Some regions are particularly vulnerable to reduced precipitation. A significant part of the African continent is dry. The climate trend is longer periods of drought, shorter periods of heavy rain. Arid, semiarid, and dry sub humid areas include those lands where the ratio of precipitation to potential evaporation (PET) ranges from 0.05 to 0.65. In Africa, these conditions cover 13 million km2 or 43% of the continent's land area, where 270 million people, or 40% of the continent’s population, live (UNDP, 1997).
Areas particularly at risk include the Sahel – a 3.5 million-km2 band of semi-arid lands stretching along the southern margin of the Sahara Desert – and some nations that consist entirely of dry lands (e.g., Botswana and Eritrea). Desertification in Africa has reduced by 25% the potential vegetative productivity of more than 7 million km2, or one-quarter of the continent's land area (UNEP, 1997). Desertification consists more of degradation of the productive capacity of patches well outside open sand deserts rather than the inexorable encroachment of open sand onto greenlands. Arid lands can respond quickly to seasonal fluctuations. Indeed, analysis of 1980-1990 NDVI data to track the limit of vegetative growth along the Sahara-Sahel margin revealed wide fluctuations.
The 1990 limit of vegetative growth lay 130 km south of its 1980 position (Tucker et al., 1991). Unsustainable agricultural practices, overgrazing, and deforestation constitute the major anthropogenic factors that drive desertification. Unsustainable agricultural practices include short rotation of export crops, undisciplined use of fire, and removal of protective crop residues. Overgrazing consists of running livestock at higher densities or shorter rotations than an ecosystem sustainably can support. Finally, deforestation consists of permanent clearing of closed-canopy forests and cutting of single trees outside forests.
Forest area in Africa decreased by approximately 37,000-km2 yr-1 from 1990 to 1995 (FAO, 1999a). UNEP (1997) attributes two-thirds of the area already desertified in Africa to overgrazing and the remaining third to unsustainable agricultural and forestry practices. Population growth ultimately can drive desertification if it intensifies agrosylvopastoral exploitation or if it increases the land area subjected to unsustainable agricultural practices, overgrazing, or deforestation. The total population of Africa grew from 220 million in 1950 to 750 million in 1998-a rate of 2.5% yr- 1 (United Nations, 1999). Increasing food, wood, and forage needs accompanying this growth place an inordinate burden on the region's natural resources.