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

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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.