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water would seem like a mirage in the sweltering Sahara desert today, but climatologist John Kutzbach of the University of Wisconsin, Madison, us , reports that the Sahara and Sahel regions of northern Africa were much wetter and greener.
The Sahara, in the past 5,000 to 12,000 years (Holocene period) was vastly different from what it is today. Lake Chad, presently the desert's largest lake, was massive in that period, covering a region roughly the size of the current Caspian Sea. The grassland region that now borders the fringes of the Sahara once extended 300 miles further into the desert.
Researchers have collected evidences of this strikingly different landscape in the Sahara. They found gigantic dried-up lake beds in the parched desert, whose shallow depressions held fossils of fish and snails. Archaeologists discovered old fishing villages on the fringes of these ancient lakes. Fossilised pollen of plants that today exist hundreds of miles away were also discovered. According to the present belief, a slight shift in the Earth's orbit forced these changes, causing stronger summer monsoons to sweep through the region.
But the study by Kutzbach found a greater change. The vegetation and soil changes during that period actually enhanced the effects of orbital shift, increasing the annual precipitation by as much as 10 per cent. The increase occured because the vegetation and soil encouraged water retention and recycling, rather than water loss by run-off.
Combined with the orbital change -- Kutzbach estimates -- southern Sahara received about 25 per cent more rainfall than it does today. All this points to the fact that the role of vegetation in effecting climate change cannot be ignored. Kutzbach points out, "Better understanding of landcover changes help predict future climate change. One simply cannot look at global warming and the resulting impact on rainfall and temperatures; one must also consider what is happening on the ground -- the changes in soil and vegetation -- that could add to climate change."