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The transformation of the Sahara from a tropical grassland to the desert it is today was a slow process which took thousands of millennia, new research reveals, the findings contradict previous work which suggested that the switch from a 'green' to a 'brown' Sahara was a rapid process which occurred over just a few centuries.

The Sahara became a tropical grassland dotted with lakes around 14,800 years ago thanks to a strengthening of the monsoon. Analyses of marine sediments taken off the western coast of the Sahara seemed to suggest that the Sahara dried out rather suddenly some 5,500 years ago. This rapid change in the environment was attributed to positive feedback processes between rainfall patterns and vegetation changes.

In this latest study, an international team of scientists led by Dr Stefan Kr�¶pelin of the University of Cologne in Germany studied sediments from Lake Yoa in northern Chad. Unlike most other lakes in the region, Lake Yoa has never dried out as it is fed by an underground aquifer. Every summer and winter for the last 6,000 years, a layer of sediment has been deposited on the lake bed.

By studying the geochemical composition of these sediments, as well as the remains of plants and animals found in them, the scientists were able to reconstruct a detailed, high resolution record of the Saharan environment over the last 6,000 years. Their findings are published in the latest edition of the journal Science.

The results show that the drying out of the Sahara was a gradual process occurring between 5,600 and 2,700 years ago in response to a gradual decline in the strength of the monsoon. As the rains decreased, the trees and plants of the tropical grassland disappeared and were replaced with typical Sahel vegetation. Eventually, the grass cover also disappeared, and plants specialised for desert survival moved in.

The findings are important because until now, the case of the drying out of the Sahara has been held up as an example of how quickly environments can change if certain tipping points are reached.

Furthermore, the scientists emphasise that information on the speed of these changes is crucial for understanding the interaction between tropical and mid-latitude weather systems.

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http://www.sciencemag.org
http://www.uni-koeln.de