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Climate-simulation models indicate substantial future increases in soil erosion. Desertification will be exacerbated by reductions in average annual rainfall and increased evapotranspiration, especially in soils that have low levels of biological activity, organic matter, and aggregate stability. Desertification and climate can form a "feedback loop," with the loss of vegetation caused by desertification reducing carbon sinks and increasing emissions from biodegrading plants.

Soil degradation begins with removal of vegetation. Unprotected, dry soil surfaces are readily eroded by rain and wind, leaving infertile lower soil layers that bake in the sun and become an unproductive hard-pan. Sand dunes may form where the blown surface material accumulates. Water is a defining constraint of the drylands. Drought avoidance and coping strategies are imperative, such as choosing drought-tolerant crops, low plant densities, water conservation, and water harvesting. While water shortage is a constant concern, much of the water that is available is not effi ciently used. In the Sahel, degraded soils often exhibit impeded water infiltration, so much is lost as runoff.

Given the uncertainty in the models, some models still project other outcomes, such as the regreening of the Sahel zone. Nevertheless, a loss of vegetative productivity can lead to long-term declines in agricultural yields, livestock yields, plant standing biomass and plant biodiversity, changes that reduce the ability of the land to support people. While before, even in adverse areas people in drylands were able to cope with the cycles of droughts without depleting soils, now, with the extension of droughts due to climate change, these systems became vulnerable to breakdown, often sparking an exodus of rural people to urban areas. Breaking the strong connection of people to the land produces profound changes in social structure, cultural identity, and political stability.

Desertification also impacts remote areas, and has silted-up rivers and retreated lake surfaces far away from its origin. Since the early 1950s, 670,000 ha. of arable land and 2.35 million ha. of rangeland, steppe, and grassland were invaded by shifting sands. Dust storms created as a result of destroyed vegetative cover led to air quality problem, and acid, rain elsewhere.

SEE ALSO: Agriculture; Deforestation; Deserts; Rain; Rainfall Patterns.

BIBLIOGRAPHY. A. Monem Balba, Management of Problem Soils in Arid Ecosystems (CRC Press, 1995); IPCC 2007, CLIMATE Change 2007—The Physical Science Basis. Contributions of Working Group I to the Fourth Assessment Report of the IPCC, (cited November 2007); M.K.B. Lüdeke, G. Petschel-Held, H.J. Schellnhuber, "Syndromes of Global Change: The First Panoramic View," GAIA (v.13/1, 2004); Maryam Niamir-Fuller, ed., Managing Mobility in African Rangelands. The Legitimization of Transhumance (Intermediate Technology Publications Ltd., 1999); L.R. Oldeman, Soil Degradation: A Threat to Food Security? Report 98/01 (International Soil Reference and Information Centre, 1998); L.R. Oldeman, et al., World Map of The Status of Human Induced Soil Degradation: An Explanatory Note (International Soil Reference and Information Centre and United Nations Environment Programme, 1991); J. Skuijns, Semiarid Lands and Deserts: Soil Resource and Reclamation (Marcel Dekker, 1991); United Nations Environment Programme, (cited November 2007); S. Uriel, et al., Drylands Systems. Millennium Assessment, Condition and Trends Vol. III (Millennium Assessment, 2005); Walter Whitford, Ecology of Desert Systems (Elsevier, 2002); M. Winslow, et al., Desertification, Drought, Poverty and Agriculture: Research Lessons and Opportunities (International Center for Agricultural research in The Dry Areas, International Crops Research Institute for the Semi-Arid Tropics, and the UNCCD Global Mechanism, 2004).

Ingrid Hartmann Independent Scholar

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