Washington, May 9 : A new report has suggested that the grassy prehistoric Sahara turned into Earth's largest hot desert more slowly than previously thought, and the fact that global warming may turn it green once again.

According to a report in National Geographic News, the new research is based on deposits from Lake Yoa, 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, according to study lead author Stefan Kropelin 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.

About 20 feet (6 meters) of water evaporate from the lake every year, which is equivalent to the annual water consumption of about a million people.

The new study contradicts past research that suggested the Sahara region dried up within a few hundred years. That research was based on windblown Saharan dust found in Atlantic Ocean sediments.

The new study 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.

Sand particles in the lake show that fierce desert winds didn't start picking up until about 3,700 years ago, the study found.

The only rapid change noted was in the lake itself, which switched from a freshwater to a salt lake between 4,200 and 3,900 years ago.

According to Kropelin, the transformation happened exactly in the time period when monsoon rains began moving away to the south.

This meant there was no longer surface water flowing in to counter salinity caused by evaporating water.

"To a large degree we can now show that such an abrupt drying out of the Sahara was a myth," said Kropelin.

As for global warming turning the Sahara green once again, Kropelin said that since 1988, there has been a strong indication of a return of increasing rains in the eastern Sahara.

"Already in some areas you can see slight changes in the vegetation," he said.