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Desertification

Desertification

Desertification is the degradation of grasslands, savannas , and woodlands to a more desert-like condition, with resulting decrease in plant production and the land's ability to support livestock grazing or other human uses. Vegetation becomes sparse; exposed soil becomes more vulnerable to erosion; and yields from cropland or grazing are reduced. The margins of most semiarid regions (in North and South America, much of central Asia, the African Sahel, South Africa, and Australia) are at high risk of desertification. Estimates of land degradation rates range from 50,000 to 120,000 square kilometers per year, affecting up to 60 percent of semi-arid rangeland and cropland.

Multiple causes may trigger desertification. Climatic shifts, especially long-lasting drought cycles, can drive ecosystems to more desert-like conditions. Over the past 40,000 years many regions have experienced repeated shifts in vegetation from semiarid to desert and back again in response to natural environmental variation. Semiarid ecosystems contain organisms well adapted to tolerate drought under natural conditions. Human activities such as woodland clearance, severe soil disturbance, or inappropriate cultivation practices have clearly contributed to desertification in many regions, and human disturbance makes semi-arid systems vulnerable to further degradation.

Frequently, desertification is marked by the decline of grasses and the replacement of continuous grasslands by scattered shrubs and thorny vegetation, leaving much bare soil. One result is that soil resources become more concentrated around the large plants, and conditions grow increasingly difficult for most organisms in the bare areas. These exposed surfaces are then vulnerable to further degradation through erosion, evaporation, and high temperatures. Desertification can also result when cultivated areas are abandoned and soil conditions have been so altered as to impede recovery of natural ecosystems. Such alterations include erosion, increased salt from irrigation, and loss of soil organisms.

Desertification is a challenge to developed as well as developing nations. Because semi-arid ecosystems have historically been important as livestockproducing areas, desertification has negative consequences for human populations. Desertification may also trigger further aspects of global environmental change. The increased proportion of bare soil relative to green vegetation can change Earth's radiation balance (the balance between absorbed and reflected solar energy) and thus temperatures. Dust eroded from exposed soil can be transported long distances, affecting other ecosystems and altering air quality.

Minor changes in average climate may have potentially large effects on semi-arid vegetation; hence "global warming" could exacerbate desertification. Because air temperature, carbon dioxide (CO2) concentrations, and relative humidity affect plant growth and water use in complex, interacting ways, it is difficult to predict the net effect of atmospheric and climatic changes on dryland vegetation. Even if warming climate were to result in greater moisture and hence more precipitation in some areas, some areas, such as continental interiors, would likely experience warming without significant additions of precipitation; hence concerns about desertification may be well founded. However, intensified land use, higher numbers of grazing livestock, and other pressures resulting from growing human populations are likely to be far more significant drivers of desertification in the near future than any climatic shifts.

see also Desert; Global Climate Change; Grassland

Laura F. Huenneke

Bibliography

Allan, T., and A. Warren, eds. Deserts: The Encroaching Wilderness. New York: Oxford University Press, 1993.

Schlesinger, William H., et al. "Biological Feedbacks in Global Desertification." Science 247 (1990): 10431048.

United Nations Environment Program. World Atlas of Desertification. London: Edward Arnold, 1992.

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desertification

desertification The gradual conversion of fertile land into desert, usually as a result of human activities. Loss of topsoil leads to further soil erosion until the land can no longer be used to grow crops or support livestock. A major factor contributing to desertification is bad management of farmland. Overgrazing of livestock removes the plant cover and exposes the soil, making it vulnerable to erosion. Overintensive cultivation of crop plants, especially monoculture (see agriculture), depletes the soil of nutrients and organic matter, resulting in loss of fertility and increasing its susceptibility to erosion. In many Third World countries it is difficult to control the process of desertification as the livelihood of the people often depends on practices that contribute to soil erosion. Another major cause of desertification is deforestation.

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Desertification

Desertification

Human survival and prosperity are dependent ultimately on the productivity of the lands on which populations reside. In many parts of the world, however, previously productive lands have become less fertile or completely sterile , failing to meet the basic needs of local populations. Desertification has widely been recognized as one of the several major global environmental problems since the 1970s. According to the United Nations Environmental Programme, drylands that are susceptible to desertification account for more than one-third of the world land area and support more than 20 percent of the global human population. As the rapid growth of the human population continues, demands for resources from these fragile environments increase as well. Therefore, understanding the scope, causes, and mechanisms of desertification and developing sound and effective management and mitigation plans are extremely important for maintaining the ecological, socioeconomic, and political stability of both the dryland areas and the entire world.

Degradation and Loss of Productivity

The term desertification was first used by two French ecologists: L. Lavauden in 1927 and A. Aubreville in 1949, who then eyewitnessed the land degradation occurring in north and west Africa. Since then, more than one hundred definitions have appeared in the English literature. Desertification sometimes has been used interchangeably with desertization, which refers to desert encroachment into previously nondesert areas driven by human activities. A widely used definition for desertification is land degradation in arid, semiarid, and dry subhumid regions due to human activities and climate variations, which may lead to the permanent loss of land productivity. This definition was accepted at the United Nations Conference on Desertification in 1977, and later adopted by the Earth Summit, the United Nations Conference on Environment and Development in 1992, and the Intergovernmental Convention to Combat Desertification in 1994.

WORLD DISTRIBUTION OF DRYLANDS, 1996
Bioclimatic Zones Extent (in thousands of square kilometers) Percentage of World Land Area P/PET Ratio*
Dry-subhumid land 12,947 9.9 0.45-0.65
Semiarid land 23,053 17.7 0.20-0.45
Arid land 15,692 12.1 0.05-0.20
Total drylands susceptible to desertification 51,692 39.7
Hyperarid land (extremely harsh environment and thus not susceptible to desertification) 9,781 7.5 0.05
Total world dryland area 61,473 47.2
* P is the mean annual precipitation, and PET is the mean annual potential evapotranspiration, which is a combined term for water lost as vapor from soil surface (evaporation) and from the surface of plants mainly via stomata (transpiration). P/PET ratio is also called aridity index (I) and is often used to classify bioclimatic zones. Smaller values of the ratio correspond to drier areas.
source: Data from United Nations Environmental Programme, 1992; adapted from H. N. Le Houérou, "Climate Change, Drought, and Desertification," Journal of Arid Environments 34 (1996): 133 -85.
DESERTIFICATION EXTENT AND SEVERITY IN WORLD REGIONS, 1995
Desertified Area (in thousands kilometers)
Region Total Dryland Area (in thousands of square kilometers) Light and Moderate Strong and Extreme Total Area of Desertified Land
Asia 16,718 3,267 437 3,704
Africa 12,860 2,453 740 3,193
Europe 2,997 946 49 995
Australasia 6,633 860 16 876
North America 7,324 722 71 793
South America 5,160 728 63 791
Total 51,692 8,976 1,376 10,352
source: Data from D. S. G. Thomas, "Desertification: Causes and Processes." In Encyclopedia of Environmental Biology, Vol. 1, edited by W. A. Nierenberg (San Diego: Academic Press, 1995), 463 -73.

Desertification may be viewed as the worst form of land degradation, the general process of declining soil fertility, impairing ecosystem structure and function, decreasing biodiversity, and diminishing economic viability. After an ecosystem is severely desertified, its full recovery may not be achieved even during relatively moist conditions without intensive rehabilitation efforts. Natural deserts, without human disturbances, are healthy and relatively stable ecosystems that support a variety of life formssometimes spectacularlike the saguaro in the Sonoran Desert. The simplistic view that desertification is a process that transforms nondesert lands into desert-like lands may thus be too superficial and misleading. Also, deserts do emerge independent of human activities, and the term aridization refers to this natural development of deserts through evolution of drier climates, which takes place much more slowly than desertification.

Causes of Desertification

Human abuses of the land (e.g., overcultivation, overgrazing, and urbanization ) are the primary causes for desertification, whereas adverse climate variations (e.g., droughts) may accelerate or trigger the process. By drastically reducing or destroying vegetation cover and soil fertility, human activities can result in desertification without drought, but not vice versa. For example, overgrazing reduces both productivity and biodiversity of grasslands and can lead to a grassland-to-shrub land transition. Overcultivation completely destroys natural vegetation and can eventually exhaust soil resources. In both cases, human activities can transform drylands into un-productive wastelands through the processes of soil erosion (by wind and water), salinization , and alkalinization .

Desertification often is a result of the interactions between human and climate factors. Since human actions are tied to many social, economic, political, and environmental processes, the relative importance of major causes for desertification varies from one region to another. For example, the most dominant cause for desertification in China is overcultivation, but in north Africa and the Near East it is overgrazing. Besides droughts, global climate change may also affect desertification. Studies have suggested that global warming may reduce soil moisture over large areas of semiarid grasslands

CAUSES OF DESERTIFICATION IN WORLD REGIONS, 1996
Regions or Countries Overcultivation Overstocking Fuelwood Collection Salinization Urbanization Other
Northwest China 45* 16 18 2 3 14
North Africa and Near East 50 26 21 2 1 -
Sahel and East Africa 25 65 10 - -
Middle Asia 10 62 - 9 10 9
United States 22 73 - 5 N/A -
Australia 20 75 - 2 1 -
* The numbers are in percentage of the total desertified area in the corresponding region.
source: Data from H. N. Le Houérou, "Climate Change, Drought, and Desertification," Journal of Arid Enviroments34 (1996): 133-85.

and thus increase the extent of desertified lands in North America and Asia. The possible effects of climate change on desertification, however, seem much smaller than the impact of land use activities by humans.

Dry-subhumid, semiarid, arid, and hyperarid areas together form the world drylands, covering as much as 47 percent of the total land area. Dry forest, grassland, and shrub land ecosystems are found in drylands except in hyperarid land (the true desert), which experiences extreme dry conditions and usually seems lifeless (e.g., central Sahara and the Namib Desert of Africa, the Hizad on the Arabian Peninsula, the Taklimakan and Turpan Depressions in central Asia, and Death Valley in the United States). Desertification occurs primarily in all drylands except hyperarid lands because climatic and ecological conditions make them more susceptible to land degradation than more humid regions. It is hard for hyperarid lands to become more desertlike, and thus they are usually excluded from the consideration of desertification.

Desertification has been occurring at an astonishing rate over six continents. Most of the desertified lands are found in Asia and Africa, while the problem also has become significant in Europe, Australasia, North America, and South America. Approximately 25 percent of the irrigated land (3 percent of the drylands), 50 percent of the rain-fed cropland (9 percent of the drylands), and 75 percent of the rangeland (88 percent of the drylands) have been desertified to different degrees. Although the accuracy of estimating the exact extent and rate of desertification needs to be improved with the aid of advanced technologies such as satellite remote sensing and geographic information systems (computer systems for storing, retrieving, and manipulating spatial or geographic data), there is little doubt that extensive areas of the world's drylands have increasingly experienced some form of chronic land degradation since the early 1900s.

Desertification has affected more than one hundred countries and resulted in profound ecological, social, and economic consequences throughout the world. Combating desertification is an urgent and grand challenge facing humanity today. Global efforts and local solutions are both needed. Preventive and rehabilitation measures must be undertaken simultaneously based on scientific findings and socioeconomic considerations.

see also Deserts; Global Warming; Human Impacts.

Jianguo Wu

Bibliography

Dregne, H. E., ed. Degradation and Restoration of Arid Lands. Lubbock, TX: International Center for Arid and Semiarid Land Studies, Texas Tech University, 1992.

. "Desertification: Challenges Ahead." Annals of Arid Zone 35, no. 4 (1996): 305-11.

Schlesinger, W. H., et al. "Science and the Desertification Debate." Journal of Arid Environments 37 (1997): 599-608.

United Nations Environmental Programme. World Atlas of Desertification. London: Arnold, 1992.

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desertification

desertification Process by which a desert gradually spreads into neighbouring areas of semi-desert. The change may result from a natural event, such as fire or climatic change, but occurs most frequently as a result of human activity. Once vegetation is removed (usually by over-grazing or for firewood), the soil is easily eroded and the land rendered infertile.

http://www.unccd.int; http://www.fao.org/desertification

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desertification

desertification The process of desert expansion or formation, which may occur as a direct consequence of climatic change (e.g. shifts in the location of the major planetary pressure and wind systems), as the result of poor land-use policy (e.g. overgrazing), or owing to some complex interaction of these factors (e.g. overgrazing leading to albedo change, favouring climatic change in the form of increased dryness).

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desertification

desertification The process of desert expansion or formation, which may occur as a direct consequence of climatic change (e.g. shifts in the location of the major planetary pressure and wind systems), poor land-use policy (e.g. overgrazing), or some complex interaction of these factors (e.g. overgrazing leading to albedo change, favouring climatic change in the form of increased dryness).

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desertification

desertification The spread of desert-like conditions, particularly in arid or semi-arid areas, due to the influence of human activity and climatic change.

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