Sahara dried out slowly, not abruptly: study

By Alister Doyle, Environment Correspondent

OSLO (Reuters) - The once-green Sahara turned to desert over thousands of years rather than in an abrupt shift as previously believed, according to a study on Thursday that may help understanding of future climate changes.

And there are now signs of a tiny shift back towards greener conditions in parts of the Sahara, apparently because of global warming, said the lead author of the report about the desert's history published in the journal Science.

The study of ancient pollen, spores and aquatic organisms in sediments in Lake Yoa in northern Chad showed the region gradually shifted from savannah 6,000 years ago towards the arid conditions that took over about 2,700 years ago.

The findings, about one of the biggest environmental shifts of the past 10,000 years, challenge past belief based on evidence in marine sediments that a far quicker change created the world's biggest hot desert.

"The hypothesis (of a sudden shift) was astonishing but it was still taken up," said Stefan Kropelin of the University of Cologne in Germany, lead author of the study with scientists in Belgium, Canada, the United States, Sweden and France.

The scientists, studying the remote 3.5 sq km (1.4 sq mile) Lake Yoa, found the region had once had grasses and scattered acacia trees, ferns and herbs. The salty lake is renewed by groundwater welling up from beneath the desert.

A gradual drying, blamed on shifts in monsoon rains linked to shifts in the power of the sun, meant large amounts of dust started blowing in the region about 4,300 years ago. The Sahara now covers an area the size of the United States.

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