for National Geographic News
The grassy prehistoric Sahara turned into Earth's largest hot desert more slowly than previously thought, a new report says—and some say global warming may turn the desert green once again.
The new research is based on deposits from a unique desert lake in remote northern Chad.
Lake Yoa, sustained by prehistoric groundwater, has survived for millennia despite constant drought and searing heat.
The body of water contains an unbroken climate record going back at least 6,000 years, said study lead author Stefan Kröpelin of the Institute of Prehistoric Archaeology at the University of Cologne in Germany.
Ancient pollen, insects, algae, and other fossil clues preserved in the lake's sediments point to a gradual transformation to a desert environment.
(See photos of the Sahara today.)
The study contradicts past research that suggested the region dried up within a few hundred years. That research was based on windblown Saharan dust found in Atlantic Ocean sediments.
"This was a hypothesis used by most of the modelers and many of the scientific community who were not working themselves in the Sahara," Kröpelin said.
"To a large degree we can now show that such an abrupt drying out of the Sahara was a myth," he said.
The new study, which appears tomorrow in the journal Science, instead found evidence for a slow decline in tropical plants, followed by the gradual loss of savanna-type grasslands, and then the eventual spread of desert species.
Pollen samples revealed, for example, that the decrease in tropical trees accelerated after 4,800 years ago, while desert plants took root between 3,900 and 3,100 years ago.
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