Valle de la Luna ("Valley of the Moon") in the Atacama Desert of Chile, the world's driest non-polar desert
Sand dunes in the Rub' al Khali ("Empty quarter") in the United Arab Emirates
A desert is a barren area of landscape where little precipitation
occurs and, consequently, living conditions are hostile for plant and
animal life. The lack of vegetation exposes the unprotected surface of
the ground to the processes of denudation. About one-third of the land surface of the world is arid or semi-arid. This includes much of the polar regions where little precipitation occurs and which are sometimes called polar deserts
or "cold deserts". Deserts can be classified by the amount of
precipitation that falls, by the temperature that prevails, by the
causes of desertification or by their geographical location.
Deserts are formed by weathering
processes as large variations in temperature between day and night put
strains on the rocks which consequently break in pieces. Although rain
seldom occurs in deserts, there are occasional downpours that can result
in flash floods. Rain falling on hot rocks can cause them to shatter
and the resulting fragments and rubble strewn over the desert floor are
further eroded by the wind. This picks up particles of sand and dust and
wafts them aloft in sand or dust storms. Wind-blown sand grains striking any solid object in their path can
abrade the surface. Rocks are smoothed down, and the wind sorts sand
into uniform deposits. The grains end up as level sheets of sand or are
piled high in billowing sand dunes. Other deserts are flat, stony plains where all the fine material has been blown away and the surface consists of a mosaic of smooth stones. These areas are known as desert pavements and little further erosion
takes place. Other desert features include rock outcrops, exposed
bedrock and clays once deposited by flowing water. Temporary lakes may
form and salt pans may be left when waters evaporate. There may be
underground sources of water in the form of springs and seepages from aquifers. Where these are found, oases can occur.
Plants and animals living in the desert need special adaptations
to survive in the harsh environment. Plants tend to be tough and wiry
with small or no leaves, water-resistant cuticles and often spines to deter herbivory. Some annual plants germinate,
bloom and die in the course of a few weeks after rainfall while other
long-lived plants survive for years and have deep root systems able to
tap underground moisture. Animals need to keep cool and find enough food
and water to survive. Many are nocturnal
and stay in the shade or underground during the heat of the day. They
tend to be efficient at conserving water, extracting most of their needs
from their food and concentrating their urine. Some animals remain in a state of dormancy for long periods, ready to become active again during the rare rainfall. They then reproduce rapidly while conditions are favorable before returning to dormancy.
People have struggled to live in deserts and the surrounding semi-arid lands for millennia. Nomads
have moved their flocks and herds to wherever grazing is available and
oases have provided opportunities for a more settled way of life. The
cultivation of semi-arid regions encourages erosion of soil and is one
of the causes of increased desertification. Desert farming is possible with the aid of irrigation, and the Imperial Valley
in California provides an example of how previously barren land can be
made productive by the import of water from an outside source. Many trade routes have been forged across deserts, especially across the Sahara Desert, and traditionally were used by caravans of camels carrying salt, gold, ivory and other goods. Large numbers of slaves
were also taken northwards across the Sahara. Some mineral extraction
also takes place in deserts, and the uninterrupted sunlight gives
potential for the capture of large quantities of solar energy.
Etymology
English desert and its Romance cognates (including Italian and Portuguese deserto, French désert and Spanish desierto) all come from the ecclesiastical Latin dēsertum (originally "an abandoned place"), a participle of dēserere, "to abandon".
The correlation between aridity and sparse population is complex and
dynamic, varying by culture, era, and technologies; thus the use of the
word desert can cause confusion. In English before the 20th century, desert was often used in the sense of "unpopulated area", without specific reference to aridity; but today the word is most often used in its climate-science sense (an area of low precipitation). Phrases such as "desert island" and "Great American Desert", or Shakespeare's "deserts of Bohemia" (The Winter's Tale) in previous centuries did not necessarily imply sand or aridity; their focus was the sparse population.
Physical geography
A desert is a region of land that is very dry because it receives low amounts of precipitation
(usually in the form of rain, but it may be snow, mist or fog), often
has little coverage by plants, and in which streams dry up unless they
are supplied by water from outside the area. Deserts generally receive less than 250 mm (10 in) of precipitation each year. The potential evapotranspiration may be large but (in the absence of available water) the actual evapotranspiration may be close to zero. Semideserts are regions which receive between 250 and 500 mm (10 and 20 in) and when clad in grass, these are known as steppes.
Classification
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The Sahara is the largest hot desert in the world
Deserts have been defined and classified in a number of ways,
generally combining total precipitation, number of days on which this
falls, temperature, and humidity, and sometimes additional factors. For example, Phoenix, Arizona,
receives less than 250 mm (9.8 in) of precipitation per year, and is
immediately recognized as being located in a desert because of its
aridity-adapted plants. The North Slope of Alaska's Brooks Range also receives less than 250 mm (9.8 in) of precipitation per year and is often classified as a cold desert. Other regions of the world have cold deserts, including areas of the Himalayas and other high-altitude areas in other parts of the world. Polar deserts cover much of the ice-free areas of the Arctic and Antarctic.
A non-technical definition is that deserts are those parts of the
Earth's surface that have insufficient vegetation cover to support a
human population.
Potential evapotranspiration
supplements the measurement of 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 the amount of surface storage of water. Evapotranspiration is the combination of water loss through atmospheric evaporation and through the life processes of plants. Potential evapotranspiration, then, is the amount of water that could evaporate in any given region. As an example, Tucson, Arizona
receives about 300 mm (12 in) of rain per year, however about 2,500 mm
(98 in) of water could evaporate over the course of a year.
In other words, about eight times more water could evaporate from the
region than actually falls as rain. Rates of evapotranspiration in cold
regions such as Alaska are much lower because of the lack of heat to aid
in the evaporation process.
Deserts are sometimes classified as "hot" or "cold", "semiarid" or "coastal".
The characteristics of hot deserts include high temperatures in summer;
greater evaporation than precipitation usually exacerbated by high
temperatures, strong winds and lack of cloud cover; considerable
variation in the occurrence of precipitation, its intensity and
distribution; and low humidity. Winter temperatures vary considerably
between different deserts and are often related to the location of the
desert on the continental landmass and the latitude. Daily variations in
temperature can be as great as 22 °C (40 °F) or more, with heat loss by
radiation at night being increased by the clear skies.
Cold desert: snow surface at Dome C Station, 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.
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 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.
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.
Arid deserts receive between 25 and 200 mm (1 and 8 in) in a year and
semiarid deserts between 200 and 500 mm (8 and 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.
A semi-arid desert or a steppe is a version of the arid desert with much more rainfall, vegetation and higher humidity. These regions feature a semi-arid climate and are less extreme than regular deserts.
Like arid deserts, temperatures can vary greatly in semi deserts. They
share some characteristics of a true desert and are usually located at
the edge of deserts and continental dry areas. They usually receive
precipitation from 250 mm (10 in) to 500 mm (20 in) but this can vary
due to evapotranspiration and soil nutrition. Semi deserts can be found
in the Tabernas Desert (and some of the Spanish Plateau), The Sahel, The Eurasian Steppe, most of Central Asia, the Western US, most of Northern Mexico, portions of South America (especially in Argentina) and the Australian Outback. They usually feature BSh (hot steppe) or BSk (temperate steppe) in the Köppen climate classification.
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 Namibia, 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.
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.
The Agasthiyamalai hills cut off Tirunelveli in India from the monsoons, 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 deserts. Trade wind deserts occur either side of the horse latitudes
at 30° to 35° North and South. These belts are associated with the
subtropical anticyclone and the large-scale descent of dry air moving
from high-altitudes toward the poles. The Sahara Desert is of this type.
Mid-latitude deserts occur between 30° and 50° North and South. They
are mostly in areas remote from the sea where most of the moisture has
already precipitated from the prevailing winds. They include the Tengger and Sonoran Deserts.
Monsoon deserts are similar. They occur in regions where large
temperature differences occur between sea and land. Moist warm air rises
over the land, deposits its water content and circulates back to sea.
Further inland, areas receive very little precipitation. The Thar Desert near the India/Pakistan border is of this type.
In some parts of the world, deserts are created by a rain shadow effect. Orographic lift
occurs as air masses rise to pass over high ground. In the process they
cool and lose much of their moisture by precipitation on the windward slope of the mountain range. When they descend on the leeward side, they warm and their capacity to hold moisture increases so an area with relatively little precipitation occurs. 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.
Other areas are arid by virtue of being a very long way from the nearest available sources of moisture.
Montane deserts are arid places with a very high altitude; the most prominent example is found north of the Himalayas, in the Kunlun Mountains and the Tibetan Plateau. Many locations within this category have elevations exceeding 3,000 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.
Polar deserts such as McMurdo Dry Valleys remain ice-free because of the dry katabatic winds that flow downhill from the surrounding mountains. Former desert areas presently in non-arid environments, such as the Sandhills in Nebraska, are known as paleodeserts. In the Köppen climate classification system, deserts are classed as BWh (hot desert) or BWk (temperate desert). In the Thornthwaite climate classification system, deserts would be classified as arid megathermal climates.
Weathering processes
Exfoliation of weathering rocks in Texas, USA.
Deserts usually have a large diurnal
and seasonal temperature range, with high daytime temperatures falling
sharply at night. The diurnal range may be as much as 20 to 30 °C (36 to
54 °F) and the rock surface experiences even greater temperature
differentials. During the day the sky is usually clear and most of the sun's
radiation reaches the ground, but as soon as the sun sets, the desert
cools quickly by radiating heat into space. In hot deserts, the
temperature during daytime can exceed 45 °C (113 °F) in summer and
plunge below freezing point at night during winter.
One square centimeter (0.16 sq in) of windblown sand from the Gobi Desert
Such large temperature variations have a destructive effect on the
exposed rocky surfaces. The repeated fluctuations put a strain on
exposed rock and the flanks of mountains crack and shatter. Fragmented
strata slide down into the valleys where they continue to break into
pieces due to the relentless sun by day and chill by night. Successive
strata are exposed to further weathering. The relief of the internal
pressure that has built up in rocks that have been underground for aeons
can cause them to shatter. Exfoliation
also occurs when the outer surfaces of rocks split off in flat flakes.
This is believed to be caused by the stresses put on the rock by
repeated expansions and contractions which induces fracturing parallel
to the original surface.
Chemical weathering processes probably play a more important role in
deserts than was previously thought. The necessary moisture may be
present in the form of dew or mist. Ground water may be drawn to the
surface by evaporation and the formation of salt crystals may dislodge
rock particles as sand or disintegrate rocks by exfoliation. Shallow
caves are sometimes formed at the base of cliffs by this means.
As the desert mountains decay, large areas of shattered rock and
rubble occur. The process continues and the end products are either dust
or sand. Dust is formed from solidified clay or volcanic deposits
whereas sand results from the fragmentation of harder granites,
limestone and sandstone.
There is a certain critical size (about 0.5 mm) below which further
temperature-induced weathering of rocks does not occur and this provides
a minimum size for sand grains.
As the mountains are eroded, more and more sand is created. At
high wind speeds, sand grains are picked up off the surface and blown
along, a process known as saltation. The whirling airborne grains act as a sand blasting mechanism which grinds away solid objects in its path as the kinetic energy of the wind is transferred to the ground. The sand eventually ends up deposited in level areas known as sand-fields or sand-seas, or piled up in dunes.
Dust storms and sandstorms
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, saltation
or creep. Suspension is only possible for particles less than 0.1 mm
(0.004 in) in diameter. In a dust storm, these fine particles are lifted
up and wafted aloft to heights of up to 6 km (3.7 mi). They reduce
visibility and can remain in the atmosphere for days on end, conveyed by
the trade winds for distances of up to 6,000 km (3,700 mi).
Denser clouds of dust can be formed in stronger winds, moving across
the land with a billowing leading edge. The sunlight can be obliterated
and it may become as dark as night at ground level.
In a study of a dust storm in China in 2001, it was estimated that 6.5
million tons of dust were involved, covering an area of 134,000,000 km2 (52,000,000 sq mi). The mean particle size was 1.44 μm.
A much smaller scale, short-lived phenomenon can occur in calm
conditions when hot air near the ground rises quickly through a small
pocket of cooler, low-pressure air above forming a whirling column of
particles, a dust devil.
Wind-blown particles: 1=Creep 2=Saltation 3=Suspension 4=Wind current
Sandstorms occur with much less frequency than dust storms. They are
often preceded by severe dust storms and occur when the wind velocity
increases to a point where it can lift heavier particles. These grains
of sand, up to about 0.5 mm (0.020 in) in diameter are jerked into the
air but soon fall back to earth, ejecting other particles in the
process. Their weight prevents them from being airborne for long and
most only travel a distance of a few meters (yards). The sand streams
along above the surface of the ground like a fluid, often rising to
heights of about 30 cm (12 in).
In a really severe steady blow, 2 m (6 ft 7 in) is about as high as the
sand stream can rise as the largest sand grains do not become airborne
at all. They are transported by creep, being rolled along the desert
floor or performing short jumps.
During a sandstorm, the wind-blown sand particles become
electrically charged. Such electric fields, which range in size up to 80
kV/m, can produce sparks and cause interference with telecommunications
equipment. They are also unpleasant for humans and can cause headaches
and nausea.
The electric fields are caused by the 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.
Major deserts
The world's largest non-polar deserts
Deserts take up about one third of the Earth's land surface.
Bottomlands may be salt-covered flats. Eolian processes
are major factors in shaping desert landscapes. Polar deserts (also
seen as "cold deserts") have similar features, except the main form of
precipitation is snow rather than rain. Antarctica is the world's largest cold desert (composed of about 98% thick continental ice sheet and 2% barren rock). Some of the barren rock is to be found in the so-called Dry Valleys of Antarctica that almost never get snow, which can have ice-encrusted saline lakes that suggest evaporation far greater than the rare snowfall due to the strong katabatic winds that even evaporate ice.
| Rank | Desert | Area (km2) | Area (mi²) |
|---|---|---|---|
| 1 | Antarctic Desert (Antarctica) | 14,200,000 | 5,500,000 |
| 2 | Arctic Desert (Arctic) | 13,900,000 | 5,400,000 |
| 3 | Sahara Desert (Africa) | 9,100,000 | 3,500,000 |
| 4 | Arabian Desert (Middle East) | 2,600,000 | 1,000,000 |
| 5 | Gobi Desert (Asia) | 1,300,000 | 500,000 |
| 6 | Patagonian Desert (South America) | 670,000 | 260,000 |
| 7 | Great Victoria Desert (Australia) | 647,000 | 250,000 |
| 8 | Kalahari Desert (Africa) | 570,000 | 220,000 |
| 9 | Great Basin Desert (North America) | 490,000 | 190,000 |
| 10 | Syrian Desert (Middle East) | 490,000 | 190,000 |
Deserts, both hot and cold, play a part in moderating the Earth's
temperature. This is because they reflect more of the incoming light and
their albedo is higher than that of forests or the sea.
Features
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, but deserts do not always look like this.
Across the world, around 20% of desert is sand, varying from only 2% in
North America to 30% in Australia and over 45% in Central Asia. Where sand does occur, it is usually in large quantities in the form of sand sheets or extensive areas of dunes.
A sand sheet is a near-level, firm expanse of partially
consolidated particles in a layer that varies from a few centimeters to a
few meters thick. The structure of the sheet consists of thin
horizontal layers of coarse silt and very fine to medium grain sand,
separated by layers of coarse sand and pea-gravel which are a single
grain thick. These larger particles anchor the other particles in place
and may also be packed together on the surface so as to form a miniature
desert pavement.
Small ripples form on the sand sheet when the wind exceeds 24 km/h
(15 mph). They form perpendicular to the wind direction and gradually
move across the surface as the wind continues to blow. The distance
between their crests corresponds to the average length of jumps made by
particles during saltation. The ripples are ephemeral and a change in
wind direction causes them to reorganise.
Diagram showing barchan dune formation, with the wind blowing from the left
Sand dunes are accumulations of windblown sand piled up in mounds or
ridges. They form downwind of copious sources of dry, loose sand and
occur when topographic and climatic conditions cause airborne particles
to settle. As the wind blows, saltation and creep take place on the
windward side of the dune and individual grains of sand move uphill.
When they reach the crest, they cascade down the far side. The upwind
slope typically has a gradient of 10° to 20° while the lee slope is
around 32°, the angle at which loose dry sand will slip. As this
wind-induced movement of sand grains takes place, the dune moves slowly
across the surface of the ground.
Dunes are sometimes solitary, but they are more often grouped together
in dune fields. When these are extensive, they are known as sand seas or
ergs.
The shape of the dune depends on the characteristics of the prevailing wind. Barchan
dunes are produced by strong winds blowing across a level surface and
are crescent-shaped with the concave side away from the wind. When there
are two directions from which winds regularly blow, a series of long,
linear dunes known as seif
dunes may form. These also occur parallel to a strong wind that blows
in one general direction. Transverse dunes run at a right angle to the
prevailing wind direction. Star dunes are formed by variable winds, and
have several ridges and slip faces radiating from a central point. They
tend to grow vertically; they can reach a height of 500 m (1,600 ft),
making them the tallest type of dune. Rounded mounds of sand without a
slip face are the rare dome dunes, found on the upwind edges of sand
seas.
Windswept desert pavement of small, smooth, closely packed stones in the Mojave desert
A large part of the surface area of the world's deserts consists of
flat, stone-covered plains dominated by wind erosion. In "eolian
deflation", the wind continually removes fine-grained material, which
becomes wind-blown sand. This exposes coarser-grained material, mainly pebbles with some larger stones or cobbles, leaving a desert pavement, an area of land overlaid by closely packed smooth stones forming a tessellated
mosaic. Different theories exist as to how exactly the pavement is
formed. It may be that after the sand and dust is blown away by the wind
the stones jiggle themselves into place; alternatively, stones
previously below ground may in some way work themselves to the surface.
Very little further erosion takes place after the formation of a
pavement, and the ground becomes stable. Evaporation brings moisture to
the surface by capillary action and calcium salts may be precipitated,
binding particles together to form a desert conglomerate.
In time, bacteria that live on the surface of the stones accumulate a
film of minerals and clay particles, forming a shiny brown coating known
as desert varnish.
Other non-sandy deserts consist of exposed outcrops of bedrock, dry soils or aridisols, and a variety of landforms affected by flowing water, such as alluvial fans, sinks or playas, temporary or permanent lakes, and oases. A hamada is a type of desert landscape consisting of a high rocky plateau where the sand has been removed by aeolian processes.
Other landforms include plains largely covered by gravels and angular
boulders, from which the finer particles have been stripped by the wind.
These are called "reg" in the western Sahara, "serir" in the eastern
Sahara, "gibber plains" in Australia and "saï" in central Asia. The Tassili Plateau
in Algeria is an impressive jumble of eroded sandstone outcrops,
canyons, blocks, pinnacles, fissures, slabs and ravines. In some places
the wind has carved holes or arches, and in others, it has created
mushroom-like pillars narrower at the base than the top. In the Colorado Plateau it is water that has been the eroding force. Here the Colorado River has cut its way over the millennia through the high desert floor creating a canyon that is over a mile (6,000 feet or 1,800 meters) deep in places, exposing strata that are over two billion years old.
Water
Atacama, the world's driest non-polar desert, part of the Arid Diagonal of South America.
One of the driest places on Earth is the Atacama Desert. It is virtually devoid of life because it is blocked from receiving precipitation by the Andes mountains to the east and the Chilean Coast Range to the west. The cold Humboldt Current and the anticyclone of the Pacific are essential to keep the dry climate of the Atacama. The average 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). Nevertheless, there is some plant life in the Atacama, in the form of specialist plants that obtain moisture from dew and the fogs that blow in from the Pacific.
Flash flood in the Gobi
When rain falls in deserts, as it occasionally does, it is often with
great violence. The desert surface is evidence of this with dry stream
channels known as arroyos or wadis meandering across its surface. These can experience flash floods,
becoming raging torrents with surprising rapidity after a storm that
may be many 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. 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. 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. Seepages may occur in the walls of canyons and pools may survive in deep shade near the dried up watercourse below.
Lakes may form in basins where there is sufficient 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. These include the Great Salt Lake, Utah Lake, Sevier Lake and many dry lake beds. The smooth flat surfaces of playas have been used for attempted vehicle speed records at Black Rock Desert and Bonneville Speedway and the United States Air Force uses Rogers Dry Lake in the Mojave Desert as runways for aircraft and the space shuttle.
Biogeography
Flora
Xerophytes: Cardón cacti in the Baja California Desert, Cataviña region, Mexico
Plants face severe challenges in arid environments. Problems they
need to solve include how to obtain enough water, how to avoid being
eaten and how to reproduce. Photosynthesis
is the key to plant growth. It can only take 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, or by using C4 carbon fixation.
Many desert plants have reduced the size of their leaves or
abandoned them altogether. Cacti are desert specialists, and in most
species, the leaves have been dispensed with and the chlorophyll
displaced into the trunks, the cellular structure of which has been
modified to allow them to store water. When rain falls, the water is
rapidly absorbed by the shallow roots and retained to allow them to
survive until the next downpour, which may be months or years away. The giant saguaro cacti of the Sonoran Desert
form "forests", providing shade for other plants and nesting places for
desert birds. Saguaro grows slowly but may live for up to two hundred
years. The surface of the trunk is folded like a concertina, allowing it to expand, and a large specimen can hold eight tons of water after a good downpour.
Cacti are present in both North and South America with a post-Gondwana origin. Other xerophytic plants have developed similar strategies by a process known as convergent evolution.
They limit water loss by reducing the size and number of stomata, by
having waxy coatings and hairy or tiny leaves. Some are deciduous,
shedding their leaves in the driest season, and others curl their leaves
up to reduce transpiration. Others store water in succulent leaves or
stems or in 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. The saltbush in Australia has succulent leaves and secretes salt crystals, enabling it to live in saline areas. In common with cacti, many have developed spines to ward off browsing animals.
The camel thorn tree (Acacia erioloba) in the Namib Desert is nearly leafless in dry periods.
Some desert plants produce seed which lies dormant in the soil until sparked into growth by rainfall. With annuals,
such plants grow with great rapidity and may flower and set seed within
weeks, aiming to complete their development before the last vestige of
water dries up. For perennial plants, reproduction is more likely to be
successful if the seed germinates in a shaded position, but not so close
to the parent plant as to be in competition with it. Some seed will not
germinate until it has been blown about on the desert floor to scarify the seed coat. The seed of the mesquite
tree, which grows in deserts in the Americas, is hard and fails to
sprout even when planted carefully. When it has passed through the gut
of a pronghorn it germinates readily, and the little pile of moist dung provides an excellent start to life well away from the parent tree.
The stems and leaves of some plants lower the surface velocity of
sand-carrying winds and protect the ground from erosion. Even small
fungi and microscopic plant organisms found on the soil surface
(so-called cryptobiotic soil)
can be a vital link in preventing erosion and providing support for
other living organisms. Cold deserts often have high concentrations of
salt in the soil. Grasses and low shrubs are the dominant vegetation
here and the ground may be covered with lichens. Most shrubs have spiny leaves and shed them in the coldest part of the year.
Fauna
Animals adapted to live in deserts are called xerocoles.
There is no evidence that body temperature of mammals and birds is
adaptive to the different climates, either of great heat or cold. In
fact, with a very few exceptions, their basal metabolic rate is determined by body size, irrespective of the climate in which they live.
Many desert animals (and plants) show especially clear evolutionary
adaptations for water conservation or heat tolerance and so are often
studied in comparative physiology, ecophysiology, and evolutionary physiology. One well-studied example is the specializations of mammalian kidneys shown by desert-inhabiting species. Many examples of convergent evolution have been identified in desert organisms, including between cacti and Euphorbia, kangaroo rats and jerboas, Phrynosoma and Moloch lizards.
The cream-colored courser, Cursorius cursor, is a well-camouflaged desert resident with its dusty coloration, countershading, and disruptive head markings.
Deserts present a very challenging environment for animals. Not only
do they require food and water but they also need to keep their body
temperature at a tolerable level. In many ways, birds are the ablest to
do this of the higher animals. They can move to areas of greater food
availability as the desert blooms after local rainfall and can fly to
faraway waterholes. In hot deserts, gliding birds can remove themselves
from the over-heated desert floor by using thermals to soar in the
cooler air at great heights. In order to conserve energy, other desert
birds run rather than fly. The cream-colored courser
flits gracefully across the ground on its long legs, stopping
periodically to snatch up insects. Like other desert birds, it is well-camouflaged by its coloring and can merge into the landscape when stationary. The sandgrouse is an expert at this and nests on the open desert floor dozens of kilometers (miles) away from the waterhole
it needs to visit daily. Some small diurnal birds are found in very
restricted localities where their plumage matches the color of the
underlying surface. The desert lark takes frequent dust baths which ensures that it matches its environment.
Water and carbon dioxide are metabolic end products of oxidation of fats, proteins, and carbohydrates.
Oxidising a gram of carbohydrate produces 0.60 grams of water; a gram
of protein produces 0.41 grams of water; and a gram of fat produces 1.07
grams of water, making it possible for xerocoles to live with little or no access to drinking water. The kangaroo rat for example makes use of this water of metabolism and conserves water both by having a low basal metabolic rate and by remaining underground during the heat of the day, reducing loss of water through its skin and respiratory system when at rest. Herbivorous mammals obtain moisture from the plants they eat. Species such as the addax antelope, dik-dik, Grant's gazelle and oryx are so efficient at doing this that they apparently never need to drink. The camel is a superb example of a mammal adapted to desert life. It minimizes its water loss by producing concentrated urine and dry dung, and is able to lose 40% of its body weight through water loss without dying of dehydration. Carnivores can obtain much of their water needs from the body fluids of their prey. Many other hot desert animals are nocturnal,
seeking out shade during the day or dwelling underground in burrows. At
depths of more than 50 cm (20 in), these remain at between 30 to 32 °C
(86 to 90 °F) regardless of the external temperature. Jerboas, desert rats,
kangaroo rats and other small rodents emerge from their burrows at
night and so do the foxes, coyotes, jackals and snakes that prey on
them. Kangaroos keep cool by increasing their respiration rate, panting,
sweating and moistening the skin of their forelegs with saliva. Mammals living in cold deserts have developed greater insulation through warmer body fur and insulating layers of fat beneath the skin. The arctic weasel
has a metabolic rate that is two or three times as high as would be
expected for an animal of its size. Birds have avoided the problem of
losing heat through their feet by not attempting to maintain them at the
same temperature as the rest of their bodies, a form of adaptive
insulation. The emperor penguin
has dense plumage, a downy under layer, an air insulation layer next
the skin and various thermoregulatory strategies to maintain its body
temperature in one of the harshest environments on Earth.
The desert iguana (Dipsosaurus dorsalis) is well-adapted to desert life.
Being ectotherms, reptiles
are unable to live in cold deserts but are well-suited to hot ones. In
the heat of the day in the Sahara, the temperature can rise to 50 °C
(122 °F). Reptiles cannot survive at this temperature and lizards will
be prostrated by heat at 45 °C (113 °F). They have few adaptations to
desert life and are unable to cool themselves by sweating so they
shelter during the heat of the day. In the first part of the night, as
the ground radiates the heat absorbed during the day, they emerge and
search for prey. Lizards and snakes are the most numerous in arid regions and certain snakes have developed a novel method of locomotion that enables them to move sidewards and navigate high sand-dunes. These include the horned viper of Africa and the sidewinder of North America, evolutionarily distinct but with similar behavioural patterns because of convergent evolution. Many desert reptiles are ambush predators and often bury themselves in the sand, waiting for prey to come within range.
Amphibians
might seem unlikely desert-dwellers, because of their need to keep
their skins moist and their dependence on water for reproductive
purposes. In fact, the few species that are found in this habitat have
made some remarkable adaptations. Most of them are fossorial, spending
the hot dry months aestivating in deep burrows. While there they shed their skins a number of times and retain the remnants around them as a waterproof cocoon to retain moisture. In the Sonoran Desert, Couch's spadefoot toad
spends most of the year dormant in its burrow. Heavy rain is the
trigger for emergence and the first male to find a suitable pool calls
to attract others. Eggs are laid and the tadpoles grow rapidly as they
must reach metamorphosis
before the water evaporates. As the desert dries out, the adult toads
rebury themselves. The juveniles stay on the surface for a while,
feeding and growing, but soon dig themselves burrows. Few make it to
adulthood. The water holding frog in Australia has a similar life cycle and may aestivate for as long as five years if no rain falls. The Desert rain frog of Namibia is nocturnal and survives because of the damp sea fogs that roll in from the Atlantic.
Tadpole shrimp survive dry periods as eggs, which rapidly hatch and develop after rain.
Invertebrates, particularly arthropods, have successfully made their homes in the desert. Flies, beetles, ants, termites, locusts, millipedes, scorpions and spiders have hard cuticles
which are impervious to water and many of them lay their eggs
underground and their young develop away from the temperature extremes
at the surface. The Saharan silver ant (Cataglyphis bombycina) uses a heat shock protein in a novel way and forages in the open during brief forays in the heat of the day. The long-legged darkling beetle in Namibia stands on its front legs and raises its carapace to catch the morning mist as condensate, funnelling the water into its mouth. Some arthropods make use of the ephemeral pools that form after rain and complete their life cycle in a matter of days. The desert shrimp does this, appearing "miraculously" in new-formed puddles as the dormant eggs hatch. Others, such as brine shrimps, fairy shrimps and tadpole shrimps, are cryptobiotic and can lose up to 92% of their bodyweight, rehydrating as soon as it rains and their temporary pools reappear.
Human relations
Humans have long made use of deserts as places to live, and more recently have started to exploit them for minerals and energy capture. Deserts play a significant role in human culture with an extensive literature.
History
Shepherd near Marrakech leading his flock to new pasture
Middle Paleolithic hunter-gatherers in a desert environment, south of Iran
People have been living in deserts for millennia. Many, such as the Bushmen in the Kalahari, the Aborigines in Australia and various tribes of North American Indians, were originally hunter-gatherers.
They developed skills in the manufacture and use of weapons, animal
tracking, finding water, foraging for edible plants and using the things
they found in their natural environment to supply their everyday needs.
Their self-sufficient skills and knowledge were passed down through the
generations by word of mouth. Other cultures developed a nomadic way of life as herders of sheep, goats, cattle, camels, yaks, llamas 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.
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. Several million slaves may have been taken northwards across the Sahara between the 8th and 18th centuries. Traditional means of overland transport declined with the advent of motor vehicles, shipping and air freight, but caravans still travel along routes between Agadez and Bilma and between Timbuktu and Taoudenni carrying salt from the interior to desert-edge communities.
Round the rims of deserts, where more precipitation occurred and
conditions were more suitable, some groups took to cultivating crops.
This may have happened when drought
caused the death of herd animals, forcing herdsmen to turn to
cultivation. With few inputs, they were at the mercy of the weather and
may have lived at bare subsistence
level. The land they cultivated reduced the area available to nomadic
herders, causing disputes over land. The semi-arid fringes of the desert
have fragile soils which are at risk of erosion when exposed, as
happened in the American Dust Bowl
in the 1930s. The grasses that held the soil in place were ploughed
under, and a series of dry years caused crop failures, while enormous
dust storms blew the topsoil away. Half a million Americans were forced
to leave their land in this catastrophe.
Similar damage is being done today to the semi-arid areas that
rim deserts and about twelve million hectares of land are being turned
to desert each year. Desertification
is caused by such factors as drought, climatic shifts, tillage for
agriculture, overgrazing and deforestation. Vegetation plays a major
role in determining the composition of the soil. In many environments,
the rate of erosion and run off increases dramatically with reduced
vegetation cover.
Natural resource extraction
A mining plant near Jodhpur, India
Deserts contain substantial mineral resources, sometimes over their
entire surface, giving them their characteristic colors. For example,
the red of many sand deserts comes from laterite minerals. Geological processes in a desert climate can concentrate minerals into valuable deposits. Leaching by ground water can extract ore minerals and redeposit them, according to the water table, in concentrated form. Similarly, evaporation tends to concentrate minerals in desert lakes, creating dry lake beds or playas rich in minerals. Evaporation can concentrate minerals as a variety of evaporite deposits, including gypsum, sodium nitrate, sodium chloride and borates. Evaporites are found in the USA's Great Basin Desert, historically exploited by the "20-mule teams" pulling carts of borax from Death Valley to the nearest railway. A desert especially rich in mineral salts is the Atacama Desert, Chile, where sodium nitrate has been mined for explosives and fertilizer since around 1850. Other desert minerals are copper from Chile, Peru, and Iran, and iron and uranium in Australia. Many other metals, salts and commercially valuable types of rock such as pumice are extracted from deserts around the world.
Oil and gas form on the bottom of shallow seas when
micro-organisms decompose under anoxic conditions and later become
covered with sediment. Many deserts were at one time the sites of
shallow seas and others have had underlying hydrocarbon deposits
transported to them by the movement of tectonic plates.
Some major oilfields such as Ghawar are found under the sands of Saudi Arabia. Geologists believe that other oil deposits were formed by aeolian processes in ancient deserts as may be the case with some of the major American oil fields.
Farming
Mosaic of fields in Imperial Valley
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 practices. Once fertility has been
built up, further crop production preserves the soil from destruction by
wind and other forms of erosion.
It has been found that plant growth-promoting bacteria play a role in
increasing the resistance of plants to stress conditions and these rhizobacterial
suspensions could be inoculated into the soil in the vicinity of the
plants. A study of these microbes found that desert farming hampers
desertification by establishing islands of fertility allowing farmers to
achieve increased yields despite the adverse environmental conditions. A field trial in the Sonoran Desert which exposed the roots of different species of tree to rhizobacteria and the nitrogen fixing bacterium Azospirillum brasilense with the aim of restoring degraded lands was only partially successful.
The Judean Desert was farmed in the 7th century BC during the Iron Age to supply food for desert forts.
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 miles (800 km) of large canals and
maintained them for centuries, an impressive feat of engineering. They
grew maize, beans, squash and peppers.
A modern example of desert farming is the Imperial Valley in California, which has high temperatures and average rainfall of just 3 in (76 mm) per year.
The economy is heavily based on agriculture and the land is irrigated
through a network of canals and pipelines sourced entirely from the 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 the
evaporation of river water.
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.
Solar energy capture
Deserts are increasingly seen as sources for solar energy, partly due to low amounts of cloud cover. Many solar power plants have been built in the Mojave Desert such as the Solar Energy Generating Systems and Ivanpah Solar Power Facility. Large swaths of this desert are covered in mirrors.
The potential for generating solar energy from the Sahara Desert is huge, the highest found on the globe. Professor David Faiman of Ben-Gurion University has stated that the technology now exists to supply all of the world's electricity needs from 10% of the Sahara Desert. Desertec Industrial Initiative
was 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.
The Negev Desert, Israel, and the surrounding area, including the Arava Valley, receive plenty of sunshine and are generally not arable. This has resulted in the construction of many solar plants. David Faiman has proposed that "giant" solar plants in the Negev could supply all of Israel's needs for electricity.
Warfare
War in the desert: Battle of El Alamein, 1942
The Arabs were probably the first organized force to conduct
successful battles in the desert. By knowing back routes and the
locations of oases and by utilizing camels, Muslim Arab forces were able
to successfully overcome both Roman and Persian forces in the period
600 to 700 AD during the expansion of the Islamic caliphate.
Many centuries later, both world wars saw fighting in the desert. In 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.
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. Its most easterly point was at El Alamein in Egypt, where the Allies decisively defeated the Axis forces in 1942.
In culture
Marco Polo arriving in a desert land with camels. 14th-century miniature from Il milione.
The desert is generally thought of as a barren and empty landscape.
It has been portrayed by writers, film-makers, philosophers, artists and
critics as a place of extremes, a metaphor for anything from death, war or religion to the primitive past or the desolate future.
There is an extensive literature on the subject of deserts. An early historical account is that of Marco Polo (c. 1254–1324), who travelled through Central Asia to China, crossing a number of deserts in his twenty four year trek.
Some accounts give vivid descriptions of desert conditions, though
often accounts of journeys across deserts are interwoven with
reflection, as is the case in Charles Montagu Doughty's major work, Travels in Arabia Deserta (1888). Antoine de Saint-Exupéry described both his flying and the desert in Wind, Sand and Stars 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. 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. 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.
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."
Deserts on other planets
View of the Martian desert seen by the probe Spirit in 2004.
Mars is the only other planet in the Solar System besides earth 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 the 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. 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. The surface of Titan, a moon of Saturn, also has a desert-like surface with dune seas.
