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The Sahelian drought that began in 1968 was responsible for the deaths of between 100,000 and 250,000 people, the disruption of millions of lives, and the collapse of the agricultural bases of five countries (photograph by Daniel Stiles, UNEP). |
The world's great deserts were formed by natural processes interacting over long intervals of time. During most of these times, deserts have grown and shrunk independent of human activities. Paleodeserts, large sand seas now inactive because they are stabilized by vegetation, extend well beyond the present margins of core deserts, such as the Sahara. In some regions, deserts are separated sharply from surrounding, less arid areas by mountains and other contrasting landforms that reflect basic structural differences in the regional geology. In other areas, desert fringes form a gradual transition from a dry to a more humid environment, making it more difficult to define the desert border.
These transition zones have very fragile,
delicately balanced ecosystems. Desert fringes often are a mosaic of
microclimates. Small hollows support vegetation that picks up heat from
the hot winds and protects the land from the prevailing winds. After
rainfall the vegetated areas are distinctly cooler than the surroundings.
In these marginal areas, human activity may stress the ecosystem beyond
its tolerance limit, resulting in degradation of the land. By pounding the
soil with their hooves, livestock compact the substrate, increase the
proportion of fine material, and reduce the percolation rate of the soil,
thus encouraging erosion by wind and water. Grazing and the collection of
firewood reduces or eliminates plants that help to bind the soil.
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Overgrazing has made the Rio Puerco Basin of central New Mexico one of the most eroded river basins of the American West and has increased the high sediment content of the river (photograph by Terrence Moore). |
This degradation of formerly productive land-- desertification--is a complex process. It involves multiple causes, and it proceeds at varying rates in different climates. Desertification may intensify a general climatic trend toward greater aridity, or it may initiate a change in local climate.
Desertification does not occur in linear, easily mappable patterns. Deserts advance erratically, forming patches on their borders. Areas far from natural deserts can degrade quickly to barren soil, rock, or sand through poor land management. The presence of a nearby desert has no direct relationship to desertification. Unfortunately, an area undergoing desertification is brought to public attention only after the process is well underway. Often little or no data are available to indicate the previous state of the ecosystem or the rate of degradation. Scientists still question whether desertification, as a process of global change, is permanent or how and when it can be halted or reversed.
Information provided by A . S . Walker
http://pubs.usgs.gov