A new theory that explains how did Sahara became desert

Sahara has become the largest desert on the planet 2,700 years ago, after a very slow transformation, according to a study that call into question a theory issued in 2000, according to which it was sudden desertification, and has been published in the journal Science.
 
6,000 years ago, the Sahara was very green, covered with trees, savannahs and many lakes. Region, which is larger than Australia was inhabited by people, say the authors. Most of the physical clues that could provide information about the evolution of geographic Sahara disappeared. However, studying the layers of sediment collected from the bottom of one of the largest lakes that exist in Sahara, Yoa, located in a region of northern Chad, the team of European researchers, Canadians and Americans could reconstruct the history of the region in the last 6,000 years. Experts have analyzed sediments, geochemical tests were conducted and examined biomarkers, such as pollen from plants that could be found all over the area to settle before dessert. Researchers analyzed the remains of aquatic organisms.

The results contradict the theory that the Sahara became a desert 5,500 years ago, in a few centuries, marking the end of the African humid period when was a regular seasonal rainfall region, explained Stefan Kröpelin, a geologist at the Institute of Prehistoric Archaeology at the University of Cologne, Germany, lead author of the study. In 2000, analysis of sediments obtained from drilling conducted off the coast of western Mauritania indicated a sharp increase in the amount of dust carried by winds blowing outward Saharan Africa, according to a study by Peter deMenocal from Columbia University, New York.

Data collected from Lake Yoa shows the opposite, namely that the transition was a gradual climatic said Kröpelin. Abundant tropical vegetation gradually decreased, then disappeared grass ground cover, then install the desert, he explained. The German geologist Peter challenge deMenocal data collected, but believes that it was not interpreted correctly. Lake Yoa, with a depth of 24 meters, continue to be supplied with water from underground tanks that were filled during wet Sahara, which began about 15,000 years ago. Water reserves are rich enough to replace the six meters of water lost by evaporation from the lake annually, say researchers who add that the annual rainfall does not exceed a few millimeters per sqm. Sahara desertification resulted in the expulsion of the southern populations of North Africa and could be responsible for arrival of Egyptian civilization of the Pharaohs, researchers say.