Desert (Encyclopedia of Science)
A desert is an arid land area that generally receives less than 10 inches (250 millimeters) of rainfall per year. What little water it does receive is quickly lost through evaporation. Average annual precipitation in the world's deserts ranges from about 0.4 to 1 inch (10 to 25 millimeters) in the driest areas to 10 inches (250 millimeters) in semiarid regions.
Other features that mark desert systems include high winds, low humidity, and temperatures that can fluctuate dramatically. It is not uncommon for the temperature to soar above 90°F (32°C) and then drop below 32°F (0°C) in a single day in the desert.
Most of the world's desert ecosystems (communities of plants and animals) are located in two belts near the tropics at 30 degrees north and 30 degrees south of the equator. These areas receive little rainfall because of the downward flow of dry air currents that originate at the equator. As this equatorial air moves north and south, it cools and loses whatever moisture it contains. Once this cool, dry air moves back toward Earth's surface, it is rewarmed, making it even drier. Over the desert areas, the dry air currents draw moisture away from the land on their journey back toward the equator.
Deserts around the world
The vast Sahara Desert in northern Africa encompasses an area 3,000 miles (4,800 kilometers) wide...
(The entire section is 1173 words.)
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Desert and Desertification (World of Earth Science)
Areas that receive less than 10 inches (25.4 cm) of rain a year are generally classified as deserts. Dry (arid) regions are usually found in area of high pressure (subtropical highs, leeward sides of mountains, etc.) associated with descending divergent air masses that are common between 30 degrees N and 30 degrees S latitude.
As a consequence of low moisture, desert vegetation is sparse and specifically adapted to conserve water. Deserts are areas of high relief (e.g., mesas, buttes, etc). Desert regions typically feature well-sorted sands, often found in various dune formations shaped by sand type, moisture content, and eoilian processes.
In desert areas, change usually occurs by some form of physical weathering. The wide diurnal temperature can make the modest amounts of moisture present powerful weather factors through continual freezing and thawing cycles that can result in micro-fracturing of rock. Winds often allow high levels of physical or frictional abrasion. Oxidation and other forms of chemical weathering produce familiar reddish dessert "varnish."
Desertification refers to the gradual degradation of productive arid or semi-arid land into biologically unproductive land (e.g., a change of grassland to desert). The term desertification was first used by the French botanist Aubreville in 1949, to refer to the transformation of productive agricultural land into a desert-like condition.
However, the processes whereby arid lands are stripped of their productivity do not always result in the development of a desert. In some cases, desertification has been successfully reversed through careful land stewardship, and areas degraded by this process have been restored to a more productive condition. In the worst cases, however, semi-desert and desert lands can lose their sparse complement of plants and animals and become barren, gullied wasteland.
Desertification is sometimes caused by natural influences. This process has been ongoing for eons in some regions, in conjunction with long-term changes in climatic conditions, especially decreased precipitation. Until the twentieth century, humans were able to simply move their agricultural activity away from land rendered unusable by desertification. However, this strategy has been rendered less tenable by the immense population increase of humans during the past century, a change that has increased the attention paid to the degradation of once-productive drylands.
Desertification claimed major international attention in the 1970s. This resulted from an extended period of severe drought in the Sahel region during 1968 to 1973, affecting six African countries on the southern border of the Sahara Desert. Although international relief measures were undertaken, millions of livestock died during that prolonged drought, and thousands of people suffered or died of starvation.
Arid lands of parts of North America are among those severely affected by desertification; almost 90% of such habitats are considered to be moderately to severely desertified. The arid and semi-arid lands of the western and southwestern United States are highly vulnerable to this kind of damage. The perennial grasses and forbs that dominate arid-land vegetation can provide good forage for cattle, but if these animals are kept at too high a stocking density, they will overgraze and degrade the natural vegetation cover, contributing to erosion and desertification. In addition, excessive withdrawals of groundwater to irrigate crops and supply cities is exceeding the ability of the aquifers to replenish, resulting in a rapid decline in height of the water table. Moreover, the salts left behind on the soil surface after irrigation water evaporates results in land degradation through salinization, creating toxic conditions for crops.
Desertification is best regarded as a process of continuous ecosystem degradation, including damage to plants and animals, as well as to geophysical resources such as water and soil. Desertification is usually discussed in the context of dry regions and ecosystems, but it can also affect prairies, savannas, rain forest, and mountainous habitats. Such effects can range from minor to severe.
The physical characteristics of land undergoing desertification include the progressive loss of the natural, mature vegetation from the ecosystem; the loss of topsoil; increasing salinity of the soil that reduces crop yields and may produce a salty surface crust that hinders the seepage of water into the deeper soil; and an increasing number of gullies or sand dunes as the soil is eroded by wind action.
Among the natural forces of desertification are wind and water erosion of soil, long-term changes in rainfall patterns, and other changes in climatic conditions. The role of drought is variable and related in part to its duration; a prolonged drought accompanied by poor land management may be devastating, while a shorter drought might not be. As such, drought stresses the ecosystem without necessarily degrading it permanently. Rainfall similarly plays a variable role that depends on its duration, the seasonal pattern of its occurrence, and its spatial distribution. The list of human or cultural influences on desertification includes vegetation loss by overgrazing, the depletion of groundwater, surface runoff of rainwater, frequent burning, deforestation, the influence of invasive non-native species, physical compaction of the soil by livestock and vehicles, and damage by strip-mining.
Land management measures to combat desertification focus on improving sustainability and long-term productivity. It is not always possible to return a desertified area to its predesertified condition. As such, mitigating the effects of desertification is best achieved by converting the degraded ecosystem into a new state that can withstand cultural and climatic land-use pressures. Specific measures include developing a resilient vegetation cover of mixed trees, shrubs, and grasses suitable to local conditions. The soil must be protected against wind and water erosion, compaction, and salinization. Water diversions that excessively lower the water table must be reversed, and if possible new sources of water found for human and animal populations.
See also Adiabatic heating; Atmospheric circulation; Basin and range topography; Depositional environments; Desalination; Dune fields; Dust storms; Eolian processes; Erosion; Evaporation; Global warming; Hydrogeology; Hydrologic cycle; Landforms; Landscape evolution; Rate factors in geologic processes; Seasonal winds; Soil and soil horizons; Water table; Weathering and weathering series