Desert

Not to be confused with dessert.

This article is about dry terrain. For the act of abandoning or withdrawing support, see desertion. For other uses, see Desert (disambiguation).

Traditional concept: Sand dunes in the Rub' al Khali ("Empty quarter") of Saudi Arabia, but most deserts are rocky, not sandy.

A desert is a landscape or region of land that is very dry because of low rainfall amounts (precipitation), often has little coverage by plants, and in which streams dry up unless they are supplied by water from outside areas. Deserts can also be described as areas where more water is lost by evapotranspiration than falls as precipitation. Desert plants must have special adaptations to survive with this little water. Deserts generally receive less than 250 millimetres (10 in) of rain (precipitation) each year.Semideserts or steppes are regions which receive between 250 millimetres (10 in) and 400 to 500 millimetres (16 to 20 in).

Physical geography

Classification

Cold desert: snow surface at Dome C Station, Antarctica

Deserts have been defined and classified in a number of ways, generally combining total rainfall, number of days on which rain falls, temperature, and humidity, and sometimes additional factors. For example, Phoenix, Arizona receives less than 250 millimeters (10 in) of precipitation per year, and is immediately recognized as being located in a desert due to its arid adapted plants. The North Slope of Alaska's Brooks Range also receives less than 250 millimeters (10 in) of precipitation per year and is often classified as a cold desert. Other regions of the world have cold deserts, including areas of the Himalayas and other high altitude areas in other parts of the world. Polar deserts cover much of the ice free areas of the Arctic and Antarctic. A non-technical definition is that deserts are those parts of the Earth's surface that have insufficient vegetation cover to support a human population.

Potential evapotranspiration supplements the measurement of rainfall in providing a scientific measurement-based definition of a desert. The water budget of an area can be calculated using the formula P − PE ± S, wherein P is precipitation, PE is potential evapotranspiration rates and S is amount of surface storage of water. Evapotranspiration is the combination of water loss through atmospheric evaporation and through the life processes of plants. Potential evapotranspiration, then, is the amount of water that could evaporate in any given region. As an example, Tucson, Arizona receives about 300 millimeters (12 in) of rain per year, however about 2,500 millimeters (100 in) of water could evaporate over the course of a year. In other words, about eight times more water could evaporate from the region than actually falls as rain. Rates of evapotranspiration in cold regions such as Alaska are much lower because of the lack of heat to aid in the evaporation process.

Deserts are sometimes classified as "hot" or "cold", "semiarid" or "coastal". Hot deserts are mostly located near the Tropics of Capricorn or Cancer. The mean temperature is about 22 °C (72 °F), with a daily range of 40 °C (104 °F) to 7 °C (45 °F) or even lower. The rainfall is very low, especially in winter, and may come in the form of a very occasional heavy downpour. The soil is coarse-textured gravel or sand, shallow and well drained. Plants here tend to have deep taproots and may only open their stomata at night.

Cold deserts can be covered with snow or ice for part of the year; frozen water unavailable to plant life. They are found in Greenland, the nearctic ecozone of North America and Antarctica. The mean winter temperature is between 4 °C (39 °F) and −2 °C (28 °F) and the annual precipitation between 15 and 26 cm (6 and 10 in). The soil is fine silt, saline and heavy. Plants growing here tend to be widely separated, deciduous, low and spiny.

Semiarid deserts have long, mostly dry summers and little rainfall in winter. The temperature does not rise as high as in hot deserts, averaging 21 °C (70 °F) to 27 °C (81 °F) in summer and the evenings and nights are cool. As a result, condensation occurs and these areas may receive most of their precipitation in the form of dew. The soil ranges from a fine sand to a coarse gravel and is usually shallow and well-drained. Plants tend to be spiny or have glaucous, silvery or glossy leaves.

Coastal deserts are mostly found on the western edges of continental land masses in regions where cold currents from the polar regions approach the land or cold water upwellings rise from the ocean depths. The cool winds crossing this water pick up little moisture and the coastal regions receive only slight precipitation though they often experience mists or fogs, especially in the winter. The average summer temperature is in the range 13 °C (55 °F) to 24 °C (75 °F) and the winter temperature lower than 6 °C (43 °F). The soil is usually fine-textured and well drained. Deserts of this type occur in Chile, south-west Africa, southern California and Baja California.

In 1961, Peveril Meigs divided desert regions on Earth into three categories according to the amount of precipitation they received. In this now widely accepted system, extremely arid lands have at least twelve consecutive months without rainfall, arid lands have less than 250 mm (10 in) of annual rainfall, and semiarid lands have a mean annual precipitation of between 250 and 500 mm (10–20 in). Both extremely arid and arid lands are considered to be deserts while semiarid lands are generally referred to as steppes.

The Agasthiyamalai hills cut off Tirunelveli in India from the monsoons, creating a rainshadow region.

Deserts are also classified by their geographical location and dominant weather pattern as trade wind, mid-latitude, rain shadow, coastal, monsoon, or polar deserts. Trade wind deserts occur either side of the horse latitudes at 30° to 35° North and South. These belts are associated with the subtropical anticyclone and the large-scale descent of dry air moving from high-altitudes toward the poles. The Sahara Desert is of this type. Mid-latitude deserts occur between 30° and 50° North and South. They are mostly in areas remote from the sea where most of the moisture has already precipitated from the prevailing winds. They include the Tengger and Sonoran deserts. Monsoon deserts are similar. They occur in regions where large temperature differences occur between sea and land. Moist warm air rises over the land, deposits its water content and circulates back to sea. Further inland, areas receive very little precipitation. The Thar Desert near the India/Pakistan border is of this type.

In some parts of the world, deserts are created by a rain shadow effect. Orographic lift occurs as air masses rise to pass over high ground. In the process they cool and lose much of their moisture by precipitation on the windward slope of the mountain range. When they descend on the leeward side, they warm and their capacity to hold moisture increases so an area with relatively little rainfall occurs. The Taklamakan Desert is an example, lying in the rain shadow of the Himalayas and receiving less than 38 millimetres (1.5 in) rainfall annually. Other areas are arid by virtue of being a very long way from the nearest available sources of moisture.

Montane deserts are arid places with a very high altitude; the most prominent example is found north of the Himalayas, in the Kunlun Mountains and the Tibetan Plateau. Many locations within this category have elevations exceeding 3,000 meters (10,000 ft) and the thermal regime can be hemiboreal. These places owe their profound aridity (the average annual precipitation is often less than 40 mm or 1.5 in) to being very far from the nearest available sources of moisture and are often in the lee of mountain ranges. Montane deserts are normally cold, or may be scorchingly hot by day and very cold by night as is true of the northeastern slopes of Mount Kilimanjaro.

Polar deserts such as McMurdo Dry Valleys remain ice-free because of the dry katabatic winds that flow downhill from the surrounding mountains. Former desert areas presently in non-arid environments, such as Sandhills (Nebraska), are known as paleodeserts. In the Köppen climate classification system, deserts are classed as BWh (hot desert) or BWk (temperate desert). In the Thornthwaite climate classification system, deserts would be classified as arid megathermal climates.

Weathering processes

Exfoliation of weathered rocks in Texas

Deserts usually have a large diurnal and seasonal temperature range, with high daytime temperatures, and low nighttime temperatures. The humidity may be as low as 2 to 5% and because water vapour in the atmosphere acts to trap long wave infrared radiation from the ground, the cloudless desert sky is incapable of blocking sunlight during the day or trapping heat during the night. Thus, during daylight, most of the sun's heat reaches the ground, and as soon as the sun sets the desert cools quickly by radiating its heat into space. In hot deserts, the temperature in the daytime can exceed 45 °C (113 °F) in the summer and dip to below freezing point at night during the winter.

One square centimetre of windblown sand from the Gobi Desert

Such large temperature variations have a destructive effect on the exposed rocky surfaces. The repeated fluctuations put a strain on exposed rock and the flanks of mountains crack and shatter. Fragmented strata slide down into the valleys where they continue to break into pieces due to the remorseless sun by day and chill by night. Successive strata are exposed to further weathering. The relief of the internal pressure that has built up in rocks that have been underground for aeons can cause them to shatter. Exfoliation also occurs when the outer surfaces of rocks split off in flat flakes. This is caused by moisture that has entered through minute cracks becoming trapped. Minerals present in the rock in the form of salts and clay swell up and a strain develops which causes a thin layer of stone to break off.

As the desert mountains decay, large areas of shattered rock and rubble occur. The process continues and the end products are either dust or sand. Dust is formed from solidified clay or volcanic deposits whereas sand results from the fragmentation of harder granites, limestone and sandstone. There is a certain critical size (about 500µ) below which further temperature-induced weathering of rocks does not occur and this provides a minimum size for sand grains.

As the mountains are eroded, more and more sand is created. At high wind speeds, sand grains are picked up off the surface and blown along, a process known as saltation. The whirling airborne grains act as a sand blasting mechanism which grinds away solid objects in its path as the kinetic energy of the wind is transferred to the ground. The sand eventually ends up deposited in level areas known as sand-fields or sand-seas, or piled up in dunes.

Dust storms and sandstorms

Main article: Dust storm

Dust storm about to engulf a military camp in Iraq, 2005

Sand and dust storms are natural events that occur in arid regions where the land is not protected by a covering of vegetation. Dust storms usually start in desert margins rather than the deserts themselves where the finer materials have already been blown away. As a steady wind begins to blow, fine particles lying on the exposed ground begin to vibrate. At greater wind speeds, some particles are lifted into the air stream. When they land, they strike other particles which may be jerked into the air in their turn, starting a chain reaction. Once ejected, these particles move in one of three possible ways, depending on their size and density; suspension, saltation or creep. Suspension is only possible for particles less than 0.1 mm (0.004 in) in diameter. In a dust storm, these fine particles are lifted up and wafted aloft to heights of up to 6 kilometres (3.7 mi). They reduce visibility and can remain in the atmosphere for days on end, conveyed by the trade winds distances of thousands of kilometres (miles). Denser clouds of dust can be formed in stronger winds, moving across the land with a billowing leading edge. The sunlight can be obliterated and it may become as dark as night at ground level. In a study of a dust storm in China in 2001, it was estimated that 6.5 million tons of dust were involved, covering an area of 134,000,000 square kilometres (52,000,000 sq mi). The mean particle size was 1.44 μm.

Wind-blown particles: 1=Creep 2=Saltation 3=Suspension 4=Wind current

Sandstorms occur with much less frequency. They are often preceded by severe dust storms and occur when the wind velocity increases to a point where it can lift heavier particles. These grains of sand, up to about 0.5 millimetres (0.020 in) in diameter are jerked into the air but soon fall back to earth, ejecting other particles in the process. Their weight prevents them from being airborne for long and most only travel a distance of a few metres (yards). The sand streams along above the surface of the ground like a fluid, often rising to heights of about 30 centimetres (12 in). In a really severe steady blow, 2 metres (6 ft 7 in) is about as high as the sand stream can rise as the largest sand grains do not become airborne at all. They are transported by creep, being rolled along the desert floor or performing short jumps.

During a sandstorm, the wind-blown sand particles become electrically charged. The electric field can produce sparks and interference with telecommunications equipment and range in size between 1 and 200 kV/m. The electric fields are caused by collision between airborne particles and when saltating sand grains impact the ground. The mechanism is little understood but the particles usually have a negative charge when their diameter is under 250 μm and a positive one when they are over 500 μm.

Major deserts

The world's largest non-polar deserts

Deserts take up about one third (33%) of the Earth's land surface. Bottomlands may be salt-covered flats. Eolian processes are major factors in shaping desert landscapes. Polar deserts (also seen as "cold deserts") have similar features, except the main form of precipitation is snow rather than rain. Antarctica is the world's largest cold desert (composed of about 98% thick continental ice sheet and 2% barren rock). Some of the barren rock is to be found in the so-called Dry Valleys of Antarctica that almost never get snow, which can have ice-encrusted saline lakes that suggest evaporation far greater than the rare snowfall due to the strong katabatic winds that evaporate even ice.

See also: List of deserts by area

The ten largest deserts

Rank	Desert	Area (km²)	Area (mi²)
1	Antarctic Desert (Antarctica)	14,200,000	5,500,000
2	Arctic Desert (Arctic)	13,900,000	5,400,000
3	Sahara Desert (Africa)	9,100,000	3,500,000
4	Arabian Desert (Middle East)	2,600,000	1,000,000
5	Gobi Desert (Asia)	1,300,000	500,000
6	Patagonian Desert (South America)	670,000	260,000
7	Great Victoria Desert (Australia)	647,000	250,000
8	Kalahari Desert (Africa)	570,000	220,000
9	Great Basin Desert (North America)	490,000	190,000
10	Thar Desert (India, Pakistan)	450,000	175,000

Deserts, both hot and cold, play a part in moderating the Earth's temperature. This is because they reflect more of the incoming light and their albedo is higher than that of forests or the sea.

Features

Aerial view of Makhtesh Ramon, an erosion cirque of a type unique to the Negev

Contrary to the impression given by films such as Desert Sands (1955) or Ice Cold in Alex (1958), and books such as Wilfred Thesiger's Arabian Sands, deserts do not always consist of sand. Across the world, around 20 percent of desert is sand, varying from only two percent in North America to 30% in Australia and over 45% in Central Asia. Where sand occurs, it is usually in large quantities consisting of sand sheets or extensive areas of dunes.

A sand sheet is a near-level, firm expanse of partially consolidated particles in a layer that varies from a few centimetres to a few metres thick. The structure of the sheet consists of thin horizontal layers of coarse silt and very fine to medium grain sand, separated by layers of coarse sand and pea-gravel which are a single grain thick. These larger particles anchor the other particles in place and may also be packed together on the surface so as to form a miniature desert pavement. Small ripples form on the sand sheet when the wind exceeds 24 kph (15 mph). They form perpendicular to the wind direction and gradually move across the surface as the wind continues to blow. The distance between their crests corresponds to the average length of jumps made by particles during saltation. The ripples are ephemeral and a change in wind direction causes them to reorganise.

Diagram showing barchan dune formation, with the wind blowing from the left

Sand dunes are accumulations of windblown sand piled up in mounds or ridges. They form downwind of copious sources of dry, loose sand and occur when topographic and climatic conditions cause airborne particles to settle. As the wind blows, saltation and creep take place on the windward side of the dune and individual grains of sand move uphill. When they reach the crest, they cascade down the far side. The upwind slope typically has a gradient of 10° to 20° while the lee slope is around 32°, the angle at which loose dry sand will slip. As this wind-induced movement of sand grains takes place, the dune moves slowly across the surface of the ground. Dunes are sometimes solitary, but they are more often grouped together in dune fields. When these are extensive, they are known as sand seas or ergs.

The shape of the dune depends on the characteristics of the prevailing wind. Barchan dunes are produced by strong winds blowing across a level surface, and are crescent-shaped with the concave side away from the wind. When there are two directions from which winds regularly blow, a series of long, linear dunes known as seif dunes may form. These also occur parallel to a strong wind that blows in one general direction. Transverse dunes run at a right angle to the prevailing wind direction. Star dunes are formed by variable winds, and have several ridges and slip faces radiating from a central point. They tend to grow vertically; they can reach a height of 500 metres (1,600 ft), making them the tallest type of dune. Rounded mounds of sand without a slip face are the rare dome dunes, found on the upwind edges of sand seas.

Windswept desert pavement of small, smooth, closely packed stones in the Mojave desert

A large part of the surface area of the world's deserts consists of flat, stone-covered plains dominated by wind erosion. In "eolian deflation", the wind continually removes fine-grained material, which becomes wind-blown sand. This exposes coarser-grained material, mainly pebbles with some larger stones or cobbles, leaving a desert pavement, an area of land overlaid by closely packed smooth stones forming a tessellated mosaic. Different theories exist as to how exactly the pavement is formed. It may be that after the sand and dust is blown away by the wind the stones jiggle themselves into place; alternatively, stones previously below ground may in some way work themselves to the surface. Very little further erosion takes place after the formation of a pavement, and the ground becomes stable. Evaporation brings moisture to the surface by capillary action and calcium salts may be precipitated, binding particles together to form a desert conglomerate. In time, bacteria that live on the surface of the stones accumulate a film of minerals and clay particles, forming a shiny brown coating known as desert varnish.

Other non-sandy deserts consist of exposed outcrops of bedrock, dry soils or aridisols, and a variety of landforms affected by flowing water, such as alluvial fans, sinks or playas, temporary or permanent lakes, and oases. A hamada is a type of desert landscape consisting of a high rocky plateau where the sand has been removed by aeolian processes. Other landforms include plains largely covered by gravels and angular boulders, from which the finer particles have been stripped by the wind. These are called "reg" in the western Sahara, "serir" in the eastern Sahara, "gibber plains" in Australia and "saï" in central Asia. The Tassili Plateau in Algeria is an impressive jumble of eroded sandstone outcrops, canyons, blocks, pinnacles, fissures, slabs and ravines. In some places the wind has carved holes or arches and in others it has created mushroom-like pillars narrower at the base than the top. In the Colorado Plateau it is water that has been the eroding force. Here the Colorado River has cut its way over the millennia through the high desert floor creating a canyon that is over a mile (6,000 feet or 1,800 metres) deep in places, exposing strata that are over two billion year old.

Water

Atacama, the world's driest desert

The driest place on Earth is the Atacama Desert. It is virtually devoid of life because it is blocked from receiving precipitation by the Andes mountains to the east and the Chilean Coast Range to the west. The cold Humboldt Current and the anticyclone of the Pacific are essential to keep the dry climate of the Atacama. The average rainfall in the Chilean region of Antofagasta is just 1 mm per year. Some weather stations in the Atacama have never received rain. Evidence suggests that the Atacama may not have had any significant rainfall from 1570 to 1971. It is so arid that mountains that reach as high as 6,885 meters (22,590 feet) are completely free of glaciers and, in the southern part from 25°S to 27°S, may have been glacier-free throughout the Quaternary, though permafrost extends down to an altitude of 4,400 meters and is continuous above 5,600 meters. Nevertheless, there is some plant life in the Atacama, in the form of specialist plants that obtain moisture from dew and the fogs that blow in from the Pacific.

Flash flood in the Gobi

When rain falls in deserts, as it occasionally does, it is often with great violence. The desert surface is evidence of this with dry stream channels known as arroyos or wadis meandering across its surface. These can experience flash floods, becoming raging torrents with surprising rapidity after a storm that may be many kilometres away. Most deserts are in basins with no drainage to the sea but some are crossed by exotic rivers sourced in mountain ranges or other high rainfall areas beyond their borders. The River Nile, the Colorado River and the Yellow River do this, losing much of their water through evaporation as they pass through the desert and raising groundwater levels nearby. There may also be underground sources of water in deserts in the form of springs, aquifers, underground rivers or lakes. Where these lie close to the surface, oases may form and plant and animal life flourish.Kharga Oasis in Egypt is 150 kilometres (93 mi) long and results from the removal by wind erosion of sand and dust to expose the water table just above the bedrock. Seepages may occur in the walls of canyons and pools may survive in deep shade near the dried up watercourse below.

Lakes may form in basins where there is sufficient rainfall or meltwater from glaciers above. They are usually shallow and saline, and wind blowing over their surface can cause stress, moving the water over nearby low-lying areas. When the lakes dry up, they leave a crust or hardpan behind. This area of deposited clay, silt or sand is known as a playa. The deserts of North America have more than one hundred playas, many of them relics of Lake Bonneville which covered parts of Utah, Nevada and Idaho during the last ice age when the climate was colder and wetter. These include the Great Salt Lake, Utah Lake, Sevier Lake and many dry lake beds. The smooth flat surfaces of playas have been used for attempted vehicle speed records at Black Rock Desert and Bonneville Speedway and the United States Air Force uses Rogers Dry Lake in the Mojave Desert as runways for aircraft and the space shuttle.

Biogeography

Flora

Xerophytes: Cardón cacti in the Baja California Desert, Cataviña region, Mexico

Plants face severe challenges in arid environments. Problems they need to solve include how to obtain enough water, how to avoid being eaten and how to reproduce. Photosynthesis is the key to plant growth. It can only takes place during the day as energy from the sun is required, but during the day, many deserts become very hot. Opening stomata to allow in the carbon dioxide necessary for the process causes evapotranspiration, and conservation of water is a top priority for desert vegetation. Some plants have resolved this problem by adopting crassulacean acid metabolism or C4 carbon fixation. Both these mechanisms allow carbon dioxide to enter the plant at night and be stored inside the tissues for use during the daytime, avoiding the necessity for the stomata to be opened in the heat of the day.

Many desert plants have reduced the size of their leaves or abandoned them altogether. Cacti are desert specialists, the leaves have been dispensed with and the chlorophyll displaced into the trunks, the cellular structure of which have been modified to allow them to store water. When rain falls, the water is rapidly absorbed by the shallow roots and retained to allow them to survive until the next downpour which may be months or years away. The giant saguaro cacti of the Sonoran Desert form "forests", provide shade for other plants and nesting places for desert birds. Saguaro grow slowly but may live for up to two hundred years. The surface of the trunk is folded like a concertina allowing it to expand and a large specimen can hold eight tons of water after a good downpour.

Cacti are restricted to the New World but other xerophytic plants have developed similar strategies by a process known as convergent evolution. They limit water loss by reducing the size and number of stomata, by having waxy coatings and hairy or tiny leaves. Some are deciduous, shedding their leaves in the driest season, and others curl their leaves up to reduce transpiration. Others store water in succulent leaves or stems or in fleshy tubers. Desert plants maximise water uptake by having shallow roots that spread widely, or by developing long taproots that reach down to deep rock strata for ground water. The saltbush in Australia has succulent leaves and secretes salt crystals, enabling it to live in saline areas. In common with cacti, many have developed spines to ward off browsing animals.

Camel Thorn Tree (Acacia erioloba) in the Namib Desert is nearly leafless in dry periods

Some desert plants produce seed which lies dormant in the soil until sparked into growth by rainfall. When annuals, such plants grow with great rapidity and may flower and set seed within weeks, aiming to complete their development before the last vestige of water dries up. For perennial plants, reproduction is more likely to be successful if the seed germinates in a shaded position, but not so close to the parent plant as to be in competition with it. Some seed will not germinate until it has been blown about on the desert floor to scarify the seed coat. The seed of the mesquite tree which grows in deserts in American is hard and fails to sprout even when planted carefully. When it has passed through the gut of a pronghorn it germinates readily, and the little pile of moist dung provides an excellent start to life well away from the parent tree. The stems and leaves of some plants lower the surface velocity of sand-carrying winds and protect the ground from erosion. Even small fungi and microscopic plant organisms found on the soil surface (so-called cryptobiotic soil) can be a vital link in preventing erosion and providing support for other living organisms. Cold deserts often have high concentrations of salt in the soil. Grasses and low shrubs are the dominant vegetation here and the ground may be covered with lichens. Most shrubs have spiny leaves and shed them in the coldest part of the year.

Fauna

Main article: Xerocole

Animals adapted to live in deserts are called xerocoles. Many desert animals (and plants) show especially clear evolutionary adaptations for water conservation or heat tolerance and so are often studied in comparative physiology, ecophysiology, and evolutionary physiology. One well-studied example is the specializations of mammalian kidneys shown by desert-inhabiting species. Many examples of convergent evolution have been identified in desert organisms, including between cacti and Euphorbia, kangaroo rats and jerboas, Phrynosoma and Moloch lizards.

Cream-coloured courser, Cursorius cursor, is a well-camouflaged desert resident with its dusty coloration, countershading, and disruptive head markings.

Deserts present a very challenging environment for animals. Not only do they require food and water but they also need to keep their body temperature at a tolerable level. In many ways birds are the most able to do this of the higher animals. They can move to areas of greater food availability as the desert blooms after local rainfall and can fly to faraway waterholes. In hot deserts, gliding birds can remove themselves from the over-heated desert floor by using thermals to soar in the cooler air at great heights. In order to conserve energy, other desert birds run rather than fly. The cream-coloured courser flits gracefully across the ground on its long legs, stopping periodically to snatch up insects. Like other desert birds it is well-camouflaged by its colouring and can merge into the landscape when stationary. The sandgrouse is an expert at this and nests on the open desert floor dozens of kilometres (miles) away from the waterhole it needs to visit daily. Some small diurnal birds are found in very restricted localities where their plumage matches the colour of the underlying surface. The desert lark takes frequent dust baths which ensures that it matches its environment.

Herbivorous mammals obtain moisture from the plants they eat. The addax antelope is so efficient at doing this that it never needs to drink. The camel is a superb example of a mammal adapted to desert life. It minimizes its water loss by producing concentrated urine and dry dung, and is able to lose forty percent of its body weight through water loss without dying of dehydration. Many other hot desert animals are nocturnal, seeking out shade during the day or dwelling underground in burrows. At depths of more than 50 centimetres (20 in), these remain at between 30 and 32 °C (86 and 90 °F) regardless of the external temperature.Jerboas, desert rats, kangaroo rats and other small rodents emerge from their burrows at night and so do the foxes, coyotes, jackals and snakes that prey on them. Carnivores can obtain much of their water needs from the body fluids of their victims.

Being ectotherms, reptiles are unable to live in cold deserts but are well-suited to hot ones. In the heat of the day in the Sahara, the temperature can rise to 50 °C (122 °F). Reptiles cannot survive at this temperature and lizards will be prostrated by heat at 45 °C (113 °F). They have few adaptations to desert life and are unable to cool themselves by sweating so they shelter during the heat of the day. In the first part of the night, as the ground radiates the heat absorbed during the day, they emerge and search for prey. Lizards and snakes are the most numerous in arid regions and certain snakes have developed a novel method of locomotion that enables them to move sidewards and navigate high sand-dunes. These include the horned viper of Africa and the sidewinder of North America, evolutionarily distinct but with similar behavioural patterns because of convergent evolution. Many desert reptiles are ambush predators and often bury themselves in the sand, waiting for a victim to come within range.

Amphibians might seem unlikely desert-dwellers, because of their need to keep their skins moist and their dependence on water for reproductive purposes. In fact, the few species that are found in this habitat have made some remarkable adaptations. Most of them are fossorial, spending the hot dry months aestivating in deep burrows. While there they shed their skins a number of times and retain the remnants around them as a waterproof cocoon to retain moisture. In the Sonoran Desert, Couch's spadefoot toad spends most of the year dormant in its burrow. Heavy rain is the trigger for emergence and the first male to find a suitable pool calls to attract others. Eggs are laid and the tadpoles grow rapidly as they must reach metamorphosis before the water evaporates. As the desert dries out, the adult toads rebury themselves. The juveniles stay on the surface for a while, feeding and growing, but soon dig themselves burrows. Few make it to adulthood. The water holding frog in Australia has a similar life cycle and may aestivate for as long as five years if no rain falls. The Desert rain frog of Namibia is nocturnal and survives because of the damp sea fogs that roll in from the Atlantic.

Tadpole shrimp survive dry periods as eggs, which rapidly hatch and develop after rain.

Invertebrates, particularly arthropods, have successfully made their homes in the desert. Flies, beetles, ants, termites, locusts, millipedes, scorpions and spiders have hard cuticles which are impervious to water and many of them lay their eggs underground and their young develop away from the temperature extremes at the surface. The long-legged darkling beetle in Namibia stands on its front legs and raises its carapace to catch the morning mist as condensate, funnelling the water into its mouth. Some arthropods make use of the ephemeral pools that form after rain and complete their life cycle in a matter of days. The desert shrimp does this, appearing "miraculously" in new-formed puddles as the dormant eggs hatch. Others, such as brine shrimps, fairy shrimps and tadpole shrimps, are cryptobiotic and can lose up to 92% of their bodyweight, rehydrating as soon as it rains and their temporary pools reappear.

Human relations

Mineral extraction

A mining plant near Jodhpur, India

Deserts contain substantial mineral resources, sometimes over their entire surface, giving them their characteristic colours. For example, the red of many sand deserts comes from laterite minerals. Geological processes in a desert climate can concentrate minerals into valuable deposits. Leaching by ground water can extract ore minerals and redeposit them, according to the water table, in concentrated form. Similarly, evaporation tends to concentrate minerals in desert lakes, creating dry lake beds or playas rich in minerals. Evaporation can concentrate minerals as a variety of evaporite deposits, including gypsum, sodium nitrate, sodium chloride and borates. Evaporites are found in the USA's Great Basin Desert, historically exploited by the "20-mule teams" pulling carts of borax from Death Valley to the nearest railway. A desert especially rich in mineral salts is the Atacama Desert, Chile, where sodium nitrate has been mined for explosives and fertilizer since around 1850. Other desert minerals are copper from Chile, Peru, and Iran, and iron and uranium in Australia. Many other metals, salts and commercially valuable types of rock such as pumice are extracted from deserts around the world.

Oil and gas form on the bottom of shallow seas when micro-organisms decompose under anoxic conditions and later become covered with sediment. Many deserts were at one time the sites of shallow seas and others have had underlying hydrocarbon deposits transported to them by the movement of tectonic plates. Some major oilfields such as Ghawar are found under the sands of Saudi Arabia. Geologists believe that other oil deposits were formed by aeolian processes in ancient deserts as may be the case with some of the major American oil fields.

Solar energy capture

Desertec proposes using the Saharan and Arabian deserts to produce solar energy to power Europe and the Middle East.

Deserts are increasingly seen as sources for solar energy, partly due to lower cloud cover.

Many successful solar power plants have been built in the Mojave Desert. These plants have a combined capacity of 354 megawatts (MW) making them the largest solar power installation in the world. Large swaths of the desert are covered in mirrors (used for solar energy), including nine fields of solar collectors. The Mojave Solar Park is currently under construction and will produce 280MW when completed.

The potential of generating solar energy from the Sahara desert is immense. Professor David Faiman of Ben-Gurion University has stated that the technology now exists to supply all of the world's electricity needs with 10% of the Sahara desert.Desertec Industrial Initiative is a consortium seeking $560 billion investment in North African solar and wind installations over the next 40 years to supply electricity to Europe via cable lines running under the Mediterranean Sea. European interest in the Sahara desert stems from its two aspects: amount of sunshine and empty space. The Sahara receives more sunshine per are than the sunniest of regions in Europe. The Sahara desert also has the empty space required to house fields of mirrors for solar plants, totalling hundreds of square miles.

The Negev Desert, Israel, and the surrounding area, including the Arava Valley, receive plenty of sunshine and are generally not arable. This has resulted in the construction of many solar plants. David Faiman has proposed that "giant" solar plants in the Negev could supply all of Israel's needs for electricity.

Human life in deserts

Shepherd near Marrakech leading his flock to new pasture

People have been living in deserts for millennia. Many, such as the Bushmen in the Kalahari, the Aborigines in Australia and various tribes of North American Indians, were originally hunter-gatherers. They developed skills in the manufacture and use of weapons, animal tracking, finding water, foraging for edible plants and using the things they found in their natural environment to supply their everyday needs. Their self-sufficient skills and knowledge were passed down through the generations by word of mouth. Other cultures developed a nomadic way of life as herders of sheep, goats, cattle, camels, yaks, lamas or reindeer. They travelled over large areas with their herds, moving to new pastures as seasonal and erratic rainfall encouraged new plant growth. They took with them their tents made of cloth or skins draped over poles and they ate milk, blood and sometimes meat provided by their livestock.

Salt caravan travelling between Agadez and the Bilma salt mines

The desert nomads were also traders. The Sahara is a very large expanse of land stretching from the Atlantic rim to Egypt. Trade routes were developed linking the Sahel in the south with the fertile Mediterranean region to the north and large numbers of camels were used to carry valuable goods across the desert interior. The Tuareg were traders and the goods transported traditionally included slaves, ivory and gold going northwards and salt going southwards. Berbers with knowledge of the region were employed to guide the caravans between the various oases and wells. As many as eight million slaves may have been taken northwards across the Sahara between the 8th and 18th centuries. Traditional means of overland transport declined with the advent of motor vehicles, shipping and air freight, but caravans still travel along routes between Agadez and Bilma and between Timbuktu and Taoudenni carrying salt from the interior to desert-edge communities.

Round the rims of deserts, where more precipitation occurred and conditions were more suitable, some groups took to cultivating crops. This may have happened when drought caused the death of herd animals, forcing herdsmen to turn to cultivation. With few inputs, they were at the mercy of the weather and may have lived at bare subsistence level. The land they cultivated reduced the area available to nomadic herders, causing disputes over land. The semi-arid fringes of the desert have fragile soils which are at risk of erosion when exposed, as happened in the American Dust Bowl in the 1930s. The grasses that held the soil in place were ploughed under, and a series of dry years caused crop failures, while enormous dust storms blew the topsoil away. Half a million Americans were forced to leave their land in this catastrophe.

Similar damage is being done today to the semi-arid areas that rim deserts and about twelve million hectares of land are being turned to desert each year.Desertification is caused by such factors as drought, climatic shifts, tillage for agriculture, overgrazing and deforestation. Vegetation plays a major role in determining the composition of the soil. In many environments, the rate of erosion and run off increases dramatically with reduced vegetation cover. Unprotected dry surfaces tend to be blown away by the wind or be washed away by flash floods, leaving infertile soil layers that bake in the sun and become unproductive hardpan. Although overgrazing has historically been considered to be a cause of desertification, there is some evidence that wild and domesticated animals actually improve fertility and vegetation cover, and that their removal encourages erosive processes.

In culture

In literature

There is an extensive literature of and about deserts. Some is directly descriptive, though often literal description of travel is combined with reflection, as in the case of Charles Montagu Doughty's enormous Travels in Arabia Deserta (1888). An early historical account is that of Marco Polo (c. 1254–1324), who travelled through Central Asia to China for 24 years, crossing a number of deserts.Antoine de Saint-Exupéry described both his flying and the desert in Wind, Sand and Stars (in French: Terre des Hommes).Gertrude Bell (1868–1926) travelled extensively in the Arabian desert, becoming an expert, writing books and advising the British government on dealing with the Arabs.Freya Stark (1893–1993) travelled alone in the Middle East, visiting Turkey, Arabia, Syria, Persia, and Yemen, writing over 20 books. The German naturalist Uwe George spent several years living in deserts, recording his experiences and research as In the Deserts of this Earth. Ralph Bagnold both studied desert geomorphology and created the Long Range Desert Group in the Second World War.

Frank Herbert's Dune is a science fiction novel about a desert planet.

In poetry

T.S. Eliot's modernist 1922 poem The Waste Land has been called "one of the most important poems of the 20th century."

The American poet Robert Frost's Desert Places ends with the stanza "They cannot scare me with their empty spaces / Between stars - on stars where no human race is. / I have it in me so much nearer home / To scare myself with my own desert places."

In war

Field gun firing at night in the decisive desert battle at El Alamein, 1942

Both world wars saw fighting in the desert. In the First World War, the Ottoman Turks were defeated in a campaign that spanned the Arabian peninsula, fought by regular armies assisted on the British side by irregular Arab forces in the Arab Revolt of the Hejaz, made famous by T. E. Lawrence's Seven Pillars of Wisdom. In the Second World War, the Western Desert Campaign began in Libya, with the front swinging backwards and forwards hundreds of kilometres across the desert. Its most easterly point was at El Alamein in Egypt, where the allies decisively defeated the Axis forces in 1942.

Deserts on other planets

View of the Martian desert seen by the probe Spirit in 2004.

Mars is the only planet in the solar system on which deserts have been identified. Despite its low surface atmospheric pressure (only 1/100 of that of the Earth), the patterns of atmospheric circulation on Mars have formed a sea of circumpolar sand more than 5 million km² in area, much larger than deserts on Earth.

The Martian deserts principally consist of dunes in the form of half-moon in flat areas near the permanent polar ice caps in the north of the planet. The smaller dune fields occupy the bottom of many of the craters situated at the Martian polar regions.

Etymology

English desert and its Romance cognates (including Italian and Portuguese deserto, French désert and Spanish desierto) all come from the ecclesiastical Latin dēsertum (originally "an abandoned place"), a participle of dēserere, "to abandon". The correlation between aridity and sparse population is complex and dynamic, varying by culture, era, and technologies; thus the use of the word desert can cause confusion. In English prior to the 20th century, desert was often used in the sense of "unpopulated area", without specific reference to aridity; but today the word is most often used in its climate-science sense (an area of low precipitation). Phrases such as "desert island" and "Great American Desert" in previous centuries did not necessarily imply sand or aridity; their focus was the sparse population.

The transliteration of the Ancient Egyptian term for the Red land (i.e. the deserts on either side of the fertile Black land irrigated by the Nile) is dšrt (conventionally pronounced deshret); it has been stated that "It is not impossible that the very word deserta entered the Latin language by way of Egyptian".

See also

• Aridification
• Arid Lands Information Network
• Desert greening
• Desertification
• Deserts of Australia
• International Center for Agricultural Research in the Dry Areas
• List of deserts
• List of deserts by area
• List of North American deserts
• Sediment precipitation
• Semi-arid climate

Bibliography

• George, Uwe (1978). In the Deserts of this Earth. Hamish Hamilton. ISBN 0-241-89777-7. 
• Pye, Kenneth; Tsoar, Haim (2009). Aeolian Sand and Sand Dunes. Springer. ISBN 3540859101. 

Further reading

• Bagnold, Ralph A. (1941). The physics of blown sand and desert dunes. Methuen. 
• Macmahon, James (1988). Deserts. National Audubon Society nature guides. Random House / Chanticleer Press. ISBN 978-0-394-73139-1. 

External links

• "The Desert Biome". University of California Museum of Paleontology. 1996. 
• "Global Deserts Outlook". United Nations Environment Programme (UNEP). 2006. , a report in the Global Environment Outlook (GEO) series.
• Map with biodiversity scenarios for desert areas, from the Global Deserts Outlook.
• Finding Water in the Desert – wikiHow page