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
10.2.6.2. Nonclimatic Driving Forces of Desertification
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.