 kilometer = 0.54 nautical mile AREA 1 square inch = 6.45 cm2; 1 square centimeter = 0.16 inch2 1 square foot = 9.29 dm2; 1 square decimeter = 0.11 foot2 1 square yard = 0.84 m2; 1 square meter = 1.20 yard2 1 acre = 0.4047 ha; 1 hectare = 2.471 acres 1 square mile = 2.59 km2; 1 square kilometer = 0.39 mile2

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VOLUME 1 pint = 0.57 liter; 1 liter = 1.76 pints 1 quart = 1.14 liters; 1 liter = 0.88 quart 1 gallon (US) = 3.785 liters; 1 liter = 0.264 gallon 1 barrel (bl) = 0.159 m3; 1 cubic meter = 6.29 barrels (bbl) 1 cubic inch = 16.39 cm3; 1 cubic centimeter = 0.061 inch3 1 cubic foot = 0.028 m3; 1 cubic meter = 35.31 feet3 1 cubic yard = 0.765 m3; 1 cubic meter = 1.31 yards3 MASS 1 ounce = 28.4 g; 1 gram = 0.035 oz 1 pound = 0.454 kg; 1 kilogram = 2.2 pounds 1 ton = 0.91 t; 1 tonne = 1.1 ton FORCE 1 pound force (lbf) = 4.45 N; 1 newton = 0.225 pound force PRESSURE 1 bar = 0.1 Mpa; 1 megapascal = 10 bar 1 pound force per square inch = 6.89 kPa; 1 kilopascal = 0.145 lbf in-2 1 pound force per square foot = 47.88 Pa; 1 pascal = 0.021 lbf ft-2

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1 inch of mercury (Hg) = 3.38 kPa; 1 kilopascal = 0.29 inch mercury 1 standard atmosphere (atm) = 0.1013 Mpa; 1 megapascal = 9.87 atmospheres WORK, HEAT, OR ENERGY 1 calorie (cal) = 4.187 J; 1 joule = 0.239 calorie 1 British thermal unit (Btu) = 1.055 kJ; 1 kilojoule = 0.948 British thermal unit 1 erg = 0.1 μJ; 1 microjoule = 10 erg SPEED 1 foot per second = 0.305 m s-1; 1 meter per second = 3.28 feet s-1 1 foot per second = 1.097 km h-1; 1 kilometer per hour = 0.911 foot s-1 1 mile per hour (MPH) = 1.609 km h-1; 1 kilometer per hour = 0.6214 mile per hour 1 knot = 1.852 km h-1; 1 kilometer per hour = 0.54 knot TEMPERATURE °F = (°C ÷ 5 × 9) + 32; °C = (°F - 32) × 5 ÷ 9

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Glossary

ablation a fairly general term describing losses by melting and evaporation and also by sublimation

angle of repose the steepest angle at which loose material such as gravel or sand can remain standing

absolute humidity the amount of water present in the air expressed as the percentage of the amount needed to saturate the air at that temperature

angular momentum the momentum of a body that is rotating around an axis; it is the product of the mass of the body, the radius of its orbit, and the square of its speed of rotation (angular velocity)

absorber layer in a solar cell, a layer comprising two dissimilar materials in close contact that produce an electrical field active layer the uppermost layer of a permafrost soil that thaws in summer adiabatic a change in temperature due to compression or expansion, with no reference to the temperature of the surrounding medium adventitious root position

a plant root growing from an unusual

air mass a large body of air, covering a substantial part of a continent or ocean, in which the physical conditions are fairly constant throughout aklé dune a sand dune that forms a wavy ridge at right angles to the prevailing wind albedo the reflectiveness of a surface, measured as the proportion of light falling on the surface that is reflected alga a plantlike organism that performs photosynthesis; many algae are single-celled, but seaweeds are also algae alley cropping

see corridor farming

alluvial fan a mass of sediment deposited where the bed of a river suddenly becomes less steep (e.g., at the foot of a hill) alluvium material that has been deposited by a river or stream altiplano the high, bleak Andean plateau alveoli

small holes in a pedestal rock

andhis a severe dust storm or sandstorm in the Thar Desert

annual plant a plant that completes its life cycle, from the germination of seed to production of seed, in a single season anticyclone a region of high atmospheric pressure aphelion the point in an elliptical orbit where the orbiting body is farthest from the body about which it orbits aquiclude see aquifuge aquifer a mass of permeable material such as sand or gravel that lies above an impermeable layer and through which groundwater flows aquifuge (aquiclude) a material that will hold water but prevents water from flowing through it aquitard a material that will hold water and that allows water to flow through it, but more slowly than the water would flow through an aquifer areole a sunken pad on the surface of a cactus plant from which spines and glochids grow artesian well pumping

a well from which water flows without

autotomy detaching a part of the body by an animal that is threatened to distract an attacker (the detached part, commonly the tail, often wriggles vigorously); the lost body part later grows back autotroph an organism that is able to synthesize complex organic compounds from simple inorganic ingredients; an organism that uses carbon dioxide as its principal source of carbon awn

one of the stiff hairs on a grass seed

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back-junction layer in a solar cell, a layer of metal covering the entire base of the cell from which electrical current flows out of the cell bacteria organisms that are usually single-celled, lack a distinct cell nucleus, and reproduce by fission; depending on the taxonomic system, Bacteria are one of the domains of life or a kingdom in the superkingdom Prokarya baleen in whales of the suborder Mysticeti, sheets of keratin, their ends fringed with hairs, that hang from the roof of the mouth and are used to filter food from the water barchan dune a cresent-shaped sand dune aligned at right angles to the prevailing wind basal metabolic rate a measure of the rate at which an animal’s metabolism functions bedform the shape of the surface of a sedimentary deposit, formed by the flow of air or water across it Beltian body a food store rich in oils and proteins found on the leaves of Acacia cornigera that is used by ants berg wind a hot, dry, dusty wind that blows across southern Africa on about 50 days of the year, most frequently in winter biennial plant a plant that lives for two years, producing seed in the second biomass the total mass of all the living organisms within an ecosystem or at a particular trophic level, or some set of them (e.g., of producers, consumers, predators, or a single species) blackbody a body that absorbs all the radiation falling upon it and emits all the absorbed radiation at a wavelength inversely proportional to its absolute temperature blizzard a wind accompanied by heavy snow that is falling or blown up from the surface; by definition, the wind must be at least 35 MPH (56 km/h), the temperature no higher than 20°F (-7°C), the snow sufficient to produce a layer at least 10 inches (250 mm) thick, and the visibility less than 0.25 mile (0.4 km) blubber a layer of fat beneath the skin that provides thermal insulation in marine animals such as whales and seals bolson

a deep basin between faulted blocks

boundary layer a layer of air lying immediately adjacent to a surface and within which conditions are strongly influenced by the proximity of the surface; the boundary layer above open water is about 0.04 inch (1 mm) thick, and the planetary boundary layer above the Earth’s surface extends to about 1,650 feet (500 m)

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brigalow a type of acacia scrub found in the Australian desert butte a flat-topped hill, smaller than a mesa, formed by erosion cacimbo a mist or wet fog, sometimes with drizzle, that occurs on the coast of Angola during the dry season calcite a form of calcium carbonate (CaCO3) that is a common ingredient of the shells of marine organisms and that can be precipitated from seawater calving the formation of an iceberg by breaking away from the tip of a glacier capacity factor the ratio of the amount of power a wind generator produces in a year to the power output stated in its specification capillarity the movement of water through a narrow tube or pore due to surface tension between water molecules and the sides of the opening capillary fringe the region above the water table into which water is drawn by capillarity caravan a group of people traveling together across a desert caravansary a facility offering overnight accommodation for caravans and their animals cavitation the collapse of a bubble of gas or liquid because it is at a lower pressure than its surroundings chech a piece of colored cloth men wear in parts of the Sahara as a turban that also covers the nose and mouth to keep out the sand chelicerae the first of the six pairs of appendages possessed by arachnids (spiders, scorpions, mites, etc.); in mites they are used for piercing, but in most arachnids they resemble pincers and are used to seize and squeeze prey, for defense, and for digging; in spiders they are used to inject venom chlorophyll the pigment present in the leaves and sometimes stems of green plants that gives them their green color; chlorophyll molecules trap light, thus supplying the energy for photosynthesis chloroplast the structure in plant cells that contains chlorophyll and in which photosynthesis takes place chott

a salt lake in the Sahara

circadian rhythm the approximately 24-hour cycle of changes in the metabolism of most organisms clathrate a compound formed by a molecule of one compound held inside a cage of molecules of a different compound; methane hydrate is a clathrate compound climatic minimum a period during which average temperatures are significantly lower than those in the preceding or succeeding periods

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Glossary climatic optimum a period during which average temperatures are significantly higher than those in the preceding or succeeding periods climatologist

a scientist who studies climate

clone a group of genetically identical individuals, or one member of such a group cloud condensation nuclei minute particles present in the air onto which water vapor condenses to form cloud cold pole one of the places that experience the lowest average temperatures on Earth; there are three cold poles, at Vostok Station, Antarctica; Verkhoyansk, Siberia; and Snag, Yukon conservative margin a boundary between two tectonic plates where crustal rock is being neither formed nor destroyed; the plates slide past each other constructive margin a boundary between two tectonic plates where crustal rock is being formed continental drift the movement of continental land masses in relation to one another convection the transport of heat through a fluid (gas or liquid) by the movement of the fluid itself convection cell a closed circulation in which air or water is warmed from below, rises and cools, then subsides to be warmed again convergence see convergent evolution convergent evolution (convergence) evolution in which similar environmental challenges produce similar responses in unrelated species coppicing the technique of cutting through a broadleaved tree close to ground level in order to produce a crop of thin poles core temperature animal CorF

the internal body temperature of an

see Coriolis effect

Coriolis effect (CorF) the deflection due to the Earth’s rotation experienced by bodies moving in relation to the Earth’s surface; bodies are deflected to the right in the Northern Hemisphere and to the left in the Southern Hemisphere

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creep the movement of sand that rolls across the surface, driven by the wind cuticle a thin, waxy outer coat that protects the leaves and stem of a plant; in animals, a protective outer layer to the skin cyanobacteria bacteria that possess chlorophyll and perform photosynthesis cyclone an area of low atmospheric pressure; a tropical cyclone that occurs in the northern Indian Ocean damping depth the depth below which the soil temperature does not rise during the day and fall at night; in dry sand it is about three inches (7.6 cm) deflation the removal of surface material by the action of the wind deimatic coloration bright animal coloration that is designed to be displayed suddenly in order to startle a predator, making it recoil and allowing the potential victim to escape dendrochronology the scientific dating of material by counting annual growth rings in trees dendroclimatology the reconstruction of past climates through the study of annual growth rings in trees denkli

see shaduf

deposition

the formation of ice directly from water vapor

depression an area of low atmospheric pressure associated with a frontal system desert a region in which for most of the time less precipitation falls than could evaporate during the same period; the amount of precipitation needed for a desert to develop varies according to the temperature, but a desert is likely anywhere that the annual precipitation is less than 10 inches (250 mm) a year and highly erratic desertification land degradation in arid, semiarid, and dry subhumid areas resulting at least partly from human management and that is sustained for at least 10 years desert rose calcite or gypsum that has crystallized into a shape resembling the overlapping petals of a flower; also the shrub Adenium obesum

growing annual crops

desert varnish a thin, dark coating of iron and manganese oxides that coats exposed rock surfaces, most commonly in a desert environment

countercurrent exchange the conservation of body warmth through the exchange of heat between warm arterial and cool venous blood flowing in opposite directions through blood vessels lying side by side

destructive margin a boundary between two tectonic plates where the plates are colliding

covalent bond a chemical bond linking two atoms that share one or more electrons

dew point the temperature at which air becomes saturated; water vapor condenses if the temperature falls

corridor farming (alley cropping) between rows of trees cotyledon

a seed leaf

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detritivore an organism that feeds on dead organic material (detritus)

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below the dew point, and water evaporates if the temperature rises above the dew point disjunct distribution the occurrence of related species in places separated by major geographical barriers, such as an ocean or mountain range diurnation becoming torpid overnight or for a short period during the day divergent evolution (evolutionary divergence) evolution in which two species with a common ancestor become increasingly different over succeeding generations draa a ridge of sand or chain of dunes in the Sahara more than 1,000 feet (330 m) high and 0.3–3 miles (0.5–5 km) from its nearest neighbor; it is the largest landform in the sandy desert dreikanter

a ventifact with three facets

drift deposits material deposited by a glacier; it was originally thought to have been deposited by melting glaciers as they drifted across the sea dry valley one of the areas in Antarctica where the ground is free from ice dust devil a small, spiraling column of dust and sand dynamic soaring a technique used most notably by albatrosses that allows a bird to remain aloft for long periods with a minimum of effort by adjusting its airspeed in relation to the wind speed at different heights easterly jet a jet stream that blows from east to west in summer across India and Africa

eluviation the removal by water, in solution or suspension, of materials from the upper layers of the soil and their deposition in a lower layer elytra the modified wings that form the wing cases of a beetle endotherm a homeotherm that maintains a constant body temperature by physiological means; only birds and mammals are endotherms ENSO El Niño–southern oscillation; the full cycle of an El Niño and La Niña, linked to a southern oscillation eolian

pertaining to the action of the wind

ephemeral plant vanishes

a plant that grows rapidly and then

epiphyte a plant that grows on the surface of another plant, using its host only for support equatorial trough the belt of low atmospheric pressure that encircles the Earth where the trade winds of the Northern and Southern Hemispheres converge equinox one of the two days in the year when the noonday Sun is directly overhead at the equator; the Sun is above the horizon for 12 hours and below it for 12 hours everywhere on Earth erg

a sea of sand in a hot desert

erratic a rock that has been transported by glacial action to a place where it is entirely different from the local rock

ecology the scientific study of the relationships between different species of organisms and between those organisms and their physical and chemical surroundings

estivation animal dormancy during hot weather

ecosystem a discrete unit consisting of living organisms and their chemical and physical surroundings that interact to form a coherent system

evaporite a sedimentary rock formed from the salts deposited when all the water evaporated from a lake or shallow arm of the sea

ectotherm a homeotherm that maintains a constant body temperature by behavioral means such as basking and seeking shade

evapotranspiration evaporation and transpiration considered together

einkanter

a ventifact with a single facet

electron an elementary particle carrying a negative charge element a substance that cannot be broken down into simpler substances; all of its atoms possess the same number of protons and electrons, but the number of neutrons may vary El Niño a periodic weakening or even reversal in the direction of the ocean current in the tropical South Pacific associated with a weakening or reversal in the direction of the trade winds; it brings heavy rain to the ordinarily arid west coast of South America and dry weather, sometimes drought, to Indonesia

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evaporation phase

the change from the liquid to the gaseous

exosmosis the process of osmosis by which water flows out of plant roots due to high salt concentration in the adjacent soil exotherm an animal in which the body temperature varies with that of its surroundings fall speed

see terminal velocity

famine food a food people will eat only when no alternative is available fault the surface of a fracture in a rock where one side of the broken rock has moved in relation to the other side fell-field an arid island surrounded by sodden ground in a tundra environment

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field capacity the water that remains in the soil after excess moisture has drained freely from that soil

guttation the extrusion of water, sometimes containing dissolved salts, from the leaves or stem of a plant

firn (névé) snow that has survived through a summer without melting

gypsum an evaporite mineral, hydrated calcium sulfate (CaSO4.2H2O)

fluvial

gyre a circular or spiral system of ocean currents

fog

pertaining to a river

cloud at ground level that reduces horizontal visibility to less than 0.6 mile (1 km)

foggara

see qanāt

föhn wind a warm, dry wind, sometimes of gale force, that blows on the leeward side of a mountain range fossil anything that is ancient, especially if it is discovered below ground or embedded in rock front a boundary between two air masses one of which is warmer than the other fungi organisms belonging to the kingdom Fungi, comprising nonphotosynthesizing organisms that feed by absorbing organic substances from their surroundings and reproduce by spores geophyte a plant that spends periods when conditions are unfavorable underground, as a bulb, tuber, corm, or fleshy rhizome ghibli a hot, dry wind that blows in northern Libya gibber plain the Australian name for an extensive plain covered by loose fragments of rock glacial pertaining to a glacier; a period when ice sheets covered a substantial part of the Earth’s surface (i.e., an ice age) glacier a large mass of ice lying on the land and usually moving across it glochid one of the short, barbed hairs growing from certain cacti Great Conveyor a system of ocean currents that circulates through all the ocean basins, transporting cold water from high to low latitudes and warm water from the equator to high latitudes

Hadley cell the tropical part of the general circulation of the atmosphere; air rises over the equator, moves away from the equator at high altitude, subsides over the subtropics, and flows toward the equator at low altitude halophobe

a plant that is intolerant of salt

halophyte

a plant that tolerates salt

hammada a rocky desert surface consisting mainly of exposed bedrock and large boulders heliostat a mirror attached to a mechanism that causes it to track the Sun across the sky heterotroph an organism that consumes other organisms, living or dead hibernation

animal dormancy during cold weather

homeotherm an animal that maintains a fairly constant body temperature humidity

the amount of water vapor present in the air

hydrogen bond the attraction that links molecules in which hydrogen is bonded to nitrogen, oxygen, or fluorine hydroponics the technique of growing plants in a solution of plant nutrients without soil ice shelf the outermost part of an ice sheet that extends over the sea igneous

a rock that has crystallized from molten magma

illuvial zone the soil layer into which materials have been washed from above inselberg an isolated, steep-sided hill standing on a plain interglacial ages)

a period between two glacial periods (ice

interstade see interstadial

greenhouse effect warming of the atmosphere due to the absorption by certain gases of long-wave radiation from the surface

interstadial (interstade) a time of warmer conditions during a glacial period (ice age), but one that is shorter and usually cooler than an interglacial

greenhouse gas a gas present in the atmosphere that absorbs long-wave radiation from the surface; the principal greenhouse gases are water vapor, carbon dioxide, methane, nitrous oxide, ozone, and chlorofluorocarbons (CFCs)

Intertropical Convergence Zone a belt encircling the Earth where the trade winds from the Northern and Southern Hemispheres converge, causing air to rise and producing low atmospheric pressure at the surface

grounding line the limit of contact between a glacier and the ground surface; beyond the grounding line there is water beneath the glacier

ionic bond a chemical bond linking two or more atoms that is based on the exchange of electrons between the atoms

groundwater aquifer

isobar a line on a weather map joining places of equal atmospheric pressure

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underground water that flows through an

ion

an atom that has lost or gained one or more electrons

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isohyet a line on a weather map joining places of equal rainfall

(2,501 J/g); and for sublimation and deposition 680 cal./ g (2,835 J/g)

isotope one or more varieties of an element comprising atoms with the same number of protons and electrons as all other atoms of that element but a different number of neutrons

lead a stretch of open water on a sea otherwise covered in sea ice

jet stream a winding ribbon of strong wind about five to 10 miles (8–16 km) above the surface; jet streams are typically thousands of miles long, hundreds of miles wide, and several miles deep kanat see qanāt karaburan a hot, dry wind, sometimes of gale force, that blows from the east-northeast across the deserts of Central Asia from early spring to late summer karez

see qanāt

katabatic wind a cold wind that blows downhill across sloping ground keratin a fibrous substance, made from proteins, from which animal hair, wool, nails, hoofs, horns, antlers, and baleen are made khamsin a hot, dry wind over the Sahara that blows at 50day intervals in late winter and early spring; the name means “fifty”

lenticels pores in the stem of a woody plant leveche a hot, dry, southerly wind over Spain that brings air from the Sahara lichen a composite organism made of a fungus and an alga or cyanobacterium litter plant and animal material lying loosely on the ground surface longitudinal dune a sand dune that is aligned in the average direction of the wind macronutrient a nutrient substance that living organisms need in relatively large amounts magma hot rock containing crystals formed by the partial melting of the base of the Earth’s crust and the upper part of the mantle mallee

dwarf Eucalyptus scrub in the Australian desert

mandible in invertebrates, the part of their mouth parts with which they seize prey; in vertebrates, the lower jaw mesa

a flat-topped hill in a desert

kinetic energy energy of motion, usually defined as the amount of work a moving body could do if it were brought to rest; it is equal to mv2/2, where m is the mass of the moving body and v is its velocity

mesophyll the tissue lying just below the surface of a leaf, where photosynthesis takes place

kleptoparasitism obtaining food by harassing flying birds to make them drop food they are carrying

micronutrient a nutrient substance that living organisms need in relatively small amounts

lag deposit coarse-grained material that is left behind after wind or water have removed the finer material

mixing ratio the ratio of the mass of any gas (commonly water vapor) present in the air to a unit mass of air with that gas removed (i.e., dry air)

land and sea breezes light winds that blow along coasts in hot weather, from sea to land by day and from land to sea by night La Niña the opposite to El Niño; easterly winds and ocean currents in the tropical South Pacific intensify, producing very wet weather in Indonesia and very dry weather in western South America lapse rate the rate at which the air temperature decreases (lapses) with increasing altitude; in unsaturated air the dry adiabatic lapse rate is 5.38°F per thousand feet (9.8°C/1,000 m); in saturated air the saturated adiabatic lapse rate varies but averages 2.75°F per thousand feet (5°C/1,000 m) latent heat the heat energy that is absorbed or released when a substance changes phase between solid and liquid, liquid and gas, and solid and gas; for water at 32°F (0°C) the latent heat of melting and freezing is 80 cal./g (334 J/g); of vaporization and condensation 600 cal./g

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methane hydrate a clathrate compound consisting of a molecule of methane held inside a cage of ice

monocot a plant that produces a single cotyledon monsoon a reversal in wind direction that occurs twice every year over much of the Tropics, producing two seasons with markedly different weather moraine

rock debris deposited by a glacier

mulga vegetation comprising grasses and Acacia scrub in the Australian desert mushroom rock

see pedestal rock

mutualism a close relationship between individuals of two species that benefits both mycobiont

the fungal partner in a lichen

myrmecophily a close and mutually beneficial relationship between a plant and ants nebka

a mound of sand trapped by a plant in the Sahara

nectar

a sugary liquid secreted by a plant

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Glossary nectary

the plant organ that produces nectar

neutron an elementary particle with a mass slightly greater than that of a proton that carries no charge; it is stable while it remains in an atomic nucleus, but outside the nucleus it decays into a proton, electron, and antineutrino niche the ecological position occupied by a plant or animal that has succeeded in establishing itself in a place where it finds the resources it needs nilometer a device invented in ancient Egypt that accurately measures the water level in the Nile node

one of the swellings at intervals along a grass stem

nucleus the large body inside all plant, fungal, and animal cells, but not cells of bacteria or cyanobacteria, that contains DNA and that controls processes inside the cell; also the central part of an atom carrying positive charge, containing protons and neutrons and accounting for most of the mass of the atom nunatak a rock that projects above the surface of the Arctic or Antarctic snow and ice oasis a place in a desert where the land is fertile because the water table lies close to or above the surface throughout the year orogeny mountain-building, especially when it is due to a collision of tectonic plates orographic lifting high ground

the forced raising of air as it crosses

osmosis the movement of water or another solvent from a region of low concentration to a region of higher concentration across a membrane separating two solutions of different concentrations until the concentrations are equal on either side of the membrane; the membrane permits the passage of solvent molecules but not molecules of the solute ouadi pad

see wadi

the swollen stem, resembling a leaf, of a prickly pear cactus

paecottah

see shaduf

paleoclimatology the study of climates in the ancient past Pangaea the supercontinent that came into existence about 260 million years ago and began to break apart about 220 million years ago papyrus a perennial herb (Cyperus papyrus), also called paper-reed, from which paper was once made parabolic dune a cresent-shaped sand dune in which the horns of the crescent point upwind and enclose a hollow where the wind has blown away the sand; it is of

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a similar shape to a barchan dune but points in the opposite direction parallel evolution evolution in which two species descended from a common ancestor continue to inhabit similar environments and consequently change little over succeeding generations parent material the weathered material from which the mineral component of a soil is derived partial pressure the proportion of the total pressure exerted by a mixture of gases that can be attributed to one of those gases; for example, if the atmospheric pressure is 1,000 mb and oxygen accounts for 21 percent of the mass of the atmosphere, then the partial pressure of oxygen is 210 mb pastoralist a person who lives by herding cattle, sheep, and goats pedestal rock (mushroom rock) an unstable, mushroomshaped landform found in deserts, produced by chemical weathering around the base, where moisture is retained longest pediment a land surface, partly covered by rock debris, at the base of a mountain or scarp that bulges upward with a gentle slope pedipalps the second of the six pairs of appendages possessed by arachnids (spiders, scorpions, etc.), that in scorpions have become modified into large claws used to seize prey; in arachnids that have large chelicerae the pedipalps are used for walking pedogenesis the formation of soil perennial plant a plant that does not die down and disappear completely after one or two seasons perihelion the point in an elliptical orbit where the orbiting body is closest to the body about which it orbits permafrost permanently frozen ground; to become permafrost the ground must remain frozen throughout a minimum of two winters and the summer between permeability a measure of the ease with which water moves through a material petiole

leaf stalk

phloem tissue through which the products of photosynthesis and hormones are transported from the leaves to all parts of a vascular plant phosphorylation the addition of a phosphate group photophosphorylation the addition of a phosphate group using light as a source of energy photorespiration a light-activated process that occurs in the chloroplasts of many plants in which oxygen is absorbed and carbon dioxide released, but without providing the plant with energy

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photosphere

DESERTS the visible surface of the Sun

photosynthesis the sequence of chemical reactions in which green plants and cyanobacteria use sunlight as a source of energy for the manufacture (synthesis) of sugars from hydrogen and carbon obtained from water and carbon dioxide, respectively; the reactions can be summarized as 6CO2 + 6H2O + light → C6H12O6 + 6O2↑, the upward arrow indicating that oxygen is released into the air; C6H12O6 is glucose, a simple sugar phyllode a modified petiole that is flattened and serves as a leaf phytobiont the alga or cyanobacterium forming part of a lichen phytoplankton face waters

microscopic green algae that drift in sur-

plane of the ecliptic the imaginary disk enclosed by the path of the Earth’s orbit around the Sun plantigrade gait walking with the soles of the feet making contact with the ground plate tectonics the theory holding that the Earth’s crust is made up of a number of rigid sections, or plates, that move in relation to each other playa (salina) the lowest part of the basin between mountains; it is prone to flooding by local rainfall or by water running off the mountains polar molecule a molecule in which the electrons are shared unequally among the nuclei, so that although the molecule carries no net electrical charge, one side of the molecule carries a positive charge and the opposite side carries a negative charge; water molecules are polar pollen the grains containing male sex cells that are produced in the anthers of flowers porosity the proportion of a volume of material that consists of spaces, or pores potential evapotranspiration the amount of water that would leave the ground by evaporation and transpiration if the supply of water were unlimited pressure gradient the rate at which the pressure changes with horizontal distance prevailing wind the direction from which the wind at a particular place blows most frequently primary growth growth that extends the tips of a plant stem and branches proteoid roots small rootlets on the roots of evergreen trees and shrubs belonging to the family Proteaceae that grow rapidly into full-sized roots when there is water for them to absorb

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proton an elementary particle bearing a positive charge equal to the negative charge on an electron qanāt (foggara, kanat, kasrez) a system of underground channels that convey water from the mountains to lower ground in the desert radiation fog fog that forms on clear nights when the relative humidity is high; the ground cools rapidly as it radiates the heat it absorbed by day, chilling the air close to the ground to below its dew point and causing water vapor to condense; if the air is still the fog layer is very shallow, but air movements can lift the fog and cold air to produce a layer up to 100 feet (30 m) deep radicle the precursor of the root rain shadow an area on the leeward side of high ground that is dry because approaching air loses its moisture as it crosses the high ground ramet

a member of a clone

reg the stony surface of a desert or the layer of pebbles that covers the surface relative humidity the amount of water vapor present in the air expressed as the percentage of the water vapor that would be needed to saturate the air at that temperature renewable freshwater resources (RFWR) water circulating through rivers and aquifers that is available for sustained human use respiration the sequence of chemical reactions in which carbon in sugar is oxidized with the release of energy; the opposite of photosynthesis; the reactions can be summarized as C6H12O6 + 6O2 → 6CO2 + 6H2O + energy; C6H12O6 is glucose, a simple sugar RFWR see renewable freshwater resources rhizome a plant stem that grows horizontally below the ground surface rhizosheath a kind of artificial bark that coats the root hairs of certain desert plants; it consists of mucilage secreted by the root hairs to which sand grains adhere rhourd a large, star-shaped or pyramid-shaped sand dune, often 330–660 feet (100–200 m) high rinderpest a highly contagious and often fatal disease of cattle, sheep, and goats; it is caused by a virus, and victims suffer acute diarrhea and discharges from the eyes, nose, and mouth roaring forties 40°S–50°S

the gales that frequently occur in latitudes

Rossby wave a wave with a wavelength of 2,485–3,728 miles (4,000–6,000 km) that develops in moving air in the middle and upper troposphere

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Glossary salt weathering weathering resulting from the evaporation of water from a salt solution that has penetrated cracks in a rock; salt expands as it crystallizes, and salt crystals expand and contract more than the rock in response to heating and cooling, widening the cracks and eventually shattering the rock sandstorm a wind storm that lifts sand grains from the surface and transports them, often for long distances saturation the condition in which the quantity of a dissolved or suspended substance a medium can hold reaches a maximum saturation vapor pressure the pressure exerted on a surface by water molecules in a layer of saturated water vapor secondary growth the thickening of the stem and branches of a woody plant due to cell division beneath the bark sedimentary rock rock formed from particles that have settled to the seabed and later been compressed seif dune a curved sand dune with a shape reminiscent of a curved sword that forms where the wind blows from two principal directions

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solarization (heliosis) the inhibition of photosynthesis when the intensity of light is very high solstice one of the two days in the year when the noonday Sun is directly overhead at one or the other of the tropics; it is overhead at the tropic of Cancer on midsummer day in the Northern Hemisphere and at the tropic of Capricorn on midsummer day in the Southern Hemisphere southern oscillation a change in the distribution of air pressure over the equatorial South Pacific; ordinarily pressure is low over Indonesia and high over the eastern Pacific, but during a southern oscillation the pattern reverses specific heat capacity the amount of heat that must be applied to a substance in order to raise its temperature by one degree; it is measured in calories per gram per degree Celsius (cal/g/°C) or in the scientific units of joules per gram per kelvin (J/g/K; 1K = 1°C = 1.8°F) specific humidity the ratio of the mass of water vapor present in the air to a unit mass of that air including the water vapor

serir a thin layer of mixed sand and gravel lying on the desert surface

spore a reproductive unit, usually consisting of a single cell, that can develop into a new organism without fusing with another cell

shadoof see shaduf

stade see stadial

shaduf (shadoof, denkli, paecottah) a device used to raise water from a river and pour it into an irrigation channel; the shaduf has a long, tapering pole mounted on a horizontal crossbeam about 10 feet (3 m) above the ground, with a bucket attached to one end of the pole and a rock as a counterweight attached to the other end

stadial (stade) a prolonged period of cold weather that is shorter and milder than a glacial

shield a region, always in a continent, of very ancient, hard rocks that are no longer affected by mountain-building processes simoom a severe dust storm or sandstorm in the Sahara sirocco a hot wind that blows across countries bordering the Mediterranean, most commonly in spring soil horizon a horizontal layer in a soil profile that differs in its mineral or organic composition from the layers above and below it and from which it can be clearly distinguished visually soil moisture tension the upward force acting on water held in a container soil profile a vertical section cut through a soil from the surface to the underlying rock solar cell a device that converts light energy into electrical energy

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stele the central structure of a plant root, constituting one end of the vascular system stellar dune a sand dune made of several ridges radiating from a central point, resembling a star shape stilting a stance adopted by certain species of desert scorpions in which the scorpion stands on the tips of its limbs, raising its body as high above the ground as it can; this allows air to circulate beneath its body stipule a straight or leafy structure that grows from the base of a petiole stolon a plant stem that grows horizontally along the ground surface stoma

see stomata

stomata (sing. stoma) small openings, or pores, on the surface of a plant leaf through which the plant cells exchange gases with the outside air; stomata can be opened or closed by the expansion or contraction of two guard cells surrounding each stoma stratosphere the region of the atmosphere that extends from the tropopause to an altitude of about 31 miles (50 km)

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subduction the sinking of one crustal plate beneath another at a destructive plate margin sublimation the direct change from ice to water vapor succulent a plant that stores water in its tissues to provide a supply for maintaining photosynthesis sucker

a shoot that emerges from a stolon or rhizome

sunspot cycle an increase and then decrease in the number of sunspots visible on the Sun’s surface over a period of approximately 11 years supercontinent a landmass that existed in the past and subsequently broke into sections the size of continents; all of Earth’s present continents were once joined together in the supercontinent Pangaea suture a joint marking the line where an ocean basin has closed and two tectonic plates have become permanently joined tafoni a large hole in a pedestal rock caused by salt weathering teleconnections linked atmospheric events that occur in widely separated parts of the world temperature inversion a layer of the atmosphere in which the temperature increases with increasing altitude terminal velocity (fall speed) ing body can attain

the maximum speed a fall-

thermal equator the region between latitudes 23°N and 10–15°S where the surface temperature is highest; its mean position is about 5°N thermal wind a wind that is generated when the air temperature changes by a large amount over a short horizontal distance; the jet stream is a thermal wind thermohaline circulation the exchange of surface and deep ocean water due to differences in temperature and salinity that takes place where surface water is freezing tillers a number of stems arising from the lowest node on a grass stem top-junction layer in a solar cell, a metal grid that forms one of the two electrical contacts allowing a current to flow tracheid a long, cylindrical cell with a tapering, perforated end; tracheids join end to end to form the xylem tissue in gymnosperms

transform fault a type of fault that occurs in rocks on the ocean floor where two adjacent crustal plates meet at a conservative plate margin; compared with similar faults on land, the direction of movement is reversed, or “transformed” transhumance the practice of farming in the lowlands through the winter and spending the summer in the high meadows with sheep and goats transpiration the evaporation of water through leaf stomata when these are open for the exchange of gases transverse dune a long sand dune, with the gradual slope on the side facing the wind and the line of the dune at right angles to the wind direction, that forms where the wind almost always blows from a particular direction trichomes outgrowths from a leaf in the form of hairs or scales trophic

pertaining to feeding

tropics two lines of latitude at 23.5°N (tropic of Cancer) and 23.5°S (tropic of Capricorn) where the Sun is directly overhead at noon at one of the solstices; that part of the Earth lying between the tropics tropopause the boundary separating the troposphere from the stratosphere; it occurs at a height of about 10 miles (16 km) over the equator, 7 miles (11 km) in middle latitudes, and 5 miles (8 km) over the North and South Poles troposphere the layer of the atmosphere that extends from the surface to the tropopause; it is the region where all weather phenomena occur tsunami a shock wave caused by a submarine landslide or an earthquake in the ocean floor that produces a tremor in the entire depth of water; the tremor crosses the ocean at high speed, producing a small wave at the surface, but when it reaches shallow water the wave slows and its height increases tubercle in some cactus species a raised structure, resembling a wart, that bears a single areole tube well a well comprising a steel tube with a pump to raise the water tundra a treeless plain in the Arctic or Antarctic where the vegetation is dominated by grasses, sedges, rushes and wood rushes together with dwarf shrubs, lichens, and mosses

trade wind inversion a temperature inversion associated with the subsidence of air on the side of the Hadley cell circulation farthest from the equator

turgor rigidity of plant tissues due to water held under pressure in the cells

trade winds the winds that blow toward the equator in equatorial regions, from the northeast in the Northern Hemisphere and from the southeast in the Southern Hemisphere

upwelling the rising of water from close to the ocean floor all the way to the surface that occurs in particular ocean areas due to the combined effects of prevailing winds, friction, and the Coriolis effect

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Glossary valley glacier a long, narrow glacier that is confined between rocks on either side vapor pressure the partial pressure exerted on a surface by water vapor present in the air vascular system the vessels consisting of phloem and xylem cells through which water and nutrients are transported to all parts of a plant ventifact a desert pebble that has been worn away by wind-blown sand in such a way as to produce clearly defined faces vessel element one of the cells forming the xylem tissue in flowering plants virga precipitation that falls from the base of a cloud but evaporates before reaching the ground; it is visible as a gray, veil-like extension below the cloud vorticity the tendency of a mass of fluid that is moving in relation to the Earth’s surface to rotate about a vertical axis wadi (ouadi)

the Arabic name for a dried-up riverbed

Walker circulation a slight but continuous latitudinal movement of tropical air that is superimposed on the Hadley cells; it was first described in 1923 by Sir Gilbert Walker (1868–1958)

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water table the upper margin of the groundwater; soil is fully saturated below the water table but unsaturated above it weathering the breaking down of rocks by physical and chemical processes weathering rind a layer of red, orange, or yellow material, one inch (2.5 cm) or more thick, on the exposed surface of a desert rock; it develops where weathering removes a surface layer of rock, exposing the rock below, thus allowing iron in the rock to oxidize and produce the red (rust) color whirlwind a column of air spiraling upward usually to about 100 feet (30 m) but sometimes higher that appears without warning in a desert wilting the limpness that occurs in plants when the plant cells contain too little water to maintain their rigidity xerophyte

a plant that tolerates drought

xylem plant tissue through which water entering at the roots is transported to all parts of the plant ziggurat

a stepped pyramid

zooplankton microscopic and very small animals, including the larvae of many species, that drift in surface waters

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Bibliography and Further Reading

BOOKS AND ARTICLES Allaby, Michael. Droughts. Rev. ed. New York: Facts On File, 2003. A general description of dry climates that includes an account and explanation of the dust bowl. ———. A Chronology of Weather. Rev. ed. New York: Facts On File, 2004. A general account that also includes two chronological lists, one of climatic disasters and the other of significant events in the development of the sciences of climatology and meteorology as well as brief descriptions of some of the persons involved. ———. Air, the Nature of Atmosphere and the Climate. New York: Facts On File, 1992. Includes a discussion on wind power. ———. Biomes of the World: Deserts (Vol. 2) and Tropical Grasslands (Vol. 9). Danbury, Conn.: Grolier Educational. 1999. These short books (part of a nine-volume series) provide a simple explanation of the problems of erosion and ways of reducing them. ———. Floods. Rev. ed. New York: Facts On File, 2003. Contains descriptions of nilometers and the Aswān Dams. Banister, Keith, and Andrew Campbell. The Encyclopedia of Aquatic Life. New York: Facts On File, 1985. Brewer, Richard. The Science of Ecology. 2d ed. Fort Worth: Saunders College Publishing, 1988. A comprehensive textbook on all aspects of ecology. Cross, Nigel, and Rhiannon Barker, eds. At the Desert’s Edge. London: Panos Publications, 1992. Interviews from the Sahel Oral History Project in which people living in the region talk about their own lives. Glenn, Edward P., J. Jed Brown, and James W. O’Leary. “Irrigating Crops with Seawater.” Scientific American (August 1998): 56–61. The article describes research into the use of seawater for growing farm crops. Halliday, Tim, and Kraig Adler, eds. The Encyclopedia of Reptiles and Amphibians. New York: Facts On File, 1986. A comprehensive reference.

Hattersley, Lia. “Electric Dreams.” New Scientist (March 6 1999): 30–33. Describes solar chimneys. Henderson-Sellers, Ann, and Peter J. Robinson. Contemporary Climatology. 2d ed. Harlow, England: Prentice Education, 1999. A textbook that is well written, not too mathematical, and makes the subject easy to understand. Heywood, V. H., R. K. Brumitt, A. Culham, and O. Seberg. Flowering Plant Families of the World. Richmond, England: Royal Botanic Gardens, Kew, 2007. Heywood, V. H. Flowering Plants of the World. New York: Oxford University Press, 1993. Botanical descriptions of all the families of flowering plants. Hidore, John J., and John E. Oliver. Climatology: An Atmospheric Science. 2d ed. Upper Saddle River, N.J.: Prentice Hall, 2002. An introductory textbook, written very simply and clearly. Kerr, Richard E. “Big El Niños Ride the Back of Slower Climate Change.” Science 283 (February 19, 1999): 1,108–1,109. Lamb, H. H. Climate, History and the Modern World. 2d ed. New York: Routledge, 1995. A clear, simply written, and highly readable account of the history of climate by one of the leading world authorities on the subject. Lutgens, Frederick K., and Edward J. Tarbuck. The Atmosphere. 7th ed. Upper Saddle River, N.J.: Prentice Hall, 1998. An introductory textbook to meteorology, written simply and clearly. Macdonald, David, ed. The Encyclopedia of Mammals. 2d ed. New York: Oxford University Press, 2006. A very comprehensive reference. McIlveen, Robin. Fundamentals of Weather and Climate. New York: Chapman & Hall, 1992. A textbook on climatology and meteorology written for more advanced students but containing simple explanations of many phenomena.

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Bibliography and Further Reading More, David M., ed. Green Planet: The Story of Plant Life on Earth. New York: Cambridge University Press, 1982. A wide-ranging account of plants and biomes. Oki, Taikan, and Shinjiro Kanae. “Global Hydrological Cycles and World Water Resources.” Science 313 (August 25, 2006): 1,068–1,072. O’Toole, Christopher, ed. The Encyclopedia of Insects. New York: Facts On File, 1986. A comprehensive reference. Perrins, Christoper M., and Alex L. A. Middleton. The Encyclopedia of Birds. New York: Facts On File, 1985. A comprehensive reference to all the families of birds. Service, Robert F. “Desalination Freshens Up.” Science 313 (August 25, 2006): 1,088–1,090. Tal, Alon. “Seeking Sustainability: Israel’s Evolving Water Management Strategy.” Science 313 (August 25, 2006): 1,081–1,084.

WEB SITES The content of most Web sites is evident from the title. A little more information has been added where this might help. “Çatalhöyük: Excavations of a Neolithic Anatolian Höyük.” Available online. URL: www.catalhoyuk.com/. Accessed August 15, 2006. A Web site designed for people interested in the ongoing excavations at this important archaeological site.

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Focusmm.com. “Focus on Catal Hoyuk.” Available online. URL: www.focusmm.com/civcty/cathyk00.htm. Accessed August 15, 2006. Describes Çatal Höyük, with details of its architecture and construction, the lives of its inhabitants, and the excavation and research of the site. Geoscience Australia. “Deserts.” Australian Government. Available online. URL: www.ga.gov.au/education/facts/ landforms/geogarea.htm. Accessed March 21, 2005. Hopkins, Angas. “Tirari-Sturt Stony Desert.” Worldwildlife. Available online. URL: www.worldwildlife.org/wild world/profiles/terrestrial/aa/aa1309_full.html. Accessed March 22, 2005. ———. “Great Sandy-Tanami Desert.” Worldwildlife. Available online. URL: www.worldwildlife.org/wildworld/ profiles/terrestrial/aa/aa1304_full.html. Accessed March 22, 2005. Hulbe, Christina. “Larsen Ice Shelf 2002.” Portland State University. Available online. URL: http://web.pdx. edu/~chulbe/science/Larsen/larsen2002. html. Accessed May 20, 2005. International Whaling Commission (IWC). “International Whaling Commission.” Available online. URL: www. iwcoffice.org/commission/iwcmain.htm. Accessed September 20, 2006. The home page of the IWC. Leslie, John. “New Iceberg Breaks Off of Larsen Ice Shelf.” NOAA. Available online. URL: www.noaanews.noaa. gov/stories2005/s2384.htm. Posted February 4, 2005. Accessed May 20, 2005.

Colmer, Tim. “Salt-tolerant Wheats Ready for Field Trials.” University of Western Australia. Available online. URL: http://www.uwa.edu.au/media/statements/ media_ statements_2006/march/salttolerant_wheats_ready_ for_field_trials_ (27_march). Posted March 27, 2006. Accessed September 12, 2006.

McDonald, James E. “Climatology of Arid Lands.” University of Arizona. Available online. URL: http://alic.arid. arizona.edu/sonoran/documents/alc_climatology_ aridlands.html. Last updated October 29, 2002. Accessed May 27, 2005. A description of the world’s deserts and explanation of their climates by a meteorologist.

CSIRO Australia. “Drought: Air Pollution Link Found.” CSIRO Australia. Available online. URL: www.dar. csiro.au/news/2002/MR05.html. Modified May 16, 2002. Accessed September 5, 2006.

Morton, S. R., J. Short, and R. D. Parker. “Refugia for Biological Diversity in Arid and Semiarid Australia; Little Sandy Desert.” Biodiversity Series Paper 4, Biodiversity Unit. Dept. of the Environment and Heritage, Australian Government. Available online. URL: www.deh.gov. au/biodiversity/publications/series/paper4/lsd.html. Last updated June 20, 2004. Accessed March 22, 2005.

Dillon, William. “Gas (Methane) Hydrates—A New Frontier.” U.S. Geological Survey. Available online. URL: http:// marine.usgs.gov/fact-sheets/gas-hydrates/title.html. Posted September 1992. Accessed August 30, 2006. Environmental Technologies Action Plan. “Water Desalination Market Acceleration.” European Union. Available online. URL: http://ec.europa.eu/environment/etap/ pdfs/waterdesalination.pdf. Posted April 2006. Accessed August 3, 2007. Flinders Ranges Research. “Captain Charles Sturt.” Available online. URL: www.southaustralianhistory.com/au/ sturt.htm. Accessed March 22, 2005.

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National Renewable Energy Laboratory (NREL). “HighFlux Solar Furnace.” Available online. URL: www.nrel. gov/facilities/highflux.html. Updated January 16, 2006. Accessed September 1, 2006. National Snow and Ice Data Center (NSIDC). “Larsen B Ice Shelf Collapses in Antarctica.” Available online. URL: http://nsidc.org/iceshelves/larsenb2002/. Posted March 18, 2002. Updated March 21, 2002. Accessed May 20, 2005.

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Oak Ridge National Laboratory. “Methane Hydrates.” Available online. URL: www.ornl.gov/info/reporter/ no16/methane.htm. Accessed August 30, 2006.

———. “Antarctic Explorers: James Clark Ross (1800– 1862).” Available online. URL: www.south-pole.com/ p0000081.htm. Accessed August 29, 2006.

Petniunass, Anthony. “Mount Kosciuszko.” Available online. URL: www.poles.org/strzelecki.html. Accessed March 22, 2005. Describes the national park.

Strzelecki Committee. “Sir Paul Edmund Strzelecki.” Available online. URL: www.poles.org/strzelecki.html. Accessed March 22, 2005.

Robert, J. F., and J. Giral. “1000 kW Solar Furnace.” CNRS. Available online. URL: www.imp.cnrs.fr/foursol/1000_ en.shtml. Accessed September 1, 2006.

U.S. Department of Energy. “Methane Hydrate—The Gas Resource of the Future.” Available online. URL: www. fossil.energy.gov/programs/oilgas/hydrates/index.html. Accessed August 30, 2006.

South-pole.com. “Antarctic Explorers: Lincoln Ellsworth (1880–1951).” Available online. URL: www.south-pole. com/p0000110.htm. Accessed August 15, 2006.

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Index

Note: Italic page numbers indicate illustrations; m refers to maps

A ablation 36, 40 aboriginal peoples 29m, 264–265 absolute humidity 7 Abu Simbel 274 Acosta, José de 192 addaxes 155–156 adiabatic cooling 45 adiabatic lapse rate 37 adiabatic warming 45 adobe 210 Africa. See also North Africa buildings in 211–212 explorers in 217–219 agamid lizards 151–152 Agassiz, Louis 45–46 agaves 110, 110–111 agriculture 173–176, 178–179 Cahokia 175–176 corridor farming 258 dry farming 258 Egypt and North Africa 173–175, 189–190 farmers v. pastoralists 268–269 Indus Valley 175 new desert crops 258–260 oasis farming 280–281 origin of wheat 178–179 water needed for 185 Ahuítzotl 193 air heat and aridity 7 transporting of heat by 78 air mass 31 air pollution 241 Akkad 176 aklé dunes 61

albatrosses 170–171 albedo 36 Aleut 206–207 algae 55 Algeria 224 Allerød Interstadial 179 alley cropping. See corridor farming alluvial fan 63 almonds 113 altiplano 26 alveoles 64 amaranths 259 American Desert, peoples of the 202–205 Amundsen, Roald 216, 217 Anasazi 183–184, 203, 209 angle, of Sun’s rays 4. See also tilt, of Earth’s axis angular momentum 78, 79 animal life 122–172 adaptation to desert heat 123–128 in the Antarctic 167–170 in the Arctic 161–167 avoidance of freezing 128–131 birds 156–158 camels 138–142, 196–197 domesticated animals 180–181 estivation 132–134 evolution 142–145 exotherms v. homeotherms 122 finding and conserving water 136–138 hibernation 134–136 invertebrates 145–150 locusts 147–150 mammals 153–156 metabolic rate 123 reptiles 150–153 anorak 206 Antarctica 35m, 35–42 animal life 167–170 dryness of 6

factors affecting climate 36–37 as food source 289–291 ice formations 38–42 people in 208–209 temperature 38 wind in 93 Antarctic cod 291 antelope 125 anticyclones 26, 86–87 ant-lions 146, 146–147 ants 114–115 AO (Arctic Oscillation) 243 aphelion 251 apple-ring acacia 258, 260 aquifer 70, 70 aquifer depletion 184–186 Arabian Desert 4, 15, 15–16, 196–198 Arabian Peninsula explorers in 219–220 minerals in 234–235 Arabian toad-headed agamid 151 Arab nomads 195–196, 196m Aral Sea 185, 185–186, 277–279, 278 Archimedes 191 Arctic 37m animal life of 161–167 dryness of 6 food resources 289–291 peoples of 205–208 temperature in 38 weather of 37–38 arctic fox 129–130, 131, 143, 163–164 Arctic Oscillation (AO) 243 Arctic seals 172 armadillo lizard 152 arousal (from hibernation) 135 Arrhenius, Svante 244 arsenic 72 artesian well 71–72 artificial oases 281–282 Asia, explorers in 220–221

■ 311 ■

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asphalt roads 222–223 Assyrian Empire 181 Aswān Dam 274 Atacama Desert 5, 6, 24m, 24–27, 211 Atbara River 272 atmosphere and the desert 73–95 air mass 82–87 circulation of the atmosphere 77–79 fronts 82–87 Hadley cells 73–77 ITCZ 79–82 oceans and climates 87–90 weather 89–93 wind 72–77, 92–94 aurochsen 181 Australia 28–32, 264–265 Australian desert 4, 5, 28–32, 29m Aztecs 192–193

B Babur 177 Babylon 176, 182 Bactrian camel 139, 141 Baghdad, Iraq 177 bannertail kangaroo rat 136–137 barbarians 194 barchan dunes 62 barley 179 barrel cactus 117 Barth, Heinrich 218 basal metabolic rate (BMR) 123 basking 126–127, 127 bears, hibernation of 135, 136 bedforms 60 Bedouin 197–198 beetles 12, 146 Belgica (ship) 214–215 Bellingshausen, Fabian Gottlieb von 215 beluga 171 Benguela Current 21 Bennett, Floyd 213 Berbers 194 Bergen School 83 Bergeron-Findeisen process 253–254 Bergmann, Karl Georg 130 biennial plant 121 biology, of deserts 96–175 animal life 122–172 Antarctic animal life 167–170 Arctic animal life 161–167 ecology 158–161 life on bare rock 55–56 photosynthesis 96–101 plant life varieties 108–122 Polar Sea life 170–171 water for plant life 101–103

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biomass 159 bipinnate leaves 114 birds 156–158 of the Antarctic 167–170 of the Arctic 162–163 and Salton Sea 279, 280 birth rate 227–228 Bjerknes, Jacob 76 Bjerknes, Vilhelm 83, 84 blizzards 36 blocking 86–87 blood flow, regulation of 127 Blue Nile 190, 272, 273 BMR (basal metabolic rate) 123 Bolivia 26 bolson 26 Bonpland, Aimé 25 Borchgrevink, Carsten Egeberg 215 boundary currents 22, 23m boundary layer 7 breathing, moisture loss and 137–138 bricks 210–211 bristlecone pine 9–10, 10 British Petroleum 223 buffalo gourd 259–260 buildings, desert 209–212 adobe 210 Africa 211–212 bricks 210–211 Çatal Hüyük 209–210 cliff dwellings 211 covered pits 209 igloo 209 pueblo apartment buildings 211 Bukhara carpets 236–237 bull horn acacia 114–115 bulls 181 burrows life in 125 storing food in 136–137 butte 64 butterflies 147 buttes 30 Byblos 181–182 Byrd, Richard E. 213, 215

C C3 pathway 99 C4 pathway 99–100 cacimbo 24 cacti 116–118, 117, 118 cactus moth 117 Cahokia 175–176 Caillé, René-Auguste 218 Cairo, Egypt 189 calcite 66

Caledonian orogeny 54–55 Calvin, Melvin 98 Calvin cycle 98–100 CAM (crassulacean acid metabolism) 100 camels 138, 138–142 in caravans 200–201 domestication of 196–197 endurance of 141 and explosive heat death 140–141 feet and limbs of 141 heat adaptation by 125 humps of 139 insulation in 140 stomach of 139 temperament of 141–142 water storage by 139–140 candelabra tree 115 capelin 291 capillarity 68–69, 69 caravans 200–201 caravansary 201 carbon-14 248 caribou 164 carpets and rugs 236–237 cashmere 235–236 Çatal Hüyük, Turkey 178, 209–210 cats 154–155 cattle 181 Cave of the Thousand Buddhas 221 CCD (UN Convention to Combat Desertification) 256 CCN. See cloud condensation nuclei cell (atmospheric) 16, 26, 73–77 cell (biology) 133–134 cell (solar) 232 Central Asian desert, plant life of 121 Chad, Lake 8–9, 9 cheetahs 154, 154–155, 155 Chile 26–27 China 201–202, 220–221, 270, 271, 279 Chinampas 192–193 chlorophyll 96–97 chloroplast 97–98 chott 13 chuckwalla 152–153, 153 circadian rhythm 97 civilizations, desert 176–184 Akkad 176 and animal domestication 180–181 Babylon 176 and climate change 176–178 Egypt 176–177 Fertile Crescent 181–182 irrigation and flood control 182–184

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Index Mali Empire 177–178 Mongol Empire 177 Sumer 176 Turkey 178–180 Clapperton, Hugh 218 classification, of soils 57 cliff dwellings 211 climate of Antarctica 36–38 of Arabian Desert 16 of Australian desert 29–30 of Kalahari Desert 20 oceans and 87–90 optima and minima 8–11 of Sahara 15 climate change and air pollution 241 in early desert history 174–177 and El Niño 242 and future of deserts 240–246 global dimming 242 global warming 244–245 greenhouse effect 243–244 increasing wetness 245 in Indus Valley 175 and La Niña 243 in North Africa 174, 176–177 in North America 175–176 North Atlantic climate cycles 243 North Pacific climate cycles 243 prehistoric 179–180 in Sahara 175 and Sahel drought 241 and sea levels 246 uncertainties in 245–246 and Walker circulation 242 and weakening of Great Conveyer 246 climatic minimum 8–11 climatic optimum 8–11 climatologists 6 cloud condensation nuclei (CCN) 94, 241 clouds, and global dimming 242 cloud seeding 254–255 cod 291 cold and animal life 128–131 Antarctica v. the Arctic 38 in Gobi Desert 199 penguins’ adaptation to 168 cold climates, hibernation in 134–136 Colorado Desert 32–34 color changes, in desert reptiles 127, 128 conductivity 91 conservative margins 54

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constructive margins 53 consumers 159–160 contaminated wells 72 continental climate 31 continental desert, plant life of 199–122 continental drift 51–55 continental interior deserts 5 convection 74 convection cell 75 Cook, James 25, 208, 215 cooling (by mammals) 124 coppicing 258 coral snakes 150–151 core temperature, maintaining 129–130 Coriolis effect 22, 79, 94, 242 corridor farming 258 cosmic radiation 247, 248 cotton 275, 277 countercurrent exchange 131 covered pits 209 crabeater seal 171 cranes 163 crassulacean acid metabolism (CAM) 100 creosote bush 105, 110 crested lizard 128 Croesus 196 crop milk 156 cyanobacteria 55 cyclones 86–87 Cyrus the Great (king of Persia) 196

D Dalton, John 74 damping depth 91 dams 271–274 D’Arch, William Knox 223 darkling beetles 147 date palms 112, 262 death rate 228–229 Death Valley, California 32, 91 December 26, 2004, tsunami 54 deflation 14 dendrochronology 10 dendroclimatology 10 deposition 36 depressions 83–85 desalination 284–288 desert(s) advances/retreats of 264–266 distribution of 257m expansion of 256 future of 292 as portion of Earth’s land surface xiii desert fox 142–143

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desertification 256, 268–269 desert iguana 128 desert locust 149, 150 desert pavement 63, 63 desert rose 65–66, 66 desert varnish 65 destructive margins 53 detritivores 161 Dias, Bartolomeu 208 diet Bedouin 198 Inuit 207 Mongol 199 and rising demand for meat 268 distillation 285–288 diurnation 132–133 divergent evolution 142 Dogon people 212 dogs 165–166, 180, 206 dolphins 170 domestication of animals 180–181, 196–197 donkeys 141 Doughty, Charles Montagu 219 draa 60 Drake, Edward L. 222 dreikanter 64 dress, Bedouin 198 drift deposits 46 drinking water, contamination of 72 drip irrigation 261–262 dromedary 139, 141, 196 drought 240, 241, 267 dry farming 258 dry valleys 58, 120 Dumont d’Urville, Jules-Sébastien César 215 dunes in Australian desert 32 formation of 60, 60–61 geology of 59–62 in Sahara 13–14 types of 61, 61–62 durum wheat 179 dust bowl 240 dust devil 32, 95 dust storm 94–95 Duveyrier, Henri 218

E ears, of mammals 153 Earth. See also tilt, of Earth’s axis “axial wobble” of 249, 250 desert as portion of land surface xiii eccentric orbit of 250–252 easterly jet 82

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ecological pyramid 160, 160 ecology 158–161 economics of the desert 222–239 carpets and rugs 236–237 cashmere 235–236 methane hydrate 226 minerals 233–235 oil 222–226 process of development 226–229 and quality of life 229 solar energy 229–233 tourism 237–239 ecosystem 159 ectotherm 122–123 Egypt 176–177, 189–191, 238, 270. See also Nile River Eielson, Carl Ben 213 einkanter 64 Ellsworth, Lincoln 214, 215 El Niño 76–77, 242 El Niño-Southern Oscillation (ENSO) 242, 266 eluviation 56 Emi Koussi (extinct volcano) 175 emmer 179 emperor penguins 168, 168 Enchanted Mesa, New Mexico 203 endotherm 122, 123 ENSO. See El Niño-Southern Oscillation eolian processes 63 equatorial trough 82 erg 13, 14, 14, 60 erosion 65 erratics 45–46 Eskimo 206. See also Inuit estivation 132–134 euphorbias 115–116 evaporation 7, 8, 67 evaporation pan 6 evaporite deposits 65 evapotranspiration 8 evolution 142–145, 143 of desert foxes 142–143 divergent v. parallel 142 of kangaroo rats and jerboas 143–144 of marsupials 144 of snakes 144–145 exomosis 188 exotherm 122 exploration 212–221 Africa 217–219 Arabia 219–220 Asia 220–221 Polar deserts 212–217 explosive heat death 124–125, 140–141

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F falcons 157–158 Falkland Current 28 farming. See agriculture feathers 157 feeding 126 feeding pyramids 160, 160 Ferrel, William 75, 78 Fertile Crescent 181–182 fertilizer 262 field capacity 104 figs 112–113 firn 44 fish(es) drought survival by 133 as food source 291 in Salton Sea 279–280 fishery regulation 291 flood irrigation 273 floods ancient 9 control of 182–183 of Nile River 190–191 flower 108 Flowers, Tim 262 fluvial processes 63 flying, as heat adaptation 156 fog 26, 253 föhn winds 44 food. See also agriculture from Arctic/Antarctic 290–291 making water from 137 food chains 161 food webs 161 Forster, Georg 25 fossils 12 Fourier Jean-Baptiste-Joseph 244 foxes 129–130, 131, 142–143, 143, 163–164 Fram (ship) 212, 217 freezing and animal life 128–131, 134 distillation by 286, 287 effect on cells 133–134 frogs 133 frontal depressions 83–85, 85 fronts 82–87 frost, wind machines to protect against 253 Fulani 195

G gales 36 Galilei, Galileo 247 gardens 182, 193

gazelles 125, 155 GDP (gross domestic product), oil and 224–225 GEF. See Global Environment Facility genetic engineering 260 Genghis Khan (Temujin) 177, 237 geography, of deserts 1–49 Arabian Desert 15, 15–16 Atacama Desert 24m, 24–27 Australian desert 28–32 causes of desert conditions 6–8 desert pavement 63, 63 Gobi Desert 16–17 Greenland 42–45 ice ages 45–47 ice formations 38–42 Kalahari Desert 18–20 location 1–6, 2m North/Central American deserts 32–34 Patagonia 27m, 27–28 polar deserts 34–38 Sahara 11, 13–15 subtropical deserts 1, 3–5 Takla Makan 17 Thar (Great Indian) Desert 17–18 ventifact 63–64 geology, of deserts 50–72 Atacama Desert 26 desert soils 57–59 orogeny 54–55 plate tectonics 52–54 sand dunes 59–62 soils 55–59 and water 66–72 water in desert soils 69–72 ghibli 14 gibber 32 gibber plain 32 Gibson Desert 31 Gila monster 151 glaciers 40, 43–44 glassworts 262 global dimming 242 Global Environment Facility (GEF) 278–279 Global Mechanism (GM) 256 global warming 244–245. See also climate change GM (Global Mechanism) 256 goats 180 Gobi Desert 5, 16–17, 17m, 198–200 Golden Horde 177 golden mole 153 grasses 109, 109–110, 126, 259 grasshopper mouse 153 grasshoppers 148–149

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Index grazing animals 199–200. See also overgrazing Great Basin 184 Great Conveyor 88–89, 89m, 246 Great Ice Age 46 Great Sandy Desert 31 greenhouse effect 243–244 greenhouse gases 244–245, 267 Greenland 42, 42–45, 44m angle of Sun’s rays 4 geographical characteristics 42–45 ice sheet 246 Inuit colonization of 205–206 modern economy of 208 Fridtjof Nansen’s exploration of 212–213 temperature of 42–44 tourism in 237 whaling in 290 gross domestic product (GDP), oil and 224–225 grounding line 41 groundwater 67–68 G/wi people 265–266 gypsum 66, 233 gyres 22, 23m, 87–88

H Hadley cells 16, 26, 73–77 Haeckel, Ernst Heinrich 159 hajj 200–201 Halley, Edmund 74–75 halophytes 262–263 hammada 14–15, 63 Hammurabi (king of Babylon) 182 hanging gardens of Babylon 182 Hannibal 8–9 Hardin, James 270 harmattan 94 health of deserts 240–263 and climate change 240–246 and improved irrigation 261–263 and land management 255–258 and Milankovitch cycles 249–253 and new desert crops 258–260 and sunspot cycles 246–249 and weather manipulation (rainmaking) 253–255 heat animal adaptation to 123–128 and aridity of air 7 benefits to animal life 125–128 flying as adaptation to 156 for photosynthesis 101 and soil development 58 transporting of, by air 78

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heat exhaustion 124 heat stroke 124 hedgehogs 153 Hedin, Sven Anders 220 heliostats 231 hematite 50 Herodotus 196, 222 Herschel, Sir Frederick William 248 Hess, Harry Hammond 52 hibernation 134–136 Himalaya Mountains, formation of 51 history, deserts and 173–221 agriculture 173–176, 178–179 American Desert, peoples of the 202–205 aquifer depletion 184–186 Arabian Desert, peoples of the 196–198 Arctic and Antarctic deserts 205–209, 212–217 Aztecs 192–193 buildings 209–212 caravans and Silk Road 200–202 civilizations 176–178, 181–184 climate change 174–177, 179–180 domesticated animals 180–181 Egypt 189–191 explorers 212–221 Gobi Desert, peoples of the 198–200 irrigation and flood control 182–184 Sahara, nomadic peoples of the 194–196 salination 187–189 waterlogging 186–187 Hohokam 183, 203, 209 Holmes, Arthur 52 homeotherm 122 Hopi 204–205 horses 198–199 Humboldt, Alexander von 25 Humboldt Glacier 45 humidity, and saturation 7–8 hunting 207 huskies 206 hydrogen bond 39 hydroponics 101

I Ibn Sa’ūd, ‘Abd al’-’Azīz (ruler of Saudi Arabia) 219, 223 ice, from snow 38–40 ice ages 45–47 icebergs of Antarctica 41–42

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in Greenland 43–44 mining of 288–289 icefish 291 ice plant 115 ice sheets defined 40 of Greenland 42 melting of 246 ice shelf, Antarctic 40–41 igloo 209 igneous rocks 64 iguanas 128, 152 illuvial zone 57 Imazighen 194, 195 index cycle 86 Indian Ocean tsunami (2004) 54 industrial minerals 233–234 Indus Valley 175 insects 131 inselbergs 24, 64 insulation 130–131 insulation, of desert animals 140 interglacials 46, 47 International Whaling Commission (IWC) 289, 290 interstades 46 Intertropical Convergence Zone (ITCZ) 18, 79–82, 80m, 241 Inuit 205–208 invertebrates 145–150 ant-lions 146–147 butterflies 147 darkling beetles 147 locusts 147–150 robber flies 147 scarabs 147 scorpions 145–146 sun spiders 146 tiger beetles 146 wolf spiders 146 worm-lions 147 irrigation and flood control 182–184 drip irrigation 261–262 flood irrigation 273 improved irrigation 261–263 rising water table due to 186 and soil degradation 188 waterlogging 186–187, 187 Irtysh River 275, 276 Isaac (biblical figure) 282 Islam, rise of 197 isobars 82, 94, 94 isotopes 12, 39 ITCZ. See Intertropical Convergence Zone (ITCZ) IWC. See International Whaling Commission

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J jerboas 143–144 jet stream 82 jojoba 260 Jordan River 282 Joshua tree 111

K Kalaallit Nunaat. See Greenland Kalahari Desert 4, 19m climate 20 dwellers of 265–266 exploration of 219 geographical characteristics 18–20 kangaroo rats 129, 143–144 Kanniyākumāri, India 284–285 Kara Sea 275 katabatic winds 93 kayak 206 Kazakhs 199, 200 khamsin 14, 94 kidneys 138 king penguins 169 kit fox 143, 143 kowari 144 krill 171, 290–291 Kublai Khan 177, 202 !Kung 265 Kuwait 229

L Lagash 282 lag deposit 63 Laing, Alexander Gordon 217 land breeze 21–24, 23 land hunger 269 land management 255–258 Langmuir, Irving 255 La Niña 77, 243 Larsen Ice Shelf 41 last glacial maximum (LGM) 47–49 Lawrence of Arabia (Thomas Edward Lawrence) 219–220 lemmings 161–162 leopard seal 170 leveche 14 LGM. See last glacial maximum Libya 223–224 lichens 119–120 light and flowers 120 for photosynthesis 101 lions 155 litter 56

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Little Ice Age 11, 175, 240, 245, 248–249 Little Sandy Desert 31 Liu Ch’e (emperor of China) 201 livestock 268. See also overgrazing Livingstone, David 19–21, 219 lizards 127, 128, 151–153, 152 Lloyd, William Forster 270 locusts 147–150, 150 desert locust 149 as grasshoppers 148–149 plagues of the past 148 traveling swarms 149–150 2004–2005 plague 147–148 longitudinal dunes 61–62, 62 Lowe, Fritz 43 lynxes 154

M macroclimates 91–92 Magellan, Ferdinand 208 magma 53 magnetic field, of Earth 52 Makarikari Pan 19 Mali Empire 177–178 mammals 153–156 addaxes 155–156 of the Arctic 163–167 cats 154–155 cooling by 124 ears of 153 gazelles 155 oryx 156 in Polar Seas 170–171 rodents 153 management of the desert 264–291 advances/retreats 264–266 Arctic/Antarctic food sources 289–291 overgrazing 268–269 pastoralism 266–269 water 269–289. See also water management Mansa Musa 177 mantle hot spots 53m manufacturing 227 marmots 134 marsupials 144 mastigure 152 Maunder, Edward Walter 247–249 Maunder Minimum 247–248 Mauritania 229 Maussa, Fauré 267 McKenzie, Dan 52–53 meat 268 Medieval Warm Period 11 membrane distillation 288

mesas 30, 64, 64 metabolism, of animals 123 methane hydrate 226 Mexican beaded lizard 151 Mexico 4, 192–193, 235 mice 153 microclimates 91–92 Middle Ages, climate of 11 Middle East minerals in 234–235 oil from 223–224 tourism in 238 Milankovitch, Milutin 249 Milankovitch cycles 249–253, 250, 251 and axial tilt 249 and Earth’s eccentric orbit 250–252 and perihelion/aphelion 251 and precession 249–250 and snowblitz theory 252–253 mineral resources 233–235 adding value to 235 of Arabian Peninsula/Middle East 234–235 industrial minerals 233–234 of Mexico 235 mining of 234 of Mongolia 235 of Namibia 235 of North Africa 234 of Southern Sahara/Sahel 234 minke whales 289 mixing ratio 7 Mohamed el Mokhtar, Fatimetou Mint 266–267 Mohammed 197 Mojave Desert 6, 32, 33, 107 Mojave ground squirrel 132, 137 moles 153 Mongol Empire 177 Mongolia minerals in 235 nomadic peoples in 198–200 tourism in 237 monitors 153 monsoon rains, capturing 261 monsoons 9, 81m, 81–82 Montezuma I (Aztec ruler) 193 Montezuma’s Well 259 Moon, Lake of the 192–193 Morocco 196 mountain avens 12, 12 mourning doves 156 mulberries 113 mulgara 144 multistage flash evaporation 285–286, 287 musk oxen 164

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Index

N NAC (North Atlantic Oscillation) 243 Nachtigal, Gustav 218–219 NADP (nicotinamide adenine dinucleotide phosphate) 98 Namib Desert 6, 19m, 20–21, 24, 219 Namibia 235 Nansen, Fridtjof 212–213 narwhal 171 Nasser, Gamal Abdel 274 Nasser, Lake 274 natural resources. See also water management of Arctic desert 206 securing access to 257–258 Nebuchadnezzar II (king of Babylon) 182 Negev Desert 183 Nestorians 201–202 névé 44 Nezahualcóyotl 193 Ngamiland 18–19 nicotinamide adenine dinucleotide phosphate (NADP) 98 Niebuhr, Carsten 219 Nile River 190–191, 271–275, 283 nilometer 190–191, 191 nomadic peoples of Arabian Desert 197–198 and pastoralism 266–269 of Sahara 194–196 North Africa. See also Egypt climate change in 174, 176–177 as granary of Rome 174 minerals in 234 tourism in 237–238 North America climate change in 175–176 deserts in 32–34, 33m Pleistocene ice sheet 48m North American kangaroo rat 143–144 North Atlantic, climate cycles over 243 North Atlantic Oscillation (NAC) 243 North Pacific, climate cycles over 243 North Pole, flights to 213 Nullarbor Plain 31–32 nunatak 120 Nunavut 205

O oasis 71, 71, 280–281 Ob River 275, 276 ocean currents, and weather 89–90 ocean gyre 87–88

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oceanic conveyor belt. See Great Conveyor oceans, and climate 87–90 ocotilla 142 Ogallala aquifer 186 oil 222–226 and asphalt roads 222–223 birth of oil industry 222 in Middle East 223–224 and national wealth 224–225 and OPEC 225 reserves of 225–226 rock 223 Oraibi pueblo 203 Organization of Petroleum Exporting Countries (OPEC) 224–226 orogeny 54–55 orographic lifting 32 oryxes 156 osmosis 97, 188, 287 ouadi. See wadi overgrazing 268–269 oxygen 12, 39

P Painted Desert 34, 34 paleoclimatology 12 Pallas’s cat 154 Palmer, Nathaniel Brown 215–216 palms 112, 112 Pangaea 51 parabolic dunes 62 parallel evolution 142 parent material 57 parka 206 partial pressure 7 pastoralism 194, 199–200, 266–269 Patagonia 27m, 27–28, 121–122 PE (potential evapotranspiration) 8 pedestal rock 64–65 Pedirka Desert 31 pedogenesis 56 penguins 168–169 perihelion 251 permafrost 59 permeability 69–70 Persian carpets 236 Peru Current 25–26 Petra, Jordan 174, 174, 174m Philby, Harry Saint John Bridger 219 phloem 102 phosphate 233–235 photoelectric cells 232 photorespiration 99 photosynthesis 96–101 and C3/C4 plants 99–100

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317

CAM and 100 and chloroplasts 97–98 essential nutrients for 100–101 heat/light for 101 and lichen 55 light-dependent reactions 98 light-independent reactions 98–99 and photorespiration 99 and respiration 100 and stomata 97 water for 185 phytobiont 55 pigeon milk 156 pigs 180–181 Pima 203–204 pits, covered 209 pit vipers 151 plant life of Australian desert 32 blooming of 106, 106–108, 107 cacti 116–118 of Central Asian deserts 121 of continental/polar deserts 199–122 flowering of 120 of Patagonia 121–122 reproduction in cold climate 120–121 salt-tolerant plants 262–263 survival tactics in desert 105–108 trapping of sand by 111–112 varieties of 108–122 water conservation by 103–105 water for 101–103, 137 plateau 26, 30 plate tectonics 52–54 Pleistocene ice sheet 48m polar bear 136, 136, 167 polar deserts climate of 8 explorers of 212–217 geographical characteristics 34–38 plant life 199–122 plant reproduction in 120–121 soils of 58 polar molecules 39, 68 polar seas, life in 170–171 Polisario Front 196 pollen 12 Polo, Marco 202 porosity 69–70 potential evapotranspiration (PE) 8 PPP (purchasing power parity), oil and 224 precession of the equinoxes 249–250 precipitation. See rain(s); rainfall pressure gradient 82

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prevailing winds 61 prickly pear 116, 116–117 primary consumers 159–160 primary growth 10 producers 159–160 ptarmigans 162–163 pueblo architecture 211 puna 26 purchasing power parity (PPP), oil and 224 pygmy planigale 144

Q qanāts 281–282 quartz 233 Quatar 229 quinoa 259

S R

radiation fog 26 rain(s) monsoon 261 vaporization of 66 raindrops, speed of 66 rainfall in Antarctica 35m in Australian desert 28, 30m rainmaking. See weather manipulation rain shadow 6, 92 ramón 260 Ramses II (king of Egypt) 274 rattlesnakes 151 reg 14, 63 reindeer 164, 289 relative humidity 7–8 reptiles 150–153 in cold climates 131 coral snakes 150–151 heat adaptation 125 lizards 151–153 pit vipers 151 resources. See natural resources respiration 99, 100 reverse osmosis (RO) 287 rhourd 62 Richtofen, Ferdinand Paul Wilhelm von 220 right whale 170, 289 rinderpest 265 rivers diversion of 275–280 reliability of 270 RO (reverse osmosis) 287 roadrunners 158 roads, asphalt 222–223

311-320_Eco-Deserts_idx.indd 318

robber flies 147 Rockefeller, John Davison 224 rock oil 223 rocks 55–56, 64–65 rodents 153 Rohlfs, Gerhard Friedrich 218 Rome, ancient 174 root hair 101–102 roots 101–102 rose of Jericho 108–109 Ross, James Clark 85, 215, 216 Rossby waves 85–86 Ross seal 171 rugs. See carpets and rugs rye 179

saguaro cacti 117, 118 Sahara 13m climate 9, 15 climate change in 175 geographical characteristics 4, 11, 13–15 minerals in 234 nomadic peoples of 194–196 oases in 281 structure of 13–14 Sahel drought in 241, 267 minerals in 234 saiga 164–165 salination 187–189 salt(s) as commodity 201 and oil 222 removing 189. See also desalination Salton Sea 279–280 salt-tolerant plants 262–263 salt weathering 64 Samarkand 237 San (Basarwa) 265, 266 sand conductivity/damping depth 91 trapping by plant life 111–112 sand cat 154 sand dunes. See dunes sandfish 151 sand grouse 157 sandstorm 14, 94–95, 95 Sarakolé Empire 177 Sargon (king of Sumer and Akkad) 176 saturation 7–8 saturation vapor pressure 7 Saudi Arabia 223 saxaul 121 scarabs 147

scavengers 161 Schaefer, Vincent 255 Schwabe, Samuel Heinrich 247 scorpions 145–146 Scott, Robert Falcon 213, 216–217 screw of Archimedes 191, 192 sea barley 262 seabirds 163 sea breeze 21–24, 23 seafloor spreading 52 sea levels, rising 246 seals 171–172 seas of sand 14. See also erg secondary growth 10 secretary birds 158 sedimentary rocks 64 seeding, cloud 254–255 seeds 106, 108 seif dune 62 selenium 279 serir 14, 63 Shackleton, Sir Ernest Henry 213–214, 217 shaduf 191, 273 shamanism 199, 200 shape changes, in desert reptiles 127 Sharru-ken (king of Sumer and Akkad) 176 sheathbills 168 Sheba, kingdom of 183 sheep 180, 200 sidewinders 144, 144–145 silica 50 Silk Road 201–202, 220 Simpson Desert 31 sirocco 14 skinks 151 snakes coral snakes 150–151 evolution of 144–145 pit vipers 151 snow and hibernation 135 ice formation from 38–40 snowblitz 252–253 snowy owls 162 soil 55–59 aging of 57 classification of 57 degradation of, from irrigation 188 of desert 57–59 formation of 56–57 of hot deserts 59 permafrost 59 in polar regions 58 of tundra 58–59 water in 69–72

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Index soil horizons 56, 56 soil moisture tension 103, 103–104 soil profile 56, 56 solar cells 232 solar chimney 229–231, 230 solar energy 229–233 solar furnace 231, 231 Solar One furnace 231 solar ponds 232–233 solar power, for distillation of water 288 solar wind 247 Somolo people 212 Songhai 177, 178 Sonoran (Yuma) Desert 34 southern oscillation 242 species distribution 51–52 specific heat capacity 90–91 specific humidity 7 spelt wheat 179 spiders 146 spiny lizard 152 Spörer, Gustav 247, 248 Spörer Minimum 247 spores 55 springbok 155 stades 46 Standard Oil 223 starbursts 253 Stein, Sir Aurel 220–221 stomata 97 storms, taming of 254–255 stratosphere 78 Strzelecki Desert 31 Sturt’s desert rose 113 Sturt Stony Desert 31 subduction 52 sublimation 8 subtropical deserts 1, 3–5 sulfur dioxide 241 Sumer 176 sun spiders 146 sunspot cycles 246–249 sunspots 246–249 surface, of desert 62–66 sutures 53 Svalbard 37

T tafoni 64 Takla Makan 5, 17 tamarisks 121 Tamerlane 237 Tanami Desert 31 Tarahumara 204 tectonic plates 53m

311-320_Eco-Deserts_idx.indd 319

teleconnections 76 temperature. See also cold; heat of Atacama Desert 24–26 and basking 126–127 core 129–130 of Greenland 42–44 and metabolic rate 123 and water vapor 76 temperature inversion 26 tents 198, 199 Teotihuacán 192–193 Thar (Great Indian) Desert 17–18, 18m thermal equator 82 thermal wind 14, 83 thermohaline circulation 88. See also Great Conveyor Thesiger, Sir Wilfred 220 thorn trees 113–114 thorny devil 152 thorny lizards 152–153 Three Gorges Dam (Yangtze River, China) 271 Tibesti Mountains 9 tiger beetles 146 Tigris River 283 tilt, of Earth’s axis 2, 3, 249–250, 250 Timbuktu 217–218 Timur 237 Tirari Desert 31 toads 133 Tohono O’odham 204 tourism Antarctic 208–209 destructiveness of 239 and economic development 237–239 trade winds 22, 73–74, 242 tragedy of the commons 270 transhumance 194 transpiration 8, 101–103, 185 transverse dunes 61 tree rings 9–11, 10 Tropics 1, 3 tropopause 78 troposphere 77–78 tsunami 54 Tuareg 194–195, 195, 212, 267 tumble grass 108 tundra 58–59, 119, 206 tundra wolf 165–166 Turkey 178–180, 209–210

U Ulaanbaatar, Mongolia 237 Umma 282

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319

UN Convention to Combat Desertification (CCD) 256 United Arab Emirates 229 urine 138

V valley glacier 40 vapor compression distillation 287–288 vapor pressure 7 vegetation. See plant life ventifacts 63–64 veterinary care 268 vetiver 258–259 Villumsen, Rasmus 43 Vonnegut, Bernard 255 vorticity 78, 86 Vostok Station 37, 38 vultures 158

W wadi 13 Walker, Sir Gilbert 75–76, 242 Walker circulation 242 walrus 172 water. See also water management for animals 136–138 for birds 156–157 for camels 139–140 and capillarity 68–69 chemistry/physics of 6–7 in desert soil 69–72 downhill flow of 67 and geology of deserts 66–72 for plants 101–105, 137 and soil development 58 waterlogging 186–187, 187 water management 269–289. See also irrigation and flood control aquifer depletion 184–186 artificial oases 281–282 conflict over water resources 282–284 dams 271–274 desalination 284–288 distillation 285–288 iceberg mining 288–289 irrigation and flood control 182–184 oasis farming 280–281 river diversion 275–280 water molecule 40, 67 water stress 184 water table 67–68 water vapor 76

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320

✦

DESERTS

weather 89–93 of the Arctic 37–38 ocean currents and 89–90 weathering 55, 64 weathering rind 65 weather manipulation 253–255 Weddell, James 215 Weddell seal 171–172 Wegener, Alfred Lothar 42–43, 51, 52 wells 71, 72 welwitschia 116 west coast deserts 5–6 wetness, increasing 245 whales 170–171 whaling 207, 289–290 wheat, origin of 178–179 whirlwind 95 White Nile 190, 272, 273 wild boar 180, 181 Wilkes, Charles 215 Wilkins, Sir George Hubert 213

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Wilson, Edward 213 wind 72–77 in Antarctica 93 circular flow of 94, 94 dust devil 95 dust storm 94–95 in hot deserts 92–93 and sand dunes 14 sandstorm 94–95 thermal 83 whirlwind 95 wind machines 253 wind power 231–232 winter, in Gobi Desert 199 winter fat 121 Wolf, Rudolf 247 wolf spiders 146 wolverines 166–167 wolves 165–166, 180 woody perennials 121 worm-lions 147 Wu-ti (emperor of China) 201

X xylem 102

Y yak 200 Yangtze River 270, 271, 279 Yellow River 271, 279 Yesugei Ba’atur 177 Younger Dryas 179–180, 252 Yucca moth 111 yuccas 111, 111 Yuma 204

Z Zahir ud-Din Mohammed 177 ziggurat 182

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