A DESERT is a barren area of landscape where little precipitation occurs and consequently living conditions are hostile for plant and animal life. The lack of vegetation exposes the unprotected surface of the ground to the processes of denudation . About one third of the land surface of the world is arid or semi-arid. This includes much of the polar regions where little precipitation occurs and which are sometimes called polar deserts or "cold deserts". Deserts can be classified by the amount of precipitation that falls, by the temperature that prevails, by the causes of desertification or by their geographical location.
Deserts are formed by weathering processes as large variations in temperature between day and night put strains on the rocks which consequently break in pieces. Although rain seldom occurs in deserts, there are occasional downpours that can result in flash floods. Rain falling on hot rocks can cause them to shatter and the resulting fragments and rubble strewn over the desert floor is further eroded by the wind. This picks up particles of sand and dust and wafts them aloft in sand or dust storms . Wind-blown sand grains striking any solid object in their path can abrade the surface. Rocks are smoothed down, and the wind sorts sand into uniform deposits. The grains end up as level sheets of sand or are piled high in billowing sand dunes. Other deserts are flat, stony plains where all the fine material has been blown away and the surface consists of a mosaic of smooth stones. These areas are known as desert pavements and little further erosion takes place. Other desert features include rock outcrops, exposed bedrock and clays once deposited by flowing water. Temporary lakes may form and salt pans may be left when waters evaporate. There may be underground sources of water in the form of springs and seepages from aquifers . Where these are found, oases can occur.
Plants and animals living in the desert need special adaptations to survive in the harsh environment. Plants tend to be tough and wiry with small or no leaves, water-resistant cuticles and often spines to deter herbivory . Some annual plants germinate , bloom and die in the course of a few weeks after rainfall while other long-lived plants survive for years and have deep root systems able to tap underground moisture. Animals need to keep cool and find enough food and water to survive. Many are nocturnal and stay in the shade or underground during the heat of the day. They tend to be efficient at conserving water, extracting most of their needs from their food and concentrating their urine . Some animals remain in a state of dormancy for long periods, ready to become active again when the rare rains fall. They then reproduce rapidly while conditions are favorable before returning to dormancy.
People have struggled to live in deserts and the surrounding semi-arid lands for millennia. Nomads have moved their flocks and herds to wherever grazing is available and oases have provided opportunities for a more settled way of life. The cultivation of semi-arid regions encourages erosion of soil and is one of the causes of increased desertification . Desert farming is possible with the aid of irrigation and the Imperial Valley in California provides an example of how previously barren land can be made productive by the import of water from an outside source. Many trade routes have been forged across deserts, especially across the Sahara Desert , and traditionally were used by caravans of camels carrying salt, gold, ivory and other goods. Large numbers of slaves were also taken northwards across the Sahara. Some mineral extraction also takes place in deserts and the uninterrupted sunlight gives potential for the capture of large quantities of solar energy .
* 1 Etymology
* 2 Physical geography
* 2.1 Classification * 2.2 Weathering processes * 2.3 Dust storms and sandstorms * 2.4 Major deserts * 2.5 Features * 2.6 Water
* 3 Biogeography
* 3.1 Flora * 3.2 Fauna
* 4 Human relations
* 4.1 History
* 4.2 Natural resource extraction
* 4.3 Farming
* 5 Deserts on other planets * 6 See also * 7 References * 8 Bibliography * 9 Further reading * 10 External links
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 before 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
A desert is a region of land that is very dry because it receives low amounts of precipitation (usually in the form of rain but may be snow, mist or fog), often has little coverage by plants, and in which streams dry up unless they are supplied by water from outside the area. Deserts can also be described as areas where more water is lost by evapotranspiration than falls as precipitation. Deserts generally receive less than 250 mm (10 in) of precipitation each year. Semideserts are regions which receive between 250 and 500 mm (10 and 20 in) and when clad in grass, these are known as steppes.
Deserts have been defined and classified in a number of ways,
generally combining total precipitation, number of days on which this
falls, temperature , and humidity, and sometimes additional factors.
Phoenix, Arizona , receives less than 250 mm (9.8 in) of
precipitation per year, and is immediately recognized as being located
in a desert because of its aridity-adapted plants. The North Slope of
Brooks Range also receives less than 250 mm (9.8 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
Potential evapotranspiration supplements the measurement of precipitation 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 mm (12 in) of rain per year, however about 2,500 mm (98 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". The characteristics of hot deserts include high
temperatures in summer; greater evaporation than precipitation usually
exacerbated by high temperatures, strong winds and lack of cloud
cover; considerable variation in the occurrence of precipitation, its
intensity and distribution; and low humidity. Winter temperatures vary
considerably between different deserts and are often related to the
location of the desert on the continental landmass and the latitude.
Daily variations in temperature can be as great as 22 °C (40 °F) or
more, with heat loss by radiation at night being increased by the
clear skies. Cold desert: snow surface at Dome C Station,
Cold deserts, sometimes known as temperate deserts, occur at higher
latitudes than hot deserts, and the aridity is caused by the dryness
of the air. Some cold deserts are far from the ocean and others are
separated by mountain ranges from the sea and in both cases there is
insufficient moisture in the air to cause much precipitation. The
largest of these deserts are found in Central Asia. Others occur on
the eastern side of the
Rocky Mountains , the eastern side of the
Based on precipitation alone, hyperarid deserts receive less than 25
mm (1 in) of rainfall a year; they have no annual seasonal cycle of
precipitation and experience twelve-month periods with no rainfall at
A semi-arid desert or a steppe is a version of the arid desert with
much more rainfall, vegetation and higher humidity. These regions
feature a semi-arid climate and are less extreme than regular deserts.
Like arid deserts, temperatures can vary greatly in semi deserts.
They share some characteristics of a true desert and are usually
located at the edge of deserts and continental dry areas. They usually
receive precipitation from 250 mm (10 in) to 500 mm (20 in) but this
can vary due to evapotranspiration and soil nutrition. Semi deserts
can be found in the
Tabernas Desert (and some of the Spanish Plateau),
The Sahel , The Eurasian
Coastal deserts are mostly found on the western edges of continental
land masses in regions where cold currents 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 have low
temperatures and very low rainfall, the main precipitation being in
the form of fog and dew. The range of temperatures on a daily and
annual scale is relatively low, being 11 °C (20 °F) and 5 °C (9
°F) respectively in the
Atacama Desert . Deserts of this type are
often long and narrow and bounded to the east by mountain ranges. They
Peveril Meigs divided desert regions on
Deserts are also classified, according to 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
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 .
In some parts of the world, deserts are created by a rain shadow
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 precipitation
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 the Sandhills in Nebraska , are known as paleodeserts. In the
Köppen climate classification
Exfoliation of weathering rocks in Texas
Deserts usually have a large diurnal and seasonal temperature range, with high daytime temperatures falling sharply at night. The diurnal range may be as much as 20 to 30 °C (36 to 54 °F) and the rock surface experiences even greater temperature differentials. During the day the sky is usually clear and most of the sun 's radiation reaches the ground, but as soon as the sun sets, the desert cools quickly by radiating heat into space. In hot deserts, the temperature during daytime can exceed 45 °C (113 °F) in summer and plunge below freezing point at night during winter. One square centimeter (0.16 sq in) 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 relentless 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 believed to be caused by the stresses put on the rock by repeated expansions and contractions which induces fracturing parallel to the original surface. Chemical weathering processes probably play a more important role in deserts than was previously thought. The necessary moisture may be present in the form of dew or mist. Ground water may be drawn to the surface by evaporation and the formation of salt crystals may dislodge rock particles as sand or disintegrate rocks by exfoliation. Shallow caves are sometimes formed at the base of cliffs by this means.
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 0.5 mm) 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
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, shape 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 km (3.7 mi). They reduce visibility and can remain in the atmosphere for days on end, conveyed by the trade winds for distances of up to 6,000 km (3,700 mi). 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 km2 (52,000,000 sq mi). The mean particle size was 1.44 μm. A much smaller scale, short-lived phenomenon can occur in calm conditions when hot air near the ground rises quickly through a small pocket of cooler, low-pressure air above forming a whirling column of particles, a dust devil . Wind-blown particles: 1=Creep 2=Saltation 3=Suspension 4=Wind current
Sandstorms occur with much less frequency than dust storms. 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 mm (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 meters (yards). The sand streams along above the surface of the ground like a fluid, often rising to heights of about 30 cm (12 in). In a really severe steady blow, 2 m (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. Such electric fields, which range in size up to 80 kV/m, can produce sparks and cause interference with telecommunications equipment. They are also unpleasant for humans and can cause headaches and nausea. The electric fields are caused by collision between airborne particles and by the impacts of saltating sand grains landing on 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.
See also: List of deserts by area The world's largest non-polar deserts
Deserts take up about one third 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 .
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 SYRIAN DESERT (Middle East) 490,000 190,000
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.
Many people think of deserts as consisting of extensive areas of
billowing sand dunes because that is the way they are often depicted
on TV and in films, but deserts do not always look like this. Across
the world, around 20% of desert is sand, varying from only 2% in North
America to 30% in
A sand sheet is a near-level, firm expanse of partially consolidated particles in a layer that varies from a few centimeters to a few meters 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 km/h (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
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 m (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
Atacama , the world's driest non-polar desert, part of the Arid Diagonal of South America.
One of the driest places on
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 kilometers 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
Lakes may form in basins where there is sufficient precipitation 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
Xerophytes : Cardón cacti in the
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.
Many desert plants have reduced the size of their leaves or abandoned them altogether. Cacti are desert specialists and in most species the leaves have been dispensed with and the chlorophyll displaced into the trunks, the cellular structure of which has 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", providing 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 present in both North and South America with a
post-Gondwana origin. 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
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 the Americas, 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. Some plants, including the Plantago Lanceolata , have to reproduce via wind pollination due to living in the environment. 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.
Main article: Xerocole
Animals adapted to live in deserts are called xerocoles . There is no evidence that body temperature of mammals and birds is adaptive to the different climates, either of great heat or cold. In fact, with a very few exceptions, their basal metabolic rate is determined by body size, irrespective of the climate in which they live. 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. _ The cream-colored 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-colored 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 coloring and can merge into the landscape when stationary. The sandgrouse is an expert at this and nests on the open desert floor dozens of kilometers (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 color of the underlying surface. The desert lark takes frequent dust baths which ensures that it matches its environment.
Water and carbon dioxide are metabolic end products of oxidation of fats, proteins, and carbohydrates. Oxidising a gram of carbohydrate produces 0.60 grams of water; a gram of protein produces 0.41 grams of water; and a gram of fat produces 1.07 grams of water, making it possible for xerocoles to live with little or no access to drinking water. The kangaroo rat for example makes use of this water of metabolism and conserves water both by having a low basal metabolic rate and by remaining underground during the heat of the day, reducing loss of water through its skin and respiratory system when at rest. Herbivorous mammals obtain moisture from the plants they eat. Species such as the addax antelope , dik-dik , Grant\'s gazelle and oryx are so efficient at doing this that they apparently never need 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 40% of its body weight through water loss without dying of dehydration. Carnivores can obtain much of their water needs from the body fluids of their prey. Many other hot desert animals are nocturnal , seeking out shade during the day or dwelling underground in burrows. At depths of more than 50 cm (20 in), these remain at between 30 to 32 °C (86 to 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. Kangaroos keep cool by increasing their respiration rate, panting, sweating and moistening the skin of their forelegs with saliva . Mammals living in cold deserts have developed greater insulation through warmer body fur and insulating layers of fat beneath the skin. The arctic weasel has a metabolic rate that is two or three times as high as would be expected for an animal of its size. Birds have avoided the problem of losing heat through their feet by not attempting to maintain them at the same temperature as the rest of their bodies, a form of adaptive insulation. The emperor penguin has dense plumage, a downy under layer, an air insulation layer next the skin and various thermoregulatory strategies to maintain its body temperature in one of the harshest environments on Earth. _ The desert iguana (Dipsosaurus dorsalis_) is well-adapted to desert life.
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 prey 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
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 Saharan silver ant (_Cataglyphis bombycina_) uses a heat shock protein in a novel way and forages in the open during brief forays in the heat of the day. 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.
Humans have long made use of deserts as places to live, and more recently have started to exploit them for minerals and energy capture. Deserts play a significant role in human culture with an extensive literature.
People have been living in deserts for millennia. Many, such as the
Bushmen in the
Kalahari , the Aborigines in
The desert nomads were also traders. The
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
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.
NATURAL RESOURCE EXTRACTION
Deserts contain substantial mineral resources, sometimes over their
entire surface, giving them their characteristic colors. 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.
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 ,
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.
Main article: Desert farming
Traditional desert farming systems have long been established in North Africa, irrigation being the key to success in an area where water stress is a limiting factor to growth. Techniques that can be used include drip irrigation , the use of organic residues or animal manures as fertilisers and other traditional agricultural management practises. Once fertility has been built up, further crop production preserves the soil from destruction by wind and other forms of erosion. It has been found that plant growth-promoting bacteria play a role in increasing the resistance of plants to stress conditions and these rhizobacterial suspensions could be inoculated into the soil in the vicinity of the plants. A study of these microbes found that desert farming hampers desertification by establishing islands of fertility allowing farmers to achieve increased yields despite the adverse environmental conditions. A field trial in the Sonoran Desert which exposed the roots of different species of tree to rhizobacteria and the nitrogen fixing bacterium _ Azospirillum brasilense _ with the aim of restoring degraded lands was only partially successful.
A modern example of desert farming is the
Imperial Valley in
California, which has high temperatures and average rainfall of just 3
in (76 mm) per year. The economy is heavily based on agriculture and
the land is irrigated through a network of canals and pipelines
sourced entirely from the
SOLAR ENERGY CAPTURE
Deserts are increasingly seen as sources for solar energy , partly due to low amounts of 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 this desert are covered in mirrors, including nine fields of solar collectors. The Mojave Solar Park is currently under construction and will produce 280MW when completed.
The potential for generating solar energy from the
Sahara Desert is
huge, the highest found on the globe. Professor
David Faiman of
Ben-Gurion University has stated that the technology now exists to
supply all of the world's electricity needs from 10% of the Sahara
Main article: Desert warfare
The Arabs were probably the first organized force to conduct successful battles in the desert. By knowing back routes and the locations of oases and by utilizing camels, Muslim Arab forces were able to successfully overcome both Roman and Persian forces in the period 600 to 700 AD during the expansion of the Islamic caliphate .
Many centuries later, both world wars saw fighting in the desert. In
First World War
Second World War
_ Marco Polo arriving in a desert land with camels. 14th century miniature from Il milione_.
The desert is generally thought of as a barren and empty landscape. It has been portrayed by writers, film-makers, philosophers, artists and critics as a place of extremes, a metaphor for anything from death, war or religion to the primitive past or the desolate future.
There is an extensive literature on the subject of deserts. An early
historical account is that of
Marco Polo (c. 1254–1324), who
Central Asia to China, crossing a number of deserts
in his twenty four year trek. Some accounts give vivid descriptions
of desert conditions, though often accounts of journeys across deserts
are interwoven with reflection, as is the case in Charles Montagu
Doughty 's major work, _Travels in
The American poet
Robert Frost expressed his bleak thoughts in his
DESERTS ON OTHER PLANETS
Main article: Desert planets _ View of the Martian desert seen by the probe Spirit _ in 2004.
* ^ Vesilind, Priit J. (August 2003). "The Driest Place on Earth".
_National Geographic Magazine_. Retrieved 2 April 2013. (Excerpt)
* ^ "Even the Driest Place on