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 (CO ² ) 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): 1043–1048.

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