The United Nations Convention to Combat Desertification (UNCCD) defines desertification as "land degradation in arid, semi-arid, and dry subhumid areas resulting from various factors, including climatic variations and human activities" (United Nations, 1994). Furthermore, UNCCD defines land degradation as a "reduction or loss, in arid, semi-arid, and dry subhumid areas, of the biological or economic productivity and complexity of rain-fed cropland, irrigated cropland, or range, pasture, forest, and woodlands resulting from land uses or from a process or combination of processes, including processes arising from human activities and habitation patterns, such as: (i) soil erosion caused by wind and/or water; (ii) deterioration of the physical, chemical, and biological or economic properties of soil; and (iii) long-term loss of natural vegetation."
Arid, semi-arid, and dry subhumid 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 (see Figure 10-9), or 43% of the continent's land areaon which 270 million people, or 40% of the continent's population, live (UNDP, 1997). Areas particularly at risk include the Sahela 3.5 million km2 band of semi-arid lands stretching along the southern margin of the Sahara Desertand some nations that consist entirely of drylands (e.g., Botswana and Eritrea). The death of as many as 250,000 people in the Sahel drought of 1968-1973 (UNCOD, 1977) demonstrated the tragic human toll of desertification.
Figure 10-9: Aridity zones for Africa as derived from mean monthly precipitation and potential evapotranspiration surfaces included on Spatial Characterization Tool for Africa CD-ROM (UNDP, 1997).
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).
Unfortunately, the relative importance of climatic (see Section 10.2.6.3) and anthropogenic (see Section 10.2.6.2) factors in causing desertification remains unresolved. Some scientists have judged that anthropogenic factors outweigh climatic factors (Depierre and Gillet, 1971; Lamprey, 1975; Le Houérou, 1989; Westing, 1994), though others maintain that extended droughts remain the key factor (Mortimore, 1989; Hoffman and Cowling, 1990; Tucker et al., 1991; Dodd, 1994). CO2-induced climate change and desertification remain inextricably linked because of feedbacks between land degradation and precipitation (see Section 10.2.6.4).
Unsustainable agricultural practices, overgrazing, and deforestation constitute the major anthropogenic factors among the forces 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 1998a 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.
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