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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.

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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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Desertification

Desertification


About one billion people live in arid or semiarid desert lands that occupy about one third of the world's land surface. In these drier parts of the world, deserts are increasing rapidly from a combination of natural processes and human activities, a process known as desertification or land degradation. An annual rainfall of less than 10 in (25 cm) will produce a desert anywhere in the world. In the semiarid areas along the desert margins, where the annual rainfall is around 16 in (40 cm), the ecosystem is inherently fragile with seasonal rains supporting the temporary growth of plants. Recent changes in the climate of these regions have meant that the rains are now unreliable and the lands that were once semiarid are now becoming desert. The process of desertification is precipitated by prolonged droughts, causing the top layers of the soil to dry out and blow away. The eroded soils become unstable and compacted and do not readily allow for seeding. This means that desertified areas do not regenerate by themselves but remain bare and continue to erode. Desertification of grazing lands or croplands is accompanied, therefore, by a sharp drop in the productivity of the land.

Natural desertification is greatly accelerated by human activities that leave soils vulnerable to erosion by wind and water. The drier grasslands with too little rain to support cultivated crops have traditionally been used for grazing livestock. When semiarid land is overgrazed (by keeping too many animals on too little land), plants that could survive moderate grazing are uprooted and destroyed altogether. Since plant roots no longer bind the soil together, the exposed soil dries out and is blown away as dust. The destruction and removal of the topsoil means that soil productivity drops drastically. The obvious solution to desertification caused by overgrazing is to limit grazing to what the land can sustain, a concept that is easy to espouse but difficult to practice.

In the Sahel zone along the southern edge of the Sahara desert, settled agriculture and overgrazing livestock on the fragile scrublands have led to widespread soil erosion. Nomadic pastoralists, who have traditionally followed their herds and flocks in search of new pastures, are now prevented by national borders from reaching their chosen grazing grounds. Instead of migrating, the nomads have been encouraged to settle permanently and this has led to their herds overgrazing available pastures.

Other human factors leading to desertification include over-cultivation, deforestation , salting of the soil through irrigation , and the plowing of marginal land. These destructive practices are intensified in developing countries by rapid population growth , high population density, poverty, and poor land management. The consequences of desertification in some countries mean intensified drought and famine and lowered standards of living. It is estimated that desertification worldwide has claimed an area the size of Brazil (2 billion acres or 810 million ha) in the past 50 years. Each year new deserts consume an area the size of Belgium (15 million acres or 6 million ha), most of which is in the African Sahel.

In marginal areas throughout the world, traditional farming practices can lead to desertification. Plowing turns the top layer of the soil upside down, burying and killing weeds but exposing bare soil to erosion. In arid areas the exposed soil dries out rapidly and is easily lost through wind erosion.

The processes of erosion and soil formation vary with climate and with the composition of the parent material. In balanced ecosystems, soil lost to erosion is replaced by new soil created by natural processes. On average, new soil is formed at a rate of about 5 tons an acre (12.5 tons per ha) per year, which is equivalent to a layer of soil about 0.2 in (0.4 cm) thick. This means that soils can sustain an erosion rate of up to 5 tons per acre per year and still remain in balance. However, much of the world's crop and forest land is not within this balance (with erosion running at two to 10 times the tolerable rate). In the United States about 6 tons of topsoil are lost for every ton of grains produced.

Forests are cut down for many different reasons. Some are cleared for agriculture, some for construction, some for paper products, and some to meet cooking and heating needs. Unfortunately, deforestation results in more than just the loss of trees, for soil is eroded, nutrients are lost to the ecosystem, and the water cycle is disrupted. The roots of the trees serve to bind the soil together and to hold water in the ecosystem, while the leaves of the trees break the force of the rain and allow it to soak into the topsoil. The result is that surface runoff from a forested hillside is half as much as from a grass-covered slope. Additionally, water soaking into the ground (rather than running off) leads to the natural recharge of groundwater and other water resources . Water and soil runoff from deforested hillsides cause flooding and siltation of agricultural and aquatic ecosystems in adjacent lowlands. Forests are also more efficient at reabsorbing and recycling the nutrients released from decaying detritus than are grasslands. Clearing forests therefore exposes the soil to both erosion and nutrient loss and alters the recharge of water reserves in the ecosystem.

Industrialized countries experienced a period of intense deforestation during the Industrial Revolution and even today much of the land has low productivity. Fortunately, most of these countries are now reforesting faster than they are deforesting. This is possible because their population growth is low, their agricultural production per acre is high, and their need for fuelwood for cooking and heating is optional, since fossil fuels and electricity are widely available. All of these factors release the pressure to further deforest the land.

In developing countries, on the other hand, high population growth rates and widespread poverty put pressure on the forests. Trees are needed for firewood, charcoal, and export, and the land is needed for farmland. In some developing countries deforestation exceeds replanting by five times. In Ethiopia, population and economic pressures pushed people to deforest and cultivate hillsides and marginally dry lands, and more than 1 billion tons of topsoil per year are now lost, resulting in recurrent famines. In parts of India and Africa there is so little wood available that dried animal dung is used to fuel cooking fires, an act that further robs the soil of potential nutrients. In Brazil, soil erosion and desertification have resulted from the conversion of forests to cattle ranches. In China, about one-third of its agricultural land has been lost to erosion, and the story is similar for many other countries.

The answer to erosion from deforestation is reforestation, better forest conservation , and better forest management to increase productivity. Planting trees on hillsides is particularly effective. Reforesting desertified areas first requires mulching the soil to hold moisture and the protection of the seedlings for several years until natural processes can regenerate the soil. Using these methods, Israel has achieved spectacular success in bringing desertlands (a product of past desertification) back to agriculture.

Desertification and its agents, deforestation and erosion, have been powerful shapers of human history. Agriculture had its roots in the once fertile crescent of the Middle East and in the Mediterranean lands. However, deforestation, overgrazing, and poor agricultural practices have turned once-productive pastureland and farmland into the near deserts of today. It is thought by some that deforestation and desertification may have even contributed to the collapse of the Greek and Roman Empires. Similar fates may have befallen the Harappan civilization in India's Indus Valley, the Mayan civilization in Central America, and the Anasazi civilization of Chaco Canyon, New Mexico, in what is today desertland.

[Neil Cumberlidge Ph.D. ]


RESOURCES

BOOKS


Dejene, A. Environment, Famine, and Politics in Ethiopia: A View from the Village. Boulder, CO: L. Rienner Publishers, 1990.

Grainger, A. Desertification: How People Make Deserts, How People Can Stop, and Why They Don't. London: International Institute for Environment and Development, 1982.

McLeish, E. The Spread of Deserts. Austin, TX: Steck-Vaughn Library, 1991.

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Desertification

Desertification

The Sahelian drought and United Nations convention to combat desertification

Desertification in North America

Processes of desertification

Land management

Resources

Desertification is the gradual degradation of productive arid or semi-arid land into biologically unproductive land. The French botanist, Andre´ Aubreville, coined the term in 1949 to describe to the transformation of productive agricultural land in northern Africa to desertlike, uncultivable fallow land. Loss of biological and ecological viability occurs when natural variations, like extended drought related to climate change, and unsustainable human activities such as over-cultivation and poor irrigation practices, strip dry lands of their stabilizing vegetation, soil nutrients, and natural water distribution systems. Earths arid and semi-arid regions are, by definition, areas with scarce precipitation; even very small changes can quickly destroy the fragile ecosystems and soil horizons that, under normal conditions, remain productive with very little water. Desertification does not, per se, result in the development of a desert. Though decertified land and deserts are both dry, the barren, gullied wastelands left by desertification barely resemble the subtle biological productivity of healthy desert ecosystems. In some cases, careful land stewardship has successfully reversed desertification, and has restored degraded areas to a more productive condition. In the worst cases, however, semi-desert and desert lands have lost their sparse complement of plants and animals, as well as their ability to support agriculture.

Desertification is a particularly pressing social and environmental issue in regions where natural dryness and human poverty coincide. Earths deserts and

semi-arid grasslands occur in the subtropical bands between 15° and 30° north and south where extended periods of high pressure persist under the trade winds. Northern and southern Africa, the Arabian peninsula, southern Asia, Australia, southern South America and the North American Southwest lie in the subtropical zones. Desertification is usually discussed in the context of dry regions and ecosystems, but it can also affect prairies, savannas, rainforests, and mountainous habitats. Global climate change can alter the boundaries of these naturally dry regions, and change the precipitation patterns within them. Arid and semi-arid regions with large, impoverished populations of subsistence farmers, like northern Africa, or with large-scale commercial agriculture, like the American Southwest, are particularly susceptible to destructive desertification.

Sometimes desertification is the result of purely natural processes. Long-term changes in climatic conditions have led to decreased precipitation in a number of regions. The northern Sahara, for example, has experienced numerous fluctuations between arid and wet conditions over the past 10, 000 years, as have the basins of the American West. Radar images collected aboard the space shuttle Endeavor show extensive river systems buried beneath more recent Saharan sands, and preserved fossil vegetation and lake shorelines suggest that forests surrounded filled lakes in northern Nevada and Utah. Cyclical atmospheric and oceanographic variations, like the El Ninö phenomenon in the southern Pacific, may also trigger extended regional drought. Environmental scientists warn that anthropogenic (human-induced) global climate change could also lead to desertification to previously unaffected regions. Until the twentieth century, humans were able to simply move their agricultural activity away from land rendered unusable by desertification. However, rapid twentieth century population growth, and a corresponding need for high agricultural productivity, has rendered that strategy untenable.

The Sahelian drought and United Nations convention to combat desertification

Desertification first attracted major international attention in the 1970s when a decade of severe drought in the Sahel region of Africa brought starvation to the impoverished populations of countries along the southern border of the Sahara Desert. Sparse stabilizing vegetation died, and the Saharan sands encroached, covering depleted agricultural land. Reduced water volume in rivers compromised irrigation and hydroelectric generation. Dust storms brought health problems and ruined equipment and buildings. Livestock died of starvation. More than 100, 000 sub-Saharan Africans died of thirst and starvation between 1972 and 1984, and more than 750, 000 people in Mali, Mauritania, and Niger were completely dependent on international food aid during 1974.

The United Nations convened the Conference on Desertification (UNOCD) in 1977 in Nairobi, Kenya in response to the Sahelian crisis. The conference brought together 500 delegates from 94 countries. The delegates approved 28 recommendations for slowing or reversing desertification, with the hope of stabilizing the area of land degraded by desertification, and preventing further crises. Scientists advising the 1977 convention estimated that desertification threatened 11.6 million mi2 (30 million km2), or 19% of Earths land surface, in 150 nations worldwide. Despite the action undertaken in the years following the UNOCD, and numerous examples of local successes, the United Nations Environment Programme (UNEP) concluded in 1991 that the threats and effects of desertification had worsened worldwide. The 1994 United Nations Convention to Combat Desertification (UNCCD), which advocates a program of locally-implemented efforts to reverse and/or prevent the desertification, was proposed at the 1992 United Nations Conference on Desertification (UNCOD) in Rio de Janeiro. By March 2002, 179 nations had agreed to the UNCCD treaty that directly affects 250 million people, and threatens to impact at least one billion.

Desertification in North America

Arid lands in 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 shrubs that dominate arid-land vegetation provide good forage for cattle, but overstocking leads to overgrazing and degradation of the natural vegetation cover, which, in turn, contributes 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. Groundwater depletion by over-pumping of the sandstone Ogalalla aquifer in the Southwestern United States has contributed to desertification in Nebraska, Kansas, Oklahoma and Texas. Moreover, the salts left behind on the soil surface after the irrigation water has evaporated results in land degradation through salinization, creating toxic conditions for crops and contaminating groundwater. Salination resulting from decades of heavy irrigation has compromised the soil quality in Californias San Joaquin Valley, which produces much of the produce sold in the United States.

Studies of pre-industrial, aboriginal people in the western and southwestern United States suggest even small numbers of people could induce long-lasting ecological changes, including desertification. For example, Native Americans reliant on mesquite beans for food planted mesquite throughout the Chihuahuan Desert of Arizona, New Mexico, Texas, and northern Mexico. Stands of mesquite developed around campsites and watering holes, and replaced the local grasses and other vegetation. The Pueblan culture, which flourished in the southwestern United States beginning around 800 AD, used the meager stands of trees for housing material, resulting in local deforestation.

Processes of desertification

Desertification is a process of continuous, gradual ecosystem degradation, during which plants and animals, and geological resources such as water and soil, are stressed beyond their ability to adjust to changing conditions. Because desertification occurs gradually, and the processes responsible for it are understood, it can often be avoided by planning or reversed before irreparable damage occurs. The physical characteristics of land undergoing desertification include progressive loss of mature, stabilizing vegetation from the ecosystem, or loss of agricultural crop cover during periods of drought or economic infeasibility, and a resulting loss of unconsolidated topsoil. This process is called deflation. Erosion by wind and water then winnows the fine-grained silt and clay particles from the soil; dramatic dust storms like those observed during the 1930s Dust Bowl in the American midwest, and in northern Africa, were essentially composed of blowing topsoil. Continued irrigation of desertified land increases soil salinity, and contaminates groundwater, but does little to reverse the loss of productivity. Finally, ongoing wind and water erosion leads to development of gullies and sand dunes across the deflated land surface.

The forces causing these physical changes to occur may be divided into natural, human or cultural, and administrative causes. Among the natural forces 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 have lasting consequences. As such, drought thus stresses the ecosystem without necessarily degrading it permanently. Rainfall similarly plays a variable role that depends on its duration, the seasonal pattern of its occur-rence, and its spatial distribution.

The list of human or cultural influences on desertification includes vegetation loss by overgrazing, 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. Desertification caused by human influences has a long historical record; there is evidence of such damage caused around the Tigris and Euphrates rivers in ancient Mesopotamia. Administrative influences contributing to desertification include encouragement of the widespread cultivation of a single crop for export, particularly if irrigation is required, and the concentration of dense human populations in arid lands. Poor economic conditions, like the Great Depression in United States in the 1930s, also contribute to degradation of croplands. During that crisis, American farmers were simultaneously confronted with bankruptcy and a decade-long drought, and they left millions of acres of plowed, bare cropland unplanted. According to the 1934 Yearbook of Agriculture, Approximately 35 million acres of formerly cultivated land have essentially been destroyed for crop production. 100 million acres now in crops have lost all or most of the topsoil; 125 million acres of land now in crops are rapidly losing topsoil.

Considering these factors together, desertification can be viewed as a process of interwoven natural, human, and economic forces causing continuous degradation over time. Therefore, ecosystem and agricultural degradation caused by desertification must be confronted from scientific, social and economic angles. Fortunately, scientists believe that severe desertification, which renders the land irreclaimable, is rare. Most desertified areas can be ecologically reclaimed or restored to agronomic productivity, if socioeconomic and cultural factors permit restoration.

Land management

Land management measures that combat desertification focus on improving sustainability and long-term productivity. Though damaged areas cannot always be restored to their pre-desertified conditions, they can often be reclaimed by designing a new state that can better withstand cultural and climatic stresses. Specific measures include developing a resilient vegetation cover of mixed trees, shrubs, and grasses suitable to local conditions that protects the soil from wind and water erosion and compaction. Redistribution of water resources, and redesign of water delivery systems, can reduce the effects of sali-nation, groundwater depletion, and wasteful water use. Finally, limiting the agricultural demands made on drought-prone arid and semi-arid lands can be accomplished by encouraging farmers to grow drought-tolerant plants, and to move water-hungry crops, like cotton and rice, to more suitable climates.

Land management methods that halt or reverse desertification have been known in the United States since the end of the nineteenth century, but they have only recently been put into widespread use. United States federal policies do support soil conservation in croplands and rangelands, but only about one third of such land is under federal protection. The United States government has strongly supported the autonomy of private landowners and corporations who, in turn, have often traded sustainable land-use practices for short-term profits. Even the ravages of the dust bowl did not result in widespread anti-desertification measures.

In the developing world, particularly in Africa, where poverty and political unrest are common, progress toward mitigation of desertification and its devastating social costs has been slow. Many of the worlds poorest people, who live in countries with the weakest and most corrupt governments, rely on unsustainable agriculture and nomadic grazing to subsist. Many African countries, including Niger, Mali, and Senegal, have experienced positive results with implementation of a system of local self-regulation. This strategy, encouraged by the United Nations, involves a pastoral association or a community assuming responsibility for maintaining a water source and its

KEY TERMS

Arid land Land receiving less than 10 in (25 cm) of rainfall annually, with a high rate of evaporation and supporting a thin vegetation cover, chiefly of grasses and shrubs.

Gradualism A slow rate of change, implying that processes are spread out over time as well as space.

Sahel region A semi-arid region along the southern border of the Sahara Desert in western Africa with a dry savanna type of vegetation. In recent years this region has been encroached on by the Sahara Desert, partly as a result of poor land management.

surrounding rangeland, while receiving free veterinary and health services. By these and similar means, cultural habits, subsistence needs, economic concerns, and ecological conservation can be addressed in a single, integrated program. Furthermore, such programs help local communities reduce their dependence on ineffective or corrupt centralized governments, and on the international aid community. Such comprehensive anti-desertification programs have been very successful on a limited scale, but progress has been slow because of the extreme poverty and sociopolitical powerlessness of the communities involved. The key to the success of any anti-desertification program is the need to adapt to local conditions, including those associated with climate, ecology, culture, government, and historical land-use.

See also Ecological integrity; Ecological monitoring; Ecological productivity; Crop rotation; Deforestation; Forestry; Land use; Water conservation.

Resources

BOOKS

Geist, Helmut. The Causes And Progression Of Desertification (Ashgate Studies in Environmental Policy and Practice). Hants, UK: Ashgate Publishing Ltd. 2004.

Press, Frank, and Raymond Siever. Understanding Earth. Chapter 14: Winds and Deserts. New York: W.H. Feeman and Company, 2001.

OTHER

National Atmospheric and Space Administration Distributed Active Archive Centers Earth Observatory. From the Dust Bowl to the Sahel. May 18, 2001.

<http://earthobservatory.nasa.gov/Study/DustBowl> (accessed October 13, 2006).

United Nations Convention to Combat Desertification. Text of the United Nations Convention to Combat Desertification. February 14, 2002. <http://www.unccd.int/convention/text/convention.php> (accessed October 13, 2006).

Marjorie Pannell

Laurie Duncan

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Desertification

Desertification

Introduction

When vegetation is lost from a dryland area, surface soil blows away leaving bare, dry, unprotected layers that harden and becomes less infertile. This type of land degradation is known as desertification. Up to 20% of the world’s drylands (excluding areas that are already deserts) have now undergone some degree of desertification with around one billion people at risk of its effects. The term does not refer to the expansion of existing desert.

The two main causes of desertification are climate variation and human exploitation of vulnerable drylands. Desertification has many adverse effects, both economic and ecological. Loss of crops and water supplies lead to hardship, famine, and disease, while habitat loss causes loss of biodiversity. The best way to combat desertification is to try to use drylands in a more sustainable and appropriate manner, so that these fragile ecosystems are conserved.

Historical Background and Scientific Foundations

The United Nations defines desertification as the persistent degradation of land in arid, semi-arid, and dry sub-humid areas, all of which have limited water supplies. Such drylands are especially vulnerable to both climatic variation and human activity, which are the leading causes of desertification. Population growth and the resulting increased demand for food lead to cultivation of less than ideal land, such as the dryland areas. Meanwhile, although the relationships between global warming and desertification is complex increased temperature will, on the whole, encourage drought.

Loss of vegetation is the key underlying cause of desertification. Plants hold moisture in the soil and provide a natural windbreak. Without them, the soil is blown away and becomes increasingly drier and unable to support plant life, including crops, forage for livestock, and wood.

Drought tends to lead to vegetation loss and desertification, because the plants do not receive enough water. A switch from nomadic to sedentary, and increasingly intensive, farming tends to encourage overgrazing, which occurs whenever there are more livestock than the land can support without degradation. Deforestation, through logging activities or clearing land for cash crops or other export activities, also causes desertification through loss of trees. Inappropriate irrigation schemes may cause increased salinity of the soil, which also leads to extensive degradation.

There are many underlying human factors underlying the causes of desertification. Increased populations will naturally put more pressure on drylands to yield food, which can lead to overgrazing and overplanting. But underpopulation can be a problem too, in some areas. For instance, many hillside terraces in Yemen have become neglected and are at risk, because there are not enough people left to care for the land, following extensive migration to neighboring oil-rich countries. War or natural disaster can also deplete an area of those who might look after its land.

Ignorance about how to look after drylands to keep them fertile also plays a part in desertification. For instance, during the famous Dust Bowl drought in mid-America in the 1930s, many farmers used ploughs that were better suited to more temperate latitudes, resulting in extensive desertification. The choice of policies and technologies can be vital in keeping drylands productive.

Impacts and Issues

The United Nations says that 250 million people are directly affected by desertification and another one billion people in over one hundred countries are at risk. These include many of the world’s poorest. A 2007 report from the UN suggested that if urgent action against desertification is not taken, 50 million people in sub-Saharan Africa and central Asia could be displaced within the next ten years. People in drylands, the majority of which are in developing countries, are more dependent on the surrounding systems for basic needs such as water, food, and wood for building than are those in ecosystems elsewhere. Africa is the place worst affected by desertification, but there are many others. For instance, the Soviet Union created a large area of desertification in Central Asia through overfarming. Northern China, Australia, northeastern Brazil, and the Caribbean islands have also experienced desertification through overgrazing and deforestation.

The impact of desertification on a population includes crop damage, damage to water supplies, habitat loss, loss of biodiversity, displacement of the local population, poverty, famine, and disease. Sometimes people affected by desertification will make the problem worse by trying to plant or graze neighboring dry areas. If they migrate, instead, to cities or other countries, social unrest and political instability often result.

In 1977 the United Nations Conference on Desertification set up a Plan of Action in recognition of the problem. But by 1991, little had been achieved, except for some local successes. At the time of the United Nations Conference on Environment and

WORDS TO KNOW

DRYLAND: Land where freshwater supplies are limited.

OVERGRAZING: The grazing of land over extended periods of time or without sufficient recovery periods, to the detriment of vegetation and soil quality.

SOIL: Unconsolidated materials above bedrock.

Development, held in Rio de Janeiro in 1992, a new, integrated approach to the problem of desertification was proposed. The United Nations Convention to Combat Desertification was adopted in 1994, and it has now been signed by 191 countries around the world.

The convention’s work covers different geographical locations, namely Africa, Asia, Latin America and the Caribbean, northern Mediterranean, and central and eastern Europe. Projects are carried out at a national and regional level, bringing together government, non-governmental organizations, and the local community. The work has to recognize the complex political and economic factors that sometimes drive desertification; progress is stepwise, with the aim of promoting sustainable practices in those places at risk.

IN CONTEXT: GLOBAL EFFORTS TO COMBAT DESERTIFICATION

The first major international plan to prevent desertification and facilitate water conservation work by national governments and international conservation agencies was formulated by the United Nations in 1977 as the U.N. Plan of Action to Combat Desertification. Conference participants noted that issues of desertification, soil and freshwater salination, water shortages, agricultural land degradation, and political and economic turmoil often compound each other.

In June 1994, the Convention to Combat Desertification was adopted. Nearly 180 signatory parties met two years later at the first Conference of the Parties.

Local and national conditions vary, and the most appropriate solution will depend upon location. But there are three basic approaches to tackling desertification. Prevention is the most cost-effective of these. Where the earliest signs of desertification are noted, the local community and government should work together on introducing alternative farming methods or even alternatives to farming or cash crops to generate income in that area. It may also be possible to stop, or even reverse, desertification already underway by rehabilitating or restoring the land. Tree planting is one possible strategy and this is being done in China and elsewhere. However, trees need water to grow and care has to be taken not to deplete already precious supplies.

Desertification is likely to increase in the future because the underlying factors driving it are so complex and can only be tackled step by step. It is related to biodiversity loss and climate change in ways that are not yet fully understood. Loss of biodiversity in a dryland, particularly when it involves plants, can alter water regulation and local climate. This, in turn, may make desertification worse. The relationship between climat change and desertification is complex. Although warmer temperatures encourage drier conditions, increased carbon dioxide in the atmosphere may actually encourage plant growth. Desertification is one of the largest environmental challenges we face. Unless it is seriously addressed, it will be impossible to alleviate poverty among those who go without life’s basic needs.

See Also Agricultural Practice Impacts; Deserts; Drought; Human Impacts; Land Use; Natural Resource Management; Overgrazing

BIBLIOGRAPHY

Books

Cunningham, W.P., and A. Cunningham. Environmental Science: A Global Concern. New York: McGraw-Hill International Edition, 2008.

Kaufmann R., and C. Cleveland. Environmental Science. New York: McGraw-Hill International Edition, 2008.

Web Sites

Green Facts. “Desertification: Scientific Facts on Desertification.” January 23, 2008. http://www.greenfacts.org/en/desertification/index.htm (accessed February 27, 2008).

Oasis. “What Is Desertification?” http://www.oasisglobal.net/what_is.htm (accessed February 26, 2008).

United Nations. “Rethinking Policies to Cope with Desertification.” June 28, 2007. http://news.bbc.co.uk/2/shared/bsp/hi/pdfs/28_06_07unreportdesert.pdf (accessed February 27, 2008).

United Nations Convention to Combat Desertification. “Explanatory Leaflet.” http://www.unccd.int/convention/text/leaflet.php (accessed February 26, 2008).

Susan Aldridge

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Desertification

Desertification

Desertification is the gradual degradation of productive arid or semi-arid land into biologically unproductive land. The French botanist, André Aubreville, coined the term in 1949 to describe to the transformation of productive agricultural land in northern Africa to desert-like, uncultivable fallowland. Loss of biological and ecological viability occurs when natural variations, like extended drought related to climate change, and unsustainable human activities such as over-cultivation and poor irrigation practices, strip drylands of their stabilizing vegetation, soil nutrients , and natural water distribution systems. The earth's arid and semi-arid regions are, by definition, areas with scarce precipitation ; even very small changes can quickly destroy the fragile ecosystems and soil horizons that remain productive in areas with very little water. Desertification does not, per se, result in the development of a desert . Though desertified land and deserts are both dry, the barren, gullied wastelands left by desertification barely resemble the subtle biological productivity of healthy desert ecosystems. In some cases, careful land stewardship has successfully reversed desertification, and has restored degraded areas to a more productive condition. In the worst cases, however, semi-desert and desert lands have lost their sparse complement of plants and animals, as well as their ability to support agriculture.

Desertification is a particularly pressing social and environmental issue in regions where natural dryness and human poverty coincide. The earth's deserts and semi-arid grasslands occur in the subtropical bands between 15° and 30° north and south where extended periods of high pressure persist under the trade winds. Northern and southern Africa, the Arabian peninsula , southern Asia , the continent of Australia , southern South America and the North American Southwest lie in the subtropical zones. Desertification is usually discussed in the context of dry regions and ecosystems, but it can also affect prairies, savannas, rainforests , and mountainous habitats. Global climate change can alter the boundaries of these naturally dry regions, and change the precipitation patterns within them. Arid and semi-arid regions with large, impoverished populations of subsistence farmers, like northern Africa, or with large-scale commercial agriculture, like the American Southwest, are particularly susceptible to destructive desertification.

Sometimes desertification is the result of purely natural processes. Long-term changes in climatic conditions have led to decreased precipitation in a number of regions. The northern Sahara, for example, has experienced numerous fluctuations between arid and wet conditions over the past 10,000 years, as have the basins of the American West. Radar images collected aboard the space shuttle Endeavor show extensive river systems buried beneath more recent Saharan sands, and preserved fossil vegetation and lake shorelines suggest that forests surrounded filled lakes in northern Nevada and Utah. Cyclical atmospheric and oceanographic variations, like the El Niño phenomenon in the southern Pacific, may also trigger extended regional drought. Environmental scientists warn that anthropogenic (human-induced) global climate change could also lead to bring desertification to previously unaffected regions. Until the twentieth century, humans were able to simply move their agricultural activity away from land rendered unusable by desertification. However, rapid twentieth century population growth, and a corresponding need for high agricultural productivity, has rendered that strategy untenable.


The Sahelian drought and United Nations convention to combat desertification

Desertification first captured major international attention in the 1970s when a decade of severe drought in the Sahel region of Africa brought starvation to the impoverished populations of countries along the southern border of the Sahara Desert. Sparse stabilizing vegetation died, and the Saharan sands encroached, covering depleted agricultural land. Reduced water volume in rivers compromised irrigation and hydroelectric generation. Dust storms brought health problems and ruined equipment and buildings. Livestock died of starvation. More than 100,000 sub-Saharan Africans died of thirst and starvation between 1972 and 1984, and more than 750,000 people in Mali, Mauritania, and Niger were completely dependent on international food aid during 1974.

The United Nations convened the Conference on Desertification (UNOCD) in 1977 in Nairobi, Kenya in response to the Sahelian Crisis. The conference brought together 500 delegates from 94 countries. The delegates approved 28 recommendations for slowing or reversing desertification, with the hope of stabilizing the area of land degraded by desertification, and preventing further crises. Scientists advising the 1977 convention estimated that desertification threatened 11.6 million mi2 (30 million km2), or 19% of the earth's land surface, in 150 nations worldwide. Despite the action undertaken in the years following the UNOCD, and numerous examples of "local successes," the United Nations Environment Programme (UNEP) concluded in 1991 that the threats and effects of desertification had worsened worldwide. The 1994 United Nations Convention to Combat Desertification (UNCCD) was proposed at the 1992 the United Nations Conference on Desertification (UNCOD)in Rio de Janeiro. By March 2002, 179 nations had agreed to the UNCCD treaty which advocates a program of locallyimplemented efforts to reverse and/or prevent the desertification that directly affects 250 million people, and threatens at least one billion.

Desertification in North America

Arid lands in 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 shrubs that dominate arid-land vegetation provide good forage for cattle, but overstocking leads to overgrazing and degradation of the natural vegetation cover, which, in turn, contribute 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. Groundwater depletion by overpumping of the sandstone Ogalalla aquifer in the Southwestern United States has contributed to desertification in Nebraska, Kansas, Oklahoma and Texas. Moreover, the salts left behind on the soil surface after the irrigation water has evaporated results in land degradation through salinization, creating toxic conditions for crops and contaminating groundwater. Salination resulting from decades of heavy irrigation has compromised the soil quality in California's San Joaquin Valley, which produces much of the produce sold in the United States.

Studies of pre-industrial, aboriginal people in the western and southwestern United States suggest even small numbers of people could induce long-lasting ecological changes, including desertification. For example, native Americans reliant on mesquite beans for food planted mesquite throughout the Chihuahuan Desert of Arizona, New Mexico, Texas, and northern Mexico. Stands of mesquite developed around campsites and watering holes, and replaced the local grasses and other vegetation. The Pueblan culture, which flourished in the southwestern United States beginning around a.d. 800, used the meager stands of trees for housing material, resulting in local deforestation .


Processes of desertification

Desertification is a process of continuous, gradual ecosystem degradation, during which plants and animals, and geological resources such as water and soil, are stressed beyond their ability to adjust to changing conditions. Because desertification occurs gradually, and the processes responsible for it are understood, it can often be avoided by planning or reversed before irreparable damage occurs. The physical characteristics of land undergoing desertification include progressive loss of mature, stabilizing vegetation from the ecosystem, or loss of agricultural crop cover during periods of drought or economic infeasibility, and a resulting loss of unconsolidated topsoil. This process is called deflation. Erosion by wind and water then winnows the fine-grained silt and clay particles from the soil; dramatic dust storms like those observed during the 1930's Dust Bowl in the American mid-west, and in northern Africa, were essentially composed of blowing topsoil. Continued irrigation of desertified land increases soil salinity, and contaminates groundwater, but does little to reverse the loss of productivity. Finally, ongoing wind and water erosion leads to development of gullies and sand dunes across the deflated land surface.

The forces causing these physical changes to occur may be divided into natural, human or cultural, and administrative causes. Among the natural forces 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 have lasting consequences. As such, drought thus 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, 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. Desertification caused by human influences has a long historical record; there is evidence of such damage caused around the Tigris and Euphrates rivers in ancient Mesopotamia. Administrative influences contributing to desertification include encouragement of the widespread cultivation of a single crop for export, particularly if irrigation is required, and the concentration of dense human populations in arid lands. Poor economic conditions, like the Great Depression in United States in the 1930s, also contribute to degradation of croplands. During that crisis, American farmers were simultaneously confronted with bankruptcy and a decade-long drought, and they left millions of acres of plowed, bare cropland unplanted. According to the 1934 Yearbook of Agriculture, "Approximately 35 million acres of formerly cultivated land have essentially been destroyed for crop production.... 100 million acres now in crops have lost all or most of the topsoil; 125 million acres of land now in crops are rapidly losing topsoil."

Considering these factors together, desertification can be viewed as a process of interwoven natural, human, and economic forces causing continuous degradation over time . Therefore, ecosystem and agricultural degradation caused by desertification must be confronted from scientific, social and economic angles. Fortunately, scientists believe that severe desertification, which renders the land irreclaimable, is rare. Most desertified areas can be ecologically reclaimed or restored to agronomic productivity, if socioeconomic and cultural factors permit restoration.


Land management

Land management measures that combat desertification focus on improving sustainability and long-term productivity. Though damaged areas cannot always be restored to their pre-desertified conditions, they can often be reclaimed by designing a new state that can better withstand cultural and climatic stresses. Specific measures include developing a resilient vegetation cover of mixed trees, shrubs, and grasses suitable to local conditions that protects the soil from wind and water erosion and compaction. Redistribution of water resources, and redesign of water delivery systems, can reduce the effects of salination, groundwater depletion, and wasteful water use. Finally, limiting the agricultural demands made on drought-prone arid and semi-arid lands can be accomplished by encouraging farmers to grow drought-tolerant plants, and to move water-hungry crops, like cotton and rice , to more suitable climates.

Land management methods that halt or reverse desertification have been known in the United States since the end of the nineteenth century, but they have only recently have they been put into widespread use. United States federal policies do support soil conservation in croplands and rangelands, but only about one third of such land is under federal protection. The United States government has strongly supported the autonomy of private landowners and corporations who, in turn, have often traded sustainable land-use practices for short-term profits. Even the ravages of the dust bowl did not result in widespread anti-desertification measures.

In the developing world, particularly in Africa, where poverty and political unrest are common, progress toward mitigation of desertification and its devastating social costs has been slow. Many of the world's poorest people, who live in countries with the weakest and most corrupt governments, rely on unsustainable agriculture and nomadic grazing to subsist. Many African countries, including Niger, Mali, and Senegal, have experienced positive results with implementation of a system of local self-regulation. This strategy, encouraged by the United Nations, involves a pastoral association or a community assuming responsibility for maintaining a water source and its surrounding rangeland , while receiving free veterinary and health services. By these and similar means, cultural habits, subsistence needs, economic concerns, and ecological conservation can be addressed in a single, integrated program. Furthermore, such programs help local communities reduce their dependence on ineffective or corrupt centralized governments, and on the international aid community. Such comprehensive anti-desertification programs have been very successful on a limited scale, but progress has been slow because of the extreme poverty and sociopolitical powerlessness of the communities involved. The key to the success of any anti-desertification program is the need to adapt to local conditions, including those associated with climate, ecology , culture, government, and historical land-use.

See also Ecological integrity; Ecological monitoring; Ecological productivity; Crop rotation; Deforestation; Forestry; Land use; Water conservation.


Resources

books

Mainguet, M. Aridity: Droughts and Human Development. Springer Verlag, 1999.

Press, Frank, and Raymond Siever. Understanding Earth.Chapter 14: Winds and Deserts. New York: W.H. Feeman and Company, 2001.

Sheridan, David. Desertification of the United States. Washington, DC: Council on Environmental Quality, U.S. Government Printing Office, 1981.

other

National Atmospheric and Space Administration. "From the Dust Bowl to the Sahel." Distributed Active Archive Centers Earth Observatory. May 18, 2001. [cited October 24, 2002] <http://earthobservatory.nasa.gov/Study/DustBowl/>.

Public Broadcasting System/WGBH. "Surviving the Dust Bowl." The American Experience. 1999 [cited October 24, 2002]. <www.pbs.org/wgbh/amex/dustbowl/>.

United Nations Convention to Combat Desertification. "Text and Status of the Convention to Combat Desertification." February 14, 2002 [cited October 24, 2002]. <http://www.unccd.int/convention/menu.php>.


Marjorie Pannell
Laurie Duncan

KEY TERMS

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Arid land

—Land receiving less than 10 in (25 cm) of rainfall annually, with a high rate of evaporation and supporting a thin vegetation cover, chiefly of grasses and shrubs.

Gradualism

—A slow rate of change, implying that processes are spread out over time as well as space.

Sahel region

—A semi-arid region along the southern border of the Sahara Desert in western Africa with a dry savanna type of vegetation. In recent years this region has been encroached on by the Sahara Desert, partly as a result of poor land management.

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Desertification

Desertification

Desertification is a term used to describe the gradual changes that take place over a region or area of land that ultimately result in the formation of a desert. Although many places in the world are called deserts, scientists usually define a desert as a region or area that receives less than 10 inches (25.4 centimeters) of water from precipitation (rain, slow, sleet, or hail) during a year.

Desertification harms many people. During the late 1960s and early 1970s, desertification brought on by a severe drought (an extended period with little or no rain) in the Sahel region of Africa devastated the local agriculture of six African countries located on the southern border of the Sahara Desert. International relief measures were unable to prevent the death of thousands of people who suffered from the resulting famine (lack of food). Millions of animals that normally relied upon eating the grass that grew in the already dry region also died. The deaths of the animals resulted in further starvation of the people living in the region.

1930s U.S. Dustbowl

During the Great Depression of the 1930s, the United States and Europe suffered difficult economic times. Jobs were scare and unemployment was high. Farms failed and many farmers were forced to abandon the land that had once supported their families and communities. In some areas of the United States, people suffered from malnutrition and starvation.

In addition to economic hardships, there was also a severe drought (an extended period of time with little or no rain over an area) over large portions of the Midwest throughout the 1930s. The drought also contributed to farm failures.

The combination of lack of irrigation (the watering of land and crops), and natural drought resulted in the loss of topsoil (a process scientists call deflation of the soil). The topsoil dried it became lighter and easily blown by the wind. As a result, great windstorms of dry dust blew millions of pounds of topsoil from once fertile farms and the large portions of the American Midwest, particularly Oklahoma and Kansas, became known as the Dust Bowl.

In some areas, the amount of dust and topsoil in the air was so great that dust storms decreased the visibility to just a few feet. The dust in the air became so choking that it was life-threatening to babies, small children, the elderly, and the sick. According to the 1934 Yearbook of Agriculture, nearly 35 million acres of cultivated land (land normally plowed and watered by farmers) became desertlike and unable to support the growth of crops. More than 200 million additional acres of land also showed signs of becoming desert-like and were in the process of desertification.

An improved economy late in the decade and the return of normal levels of rainfall, reversed the process of desertification over most of the Dustbowl region.

Dry areas of North America—particularly in the southwestern and western United States—are also vulnerable to desertification. Some scientists estimate that up to nine out of ten such areas can be described as undergoing some form of desertification.

How desertification changes the environment

As the process of desertification takes place, the land loses its ability to support agriculture. Areas that were once arid (dry), but that could still support the growth of some crops or the grazing of animals, become barren as the desertification progresses. For example, grasslands may undergo desertification to become deserts.

During desertification, an area undergoes many changes, but it does not lose its ability to support crops or animals all at once. Neither does it completely lose its ability to support life. Even a desert can support the sparse growth of vegetation that live in areas with little moisture. As desertification proceeds, however, the land gradually loses its ability to support plants that require higher levels of water, and only plants that are able to live and thrive on less water survive.

The loss of water also changes the way the land responds to wind and occasional rains. Over time, desertification results in changes to the landscape that result in the formation of areas of high erosion (areas where soil and rock are lost or worn away by water) and the formation of dramatic land features (such as flat-topped mesas or buttes). As desertification continues, the topsoil (uppermost layer of soil) dries and becomes lighter. It is easily blown away by the wind is and the remaining sand or soil may form dunes (hills of sand created by wind). The wind over the region or area then shapes the dunes in very specific ways. For example, scientists can look at the pattern of dunes and tell how the winds blow throughout the year.

The loss of water over time also changes the chemistry of the land. As the land becomes drier, the concentration of salt increases in the water that does remain. If the concentration of salt become too high (a process termed salinization), the remaining water can become too salty for humans to drink or use for watering crops.

Causes of desertification

In addition to a decline in the amount of rain that falls each year, a number of other factors are important in determining whether a region or area will undergo desertification. Factors such as wind, amount of sunlight, and use of the land also influence how fast desertification takes place.

Desertification can be caused or reversed by natural forces and cycles in the climate such as rainfall and wind patterns. For example, rainshadow deserts may form in area with little precipitation because a barrier such as a mountain range causes moisture-rich winds to lose their moisture before reaching the area.

Improper use or overuse of water resources by humans can also cause desertification. If, for example, humans overuse water to water lawns, not enough water may remain available in local groundwater or aquifers support surrounding areas needing water to grow grasses that support cattle and other grazing animals. In addition, if too many animals are introduced into an area, they can overgraze by eating too much of the available vegetation. The loss of too much natural grass and vegetation can speed up the processes of land erosion, which will speed up the process of and desertification.

Other human activities such as deforestation (the over-cutting of trees) and mining may cause desertification. The introduction of new species of plants and animals not native (naturally occurring) to a region or area—especially plants or animals that may require more water than the native species already living in the area—may result in desertification.

Halting desertification

Under the right conditions, if the use of land is carefully regulated and special practices are started to conserve water, desertification can be greatly slowed or even reversed. In this way, water conservation helps keep lands productive.

In many dry regions, water conservation practices are new to the societies that live there and it remains the custom to simply abandon dry lands in search of areas with more water available to support crops and animals. As the world's population increases, however, this practice cannot continue indefinitely, and water conservation practices have become increasingly important.

Desertification can also be reversed by natural forces and cycles in the climate.

K. Lee Lerner

For More Information

Books

Burroughs, William, ed. Climate: Into the 21st. Century. New York: Cambridge University Press, 2003.

Bush, Mark B. Ecology of a Changing Planet. Upper Saddle River, NJ: Prentice Hall, 1997.

Websites

"From the Dust Bowl to the Sahel." National Atmospheric and Space Administration: Earth Observatory.http://earthobservatory.nasa.gov/Study/DustBowl (accessed on September 7, 2004).

"National Weather Service Climate Prediction Center." National Oceanic and Atmospheric Administration. http://www.cpc.ncep.noaa.gov/ (accessed on September 7, 2004).

"Surviving the Dust Bowl: The American Experience." Public Broadcasting System/WGBH.http://www.pbs.org/wgbh/amex/dustbowl/ (accessed on September 7, 2004).

"Water." National Environment Research Council Centre for Ecology and Hydrology.http://www.ceh.ac.uk/science_topics/water.htm (accessed on September 7, 2004).

"World Climates." Blue Planet Biomes.http://www.blueplanetbiomes.org/climate.htm (accessed on September 7, 2004).

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