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.