HOW DESERTS FORM

The answer to the question “Why do deserts form?” seems obvious—sustained lack of rainfal—but he global and local climatic conditions that lead to such aridity are complex and an understanding of them helps explain such apparent anomalies as coastal deserts.

Deserts are among some of the most alien, inhospitable landscapes on the planet. Some of their most striking features—the vast fields of rubble, austerely patterned dunes, dry or seasonal riverbeds, gleaming rinks of sun-baked salt, and the seeming near-absence of life might lead an observer to suspect they are the result of some great global catastrophe. In fact, just like any other biome, or major habitat, such as rain forest, tundra, and steppe, the world's deserts have evolved over millennia—the result of complex interactions between climate and geology.

In this chapter we look at the climatic conditions, such as global wind and ocean currents and continental rainfall patterns, that have shaped both the deserts of the prehistoric past and those of today. Later, in Chapter 8, we will see how future climate changes—some of then human-caused—might shape the deserts of tomorrow.

PREHISTORIC DESERTS

Deserts have not always been where they are today. They have grown and shrunk and shifted around the planet over millions of years—a natural consequence of the great changes wrought upon the earth through the geological ages. Continents have drifted around the globe, sea levels have risen and fallen, temperatures have fluctuated, and climate patterns have shifted. Doubtless these changes, once natural but increasingly affected by human activity, will also shape the deserts of the future.

Deserts have probably never been so extensive on earth as they were during the Permian period, the last phase of the Paleozoic era, some 290 to 245 million years ago. At this time all of the main landmasses—the continents—were butted up against each other to form one giant block of land, the supercontinent known to geologists as Pangaea. The global climate of Pangaea was in some ways uniform, without such variations in temperature from equator to poles as exist today, for example. In terms of rainfall, however, the continent was far from uniform. Winds picked up moisture as they blew over the earth's seas and dropped this as rain near the coasts. As they blew on and reached the supercontinent's vast inland regions, however, they became dry as bone. Deprived of almost all precipitation, huge tracts of the interior of Pangaea, far from the sea, were harsh, barren scrub or near-empty desert.

Geologists have surmised the existence and location of prehistoric deserts from the evidence of rocks and fossils. Rocks have been forming, breaking down, and reconstituting almost since the earth came into existence some 4,600 million years ago. Different types of rocks reflect the conditions of their formation. For example, limestones such as chalk are laid down on the beds of great seas, while coals originated as the lush, part-decomposed plants that thrived in ancient swamps. Dark basalts were once vast flows of molten rock, or lava, that oozed up from the depths of the earth and slowly cooled and solidified. The characteristic rocks that indicate the existence of prehistoric deserts are sandstones, which formed as grains of sand became buried, compacted, and “glued” together. (By a curious process of geological reversal, the sands of the “classic” modern desert usually result from the erosion of these same ancient sandstones. Some sandstones are formed in shallow seas, but their detailed makeup differs sharply from that of dry-land desert sandstones.

During the Triassic period (245–208 million years ago)—the first phase of the Mesozoic era that followed the Paleozoic—many “red-rock” deposits were laid down. These widespread features of arid conditions are found, for example, in Australia's desert interior. The red rocks include sandstones, siltstones, and shales that have been colored red by the oxidation of one of their chief iron-containing minerals, hematite (ferric oxide).

The evidence of fossils

The difference between the sea- and land-formed sandstones is made still clearer by the types of fossils that these rocks contain. While shallow-water sandstones contain the remains of fish, shell-fish, and similar sea life, embedded within desert sandstones are the fossilized bones, teeth, claws, and other parts of land-dwelling animals. Many of these were reptiles, which, as we shall see in Chapter 4, are peculiarly well-adapted to life in arid conditions. About halfway through the Triassic period the typically large reptiles known as the dinosaurs appeared. At or around the same time, the first, small, shrewlike mammals also evolved. Fossil evidence also shows that the plants that flourished in Pangaea were welladapted to its arid conditions. Ginkgoes, seed ferns, cycads, and, increasingly, conifers all
flourished.

The evidence of both rocks and the fossils that they contain has suggested to paleontologists the existence of a dry, rocky countryside with scattered patches of vegetation where reptiles, insects, and scorpions scratched a living. It was this general landscape that dominated the massive interior of the supercontinent, Pangaea. Toward the end of the Triassic period, Pangaea began to break apart into smaller blocks, and the world's oceans extended inlets and arms deep into the gaps. As the newly formed continents drifted apart and the interiors of the landmasses grew nearer the sea, moisture-laden winds were able reach inland areas. By the start of the next great time span, the Jurassic period (208–146 million years ago), the climate on land had become warm and moist. Greenery spread rapidly, and the great “Age of Deserts” drew to a close.

The formation of modern deserts

The majority of modern deserts began to take shape around 13 million years ago, while the distribution of deserts we see today seems to have been established by about three million years ago. However, many dry regions have shifted and fluctuated in size since then and continue to do so.

As we saw in the Introduction, deserts are characterized by very low rainfall and other types of precipitation. However, this feature is not caused in the same way in every desert region but is the result of a complex combination of factors. This very complexity of causality in their formation explains what might appear to be as the anomalous or surprising location of some of the world's great deserts. Close to both the Arabian Sea and the great Indus River, for example, the existence of the Thar, or Great Indian, Desert, may seem strange. A better understanding of climate and, specifically, of why aridity prevails in certain regions—on both global and local levels—helps unlock such apparent mysteries.

As we shall see, in the majority of instances the key factor in desert formation is latitude—the position up or down the globe, north or south from equator to pole—with its concomitant effects on levels of rainfall. In other deserts, however, other elements are decisive, such as their distance from the sea or the presence of nearby mountain ranges. In still others, the balance of factors is more complex—a subtle amalgam of various contributing factors. In general, however, it is possible to group deserts into one of three major categories—subtropical high-pressure deserts, rainshadow deserts, and continental deserts.

SUBTROPICAL HIGH-PRESSURE DESERTS

A large number of the world's deserts are found in regions immediately north of the tropic of Cancer (the parallel of latitude about 23?° north of the equator) or immediately south of the tropic of Capricorn (the parallel of latitude about 23?° south of the equator)—that is, in the so-called subtropics. The reasons for this arise from patterns of air and water movement across the earth, which are themselves the result of complex interactions between global phenomena, such as the earth's 24-hour, west—east rotation and solar energy.

Global wind patterns

In part, global wind patterns are caused by the way in which the sun's rays warm the earth. The part of the earth that receives most of the sun's heat is the tropics—the region about the equator that lies between the tropics of Cancer and Capricorn. In the tropics the sun's rays hit the earth almost at right angles, concentrating their heat energy on the smallest surface area. They also pass through the least depth of atmosphere, with minimal scattering and spreading, before they reach the surface. Farther north and south from the equator the sun's rays approach the earth at a slanting angle and their heat energy covers a correspondingly larger area. They also have to pass through a much greater depth of atmosphere, causing their heating effects to be spread and dissipated.

The greater heat at and near the equator means that the air there becomes hot and rises, allowing cooler air to flow in. The rising hot air might be expected to move away, due north and south. That this does not quite happen is due to the Coriolis effect, or force, named after the French civil engineer Gaspard Coriolis, who first noted the phenomenon in the 19th century. Under its influence the hot equatorial winds blowing north and south are deflected from west to east, while cooler winds drawn back to the equator are deflected from east to west. These masses of air are known as the trade winds, which blow steadily for much of the year from the northeast north of the equator and from the southeast to its south, roughly between latitudes 0 and 30°. (The term “trade” used to describe the winds derives from an obsolete meaning of the word—“in a regular course or direction”—but also reflects the winds' importance for merchant shipping.)

Wet tropics, dry subtropics

The sun's heat at the tropics not only warms air, it also evaporates ocean water into water vapor that disperses into the air. The rising moisture-laden warm air rapidly expands and cools as its pressure reduces (since atmospheric pressure is highest at the earth's surface and decrcases with height), and its moisture condenses back into water, falling as rain that is largely confined to a belt about 10° north and 5 to 10° south of the equator. This is why much of this belt, the central tropics, is extremely moist and covered with dense, lush vegetation.

The now almost moistureless air continucs to gain height and flow northeast or southeast, pushed by more hot, moist air rising at the central tropics. Gradually it moves beyond about 20 to 25° north and south of the equator, into the subtropics, and becomes cool enough to descend. As it does so, its pressure rises, rehcating the air, just as the air squeezed in a bicycle pump becomes hot. This is the warm, dry, high-pressure air found about 25 to 30° north and south of the equator. It is also the air that helps create most of the world's deserts.

Circulation of air

Most of the dry, warm air that descends at 25 to 30° north or south loops back to move at surface level in the reverse direction of' its outward journey. Gradually it is drawn back toward the tropics, is recharged with heat and moisture, and so continues its movement. The end result is a cycle of air moving from the surface at the tropics up into the atmosphere, and then either northeast or southeast, sinking back to the surface at 25 to 30° north or south; this cooler air then returns toward the tropics from the northeast or southeast in the form of the trade winds. This air circulation forms corkscrewlike spirals north and south of the equator, known to climatologists as Hadley Cells.

The Hadley Cells are not self-contained. Some of the warm air descending at 25 to 30° north or south flows, not toward the equator again, but away from it, toward the middle latitudes. There it mixes with cold air coming from the polar regions, creating “battlefields” between warm and cold known as fronts. The fronts provide many temperate regions with their changeable weather.

High-pressure deserts

We have seen how the interaction of atmosphere, winds, and ocean currents sets up belts of high atmospheric pressure at subtropical latitudes of about 25 to 30° north and south of the equator, and that most of the world's deserts are found in or next to these belts. Consequently it is can be said that there are no true deserts within 10° north or south of the equator, although there are some arid or semidesert regions, such as those that occur on the Horn of Africa. Likewise, there are no major arid areas beyond about 45° north or south, with the notable exception of the polar deserts.

Deserts that owe their formation chiefly to these 25 to 30° north/south high-pressure zones are often known as high-pressure, or subtropical, climate deserts. Other deserts are sited in or near the 25 to 30° north/south zone but are maintained chiefly by additional factors, which are discussed below.

In the northern hemisphere, the two main examples of high-pressure climate deserts are the great Sahara and its eastern neighbor, the Arabian Desert complex. Both these deserts receive air that originates from the moist tropical zone to the south but which has already given up its moisture. The center of the Sahara is made even drier because of its distance from the sea—the continental desert effect discussed below.

In the southern hemisphere the persistent high-pressure atmospheric features described above produce deserts at latitudes of around 25° south. These southern high-pressure climate deserts include the Sechura and Atacama deserts in South America, the Namib and Kalahari in Africa, and most of the deserts in Australia.

RAIN-SHADOW DESERTS

In some instances, the rain-shadow effect is the primary cause of desert formation. In others, it intensifies or confirms the arid conditions already set in place by other factors, or it may help to delineate the extent of desert areas.

Air gathers moisture from the sea in the form of water vapor and, as it moves inland, blows steadily against a mountain range. The windward slopes of the mountains — those facing the oncoming winds—make the air rise (a phenomenon known as orographic lifting; oros is the Greek word for “mountain”). Because atmospheric temperature falls with increasing height—by an average of some 6°C (11°F) for every 1,000 meters (3,280 ft.)—the air mass cools as it rises. And because cooler air can hold less moisture as water vapor compared to warm air, water vapor condenses into droplets that fall as rain on the windward side of the slopes, turning to snow with increasing altitude.

The air now moves on, over the peaks of the mountains and down the leeward slopes—those lying away from the prevailing winds. By now it has lost most of its moisture content, so that little or no rain falls, creating a dry region on the leeward side of the mountains. This area is not only drier but much less cloudy than the other side of the mountains. Without cloud cover, the warming effect of the sun's rays is maximized, and the little water that does fall is lost through evaporation. Thus, in the same way that high peaks block light coming from the sun and cast deep shadows on the opposite side, the peaks also block moist air and rain from the leeward side, hence the term rain-shadow. The rain-shadow effect is most extreme when the mountains are tall and lie perpendicular to the prevailing winds, and when these winds shift only rarely from their regular direction. The effect can extend for hundreds of kilometers beyond the mountain range before other air masses are able to dilute it.

North American rain-shadow deserts

Two deserts where the rain-shadow effect predominates are the Mojave and Patagonian deserts of America. The Mojave in California lies in the lee of the Sierra Nevada, a mountain range to its west. The range runs roughly north-south for more than 600 kilometers (370 mi.), with an average width of some 100 kilometers (62 mi.), and includes the highest mountain in the contiguous United States, Mount Whitney at 4,418 meters (14,495 ft.).

Winds blowing east from the Pacific Ocean shed their rainfall over the first line of hills, the Coastal Ranges of California, and then against the wind ward wall of the Sierra Nevada. By the time the air reaches the Mojave, it is bone dry and the land resembles a moonscape, with salt flats, rocky outcrops, and sand-covered plains. The land surface of the desert was once the bed of the Pacific Ocean, until volcanic activity and mountain-building created the Sierra Nevada and Coastal Ranges and cut it off from the sea. A northern spur of the Mojave is the infamous Death Valley, the landscape of which reflects one of the most extreme climates found on the planet.

South American rain-shadow deserts

The Patagonian Desert in South America lies in the rain-shadow of the mighty Andes Mountains that run parallel to the western edge of the continent. Prevailing winds in this temperate area, some 40 to 50° south, are from the northwest and gradually lose their moisture content over the high Andean peaks, many of which approach 4,000 meters (13,000 ft.) in altitude. Cool, dry winds—known as pamperos—sweep down the western Andes foothills at an altitude of about 1,500 meters (4,900 ft.), and across the Patagonian plains. Their moisture evaporated, they carry away any rain that has fallen, thereby accentuating the drying effect of the Andean rain-shadow.

CONTINENTAL DESERTS

Another factor in the formation and location of deserts is distance from sea. Air moving over seas and oceans collects water vapor evaporated from the surface by the sun's heat. As the moisture-laden air crosses the coast and blows over the land, it rises and cools, causing the water vapor to condense and fall as rain or snow.

The air gradually loses its moisture and moves on. Unless it crosses a major lake or river system, where more sun-powered evaporation can take place, the air's water vapor content is not replenished, and the farther it travels from the coast, the drier it becomes. Consequently, regions located deep inland, in the centers of continents, generally have dry climates; wherever winds may blow from, they have generally lost their moisture by the time they arrive. In some cases the dryness is severe enough to maintain deserts. Since these deserts are found toward the middle of major landmasses, they are termed continental deserts.

Asian continental deserts

The greatest collection of continental deserts is found in Asia—unsurprisingly, given its vast land area of some 33,391,162 square kilometers (17,139,445 sq. mi.). The Kara-Kum, Taklimakan, and Gobi deserts have all formed mainly owing to the continental effect. The formidable Gobi, for example, is more than 2,000 kilometers (1,240 mi.) from any major body of water, while the virtually mountain-locked Taklimakan to its west is at a similar distance from the sea. The Kara-Kum, it is true, lies close to the Caspian Sea, but this relatively small, and shrinking, body of water is not sufficient to compensate for the desert's remoteness from oceanic shores and moisture-bearing winds.

These Asian deserts also experience rain-shadow effects from the world's largest, tallest mountain range, the Himalayas, which lies to the south and southwest. In this temperate zone, regular winds coming from the southwest drop heavy rain and thick snow on the Indian subcontinent and the windward sides of the peaks. To the north of the Himalayas, however, the air has run out of moisture. The Taklimakan is “rain-shaded” on the west and north, too—by the lofty Hindu Kush and Tian Shan mountain ranges respectively.

Australian continental deserts

The continental effect also contributes to maintaining the great inland deserts of Australia. Indeed, this “desert continent” has contributions from all three major desert-creating processes. The southern high-pressure climate belt, centred on the 25° south latitude, runs across the landmass. The Gibson and Simpson deserts are mostly more than 1,000 kilometers (620 mi.) from any coastline, while the Great Dividing Range, which runs down Australia's east coast, has a rain-shadow effect due to prevailing easterly and southeasterly winds. This is another reason why the western half of Australia, where the Great Sandy, Gibson, and Great Victoria deserts are located, is drier than the eastern.