Several thousand years were needed so that the Sahara becomes such as it is known today. It is what reveals the analysis of sediments taken in the bottom of the lake Yoa, in Chad, by Stefan Kröpelin of the university of Cologne (Germany). 6 000 years ago, the Sahara was covered with lakes and with a tropical vegetation. But the stopping of the rains of the Atlantic monsoons transformed this “green Sahara” into hyperarid desert. Until now, the only available data on this transition came from an oceanic carrot performed off Mauritania. She pointed out a violent increase of the fluxes of continental dusts, interpreted as the installation of the desert in some dozens years only. A hypothesis which these jobs on the lake Yoa refute.“ There was not big environmental crisis where everything would have roughly been destroyed, explains Ann - Mary Lézine (NATIONAL CENTRE FOR SCIENTIFIC RESEARCH). The answer of ecosystems of the Earth to the climatic confusion of the end of humid period was progressive. Tropical kinds disappeared 4 250 years ago and desert conditions set up long afterwards, 2 500 years ago.”

Picture: © Denis Delbecq - on 2007