Khác biệt giữa bản sửa đổi của “Hoang mạc”

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Phiên bản lúc 05:43, ngày 30 tháng 6 năm 2014

Hoang mạc là vùng có lượng mưa rất ít, ít hơn lượng cần thiết để hầu hết các loại thực vật sinh trưởng. Hoang mạc được xác định là những khu vực có lượng mưa ít hơn 250 mm/năm (10in/năm)^[1]^[2], do vậy nước ở hoang mạc rất hiếm, thường không có sông và suối, sự sống hiếm hoi vì có rất ít loại động vật và thực vật có thể thích nghi với môi trường khắc nghiệt này, chỉ có ít những cây bộ gai, họ xương rồng sống được điều kiện khô cằn ít nước.

Sa mạc thường dùng để chỉ những hoang mạc cát, đôi khi cũng dùng để chỉ hoang mạc nói chung. Ở một số sa mạc nóng, khí hậu thường nóng có thể tới 58°C như ở sa mạc Mexico, Turfan (Thổ Nhĩ Kỳ) nhiệt độ ban ngày mùa hạ lên tới 82,3 °C, có nơi lại lạnh đến –45 °C như ở sa mạc Gobi thuộc Châu Á. Ở vùng sa mạc Sinai, biên độ nhiệt độ chênh lệch giữa ngày và đêm có thể đến hơn 80 °C, đất đai cằn cỗi. Sa mạc thường có lượng bức xạ mặt trời lớn, nhiều cát và gió nóng luôn thổi mạnh tạo ra rất nhiều trận bão cát, hiện nay có khoảng 1/3 diện tích trái đất (lục địa) là sa mạc. Người ta thường dùng lạc đà làm phương tiện di chuyển trong sa mạc.

Các loại hoang mạc

Vào năm 1953 Peveril Meigs đã chia những hoang mạc trên Trái Đất thành ba loại dựa theo lượng mưa mà chúng nhận được. Trong hệ thống đã được chấp nhận rộng rãi này, các hoang mạc có tối thiểu 12 tháng liên tiếp không mưa, những vùng sa mạc có lượng mưa trung bình năm ít hơn 250mm. Những bán hoang mạc có lượng mưa vào khoảng 250mm đến 500mm. Các bán hoang mạc thương được coi như những đồng cỏ khô. Phương pháp đo chỉ đơn giản dựa vào lượng mưa không thể cung cấp một định nghĩa rõ ràng về việc một sa mạc là gì vì việc khô hạn cũng phụ thuộc vào sự bay hơi, một phần phụ thuộc vào thời tiết

Phân loại

Hoang mạc được định nghĩa và phân loại theo nhiều cách, thường là kết hợp giữa giáng thủy dựa trên số ngày mưa/tuyết rơi, nhiệt độ, và độ ẩm, và đôi khi thêm các yếu tố khác.^[1] Ví dụ, Phoenix, Arizona có lượng mưa hàng năm ít hơn 250 mm (9,8 in), và ngay lập tức đu7o75c công nhận là nằm trong nhóm hoang mạc bởi vì các thực vật thích nghi khô hạn ở đó. North Slope của dãy Brooks thuộc Alaska cũng có lượng giáng thủy hàng năm nhỏ hơn 250 mm (9,8 in) và thường được xếp vào hoang mạc lạnh.^[3] Các vùng khác trên thế giới có hoang mạc lạnh, bao gồm cả Himalayas^[4] và các vùng ở độ cao lớn khaa1c thuộc những nơi khác của thế giới.^[5] Các hoang mạc vùng Cực bao gồm chủ yếu là các khu vực đóng băng Bắc Băng Dương và Nam Băng Dương.^[6]^[7] Định nghĩa phi kỹ thuật nói rằng các hoang mạc là những bộ phận trên bề mặt trái đất có không đủ thảm thực vật để hỗ trợ sự sống cho con người.^[8]

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.^[9] 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.^[10]

Deserts are sometimes classified as "hot" or "cold", "semiarid" or "coastal".^[8] 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.^[11]

Cold desert: snow surface at Dome C Station, Antarctica

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 southern Andes and in southern Australia.^[12] Polar deserts are a particular class of cold desert. The air is very cold and carries little moisture so little precipitation occurs and what does fall, usually as snow, is carried along in the often strong wind and may form blizzards, drifts and dunes similar to those caused by dust and sand in other desert regions. In Antarctica, for example, the annual precipitation is about 50 mm (2 in) on the central plateau and some ten times that amount on some major peninsulas.^[11]

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 all.^[11]^[13] Arid deserts receive between 25 và 200 mm (1 và 8 in) in a year and semiarid deserts between 200 và 500 mm (8 và 20 in). However, such factors as the temperature, humidity, rate of evaporation and evapotranspiration, and the moisture storage capacity of the ground have a marked effect on the degree of aridity and the plant and animal life that can be sustained. Rain falling in the cold season may be more effective at promoting plant growth, and defining the boundaries of deserts and the semiarid regions that surround them on the grounds of precipitation alone is problematic.^[11]

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 occur in south-west Africa, Chile, southern California and Baja California. Other coastal deserts influenced by cold currents are found in Western Australia, the Arabian Peninsula and Horn of Africa, and the western fringes of the Sahara.^[11]

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 precipitation, arid lands have less than 250 mm (10 in) of annual precipitation, 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 when they are grasslands.^[1]

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.^[14] 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.^[15] 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.^[14] 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.^[14]

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 precipitation occurs.^[16] The Taklamakan Desert is an example, lying in the rain shadow of the Himalayas and receiving less than 38 mm (1,5 in) precipitation annually.^[17] Other areas are arid by virtue of being a very long way from the nearest available sources of moisture.^[18]

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 m (9.800 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.^[19]

Polar deserts such as McMurdo Dry Valleys remain ice-free because of the dry katabatic winds that flow downhill from the surrounding mountains.^[20] Former desert areas presently in non-arid environments, such as the Sandhills in Nebraska, are known as paleodeserts.^[14] 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.^[21]^[22]

Weathering processes

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.^[23] 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.^[24]

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.^[25] 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.^[23] 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.^[23]

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.^[26] 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.^[27]

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.^[28] The sand eventually ends up deposited in level areas known as sand-fields or sand-seas, or piled up in dunes.^[29]

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).^[30] 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.^[31] 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 dặm vuông Anh). The mean particle size was 1.44 μm.^[32] 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.^[33]

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)^{[chuyển đổi: số không hợp lệ]} 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).^[30] 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.^[31]

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.^[31] 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.^[34]^[35]

Features

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,^[36] but deserts do not always look like this.^[37] Across the world, around 20% of desert is sand, varying from only 2% in North America to 30% in Australia and over 45% in Central Asia.^[38] Where sand does occur, it is usually in large quantities in the form of sand sheets or extensive areas of dunes.^[38]

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.^[39] 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.^[40]

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.^[41] 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.^[42]

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.^[42]

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,^[29]^[38] 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.^[43] 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.^[44]

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.^[38] 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.^[45] 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.^[46] 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 meters) deep in places, exposing strata that are over two billion year old.^[47]

Water

One of the driest places on Earth is the Atacama Desert.^[48] 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 precipitation in the Chilean region of Antofagasta is just 1 mm (0,039 in) 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 m (22.589 ft) 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 m (14.400 ft) and is continuous above 5.600 m (18.400 ft).^[49]^[50] 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.^[48]

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 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, wells can be dug and oases may form where plant and animal life can flourish.^[38] The Nubian Sandstone Aquifer System under the Sahara Desert is the largest known accumulation of fossil water. The Great Man-Made River is a scheme launched by Libya's Colonel Gadaffi to tap this aquifer and supply water to coastal cities.^[51] Kharga Oasis in Egypt is 150 km (93 mi) long and is the largest oasis in the Libyan Desert. A lake occupied this depression in ancient times and thick deposits of sandy-clay resulted. Wells are dug to extract water from the porous sandstone that lies underneath.^[52] Seepages may occur in the walls of canyons and pools may survive in deep shade near the dried up watercourse below.^[53]

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 Lake Bonneville which covered parts of Utah, Nevada and Idaho during the last ice age when the climate was colder and wetter.^[54] 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.^[38]

Biogeography

Flora

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, allowing them to open their stomata during the night to allow CO₂ to enter, and close them during the day,^[55] or by using C4 carbon fixation.^[56]

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.^[57] 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.^[57]

Cacti are restricted to the New World but other xerophytic plants have developed similar strategies by a process known as convergent evolution.^[58] 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 maximize water uptake by having shallow roots that spread widely, or by developing long taproots that reach down to deep rock strata for ground water.^[59] The saltbush in Australia has succulent leaves and secretes salt crystals, enabling it to live in saline areas.^[59]^[60] In common with cacti, many have developed spines to ward off browsing animals.^[57]

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.^[57] 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.^[61]

Fauna

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.^[62] 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.^[63] Many examples of convergent evolution have been identified in desert organisms, including between cacti and Euphorbia, kangaroo rats and jerboas, Phrynosoma and Moloch lizards.^[64]

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.^[65]

Water and carbon dioxide are metabolic end products of oxidation of fats, proteins, and carbohydrates.^[66] 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,^[67] making it possible for xerocoles to live with little or no access to drinking water.^[68] 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,^[69] reducing loss of water through its skin and respiratory system when at rest.^[68]^[70] Herbivorous mammals obtain moisture from the plants they eat. Species such as the addax antelope,^[71] dik-dik, Grant's gazelle and oryx are so efficient at doing this that they apparently never need to drink.^[72] 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.^[73] Carnivores can obtain much of their water needs from the body fluids of their prey.^[74] 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 and 32 °C (86 and 90 °F) regardless of the external temperature.^[74] 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.^[75] 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.^[62] 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.^[76]

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.^[77]

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.^[78] The water holding frog in Australia has a similar life cycle and may aestivate for as long as five years if no rain falls.^[79] The Desert rain frog of Namibia is nocturnal and survives because of the damp sea fogs that roll in from the Atlantic.^[80]

Invertebrates, particularly arthropods, have successfully made their homes in the desert. Flies, beetles, ants, termites, locusts, millipedes, scorpions and spiders^[81] 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.^[82] 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.^[83] 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.^[84] 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.^[85]

Human relations

Humans have long made use of deserts as places to live,^[86] and more recently have started to exploit them for minerals^[87] and energy capture.^[88] Deserts play a significant role in human culture with an extensive literature.^[89]

History

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.^[86] 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 their diet included milk, blood and sometimes meat.^[90]

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.^[91] Several million slaves may have been taken northwards across the Sahara between the 8th and 18th centuries.^[92] 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.^[93]

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.^[94]

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.^[95] 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.^[96] 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.^[97]

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.^[98] 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.^[87] 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.^[87] 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.^[87] 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.^[87] 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.^[87]

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.^[99] Some major oilfields such as Ghawar are found under the sands of Saudi Arabia.^[87] 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.^[87]

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.^[100] 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.^[100] 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.^[100]

The Judean Desert was farmed in the 7th century BC during the Iron Age to supply food for desert forts.^[101] Native Americans in the south western United States became agriculturalists around 600 AD when seeds and technologies became available from Mexico. They used terracing techniques and grew gardens beside seeps, in moist areas at the foot of dunes, near streams providing flood irrigation and in areas irrigated by extensive specially built canals. The Hohokam tribe constructed over 500 dặm (800 km) of large canals and maintained them for centuries, an impressive feat of engineering. They grew maize, beans, squash and peppers.^[102]

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.^[103] The economy is heavily based on agriculture and the land is irrigated through a network of canals and pipelines sourced entirely from the Colorado River via the All-American Canal. The soil is deep and fertile, being part of the river's flood plains, and what would otherwise have been desert has been transformed into one of the most productive farming regions in California. Other water from the river is piped to urban communities but all this has been at the expense of the river, which below the extraction sites no longer has any above-ground flow during most of the year. Another problem of growing crops in this way is the build-up of salinity in the soil caused by evaporation of river water.^[104] The greening of the desert remains an aspiration and was at one time viewed as a future means for increasing food production for the world's growing population. This prospect has proved false as it disregarded the environmental damage caused elsewhere by the diversion of water for desert project irrigation.^[105]

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.^[106] Large swaths of this desert are covered in mirrors,^[107] including nine fields of solar collectors. The Mojave Solar Park is currently under construction and will produce 280MW when completed.^[108]

The potential for 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 from 10% of the Sahara Desert.^[109] Desertec Industrial Initiative is a consortium seeking $560 billion to invest in North African solar and wind installations over the next forty 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: the almost continual daytime sunshine and plenty of unused land. The Sahara receives more sunshine per acre than any part of Europe. The Sahara Desert also has the empty space totalling hundreds of square miles required to house fields of mirrors for solar plants.^[110]

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.^[88] David Faiman has proposed that "giant" solar plants in the Negev could supply all of Israel's needs for electricity.^[109]

Warfare

Both world wars saw fighting in the desert. In the First World War, the Ottoman Turks were engaged with the British regular army in a campaign that spanned the Arabian peninsula. The Turks were defeated by the British, who had the backing of irregular Arab forces that were seeking to revolt against the Turks in the Hejaz, made famous in T. E. Lawrence's book Seven Pillars of Wisdom.^[111]^[112]

In the Second World War, the Western Desert Campaign began in Italian Libya. Warfare in the desert offered great scope for tacticians to use the large open spaces without the distractions of casualties among civilian populations. Tanks and armoured vehicles were able to travel large distances unimpeded and land mines were laid in large numbers. However the size and harshness of the terrain meant that all supplies needed to be brought in from great distances. The victors in a battle would advance and their supply chain would necessarily become longer, while the defeated army could retreat, regroup and resupply. For these reasons, the front line moved back and forth through hundreds of kilometers as each side lost and regained momentum.^[113] Its most easterly point was at El Alamein in Egypt, where the Allies decisively defeated the Axis forces in 1942.^[114]

In culture

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.^[115]

There is an extensive literature on the subject of deserts.^[89] An early historical account is that of Marco Polo (c. 1254–1324), who travelled through Central Asia to China, crossing a number of deserts in his twenty four year trek.^[116] 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 Arabia Deserta (1888).^[117] Antoine de Saint-Exupéry described both his flying and the desert in Wind, Sand and Stars^[118] and Gertrude Bell travelled extensively in the Arabian desert in the early part of the 20th century, becoming an expert on the subject, writing books and advising the British government on dealing with the Arabs.^[119] Another woman explorer was Freya Stark who travelled alone in the Middle East, visiting Turkey, Arabia, Yemen, Syria, Persia and Afghanistan, writing over twenty books on her experiences.^[120] The German naturalist Uwe George spent several years living in deserts, recording his experiences and research in his book, In the Deserts of this Earth.^[121]

The American poet Robert Frost expressed his bleak thoughts in his poem, Desert Places, which 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."^[122]

Deserts on other planets

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 (1.9 million sq mi) in area, much larger than deserts on Earth. The Martian deserts principally consist of dunes in the form of half-moons 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 in the Martian polar regions.^[123] Examination of the surface of rocks by laser beamed from the Mars Exploration Rover have shown a surface film that resembles the desert varnish found on Earth although it might just be surface dust.^[124] The surface of Titan, a moon of Saturn, also has a desert-like surface with dune seas.^[125]

Địa lý

Tuyết phủ kín bề mặt mái vòm của trạm nghiên cứu C tại Châu Nam Cực - hình ảnh đại diện cho phần lớn bề mặt của lục địa này.

Các hoang mạc chiếm khoảng 1/3 diện tích bề mặt Trái Đất.^[1] Các hoang mạc nóng thường có sự chênh lệch nhiệt độ ngày đêm và theo mùa lớn với nhiệt độ ban ngày cao và ban đêm thấp. Ở các hoang mạc nóng, nhiệt độ ban ngày có thể lên đến 45 °C/113 F hoặc cao hơn trong mùa hè, và xuống 0 °C/32 F hoặc thấp hơn vào ban đêm trong mùa đông. Hơi nước trong khí quyển đóng vai trò là một bẫy giữa các sóng hồng ngoại dài phản xạ từ mặt đất, và không khí các hoang mạc khô có khả năng ngăn chặn ánh sáng mặt trời ban ngày (trời không mây) hoặc giữa nhiệt vào ban đêm. Do đó, vào ban ngày hầy hết nhiệt từ mặt trời sẽ tiếp cận đến mặt đất, và ngay sau khi mặt trời lặn, hoang mạc lạnh rất nhanh bằng cách bức xạ nhiệt của nó vào không gian. Các khu vực đô thị trong các hoang mạc không có sự dao động nhiệt độ hàng ngày lớn (hơn 14 °C/25 F), một phần là do ảnh hưởng của đảo nhiệt đô thị.

Các hoang mạc lớn trên thế giới

Châu Nam Cực, 13.829.430 km²
Vùng Bắc Cực 13.700.000 km²
Sahara, 9.100.000 km², nằm tại Bắc Châu Phi
Hoang mạc Ả rập 2.330.000 km²
Gobi (Mông Cổ), Bắc Trung Quốc 1.300.000 km²
Patagonian, Argentina 670.000 km²
Hoang mạc Kalahari 570.000 km²
Empty Quarter, bán đảo Ả Rập: sa mạc cát lớn nhất thế giới và ở đó cũng có nhiều dầu mỏ
Great Sandy - tây bắc Châu Úc 390.500 km²
Victoria lớn - tây nam Châu Úc 390.500 km²
Gibson, Victoria lớn, Simpson - trung tâm Châu Úc
Tanami - Bắc Châu Úc
Chihuahua, Mexico/tây nam nước Mỹ 360.000 km²
Takla Makan Bắc Trung Quốc 360.000 km²
Sonoran Mexico/tây nam nước Mỹ 310.000 km²
Kalahari tây nam Châu Phi 260.000 km²
Kyzyl Kum Uzbekistan 260.000 km²
Thar Ấn Độ/Pakistan 260.000 km²
Simpson Châu Úc 100.000 km²
Mohave tây nam nước Mỹ 52.000 km²
Atacama (Chile).
Libyan, (Châu Phi).
Al-Dahna, Ả Rập
Nefud, Bắc Ả Rập
Judean - Đông Israel và Khu Bờ Tây
Negev - Nam Israel
Kavir-e Lut, Đông Nam Iran.
Dasht-e Kavir (Iran).
Kara Kum (trung tâm Châu Á)
Kyzyl Kum (Kazakhstan và Uzbekistan.)
Taklamakan - Khu tự trị Duy Ngô Nhĩ Tân Cương (Trung Quốc)
Tabernas, Almería, Tây Ban Nha

Xem thêm

Chú thích

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[usgs-1] What is a desert?

[2] According to What is a desert?, the 250 mm threshold definition is attributed to Peveril Meigs.

[WalterBreckle2002-3] Walter, Heinrich; Breckle, Siegmar-W. (2002). Walter's Vegetation of the Earth: The Ecological Systems of the Geo-biosphere. Springer. tr. 457. ISBN 978-3-540-43315-6.Quản lý CS1: nhiều tên: danh sách tác giả (liên kết)

[Negi2002-4] Negi, S.S. (2002). Cold Deserts of India. Indus Publishing. tr. 9. ISBN 978-81-7387-127-6.

[RohliVega2008-5] Rohli, Robert V.; Vega, Anthony J. (2008). Climatology. Jones & Bartlett Learning. tr. 207. ISBN 978-0-7637-3828-0.Quản lý CS1: nhiều tên: danh sách tác giả (liên kết)

[ThomasFogg2008-6] Thomas, David Neville; và đồng nghiệp (2008). The biology of polar regions. Oxford University Press. tr. 64. ISBN 978-0-19-929813-6. “Và đồng nghiệp” được ghi trong: |author= (trợ giúp)

[LyonsHoward-Williams1997-7] Lyons, W. Berry; Howard-Williams, C. and Hawes, Ian (1997). Ecosystem processes in Antarctic ice-free landscapes: proceedings of an International Workshop on Polar Desert Ecosystems : Christchurch, New Zealand, 1–4 July 1996. Taylor & Francis. tr. 3–10. ISBN 978-90-5410-925-9.Quản lý CS1: nhiều tên: danh sách tác giả (liên kết)

[encyclo-8] “Desert”. 1911 Encyclopædia Britannica, Volume 8. 1911. tr. 93. Truy cập ngày 24 tháng 9 năm 2013.

[9] Buel, S. W. (1964). “Calculated actual and potential evapotranspiration in Arizona”. Tucson, Arizona University Agricultural Experiment Station Technical Bulletin. 162: 48.

[10] Mendez, J.; Hinzman, L. D.; Kane, D. L. (1998). “Evapotranspiration from a wetland complex on the Arctic coastal plain of Alaska”. Nordic Hydrology. 29 (4–5): 303–330. ISSN 0029-1277.Quản lý CS1: nhiều tên: danh sách tác giả (liên kết)

[Laity-11] Laity, Julie J. (2009). Deserts and Desert Environments: Volume 3 of Environmental Systems and Global Change Series. John Wiley & Sons. tr. 2–7, 49. ISBN 9781444300741.

[brittanica-12] Lỗi chú thích: Thẻ <ref> sai; không có nội dung trong thẻ ref có tên brittanica

[Oliver2005-13] John E. Oliver (1 tháng 1 năm 2005). The Encyclopedia of World Climatology. Springer. tr. 86. ISBN 978-1-4020-3264-6.

[Destypes-14] “Types of deserts”. Deserts: Geology and Resources. United States Geological Survey. Truy cập ngày 11 tháng 5 năm 2013.

[15] “The formation of deserts”. Desert. Oracle ThinkQuest Education Foundation. Truy cập ngày 11 tháng 5 năm 2013.

[16] Brinch, Brian (1 tháng 11 năm 2007). “How mountains influence rainfall patterns”. USA Today. Truy cập ngày 8 tháng 5 năm 2013.

[17] “Taklamakan Desert”. Encyclopedia Britannica online. Truy cập ngày 11 tháng 8 năm 2007.

[18] Pidwirny, Michael (2008). “CHAPTER 8: Introduction to the Hydrosphere (e) Cloud Formation Processes”. Physical Geography. Truy cập ngày 1 tháng 1 năm 2009.

[19] Mares, Michael (ed.) (1999). Encyclopedia of Deserts: Deserts, Montane. University of Oklahoma Press. tr. 172. ISBN 978-0-8061-3146-7.Quản lý CS1: văn bản dư: danh sách tác giả (liên kết)

[20] Bockheim, J. G. (2002). “Landform and soil development in the McMurdo Dry Valleys, Antarctica: a regional synthesis”. Arctic, Antarctic, and Alpine Research. 34 (3): 308–17. doi:10.2307/1552489.

[Fredlund1993-21] Fredlund, D.G.; Rahardjo, H. (1993). Soil Mechanics for Unsaturated Soils (PDF). Wiley-Interscience. ISBN 978-0-471-85008-3. Truy cập ngày 21 tháng 5 năm 2008.

[22] Allaby, Michael (2004). “Thornthwaite climate classification”. A Dictionary of Ecology. Encyclopedia.com. Truy cập ngày 23 tháng 9 năm 2013.

[Briggs-23] Briggs, Kenneth (1985). Physical Geography: Process and System. Hodder & Stoughton. tr. 8, 59–62. ISBN 978-0-340-35951-8.

[Uwe11-24] George, 1978. p. 11

[Uwe21-25] George, 1978. p. 21

[Uwe22-26] George, 1978. p. 22

[27] Smalley, I. J.; Vita-Finzi, C. (1968). “The formation of fine particles in sandy deserts and the nature of 'desert' loess”. Journal of Sedimentary Petrology. 38 (3): 766–774.Quản lý CS1: nhiều tên: danh sách tác giả (liên kết)

[Pye4-28] Pye & Tsoar, 2009. p. 4

[Pye141-29] Pye & Tsoar, 2009. p. 141

[GlobalAlarm-30] Yang, Youlin; Squires, Victor; Lu, Qi biên tập (2001). “Physics, Mechanics and Processes of Dust and Sandstorms”. Global Alarm: Dust and Sandstorms from the World's Drylands (PDF) |format= cần |url= (trợ giúp). United Nations Convention to Combat Desertification. tr. 17. Đã bỏ qua tham số không rõ |chapterurl= (trợ giúp)Quản lý CS1: nhiều tên: danh sách biên tập viên (liên kết)

[Uwe20-31] George, 1978. pp. 17–20 Lỗi chú thích: Thẻ <ref> không hợp lệ: tên “Uwe20” được định rõ nhiều lần, mỗi lần có nội dung khác

[32] Gu, Yingxin; Rose, William I.; Bluth, Gregg J. S. (2003). “Retrieval of mass and sizes of particles in sandstorms using two MODIS IR bands: A case study of April 7, 2001 sandstorm in China”. Geophysical Research Letters. 30 (15). Bibcode:2003GeoRL..30.1805G. doi:10.1029/2003GL017405.Quản lý CS1: nhiều tên: danh sách tác giả (liên kết)

[33] Sinclair, Peter C. (1969). “General characteristics of dust devils”. Journal of Applied Meteorology. 8 (1): 32–45. doi:10.1175/1520-0450(1969)008<0032:GCODD>2.0.CO;2.

[34] Zheng, Xiao Jing; Huang, Ning; Zhou, You-He (2003). “Laboratory measurement of electrification of wind-blown sands and simulation of its effect on sand saltation movement”. Journal of Geophysical Research: Atmospheres. 108 (D10). Bibcode:2003JGRD..108.4322Z. doi:10.1029/2002JD002572.Quản lý CS1: nhiều tên: danh sách tác giả (liên kết)

[35] Latham, J. (1964). “The electrification of snowstorms and sandstorms” (PDF). Quarterly Journal of the Royal Meteorological Society. 90 (383): 91–95. doi:10.1002/qj.49709038310.

[36] “Misconceptions surround desert terrain, vegetation”. Ask a Scientist. Cornell Center for Materials Research. 11 tháng 7 năm 2001. Truy cập ngày 24 tháng 9 năm 2013.

[37] “Habitats: Desert”. BBC Nature. 2013. Truy cập ngày 23 tháng 5 năm 2013.

[USGS-DF-38] ^ ^a ^b ^c ^d ^e ^f “Desert Features”. United States Geological Survey. 29 tháng 10 năm 1997. Truy cập ngày 23 tháng 5 năm 2013.

[39] “Sand Plains/Sand Sheets”. Desert Guide. US Army Corps of Engineers. Truy cập ngày 23 tháng 5 năm 2013.

[40] “Ripples, Sand”. Desert Guide. US Army Corps of Engineers. Truy cập ngày 23 tháng 5 năm 2013.

[41] “Dunes, General”. Desert Guide. US Army Corps of Engineers. Truy cập ngày 23 tháng 5 năm 2013.

[USGS-TD-42] “Types of Dunes”. United States Geological Survey. 29 tháng 10 năm 1997. Truy cập ngày 23 tháng 5 năm 2013.

[43] “Desert pavement”. Encyclopædia Britannica online. Truy cập ngày 23 tháng 5 năm 2013.

[44] Perry, R. S.; Adams, J. B. (1978). “Desert varnish: evidence for cyclic deposition of manganese” (PDF). Nature. 276 (5687): 489–491. doi:10.1038/276489a0.Quản lý CS1: nhiều tên: danh sách tác giả (liên kết)

[45] “Hamada, Reg, Serir, Gibber, Saï”. Springer Reference. 2013. Truy cập ngày 23 tháng 5 năm 2013.

[Uwe30-46] George, 1978. pp. 29–30

[47] Foos, Annabelle. “Geology of Grand Canyon National Park, North Rim” (PDF). Truy cập ngày 24 tháng 9 năm 2013.

[ES-48] Westbeld, A.; Klemm, O.; Grießbaum, F.; Strater, E.; Larrain, H.; Osses, P.; Cereceda, P. (2009). “Fog deposition to a Tillandsia carpet in the Atacama Desert” (PDF). Annales Geophysicae. 27: 3571–3576. doi:10.5194/angeo-27-3571-2009.Quản lý CS1: nhiều tên: danh sách tác giả (liên kết)

[49] McKay, C. P. (May–June 2002). “Too dry for life: The Atacama Desert and Mars” (PDF). Ad Astra. NASA: 30. Truy cập ngày 16 tháng 10 năm 2010.

[TWP-50] Boehm, Richard G. (2006). The World and Its People (ấn bản 2005). Glencoe. tr. 276. ISBN 978-0-07-860977-0. Đã bỏ qua tham số không rõ |coauthors= (gợi ý |author=) (trợ giúp)

[51] Preston, Benjamin (1 tháng 4 năm 2011). “Colonel Qaddafi and the Great Man-made River”. State of the Planet. Earth Institute: Columbia University. Truy cập ngày 2 tháng 10 năm 2013.

[52] Bayfield, Su (2011). “Introduction to Kharga Oasis”. Egyptian monuments. Truy cập ngày 2 tháng 10 năm 2013.

[PBS-53] “Desert Survival”. Public Broadcasting Service. Truy cập ngày 16 tháng 10 năm 2010.

[54] “Lake Bonneville”. Utah Geological Survey. Truy cập ngày 24 tháng 5 năm 2013.

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[Uwe123-57] George, 1978. pp. 122–123

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@@ Dòng 14: / Dòng 14: @@
 Hoang mạc được định nghĩa và phân loại theo nhiều cách, thường là kết hợp giữa giáng thủy dựa trên số ngày mưa/tuyết rơi, [[nhiệt độ]], và độ ẩm, và đôi khi thêm các yếu tố khác.<ref name=usgs/> Ví dụ, [[Phoenix, Arizona]] có lượng mưa hàng năm ít hơn {{convert|250|mm|abbr=on}}, và ngay lập tức đu7o75c công nhận là nằm trong nhóm hoang mạc bởi vì các thực vật thích nghi khô hạn ở đó. [[Alaska North Slope|North Slope]] của [[dãy Brooks]] thuộc Alaska cũng có lượng giáng thủy hàng năm nhỏ hơn {{convert|250|mm|abbr=on}} và thường được xếp vào hoang mạc lạnh.<ref name="WalterBreckle2002">{{cite book |author=Walter, Heinrich; Breckle, Siegmar-W. |title=Walter's Vegetation of the Earth: The Ecological Systems of the Geo-biosphere |url=http://books.google.com/?id=SdaCSwxK5bIC&pg=PA457 |year=2002 |publisher=Springer |isbn=978-3-540-43315-6 |page=457}}</ref> Các vùng khác trên thế giới có hoang mạc lạnh, bao gồm cả [[Himalayas]]<ref name="Negi2002">{{cite book|author=Negi, S.S. |title=Cold Deserts of India|url=http://books.google.com/?id=54RgJ6FgMl0C&pg=PA9|year=2002 |publisher=Indus Publishing|isbn=978-81-7387-127-6 |page=9}}</ref> và các vùng ở độ cao lớn khaa1c thuộc những nơi khác của thế giới.<ref name="RohliVega2008">{{cite book|author=Rohli, Robert V.; Vega, Anthony J. |title=Climatology|url=http://books.google.com/?id=Zhbqbrg2XswC&pg=PA207 |year=2008|publisher=Jones & Bartlett Learning |isbn=978-0-7637-3828-0 |page=207}}</ref> Các hoang mạc vùng Cực bao gồm chủ yếu là các khu vực đóng băng Bắc Băng Dương và Nam Băng Dương.<ref name="ThomasFogg2008">{{cite book|author=Thomas, David Neville et al.|title=The biology of polar regions|url=http://books.google.com/?id=-ErSVcvhI4oC&pg=PA64 |year=2008 |publisher=Oxford University Press |isbn=978-0-19-929813-6 |page=64}}</ref><ref name="LyonsHoward-Williams1997">{{cite book|author=Lyons, W. Berry; Howard-Williams, C. and Hawes, Ian |title=Ecosystem processes in Antarctic ice-free landscapes: proceedings of an International Workshop on Polar Desert Ecosystems : Christchurch, New Zealand, 1–4 July 1996|url=http://books.google.com/?id=VKtCnLa5uaYC&pg=PA3 |year=1997 |publisher=Taylor & Francis|isbn=978-90-5410-925-9 |pages=3–10}}</ref> Định nghĩa phi kỹ thuật nói rằng các hoang mạc là những bộ phận trên bề mặt trái đất có không đủ thảm thực vật để hỗ trợ sự sống cho con người.<ref name=encyclo>{{cite web |url=http://encyclopedia.jrank.org/DEM_DIO/DESERT.html |title=Desert |year=1911 |work=1911 Encyclopædia Britannica, Volume 8 |page=93 |accessdate=2013-09-24}}</ref>
+{{Đang dịch 2 (nguồn)|ngày=30
+|tháng=06
+|năm=2014
+|1 = tiếng Anh
+}}
+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 {{convert|300|mm|abbr=on}} of rain per year, however about {{convert|2500|mm|abbr=on}} of water could evaporate over the course of a year.<ref>{{cite journal |author=Buel, S. W. |year=1964 |title=Calculated actual and potential evapotranspiration in Arizona |journal=Tucson, Arizona University Agricultural Experiment Station Technical Bulletin |volume=162 |page=48 }}</ref> 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.<ref>{{cite journal |author=Mendez, J.; Hinzman, L. D.; Kane, D. L. |year=1998 |title=Evapotranspiration from a wetland complex on the Arctic coastal plain of Alaska |journal=Nordic Hydrology |volume=29 |issue=4–5 |pages=303–330 |issn=0029-1277 }}</ref>
+Deserts are sometimes classified as "hot" or "cold", "semiarid" or "coastal".<ref name=encyclo/> 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&nbsp;°C (40&nbsp;°F) or more, with heat loss by radiation at night being increased by the clear skies.<ref name=Laity>{{cite book |title=Deserts and Desert Environments: Volume 3 of Environmental Systems and Global Change Series |last=Laity |first=Julie J. |year=2009 |publisher=John Wiley & Sons |isbn=9781444300741 |pages=2–7, 49 |url=http://books.google.co.uk/books?hl=en&lr=&id=wtAbzLLTcwcC&oi=fnd&pg=PR5&dq=%22classification+of+deserts%22+&ots=R1fE69omfL&sig=zRHGP9dQL3kXcyYQkxw_huD006g#v=onepage&q=%22classification%20of%20deserts%22&f=false }}</ref>
+[[File:AntarcticaDomeCSnow.jpg|thumb|Cold desert: snow surface at Dome C Station, Antarctica]]
+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 southern [[Andes]] and in southern Australia.<ref name="brittanica"/> Polar deserts are a particular class of cold desert. The air is very cold and carries little moisture so little precipitation occurs and what does fall, usually as snow, is carried along in the often strong wind and may form blizzards, drifts and dunes similar to those caused by dust and sand in other desert regions. In [[Antarctica]], for example, the annual precipitation is about {{convert|50|mm|0|abbr=on}} on the central plateau and some ten times that amount on some major peninsulas.<ref name=Laity/>
+Based on precipitation alone, [[hyperarid]] deserts receive less than {{convert|25|mm|0|abbr=on}} of rainfall a year; they have no annual seasonal cycle of precipitation and experience twelve-month periods with no rainfall at all.<ref name=Laity/><ref name="Oliver2005">{{cite book|author=John E. Oliver|title=The Encyclopedia of World Climatology|url=http://books.google.com/books?id=-mwbAsxpRr0C&pg=PA86|date=1 January 2005|publisher=Springer|isbn=978-1-4020-3264-6|page=86}}</ref> Arid deserts receive between {{convert|25|and|200|mm|0|abbr=on}} in a year and semiarid deserts between {{convert|200|and|500|mm|0|abbr=on}}. However, such factors as the temperature, humidity, rate of evaporation and evapotranspiration, and the moisture storage capacity of the ground have a marked effect on the degree of aridity and the plant and animal life that can be sustained. Rain falling in the cold season may be more effective at promoting plant growth, and defining the boundaries of deserts and the semiarid regions that surround them on the grounds of precipitation alone is problematic.<ref name=Laity/>
+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&nbsp;°C (20&nbsp;°F) and 5&nbsp;°C (9&nbsp;°F) respectively in the [[Atacama Desert]]. Deserts of this type are often long and narrow and bounded to the east by mountain ranges. They occur in south-west Africa, [[Chile]], southern California and [[Baja California]]. Other coastal deserts influenced by cold currents are found in [[Western Australia]], the [[Arabian Peninsula]] and [[Horn of Africa]], and the western fringes of the Sahara.<ref name=Laity/>
+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 precipitation, arid lands have less than 250&nbsp;mm (10&nbsp;in) of annual precipitation, and semiarid lands have a mean annual precipitation of between 250 and 500&nbsp;mm (10–20&nbsp;in). Both extremely arid and arid lands are considered to be deserts while semiarid lands are generally referred to as [[steppes]] when they are grasslands.<ref name=usgs/>
+[[File:Agasthiyamalai range and Tirunelveli rainshadow.jpg|thumb|left|The [[Agasthiyamalai]] hills cut off [[Tirunelveli]] in [[India]] from the [[monsoon]]s, creating a [[rainshadow]] region.]]
+Deserts are also classified, according to their geographical location and dominant weather pattern, as trade wind, mid-latitude, rain shadow, coastal, monsoon, or [[polar desert]]s.<ref name=Destypes>{{cite web |url=http://pubs.usgs.gov/gip/deserts/types/ |title=Types of deserts |work=Deserts: Geology and Resources |publisher=United States Geological Survey |accessdate=2013-05-11}}</ref> 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.<ref>{{cite web |url=http://library.thinkquest.org/26634/desert/formation.htm |title =The formation of deserts |work=Desert |publisher=Oracle ThinkQuest Education Foundation |accessdate=2013-05-11}}</ref> 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 Desert|Tengger]] and [[Sonoran Desert]]s.<ref name=Destypes/> 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.<ref name=Destypes/>
+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 precipitation occurs.<ref>{{cite news |url=http://usatoday30.usatoday.com/weather/tg/wrnshdw/wrnshdw.htm |title=How mountains influence rainfall patterns |author=Brinch, Brian |work=USA Today |accessdate=2013-05-08 |date=2007-11-01}}</ref> The [[Taklamakan Desert]] is an example, lying in the rain shadow of the [[Himalayas]] and receiving less than {{convert|38|mm|abbr=on}} precipitation annually.<ref>{{cite web |url=http://www.britannica.com/eb/article-9110530/Takla-Makan-Desert |title=Taklamakan Desert |work=Encyclopedia Britannica online |accessdate=2007-08-11 }}</ref>
+Other areas are arid by virtue of being a very long way from the nearest available sources of moisture.<ref>{{cite web|author=Pidwirny, Michael  |year=2008 |url=http://www.physicalgeography.net/fundamentals/8e.html|title=CHAPTER 8: Introduction to the Hydrosphere (e) Cloud Formation Processes |publisher=Physical Geography |accessdate=2009-01-01}}</ref>
+[[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 {{convert|3000|m|abbr=on}} and the thermal regime can be [[hemiboreal]]. These places owe their profound aridity (the average annual precipitation is often less than 40&nbsp;mm or 1.5&nbsp;in) to being very far from the nearest available sources of moisture and are often in the [[Windward and leeward|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]].<ref>{{cite book |title=Encyclopedia of Deserts: Deserts, Montane |last=Mares |first=Michael (ed.) |year=1999 |publisher=University of Oklahoma Press |isbn=978-0-8061-3146-7 |page=172 |url=http://books.google.co.uk/books?id=g3CbqZtaF4oC&pg=PA172&lpg=PA172&dq=montane+desert&source=bl&ots=uXCzhwkVQy&sig=NtF1nUxig83OXWlHk11Rd7aTCMM&hl=en&sa=X&ei=oz6OUZ2DLMKL0AXf04HYDA&ved=0CGAQ6AEwBg#v=onepage&q=%22Deserts%2C%20montane%22&f=false }}</ref>
+Polar deserts such as [[McMurdo Dry Valleys]] remain ice-free because of the dry [[katabatic wind]]s that flow downhill from the surrounding mountains.<ref>{{cite journal| last =Bockheim | first =J. G. | title =Landform and soil development in the McMurdo Dry Valleys, Antarctica: a regional synthesis | journal=Arctic, Antarctic, and Alpine Research |volume=34 |issue= 3 |pages= 308–17 |year= 2002 | doi =10.2307/1552489}}</ref> Former desert areas presently in non-arid environments, such as the [[Sandhills (Nebraska)|Sandhills in Nebraska]], are known as paleodeserts.<ref name=Destypes/> 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.<ref name=Fredlund1993>{{cite book|author = Fredlund, D.G.|author2=Rahardjo, H.|year = 1993|title = Soil Mechanics for Unsaturated Soils|publisher = Wiley-Interscience|url = http://www.soilvision.com/subdomains/unsaturatedsoil.com/Docs/chapter1UST.pdf|format = PDF|isbn = 978-0-471-85008-3|accessdate = 2008-05-21}}</ref><ref>{{cite web |url=http://www.encyclopedia.com/topic/Thornthwaite_climate_classification.aspx |title=Thornthwaite climate classification |author=Allaby, Michael |year=2004 |work=A Dictionary of Ecology |publisher=Encyclopedia.com |accessdate=2013-09-23}}</ref>
+===Weathering processes===
+[[File:GeologicalExfoliationOfGraniteRock.jpg|thumb|Exfoliation of weathering rocks in Texas]]
+Deserts usually have a large [[Diurnal temperature variation|diurnal]] and seasonal temperature range, with high daytime temperatures falling sharply at night. The diurnal range may be as much as 20 to 30&nbsp;°C (36 to 54&nbsp;°F) and the rock surface experiences even greater temperature differentials.<ref name=Briggs>{{cite book |title=Physical Geography: Process and System |last=Briggs |first=Kenneth |year=1985 |publisher=Hodder & Stoughton |isbn=978-0-340-35951-8 |pages=8, 59–62 }}</ref> 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&nbsp;°C (113&nbsp;°F) in summer and plunge below freezing point at night during winter.<ref name=Uwe11>George, 1978. p. 11</ref>
+[[File:Sand from Gobi Desert.jpg|thumb|left|upright|One square centimeter<br>(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 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.<ref name=Uwe21>George, 1978. p. 21</ref> [[Exfoliation joint|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.<ref name=Briggs/> 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.<ref name=Briggs/>
+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.<ref name=Uwe22>George, 1978. p. 22</ref> 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.<ref>{{cite journal |author=Smalley, I. J.; Vita-Finzi, C. |year=1968 |title=The formation of fine particles in sandy deserts and the nature of 'desert' loess |journal=Journal of Sedimentary Petrology |volume=38 |issue=3 |pages=766–774 |url=http://archives.datapages.com/data/sepm/journals/v38-41/data/038/038003/0766.htm }}</ref>
+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 (geology)|saltation]]. The whirling airborne grains act as a [[Abrasive blasting|sand blasting]] mechanism which grinds away solid objects in its path as the kinetic energy of the wind is transferred to the ground.<ref name=Pye4>Pye & Tsoar, 2009. p. 4</ref> The sand eventually ends up deposited in level areas known as sand-fields or sand-seas, or piled up in dunes.<ref name=Pye141>Pye & Tsoar, 2009. p. 141</ref>
+===Dust storms and sandstorms===
+{{main|Dust storm}}
+[[File:Sandstorm in Al Asad, Iraq.jpg|thumb|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, shape and density; [[Suspension (chemistry)|suspension]], [[Saltation (geology)|saltation]] or creep. Suspension is only possible for particles less than 0.1&nbsp;mm (0.004&nbsp;in) in diameter. In a dust storm, these fine particles are lifted up and wafted aloft to heights of up to {{convert|6|km|abbr=on}}. They reduce visibility and can remain in the atmosphere for days on end, conveyed by the trade winds for distances of up to {{convert|6000|km|abbr=on}}.<ref name=GlobalAlarm>{{Cite book|editor=Yang, Youlin; Squires, Victor; Lu, Qi|title=Global Alarm: Dust and Sandstorms from the World's Drylands|chapter=Physics, Mechanics and Processes of Dust and Sandstorms|chapterurl=http://archive.unccd.int/publicinfo/duststorms/part1-eng.pdf|format=PDF|publisher=United Nations Convention to Combat Desertification|year=2001|page=17}}</ref> 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.<ref name=Uwe20>George, 1978. pp. 17–20</ref> 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 {{convert|134000000|km2|abbr=on}}. The mean particle size was 1.44 μm.<ref>{{cite journal |author=Gu, Yingxin; Rose, William I.; Bluth, Gregg J. S. |year=2003 |title=Retrieval of mass and sizes of particles in sandstorms using two MODIS IR bands: A case study of April 7, 2001 sandstorm in China |journal=Geophysical Research Letters |volume=30 |issue=15 |doi=10.1029/2003GL017405 |bibcode=2003GeoRL..30.1805G}}</ref> 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]].<ref>{{cite journal |author=Sinclair, Peter C. |year=1969 |title=General characteristics of dust devils |journal=Journal of Applied Meteorology |volume=8 |issue=1 |pages=32–45 |doi=10.1175/1520-0450(1969)008<0032:GCODD>2.0.CO;2 }}</ref>
+[[File:Saltation-mechanics-i18n.png|thumb|left|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 {{convert|0.5|mm|abbr=on}} 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 {{convert|30|cm|abbr=on}}.<ref name=GlobalAlarm/> In a really severe steady blow, {{convert|2|m|abbr=on}} 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.<ref name=Uwe20 />
+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.<ref name=Uwe20>George, 1978. p. 20</ref> 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.<ref>{{cite journal |author=Zheng, Xiao Jing; Huang, Ning; Zhou, You-He |year=2003 |title=Laboratory measurement of electrification of wind-blown sands and simulation of its effect on sand saltation movement |journal=Journal of Geophysical Research: Atmospheres  |volume=108 |issue=D10 |doi=10.1029/2002JD002572 |bibcode=2003JGRD..108.4322Z}}</ref><ref>{{cite journal |author=Latham, J. |year=1964 |title=The electrification of snowstorms and sandstorms |journal=Quarterly Journal of the Royal Meteorological Society |volume=90 |issue=383 |pages=91–95 |doi=10.1002/qj.49709038310 |url=http://www.idl.ku.edu/ara/AARPS/documents/ESD-emi/Latham_1963.pdf }}</ref>
+===Features===
+[[File:Israel-2013-Aerial 00-Negev-Makhtesh Ramon.jpg|thumb|Aerial view of [[Makhtesh Ramon]], an erosion cirque of a type unique to the [[Negev]]]]
+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,<ref>{{cite web |url=http://www.ccmr.cornell.edu/education/ask/?quid=121 |title=Misconceptions surround desert terrain, vegetation |date=2001-07-11 |work=Ask a Scientist |publisher=Cornell Center for Materials Research |accessdate=2013-09-24}}</ref> but deserts do not always look like this.<ref>{{cite web | url=http://www.bbc.co.uk/nature/habitats/Deserts_and_xeric_shrublands | title=Habitats: Desert | publisher=BBC Nature | year=2013 | accessdate=2013-05-23}}</ref> Across the world, around 20% of desert is sand, varying from only 2% in North America to 30% in Australia and over 45% in Central Asia.<ref name=USGS-DF/> Where sand does occur, it is usually in large quantities in the form of sand sheets or extensive areas of [[dune]]s.<ref name=USGS-DF/>
+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.<ref>{{cite web |url=http://www2.agc.army.mil/research/products/desert_guide/lsmsheet/lsplain.htm |title=Sand Plains/Sand Sheets |work=Desert Guide |publisher=US Army Corps of Engineers |accessdate=2013-05-23}}</ref> Small ripples form on the sand sheet when the wind exceeds 24 kph (15&nbsp;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.<ref>{{cite web |url=http://www2.agc.army.mil/research/products/desert_guide/lsmsheet/lssand.htm |title=Ripples, Sand |work=Desert Guide |publisher=US Army Corps of Engineers |accessdate=2013-05-23}}</ref>
+[[File:Dune en.svg|thumb|left|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.<ref>{{cite web |url=http://www2.agc.army.mil/research/products/desert_guide/lsmsheet/lsdune.htm |title=Dunes, General |work=Desert Guide |publisher=US Army Corps of Engineers |accessdate=2013-05-23}}</ref> 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 [[Erg (landform)|ergs]].<ref name=USGS-TD>{{cite web | url=http://pubs.usgs.gov/gip/deserts/dunes/ | title=Types of Dunes | publisher=United States Geological Survey | date=1997-10-29 | accessdate=2013-05-23}}</ref>
+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 dune|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 {{convert|500|m|abbr=on}}, 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.<ref name=USGS-TD/>
+[[File:Desert Pavement Mojave 2000.jpg|thumb|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 [[pebble]]s with some larger stones or [[cobblestone|cobbles]],<ref name=Pye141 /><ref name=USGS-DF>{{cite web | url=http://pubs.usgs.gov/gip/deserts/features/ | title=Desert Features | publisher=United States Geological Survey | date=1997-10-29 | accessdate=2013-05-23}}</ref> leaving a [[desert pavement]], an area of land overlaid by closely packed smooth stones forming a [[Tessellation|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.<ref>{{cite web |url=http://www.britannica.com/EBchecked/topic/159076/desert-pavement |title=Desert pavement |work=Encyclopædia Britannica online |accessdate=2013-05-23}}</ref> 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]].<ref>{{cite journal |author=Perry, R. S.; Adams, J. B. |year=1978 |title=Desert varnish: evidence for cyclic deposition of manganese |journal=Nature |volume=276 |issue=5687 |pages=489–491 |url=https://www.psi.edu/sites/default/files/imported/about/staff/rperry/perry/Nature1978.pdf |doi=10.1038/276489a0}}</ref>
+Other non-sandy deserts consist of exposed outcrops of [[bedrock]], dry soils or [[aridisols]], and a variety of landforms affected by [[fluvial|flowing water]], such as [[alluvial fan]]s, [[Sink (geography)|sinks or playas]], temporary or permanent [[lake]]s, and oases.<ref name=USGS-DF/> 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.<ref>{{cite web | url=http://www.springerreference.com/docs/html/chapterdbid/43197.html | title=Hamada, Reg, Serir, Gibber, Saï | publisher=Springer Reference | year=2013 | accessdate=2013-05-23}}</ref> The [[Tassili n'Ajjer|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.<ref name=Uwe30>George, 1978. pp. 29–30</ref> 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 [[Grand Canyon|canyon]] that is over a mile (6,000 feet or 1,800 meters) deep in places, exposing strata that are over two billion year old.<ref>{{cite web |url=http://www.nature.nps.gov/geology/education/foos/grand.pdf |title=Geology of Grand Canyon National Park, North Rim |author=Foos, Annabelle |accessdate=2013-09-24}}</ref>
+===Water===
+[[File:Atacama1.jpg|left|thumb|[[Atacama Desert|Atacama]], the world's driest desert]]
+One of the driest places on [[Earth]] is the [[Atacama Desert]].<ref name=ES>{{cite journal |author=Westbeld, A.; Klemm, O.; Grießbaum, F.; Strater, E.; Larrain, H.; Osses, P.; Cereceda, P. |year=2009 |title=Fog deposition to a ''Tillandsia'' carpet in the Atacama Desert |journal=Annales Geophysicae |volume=27 |pages=3571–3576 |url=http://www.ann-geophys.net/27/3571/2009/angeo-27-3571-2009.pdf |doi=10.5194/angeo-27-3571-2009}}</ref> 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 [[Pacific Anticyclone|anticyclone of the Pacific]] are essential to keep the dry climate of the Atacama. The average precipitation in the Chilean region of [[Antofagasta (region)|Antofagasta]] is just {{convert|1|mm|abbr=on}} 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 {{convert|6885|m|abbr=on}} are completely free of [[glacier]]s 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 {{convert|4400|m|abbr=on}} and is continuous above {{convert|5600|m|abbr=on}}.<ref>{{cite journal|journal=Ad Astra|date=May–June 2002|page=30|author=McKay, C. P.|url=http://quest.nasa.gov/challenges/marsanalog/egypt/AtacamaAdAstra.pdf |title=Too dry for life: The Atacama Desert and Mars|publisher=NASA|format=PDF |accessdate=2010-10-16}}</ref><ref name=TWP>{{cite book|first=Richard G.|last=Boehm |coauthors=Editors and writers of ''The World and Its People''|year=2006|title=The World and Its People |edition=2005 |publisher=Glencoe |isbn=978-0-07-860977-0 |page=276}}</ref> Nevertheless, there is some plant life in the Atacama, in the form of specialist plants that obtain moisture from dew and the [[fog]]s that blow in from the Pacific.<ref name=ES/>
+[[File:GobiFlood.JPG|thumb|right|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 [[arroyo (creek)|arroyos]] or [[wadi]]s meandering across its surface. These can experience [[flash flood]]s, 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 [[Nile|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 [[Spring (hydrology)|springs]], [[aquifer]]s, underground rivers or lakes. Where these lie close to the surface, [[Water well|wells]] can be dug and [[Oasis|oases]] may form where plant and animal life can flourish.<ref name=USGS-DF/> The [[Nubian Sandstone Aquifer System]] under the Sahara Desert is the largest known accumulation of [[fossil water]]. The [[Great Man-Made River]] is a scheme launched by Libya's [[Muammar Gaddafi|Colonel Gadaffi]] to tap this aquifer and supply water to coastal cities.<ref>{{cite web |url=http://blogs.ei.columbia.edu/2011/04/01/libya/ |title=Colonel Qaddafi and the Great Man-made River |author=Preston, Benjamin |date=2011-04-01 |work=State of the Planet |publisher=Earth Institute: Columbia University |accessdate=2013-10-02}}</ref> [[Kharga Oasis]] in Egypt is {{convert|150|km|abbr=on}} long and is the largest oasis in the Libyan Desert. A lake occupied this depression in ancient times and thick deposits of sandy-clay resulted. Wells are dug to extract water from the porous sandstone that lies underneath.<ref>{{cite web |url=http://egyptsites.wordpress.com/2009/03/06/introduction-to-kharga-oasis/ |title=Introduction to Kharga Oasis |author=Bayfield, Su |year=2011 |work=Egyptian monuments |accessdate=2013-10-02}}</ref> Seepages may occur in the walls of [[canyon]]s and pools may survive in deep shade near the dried up watercourse below.<ref name=PBS>{{cite web|url=http://www.pbs.org/lawrenceofarabia/revolt/water.html |title=Desert Survival |publisher=Public Broadcasting Service |accessdate=2010-10-16}}</ref>
+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 [[Dry lake|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.<ref>{{cite web |url=http://geology.utah.gov/online_html/pi/pi-39/pi39pg01.htm |title=Lake Bonneville |publisher=Utah Geological Survey |accessdate=2013-05-24}}</ref> 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 [[Edwards Air Force Base|runways]] for aircraft and the [[space shuttle]].<ref name=USGS-DF/>
+==Biogeography==
+===Flora===
+[[File:Baja California Desert.jpg|thumb|right|[[Xerophyte]]s: [[Pachycereus pringlei|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 [[stoma]]ta 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]], allowing them to open their stomata during the night to allow CO<sub>2</sub> to enter, and close them during the day,<ref name=stomata>{{cite book|title=Stomatal function|year=1987|publisher=Stanford University Press|location=Stanford, Calif.|isbn=9780804713474|url=http://books.google.de/books?id=mp-aAAAAIAAJ&pg=PA353&dq=cactus+stomata&hl=en&sa=X&ei=IQ-3Uq_7C6PmywOq0oGoAg&ved=0CC8Q6AEwAA#v=onepage&q=cactus%20stomata&f=false|author=Eduardo Zeiger|accessdate=22 December 2013}}</ref> or by using [[C4 carbon fixation]].<ref>{{cite journal |author=Osborne, Colin P.; Beerling, David J. |year=2006 |title=Nature's green revolution: the remarkable evolutionary rise of C4 plants |journal=Philosophical Transactions of the Royal Society B |volume=361 |issue=1465 |pages=173–194 |issn=1471-2970 |doi=10.1098/rstb.2005.1737 |pmid=16553316 |pmc=1626541}}</ref>
+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.<ref name=Uwe123>George, 1978. pp. 122–123</ref> The giant [[saguaro|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.<ref name=Uwe123 />
+Cacti are restricted to the [[New World]] but other [[Xerophyte|xerophytic]] plants have developed similar strategies by a process known as [[convergent evolution]].<ref>{{cite web |url=http://biology.unm.edu/ccouncil/Biology_112/Summaries/Plant_Adaptations.html |title=Plant adaptations |author=Council-Garcia, Cara Lea |year=2002 |publisher=University of New Mexico |accessdate=2013-09-24}}</ref> 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 maximize water uptake by having shallow roots that spread widely, or by developing long [[taproot]]s that reach down to deep rock strata for ground water.<ref name=DEH/> The [[saltbush]] in Australia has succulent leaves and secretes salt crystals, enabling it to live in saline areas.<ref name=DEH>{{cite web |url=http://www.nynrm.sa.gov.au/Portals/7/pdf/biodiversity/flora.pdf |title=Desert Flora |publisher=Australian Department of the Environment and Heritage |accessdate=2013-05-13}}</ref><ref>{{cite web |url=http://www.desertmuseum.org/programs/succulents_adaptation.php |title=How Plants Cope with the Desert Climate |author=Dimmitt, Mark A. |year=1997 |publisher=Arizona-Sonora Desert Museum |accessdate=2013-05-13}}</ref> In common with cacti, many have developed spines to ward off browsing animals.<ref name=Uwe123 />
+[[File:Thorn Tree Sossusvlei Namib Desert Namibia Luca Galuzzi 2004a.JPG|thumb|left|The Camel Thorn Tree ([[Acacia erioloba]]) in the [[Namib Desert]] is nearly leafless in dry periods.]]
+Some desert plants produce seed which lies [[Dormancy|dormant]] in the soil until sparked into growth by rainfall. When [[annual plant|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 [[Feces|dung]] provides an excellent start to life well away from the parent tree.<ref name=Uwe123 />  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 ''[[Soil crust|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 [[lichen]]s. Most shrubs have spiny leaves and shed them in the coldest part of the year.<ref>{{cite web |url=http://www.ucmp.berkeley.edu/glossary/gloss5/biome/deserts.html |title=Cold Deserts |year=1996|work=The desert biome |publisher=University of California Museum of Paleontology |accessdate=2013-09-23}}</ref>
+===Fauna===
+{{Main|Xerocole}}
+Animals adapted to live in deserts are called [[xerocole]]s. 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.<ref name=Scholander>{{cite journal |author=Scholander, P. F.; Hock, Raymond; Walters, Vladimir; Irving, Laurence |year=1950 |title=Adaptation to cold in arctic and tropical mammals and birds in relation to body temperature, insulation, and basal metabolic rate |journal=Biological Bulletin |volume=50 |issue=2 |page=269 |doi= |pmid= |pmc= |url=http://www.biolbull.org/content/99/2/259.full.pdf+html }}</ref> 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.<ref name='Al-kahtani_et_al_2004'>{{cite journal|last=Al-kahtani|first=M.A.|coauthors=C. Zuleta, E. Caviedes-Vidal, and T. Garland, Jr.|year=2004|title=Kidney mass and relative medullary thickness of rodents in relation to habitat, body size, and phylogeny| url=http://www.biology.ucr.edu/people/faculty/Garland/Al-kahtaniEA2004.pdf|journal=Physiological and Biochemical Zoology |volume=77|issue=3|pages=346–365|doi=10.1086/420941|pmid=15286910|last4=Garland}}</ref> Many examples of [[convergent evolution]] have been identified in desert organisms, including between [[cacti]] and [[Euphorbia]], [[kangaroo rats]] and [[jerboas]], ''[[Phrynosoma]]'' and ''[[Moloch horridus|Moloch]]'' lizards.<ref>{{cite web |url=http://www.biologyreference.com/Co-Dn/Convergent-Evolution.html |title=Convergent Evolution |author=Pianka, Eric R. |work=Biology Reference |accessdate=2013-05-28}}</ref>
+[[File:Cream-coloured courser.jpg|thumb|The cream-colored courser, ''[[Cursorius cursor]]'', is a [[camouflage|well-camouflaged]] desert resident with its dusty [[animal coloration|coloration]], [[countershading]], and [[disruptive coloration|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|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-[[camouflage]]d 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 [[Depression (geology)|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.<ref name=Uwe141>George, 1978. p. 141</ref>
+Water and carbon dioxide are metabolic end products of oxidation of fats, proteins, and carbohydrates.<ref>{{cite book|url=http://books.google.co.uk/books?id=xIj3YPKSpJkC&pg=PA511&lpg=PA511&dq=water+of+metabolism+biochemistry&source=bl&ots=uoqSq_toEV&sig=AgrSPqebpSv5tjnpeQGlMM4ebq4&hl=en&sa=X&ei=rF9WU8SqFOeN7AbE-ICoAQ&ved=0CDEQ6AEwATgU#v=onepage&q=water%20of%20metabolism%20biochemistry&f=false |page=511 |year=2006 |title=Biochemistry (fifth edition)|first1=Mary K|last1=Campbell |first2=Shawn O|last2=Farrell |publisher=Thomson Brooks/Cole |location=USA |isbn=0-453-40521-5}}</ref> 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,<ref>{{cite journal | url=http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1366125/pdf/jphysiol01419-0196.pdf | title=A method for the calculation of metabolic water | author=Morrison, S. D. | journal=J. Physiology | year=1953 | volume=122 | issue=2 | pages=399-402}} Morrison cites Brody, S. ''Bioenergetics and Growth''. Reinhold, 1945. p. 36 for the figures.</ref> making it possible for xerocoles to live with little or no access to drinking water.<ref name=Mellanby>{{cite journal |author=Mellanby, Kenneth |year=1942 |title=Metabolic water and desiccation |journal=Nature |volume=150 |pages=21 |issn=0028-0836 |doi=10.1038/150021a0}}</ref> 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,<ref name=Best_et_al>Best, T. L., et al. (1989) Dipodomys deserti. Mammalian Species 339:1-8 [http://www.science.smith.edu/msi/pdf/i0076-3519-339-01-0001.pdf]</ref> reducing loss of water through its skin and respiratory system when at rest.<ref name=Mellanby/><ref>{{cite book |title=An Analysis of Intraspecific Variation in the Kangaroo Rat ''Dipodomus merriami'' |last=Lidicker |first=W. Z. |year=1960 |publisher=University of California Press }}</ref> [[Herbivore|Herbivorous]] mammals obtain moisture from the plants they eat. Species such as the [[Addax|addax antelope]],<ref>{{cite book |last = Lacher, Jr. |first = Thomas E. |isbn=978-0-8061-3146-7 |page=7 |title=Encyclopedia of Deserts: Addax |publisher=University of Oklahoma Press |url=http://books.google.com/books?id=g3CbqZtaF4oC&lpg=PP1&pg=PA7}}</ref> [[dik-dik]], [[Grant's gazelle]] and [[oryx]] are so efficient at doing this that they apparently never need to drink.<ref>{{cite journal | url=http://www.jstor.org/stable/76120 | title=The water metabolism of a small East African antelope: the dik-dik | author=Maloiy, G. M. O. | journal=Proceedings of the Royal Society B |date=November 1973  | volume=184 | issue=1075 | pages=167–178 | doi=10.1098/rspb.1973.0041}}</ref> The [[camel]] is a superb example of a [[mammal]] adapted to desert life. It minimizes its water loss by producing concentrated urine and dry [[feces|dung]], and is able to lose 40% of its body weight through water loss without dying of dehydration.<ref>{{cite web |url=http://www.djur.cob.lu.se/Djurartiklar/Kamel.html |title=What secrets lie within the camel's hump? |first=Kerstin |last=Vann Jones |publisher=Lund University |accessdate=2013-05-21}}</ref> [[Carnivore]]s can obtain much of their water needs from the body fluids of their prey.<ref name=Silverstein/> Many other hot desert animals are [[Nocturnality|nocturnal]], seeking out shade during the day or dwelling underground in burrows. At depths of more than  {{convert|50|cm|abbr=on}}, these remain at between 30 and 32&nbsp;°C (86 and 90&nbsp;°F) regardless of the external temperature.<ref name=Silverstein>{{cite book | publisher = Twenty-First Century Books | isbn = 978-0-8225-3434-1 | last1 = Silverstein | first1 = Alvin | first2 = Virginia B. |last2= Silverstein |first3=Virginia |last3=Silverstein |first4=Laura |last4=Silverstein Nunn | title = Adaptation | year = 2008 |pages=42–43 }}</ref> [[Jerboa]]s, [[Gerbil|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]].<ref>{{cite web |url=http://austhrutime.com/Kangaroo_red.htm |title=The Red Kangaroo |author=Monroe, M. H. |work=Australia: The Land Where Time Began |accessdate=2013-10-03}}</ref> Mammals living in cold deserts have developed greater insulation through warmer body fur and insulating layers of [[fat]] beneath the skin. The [[Least weasel|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.<ref name=Scholander/> 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.<ref name=NatGeoHile>{{cite web |url=http://news.nationalgeographic.com/news/2004/03/0329_040329_TVpenguins.html |title=Emperor Penguins: Uniquely Armed for Antarctica |accessdate=2013-10-02 |author=Hile, J.|date=2004-03-29 |work=National Geographic }}</ref>
+Being [[ectotherm]]s, [[reptile]]s 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 {{convert|50|°C}}. Reptiles cannot survive at this temperature and lizards will be prostrated by heat at {{convert|45|°C}}. 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 [[Predation|prey]]. [[Lizard]]s and [[snake]]s are the most numerous in arid regions and certain snakes have developed a [[Sidewinding|novel method of locomotion]] that enables them to move sidewards and navigate high sand-dunes. These include the [[Cerastes (genus)|horned viper]] of Africa and the [[Crotalus cerastes|sidewinder]] of North America, evolutionarily distinct but with similar behavioural patterns because of [[convergent evolution]]. Many desert reptiles are [[ambush predator]]s and often bury themselves in the sand, waiting for prey to come within range.<ref>{{cite book |last = Lacher, Jr. |first = Thomas E. |isbn=978-0-8061-3146-7 |page=108 |title=Encyclopedia of Deserts: ''Cerastes'' |url=http://books.google.com/books?id=g3CbqZtaF4oC&lpg=PP1&pg=PA7}}</ref>
+[[Amphibia]]ns 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 [[Aestivation|aestivating]] in deep burrows. While there they shed their skins a number of times and retain the remnants around them as a waterproof [[cocoon (silk)|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.<ref>{{cite web |url=http://www.desertmuseum.org/books/nhsd_spadefoot.php |title=Couch's spadefoot (''Scaphiopus couchi'') |publisher=Arizona-Sonora Desert Museum |accessdate=2013-05-21}}</ref> The [[Litoria platycephala|water holding frog]] in Australia has a similar life cycle and may aestivate for as long as five years if no rain falls.<ref>{{cite journal |author=Withers, P. C. |year=1993 |title=Metabolic Depression During Estivation in the Australian Frogs, ''Neobatrachus'' and ''Cyclorana'' |journal=Australian Journal of Zoology  |volume=41 |issue=5 |pages=467–473  |doi=10.1071/ZO9930467 }}</ref> The [[Desert rain frog]] of Namibia is nocturnal and survives because of the damp [[sea fog]]s that roll in from the Atlantic.<ref>{{cite web |url=http://amphibiaweb.org/cgi/amphib_query?where-genus=Breviceps&where-species=macrops |title=''Breviceps macrops'' |author=Castillo, Nery |date=2011-06-23 |publisher=AmphibiaWeb |accessdate=2012-10-20}}</ref>
+[[File:Triops australiensis.jpg|thumb|left|[[Triops australiensis|Tadpole shrimp]] survive dry periods as eggs, which rapidly hatch and develop after rain.]]
+Invertebrates, particularly [[arthropod]]s, have successfully made their homes in the desert. [[Fly|Flies]], [[beetle]]s, [[ant]]s, [[termite]]s, [[locust]]s, [[millipede]]s, [[scorpion]]s and [[spider]]s<ref>{{cite web |url=http://www.desertmuseum.org/books/nhsd_inverts.php |title=Invertebrates: A Vertebrate Looks at Arthropods |publisher=Arizona-Sonora Desert Museum |accessdate=2013-05-21}}</ref> have hard [[cuticle]]s 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.<ref>{{cite web |url=http://library.thinkquest.org/26634/desert/invertebrate.htm |title=Invertebrates in the Desert |work=ThinkQuest |publisher=Oracle |accessdate=2013-05-22}}</ref> 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.<ref>{{cite journal |author=Moseley, Pope L. |year=1997 |title=Heat shock proteins and heat adaptation of the whole organism |journal=Journal of Applied Physiology |volume=83 |issue=5 |pages=1413–1417 |url=http://jap.physiology.org/content/83/5/1413.full }}</ref> The [[Stenocara dentata|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.<ref>{{cite book |title=Field Guide to the Insects of South Africa |last=Picker |first=Mike |last2=Griffiths |first2=Charles | last3=Weaving | first3 =Alan |year=2004 |publisher=Struik |isbn=978-1-77007-061-5 |page=232 |url=http://books.google.co.uk/books?id=s4ke5JwbTI0C&pg=PA441&lpg=PA441&dq=Stenocara+dentata&source=bl&ots=E5rApYR5UI&sig=igb4SbNuvT2PGrX5jqhhUIETKGA&hl=en&sa=X&ei=XJ2kUc6fHsKb0wXSm4GgBQ&ved=0CHAQ6AEwDQ#v=onepage&q=Stenocara%20dentata&f=false }}</ref> 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 shrimp]]s, [[Anostraca|fairy shrimps]] and [[Notostraca|tadpole shrimps]], are [[Cryptobiosis|cryptobiotic]] and can lose up to 92% of their bodyweight, rehydrating as soon as it rains and their temporary pools reappear.<ref>{{cite web |url=http://www.nps.gov/arch/naturescience/pools.htm |title=Ephemeral Pools |work=Arches National Park, Utah |publisher=National Park Service |accessdate=2013-05-22}}</ref>
+==Human relations==
+Humans have long made use of deserts as places to live,<ref name=Fagan/> and more recently have started to exploit them for minerals<ref name=USGS-minerals/> and energy capture.<ref name=Kib/> Deserts play a significant role in human culture with an extensive literature.<ref name=Bancroft/>
+===History===
+[[File:Morroco-arid-climate.jpg|thumb|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 [[Aboriginal Australians|Aborigines]] in Australia and various tribes of [[Indigenous peoples of the Americas|North American Indians]], were originally [[hunter-gatherer]]s. 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.<ref name=Fagan>{{cite book |title=People of the Earth |last=Fagan |first=Brian M. |year=2004 |publisher=Pearson Prentice Hall |isbn=978-0-205-73567-9 |pages=169–181 }}</ref> Other cultures developed a [[Nomadic pastoralism|nomadic way of life]] as herders of [[sheep]], [[goat]]s, [[cattle]], camels, [[yak]]s, [[lama]]s 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 their diet included milk, blood and sometimes meat.<ref>{{cite journal |author= Dyson-Hudson, Rada; Dyson-Hudson, Neville |year=1980 |title=Nomadic pastoralism |journal=Annual Review of Anthropology |volume=9 |pages=15–61 |jstor=2155728 |doi=10.1146/annurev.an.09.100180.000311}}</ref>
+[[File:Bilma-Salzkarawane1.jpg|thumb|left|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. [[Trans-Saharan trade|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 [[slave]]s, [[ivory]] and [[gold]] going northwards and salt going southwards. [[Berber people|Berbers]] with knowledge of the region were employed to guide the caravans between the various oases and [[Water well|wells]].<ref>{{cite journal |author=Masonen, Pekka |year=1995 |title=Trans-Saharan trade and the West African discovery of the Mediterranean |journal=Nordic Research on the Middle East |volume=3 |pages=116–142 |url=http://www.smi.uib.no/paj/Masonen.html }}</ref> Several million slaves may have been taken northwards across the Sahara between the 8th and 18th centuries.<ref>{{cite book |title=The Trans-Saharan Slave Trade |last=Wright |first=John |year=2007 |publisher=Routledge |isbn=978-0-203-96281-7 |page=22  |url=http://books.google.co.uk/books?id=i0beGIgJF8kC&q=million+slaves#v=snippet&q=million%20slaves&f=false }}</ref> Traditional means of overland transport declined with the advent of motor vehicles, shipping and air freight, but [[Camel train|caravans]] still travel along routes between [[Agadez]] and [[Bilma]] and between [[Timbuktu]] and [[Taoudenni]] carrying salt from the interior to desert-edge communities.<ref>{{cite news |title=Sahara salt trade camel caravans |url=http://news.nationalgeographic.co.uk/news/2003/05/photogalleries/salt/ |newspaper=National Geographic News |date=2010-10-28 |accessdate=2013-09-22}}</ref>
+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.<ref>{{cite web | title=First Measured Century: Interview:James Gregory | url=http://www.pbs.org/fmc/interviews/gregory.htm |accessdate=2013-05-25 | publisher=Public Broadcasting Service }}</ref>
+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.<ref>{{cite web |url=http://www.un.org/en/events/desertificationday/background.shtml |title=Desertification: Facts and figures |author= |work= |publisher=United Nations |accessdate=2013-05-26}}</ref> [[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.<ref>{{cite book |last1=Geeson |first1=Nicola |last2=Brandt |first2=C. J. |last3=Thornes |first3=J. B. |title=Mediterranean Desertification: A Mosaic of Processes and Responses |publisher=Wiley |year=2003 |isbn=978-0-470-85686-4 |page=58 |url=http://books.google.com/books?id=G_0qg0f49GQC&pg=PA58}}</ref> 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.<ref>{{cite web|last=Savory|first=Allen |title=How to green the world's deserts and reverse climate change |date=2013-03-04 |url=https://www.youtube.com/watch?v=vpTHi7O66pI}}</ref>
+===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.<ref>van Dalen, Dorrit (2009) ''Landenreeks Tsjaad'', KIT Publishers, ISBN 978-90-6832-690-1.</ref> Geological processes in a desert climate can concentrate [[mineral]]s into valuable deposits. [[Leaching (pedology)|Leaching]] by [[ground water]] can extract [[ore]] minerals and redeposit them, according to the [[water table]], in concentrated form.<ref name=USGS-minerals>{{cite web | url=http://pubs.usgs.gov/gip/deserts/minerals | title=Mineral Resources in Deserts | publisher=US Geological Survey | date=1997-10-29 | accessdate=2013-05-24}}</ref> Similarly, evaporation tends to concentrate minerals in desert lakes, creating dry lake beds or [[Sink (geography)|playas]] rich in minerals. Evaporation can concentrate minerals as a variety of [[evaporite]] deposits, including [[gypsum]], [[sodium nitrate]], [[sodium chloride]] and [[Borate mineral|borates]].<ref name=USGS-minerals/> 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 [[railroad|railway]].<ref name=USGS-minerals/> A desert especially rich in mineral salts is the [[Atacama Desert]], [[Chile]], where sodium nitrate has been mined for [[explosive]]s and [[fertilizer]] since around 1850.<ref name=USGS-minerals/> 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.<ref name=USGS-minerals/>
+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 [[Plate tectonics|tectonic plates]].<ref>{{cite web |url=http://www.scientificamerican.com/article.cfm?id=why-is-oil-usually-found |title=Why is oil usually found in deserts and Arctic areas? |author=Anderson, Roger N. |date=2006-01-16 |work=Scientific American |accessdate=2013-05-26}}</ref>
+Some major oilfields such as [[Ghawar]] are found under the sands of Saudi Arabia.<ref name=USGS-minerals/> 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.<ref name=USGS-minerals/>
+===Farming===
+{{Main|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.<ref name=Marasco/> It has been found that plant growth-promoting bacteria play a role in increasing the resistance of plants to stress conditions and these [[rhizobacteria]]l 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.<ref name=Marasco>{{cite journal |author=Marasco, Ramona; Rolli, Eleonora; Ettoumi, Besma; Vigani, Gianpiero; Mapelli, Francesca;  Borin, Sara; Abou-Hadid, Ayman F.; El-Behairy, Usama A.; Sorlini, Claudia; Cherif, Ameur; Zocchi, Graziano; Daffonchio, Daniele |year=2012 |title=A drought resistance-promoting microbiome is selected by root system under desert farming |journal=PLOSone |url=http://www.plosone.org/article/info%3Adoi%2F10.1371%2Fjournal.pone.0048479 }}</ref> A field trial in the Sonoran Desert which exposed the roots of different species of tree to rhizobacteria and the [[Nitrogen fixation|nitrogen fixing]] bacterium ''[[Azospirillum brasilense]]'' with the aim of restoring degraded lands was only partially successful.<ref name=Marasco/>
+The Judean Desert was farmed in the 7th century BC during the Iron Age to supply food for desert forts.<ref>{{cite journal |author=Stager, Lawrence E. |year=1976 |title=Farming in the Judean Desert during the Iron Age |journal=Bulletin of the American Schools of Oriental Research |volume=221 |pages=145–158 |jstor=1356097 }}</ref> Native Americans in the south western United States became agriculturalists around 600 AD when seeds and technologies became available from Mexico. They used terracing techniques and grew gardens beside seeps, in moist areas at the foot of dunes, near streams providing flood irrigation and in areas irrigated by extensive specially built canals. The [[Hohokam]] tribe constructed over {{convert|500|mi}} of large canals and maintained them for centuries, an impressive feat of engineering. They grew maize, beans, squash and peppers.<ref>{{cite web |url=http://www.cabrillo.edu/~crsmith/southwest.html |title=Agricultural Societies In Pre-European Times: Southwestern U.S. and Northwestern Mexico |author=Smith, Chuck |date=2002-10-14 |work=Native Peoples of North America |accessdate=2013-09-28}}</ref>
+[[File:Imperial valley fields.jpg|thumb|left|Mosaic of fields in Imperial Valley]]
+A modern example of desert farming is the [[Imperial Valley]] in California, which has high temperatures and average rainfall of just {{convert|3|in|abbr=on}} per year.<ref>{{cite web |url=http://vric.ucdavis.edu/virtual_tour/imp.htm |title=Imperial County Agriculture |date=2012-02-15 |publisher=University of California Cooperative Extension |accessdate=2013-09-28}}</ref>  The economy is heavily based on agriculture and the land is irrigated through a network of canals and pipelines sourced entirely from the [[Colorado River]] via the [[All-American Canal]]. The soil is deep and fertile, being part of the river's flood plains, and what would otherwise have been desert has been transformed into one of the most productive farming regions in California. Other water from the river is piped to urban communities but all this has been at the expense of the river, which below the extraction sites no longer has any above-ground flow during most of the year. Another problem of growing crops in this way is the build-up of salinity in the soil caused by evaporation of river water.<ref>{{cite journal |author=Meadows, Robin |title=Research news: UC Desert Research and Extension Center celebrates 100 years |year=2012 |journal=California Agriculture |volume=66 |issue=4 |pages=122–126 |doi=10.3733/ca.v066n04p122  |url=http://ucanr.org/repository/cao/landingpage.cfm?article=ca.v066n04p122&fulltext=yes }}</ref> The greening of the desert remains an aspiration and was at one time viewed as a future means for increasing food production for the world's growing population. This prospect has proved false as it disregarded the environmental damage caused elsewhere by the diversion of water for desert project irrigation.<ref>{{cite book |title=Mirage: the false promise of desert agriculture |last=Clemings |first=R. |year=1996 |publisher=Sierra Club Books |isbn=0-87156-416-5 |pages=1–247 |url=http://www.cabdirect.org/abstracts/19961805097.html }}</ref>
+===Solar energy capture===
+[[File:DESERTEC-Map large.jpg|thumb|250px|[[Desertec]] proposes using the Saharan and [[Arabian desert|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 low amounts of cloud cover. Many successful solar power plants [[Solar power plants in the Mojave Desert|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.<ref name="STS" >{{cite web |url=http://www.nrel.gov/docs/legosti/fy98/22589.pdf |title=Solar Trough Systems |publisher=National Renewable Energy Laboratory |accessdate=2013-09-22}}</ref> Large swaths of this desert are covered in mirrors,<ref name=CBC>{{cite news |url=http://www.cbc.ca/news2/reportsfromabroad/parry/20070815.html |author=Parry, Tom |title=Looking to the sun |publisher=Canadian Broadcasting Corporation |date=2007-08-15 |accessdate=2013-09-22}}</ref> including nine fields of solar collectors. The [[Mojave Solar Park]] is currently under construction and will produce 280MW when completed.<ref>{{cite web|last=Sandler |first=Neal |url=http://www.businessweek.com/technology/content/feb2006/tc20060214_533101.htm |title=Israeli Solar Startup Shines |publisher=Businessweek.com |date=2006-02-14 |accessdate=2010-10-16}}</ref>
+The potential for 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 from 10% of the Sahara Desert.<ref name=Register/> [[Desertec Industrial Initiative]] is a consortium seeking $560 billion to invest in North African solar and wind installations over the next forty 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: the almost continual daytime sunshine and plenty of unused land. The Sahara receives more sunshine per acre than any part of Europe. The Sahara Desert also has the empty space totalling hundreds of square miles required to house fields of mirrors for solar plants.<ref>{{cite web |url=http://www.businessweek.com/magazine/content/10_38/b4195012469892.htm |title=Sahara Solar Energy Could Power Europe |author=Matlack, Carol |date=2010-12-16 |work=BloombergBusinessweek |publisher=Bloomberg |accessdate=2013-09-22}}</ref>
+The [[Negev Desert]], [[Israel]], and the surrounding area, including the [[Arava Valley]], receive plenty of sunshine and are generally not [[Arable land|arable]]. This has resulted in the construction of many [[Solar power in Israel|solar plants]].<ref name=Kib>{{cite news |title=Head of Kibbutz Movement: We will not be discriminated against by the government |author=Waldoks, Ehud Zion |url=http://www.jpost.com/Health-and-Sci-Tech/Science-And-Environment/Head-of-Kibbutz-Movement-We-will-not-be-discriminated-against-by-the-government |newspaper=The Jerusalem Post |date=2008-03-18 |accessdate=2013-09-22}}</ref> David Faiman has proposed that "giant" solar plants in the Negev could supply all of Israel's needs for electricity.<ref name=Register>{{cite news |title=Giant solar plants in Negev could power Israel's future |author=Lettice, John |url=http://www.theregister.co.uk/2008/01/25/faiman_negev_solar_plan/ |newspaper=The Register |date=2008-01-25 |accessdate=2013-09-22}}</ref>
+[[File:2 Battle of El Alamein 001.png|thumb|left|Deployment of forces on the eve of the [[Second Battle of El Alamein]] in 1942]]
+===Warfare===
+{{Main|Desert warfare}}
+Both world wars saw fighting in the desert. In the [[First World War]], the [[Ottoman Empire|Ottoman]] [[Turkey|Turks]] were engaged with the British regular army in a campaign that spanned the Arabian peninsula. The Turks were defeated by the British, who had the backing of irregular Arab forces that were seeking to [[Arab Revolt|revolt]] against the Turks in the [[Kingdom of Hejaz|Hejaz]], made famous in [[T. E. Lawrence]]'s book ''[[Seven Pillars of Wisdom]]''.<ref>{{cite book | title=Seven Pillars of Wisdom | publisher=Private edition | author=Lawrence, T. E. | year=1922 }}</ref><ref>{{cite book | title=The Arab Revolt 1916–18: Lawrence Sets Arabia Ablaze | publisher=Osprey |isbn=978-1-84603-339-1| author=Murphy, David | year=2008}}</ref>
+In the [[Second World War]], the [[Western Desert Campaign]] began in [[Italian Libya]]. Warfare in the desert offered great scope for tacticians to use the large open spaces without the distractions of casualties among civilian populations. [[Tank]]s and [[Armoured fighting vehicle|armoured vehicles]] were able to travel large distances unimpeded and [[land mine]]s were laid in large numbers. However the size and harshness of the terrain meant that all supplies needed to be brought in from great distances. The victors in a battle would advance and their [[supply chain]] would necessarily become longer, while the defeated army could retreat, regroup and resupply. For these reasons, the [[front line]] moved back and forth through hundreds of kilometers as each side lost and regained momentum.<ref name=Woolley>{{cite journal |author=Woolley, Jo |year=2008 |title=Desert warfare |journal=History Today |volume=52 |issue=10 |url=http://www.historytoday.com/jo-woolley/desert-warfare }}</ref>  Its most easterly point was at [[El Alamein]] in [[Egypt]], where the Allies decisively defeated the Axis forces in 1942.<ref>{{cite book | title=Alamein | publisher=John Murray | author=Latimer, Jon | year=2002 | isbn=978-0-7195-6203-7}}</ref>
+===In culture===
+[[File:Marco Polo traveling.JPG|thumb|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.<ref>{{cite web |url=http://www.strath.ac.uk/humanities/courses/english/courses/literaturecultureplacemasters/options/writingthevoiddesertliterature/ |title=Writing the Void: Desert Literature |publisher=University of Strathclyde |accessdate=2013-09-24}}</ref>
+There is an extensive literature on the subject of deserts.<ref name=Bancroft>{{cite journal |author=Bancroft, John (ed.) |year=1994 |title=The Deserts in Literature |journal=The Arid Lands Newsletter |issue=35  |issn=1092-5481  |url=http://ag.arizona.edu/OALS/ALN/aln35/TOC35.html }}</ref> An early historical account is that of [[Marco Polo]] (c. 1254–1324), who travelled through Central Asia to China, crossing a number of deserts in his twenty four year trek.<ref>{{cite book |author=Bergreen, Laurence |authorlink=Laurence Bergreen |title=Marco Polo: From Venice to Xanadu |year=2007 |publisher=Quercus |pages=1–415 |isbn=978-1-84724-345-4}}</ref> 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 Arabia Deserta'' (1888).<ref>{{cite book | title=God's Fugitive | publisher=Harper Collins | author=Taylor, Andrew | year=1999 |pages=295–298| isbn=978-0-00-255815-0}}</ref> [[Antoine de Saint-Exupéry]] described both his flying and the desert in ''[[Wind, Sand and Stars]]''<ref>{{cite web | url=http://ag.arizona.edu/OALS/ALN/aln35/Hutchinson.html | title=Wind, Sand and Stars Revisited | year=1994 | accessdate=2013-05-26 | author=Hutchinson, Charles F. }}</ref> and [[Gertrude Bell]] travelled extensively in the Arabian desert in the early part of the 20th century, becoming an expert on the subject, writing books and advising the British government on dealing with the Arabs.<ref>{{cite book | title=Gertrude Bell: Queen of the Desert, Shaper of Nations | publisher=Farrar, Straus and Giroux | author=Howell, Georgina | year=2007 | isbn=978-0-374-16162-0}}</ref> Another woman explorer was [[Freya Stark]] who travelled alone in the Middle East, visiting [[Turkey]], [[Arabia]], [[Yemen]], [[Syria]], [[Iran|Persia]] and [[Afghanistan]], writing over twenty books on her experiences.<ref>{{cite book | title=Freya Stark | publisher=Penguin | author=Moorehead, C. | year=1985 | isbn=978-0-14-008108-4}}</ref> The German naturalist [[Uwe George]] spent several years living in deserts, recording his experiences and research in his book, ''In the Deserts of this Earth''.<ref name=Uwe>George, 1978.</ref>
+The American poet [[Robert Frost]] expressed his bleak thoughts in his poem, ''Desert Places'', which 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."<ref>{{cite web | url=http://www.poemhunter.com/poem/desert-places/ | title=Desert Places | publisher=Poemhunter | accessdate=2013-05-26 | author=Frost, Robert}}</ref>
+==Deserts on other planets==
+[[File:Gusev Spirit 01.jpg|thumb|right|View of the Martian desert seen by the probe ''[[Spirit rover|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 (1.9&nbsp;million sq mi) in area, much larger than deserts on Earth. The Martian deserts principally consist of dunes in the form of half-moons 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 in the Martian polar regions.<ref>{{cite web |url=http://www.thelivingmoon.com/43ancients/02files/Mars_Blue_Bird_Landforms_01.html |title=Strange Land Formations on Mars |date=2007-04-11 |work=The Blue Bird Files |accessdate=2013-09-27}}</ref> Examination of the surface of rocks by [[laser]] beamed from the [[Mars Exploration Rover]] have shown a surface film that resembles the desert varnish found on Earth although it might just be surface dust.<ref>{{cite web |url=http://www.astrobio.net/pressrelease/5409/do-mars-rocks-have-desert-varnish |title=Do Mars Rocks Have Desert Varnish? |date=2013-03-23 |work=Astrobiology |accessdate=2013-09-27}}</ref> The surface of [[Titan (moon)|Titan]], a moon of [[Saturn]], also has a [[Titan (moon)#Dark terrain|desert-like surface]] with dune seas.<ref>{{cite web |author=Arnold, K.; Radebaugh, J.; Savage, C. J.; Turtle, E. P.; Lorenz, R. D.; Stofan, E. R.; Le-Gall, A. |title=Areas of Sand Seas on Titan from Cassini Radar and ISS: Fensal and Aztlan |url=http://www.lpi.usra.edu/meetings/lpsc2011/pdf/2804.pdf |work=42nd Lunar and Planetary Science Conference, March 7–11, 2011 at The Woodlands, Texas. LPI Contribution No. 1608 |publisher=Lunar and Planetary Institute }}</ref>
 == Địa lý ==

x t s Các hoang mạc trên thế giới
Hoang mạc Hoang mạc hóa Danh sách hoang mạc
Châu Phi	Algeria Bayuda Xanh Chalbi Danakil Djurab Đông Ferlo Farafra Kalahari Libya Moçâmedes Namib Nubia Nyiri Owami Richtersveld Sahara Tanezrouft Ténéré
Châu Á	Ad-Dahna Akshi Ả Rập Aral Karakum Aralkum Badain Jaran Betpak-Dala Cholistan Dasht-e Kavir Dasht-e Khash Dasht-e Leili Dasht-e Loot Dasht-e Margo Dasht-e Naomid Gurbantünggüt Gobi Hami Lưu vực sông Ấn Judea Karakum Kharan Kumtag Kyzylkum Liwa Lop Maranjab Muyunkum Nefud Negev Ordos Qaidam Ramlat al-Sab'atayn Rub' al Khali Nga thuộc Vùng Bắc Cực Registan Saryesik-Atyrau Syria Taklamakan Tengger Thal Thar Cao nguyên Ustyurt Ramlat al-Wahiba
Châu Úc	Gibson Sandy Lớn Victoria Lớn Sandy Nhỏ Đồng bằng Nullarbor Painted Pedirka Simpson Strzelecki Sturt Tanami Tirari
Châu Âu	Accona Bardenas Reales Błędów Cabo de Gata Deliblatska Peščara Hálendi Monegros Oleshky Sahara Oltenia Ryn Stranja Tabernas
Bắc Mỹ	Alvord Amargosa Baja California Black Rock Carcross Carson Channeled scablands Chihuahuan Colorado Escalante Forty Mile Gran Desierto de Altar Bồn địa Lớn Hồ Muối Lớn Sandy Lớn Jornada del Muerto Kaʻū Lechuguilla Mojave Bắc Mỹ thuộc vùng Bắc Cực Owyhee Painted Đỏ Sevier Smoke Creek Sonora Tule (Arizona) Tule (Nevada) Yp Yuha Yuma
Nam Mỹ	Atacama La Guajira Los Médanos de Coro Monte Patagonia Sechura Tatacoa
Châu Đại Dương	Châu Zealand: Hoang mạc Rangipo
Vùng cực	Châu Nam Cực Vùng Bắc Cực Greenland Bắc Mỹ thuộc vùng Bắc Cực Nga thuộc vùng Bắc Cực
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