In the Western imagination the word “desert” most often evokes a landscape of endless gigantic sand dunes, dazzling white under a cloudless hot-blue sky and a blazing sun. This landscape of the imagination is likely to be empty—deserted—except, perhaps, for a caravan of nomads and camels that inches slowly across the horizon, or a lone man stumbling, sun-blackened and sun-parched, through the heat haze. Or there may even be an emerald-green oasis, where tents are set out in the shade of a palm grove—though this, of course, may be nothing but a tantalizing mirage. This is the magnificent and exotic landscape of movies such as David Lean's Lawrence of Arabia (1962) and Bernardo Bertolucci's The Sheltering Sky (1990), and of countless adventure stories of intrepid travelers and explorers.

This idealized or classic landscape is not pure fantasy: parts of the Sahara, Arabian, and other deserts fit quite well with this image—though perhaps with less Technicolor vibrancy. The stereotype does, however, contain some misleading notions, of which the most notable is that all deserts are hot, and that heat is crucial in defining what constitutes a desert. Temperature actually plays a secondary role or no role in such definitions—not all deserts are hot, and even so-called hot deserts are not hot all the time. The Gobi Desert deep within Central and East Asia, for example, has relatively cool but erratic temperatures even in summer and can be brutally cold in winter, and in the Sahara temperatures can easily plummet to 4°C (39°F) at night. Modern geographers also recognize the category of the polar desert, applying it to all of Antarctica and parts of the Arctic (notably Greenland), where temperatures day and night stand at the opposite extreme to those of daytime hot deserts.

Even a brief perusal of the photographs included in this book will suggest a much more varied, and even nebulous, notion of what is—or is sometimes—meant by the term “desert.” There are vast gravel plains, gleaming expanses of sun-baked salt, and rugged, eroded landscapes of pinnacles, canyons, and rock arches. There are deserts smothered with flowers and blooming cacti; there are others studded with oil wells or scarred by quarries. Some are washed by the ocean and bathed in fog, and some are ice-encrusted polar wildernesses. One of the surprising facts encountered in this book is that only 20 to 30 percent of the world's deserts are covered by sand, and that the world's great deserts in fact encompass a huge variety of terrains, not only relative to each other but sometimes within their own boundaries. There is, moreover, little exotic about the desert biome—almost 20 percent of the earth's land surface is desert, and there are deserts in almost every continent and at every latitude. Of the continents only Europe has no desert area. For many peoples of the world the desert is not a remote fantasy but a reality that impinges on their everyday lives.

Defining the desert

Definitions of the term “desert” are neither static nor absolute. All over the world the term “desert” and its foreign-language equivalents are culturally and topographically specific. European words such as “desert,”“desert” and “Wuste” emphasize the sense of abandonment that is the standard Western response to the desert landscape—an idea that is also reflected in the etymology of the name of the Namib Desert in southern Africa—“the place where there is nothing.” Arabic has not one but several words for “desert,” including erg (applied to large areas of sand or “sand seas”) and hammada (applied to stony plains), as well as the more general sahra, from whose plural form—sahara—the world's largest desert takes its name. The Turkic kum means literally “sand,” reflecting the sandy wastes of Central Asia—hence the Kara-Kum, or “Black Sand,” of Turkmenistan and the Kyzyl-Kum, or “Red Sand,” of neighboring Uzbekistan and Kazakhstan—while the Persian dasht means “plain” as well as “desert,” in reference to the plateau deserts that dominate central Iran.

Physical geographers and geologists must at least attempt to be more scientific in their definitions of what constitutes a desert, and they have debated and extended the possible meanings. Today they agree that the key determining factor is aridity, or the lack of plentiful and consistent rainfall—generally defined as less than 250 millimeters (10 in.) of annual precipitation. Such a definition extends the meaning of desert well beyond its traditional confinement to the hot deserts that have so exercised the European imagination. As Chapter 1 shows, low rainfall is a characteristic not only of the subtropical regions where most of the hot deserts—the Sahara, Arabian, and Australian deserts, for example—are located, but also of continental interiors, the western sides of continents, the leeward side of high mountain ranges, and parts of the Arctic and Antarctic regions.

Even this definition is by no means watertight; strict definitions always create seeming anomalies. The Kalahari in southern Africa is labeled a desert in every atlas, and its very name—meaning “the Great Thirst”—would appear to confirm this status. But most of the Kalahari receives roughly twice the amount of the annual maximum allowable precipitation and has a relatively rich vegetation, and therefore for some scholars this would-be desert falls outside the strict definition of the term. However, more complex definitions of aridity take into account the rate of evaporation as well as the amount of precipitation, and the Kalahari, despite its rainfall, has little standing water due to the dry heat that rapidly evaporates much of the land's moisture. In their pursuit of exact definitions, experts have sometimes devised formulas to indicate a particular region's “Index of Aridity.” One of the simplest, the Lang Rain Factor, for example, divides the annual precipitation (in millimeters) by the mean annual temperature (in centigrade). Other arid regions, while not generally called deserts and often receiving slightly more than the regulation 250 millimeters (10 in.) of rainfall, display some of the characteristics of deserts.

Such borderline “semiarid” regions are often covered by the terms “semidesert” or “drylands.” The Sahel in sub-Saharan Africa is one important area of semidesert. In recent years this vast region has come under close scrutiny as its poor but locally crucial arable and pastoral lands have become degraded and the Sahara Desert has crept southward.

The living landscape

Surprisingly perhaps, water plays a key role in shaping the desert terrain. This is because, when water does finally make its appearance in the desert, it usually does so in torrential form—powerful, destructive floods that rip through the land, sweeping away any debris or loose vegetation and over the centuries cutting channels—called “wadis” in North Africa and Arabia and “arroyos” in the Americas—deep into the landscape. Despite appearances, deserts are often mobile, changing landscapes, uniquely vulnerable to the often dramatic metamorphoses worked by weathering agents such as water, heat, and wind. Sand dunes slowly shift and grow; glistening salt pans become lakes and then dry hard again within weeks or days; and over millennia rocks are scoured and eroded into dramatic or bizarre forms, such as flat-topped mesas, mushroom-shaped zeugens, and awe-inspiring rock arches. The metamorphoses of the desert terrain form the subject of Chapter 2.

Life in the desert

Conventional wisdom depicts the desert as almost devoid of vegetation or wildlife, save perhaps for a sidewinding snake or rearing scorpion. It is seen as abandoned by human beings, who in this hostile environment are thought of as interlopers or aliens, there only because they are on their way to somewhere else or because they have fatally lost their way. In Chapters 3, 4, and 5 we shall see how many deserts, despite their dearth of water—the precondition for the survival of life—in fact provide a remarkably fertile habitat for plants, animals, and humans alike, each of which have found ingenious ways of making the best of the desert. Plants store water through months of drought or blossom and seed after rare rainfall in a matter of days, transforming bare landscape into dazzling fields of color. Animals live by night or burrow deep underground, or—as in the case of reptiles—are physiologically adapted to withstand the desert's temperature extremes. Humans living in and on the fringes of deserts have developed unique lifestyles that usually feature nomadism—a fluid way of life that is able to adapt swiftly and creatively to the vicissitudes of this harsh environment. Some of the world's earliest civilizations—including those of ancient Egypt and Mesopotamia—formed on the margins of great deserts, where the strenuousness of life called for the utmost in human endeavor.

The changing desert

Traditional nomadism is in most deserts a dying way of life. Colonialism, urban-biased political structures, and modern lifestyles and technologies derived from the West have proved antipathetic to traditional peoples everywhere, and the peoples of the desert are no exception. In Chapters 6, 7, and 8 we shall see how the desert, like every other biome, is facing unprecedented challenges to its very survival. In the late 19th and 20th centuries many deserts were found to harbor important mineral reserves, most significantly oil and natural gas, and consequently were and continue to be subject to intensive exploitation. Perhaps more damaging still has been the ambition to “make the deserts bloom”—to irrigate formerly arid land, often by exploiting nearby rivers or the water table. The Negev Desert in Israel, the Kara-Kum in Turkmenistan, and the Libyan Desert have all been subject to grandiose and often ill-considered schemes motivated by a mixture of political or propagandistic concerns as well as by more humanitarian considerations. The effects of such programs have often been catastrophic, best exemplified by the demise of the Aral Sea following the development of the Kara-Kum Canal during the Soviet period. Of more general environmental concern is the global problem of desertification—the degradation of the semidesert lands and drylands that border established, “natural” deserts through poor agricultural practices—a development that threatens millions of people with poverty and hardship. It is with this looming global catastrophe that the final chapter of this book is concerned.

The desert atlas

Interspersed with the text chapters of the book are atlas sections, arranged broadly by continent. All the world's great deserts are represented, including the polar deserts of the Arctic and Antarctic, and are richly annotated in order to give the reader a detailed understanding of their geography, ecology, and history. The maps are oriented to the north and, in addition to physical features—such as mountains, rivers, areas of sand, cities, towns, major roads, and railroads—include major political features such as national borders. With the exception of the United States and Australia, however, provincial or state borders are not shown. At the end of the book a glossary defines semispecialist terms used in the text. There is also a bibliography of the sources used for this book and recommended further reading for the reader who wishes to pursue a particular subject more deeply. The references include recent scientific and scholarly publications and websites; the latter are particularly useful for up-to-date information on changing data, such as rates of desertification.

This book draws on current scholarship, but it is not aimed at specialists. Instead it aims to give the general reader an insight into one of the world's least known habitats. The environmental concerns of recent years have tended to cluster around more appealing habitats, leaving the desert—too often perceived as some kind of ecological vacuum—largely overlooked. In its small way this book may help to rectify this injustice.