Read at : Google Alert – desertification
http://quiethands.blogspot.com/2008/02/sahel.html
Around 20 000 years ago, the Sahara had a similar extension to today. But during the subsequent wet period, it contracted to small surviving areas of North Africa. To the south, major river systems developed with extensive areas of fine sediments like those of the Niger Inland Delta and Lake Chad. The many mobile dunes were consolidated by the growth of natural vegetation. From around 3000 BC, however, the climate in the area of the northern tropic changed and the land reverted to its arid state. The Sahara expanded again. On its southern fringe, the current Sahel Zone, there have since been regular periods of drought.
The ecosystems of the savannas and the traditional forms of human economic activity have adapted to these climatically induced droughts, which have led to periodic desert-like phenomena in one region after another. The vegetation and the human and animal populations have recovered time and again from these dry periods, which have generally lasted for several years.
Desertification
The Sahel is the transition zone between the Sahara desert and the more humid tropical regions of Africa. Sahel comes from the Arabian “sahil”, meaning “edge” or “coastline”. The Sahel is the northern part of the transitional climatic zone with a precipitation of 150 to 450 mm and 8 to 10 months without any rainfall. The area ranges from the Atlantic Ocean across Africa to the Red Sea north of Ethiopia. The width varies from 300 to 500 km. As regards vegetation, the Sahel consists of semi-desert in the north and the Sahel savanna in the south. In this area the plant life is predominantly briars, shrubs and single trees, mostly Acacia albida and Acacia senegal. More vegetation grows after rainfall, but the plants very soon wither in the desiccated soil.The biggest problem of the Sahel region is desertification; the expansion of deserts into natural barriers, the savannas and steppes. There are several reasons for this. In past years there have been huge fluctuations in the amount of precipitation, with severe droughts (1912-15, 1941-42, 1968-74, 1983-85). The vegetation cover disappears and, without natural barriers, the desert moves forward – assisted by the North-East Trade Wind, which also dries up the ground. Even if there were more rain, the sandy ground is hard to cultivate again. An even more destructive contribution to the desertification is the increasing population in this sensitive area. Modern technologies allow for the construction of deeper and more efficient wells. This encourages the stock farmers to maintain more and more livestock. This stock then needs more and more pasture and the water table goes down. Shrubs and trees die off and the ground desiccates. The wind then blows away the agriculturally important soil components, leaving sandy and rocky ground.
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from http://www.eduspace.esa.int/Background/default.asp?document=532
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DESERTIFICATION 