Sahara’s metamorphosis from savannah to desert

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.

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.)

Sahara Myth

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.

First Reliable Record

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, Kröpelin noted.

"No team had ever succeeded in getting geological and paleoclimate information for the past 4,000 years since practically all the lakes had dried up, so there were no more geological archives available," he said.

The Lake Yoa data represent the first "reliable and high-resolution record" in the Sahara for verifying climate models, he added.

Such checks are important, he argues, "because if climate computer models don't work for the past, they probably won't work for the future."

Understanding climatic effects in the Sahara are especially important, since the region covers an area larger than the United States, Kröpelin said.

"Climate evolution in the Sahara reflects to a very large extent climate evolution on the African continent and beyond," he added.

Six thousand years ago, northern Africa was a place of trees, grasslands, lakes and people. Today, it is the Sahara – a desolate area larger than Australia.

Lake Yoa, in northeastern Chad, has remained a lake through the millenniums and is still a lake today, surrounded by hot desert. Although little rain falls, Lake Yoa's water is replenished from an underground aquifer.

By analyzing thousands of layers of sediment in a core, which is a column of sediment drilled from the lake bottom, a team of scientists has reconstructed the region's climate as the savannah changed to the Sahara.

In the journal Science, the researchers, led by Stefan Kroepelin, a geologist with the Institute of Prehistoric Archaeology at the University of Cologne in Germany, report that the climate transition occurred gradually.

In particular, the changing types of pollen that fell on the water and drifted to the bottom tell a story of how the terrain shifted from trees to shrubs to grasses to sand – “where today you don't find a single piece of grass,” Kroepelin said.

The findings run counter to a prevailing view that the change happened abruptly, within a few centuries, about 5,500 years ago, marking the end of the “African Humid Period” when monsoon rains poured down on the region.

That view arises from ocean sediment cores drilled off the coast, to the west of Mauritania. In 2000, analysis of the cores by researchers led by Peter B. deMenocal of Columbia University's Lamont-Doherty Earth Observatory showed a sudden rise in the dust blown off Africa at that time.

Kroepelin did not dispute the ocean core data but said it had been “over-interpreted.”

Data about what was happening on land is sparse, because blowing sands do not preserve a clear geological record the way lake sediments do. But at Lake Yoa, the water that filled underground aquifers during the humid period, which began 14,800 years ago, is still flowing into the 60-foot-deep lake. It is enough to offset the 6 meters of water that evaporate out of the lake every year, Kroepelin said. Only a few millimeters of rain fall a year.

Kroepelin said he hoped to return to Lake Yoa next year to drill a core that could trace the climate history back 12,000 years.

DeMenocal praised Kroepelin's research. “I think it's a very good body of work,” he said. “It's really the only thing of its kind from the arid interior.” But he said he wondered whether the pollen might have come mostly from the area immediately by the lake and not the larger Sahara.

Jonathan A. Holmes, director of the Environmental Change Research Center at University College London, said both sets of research had been carefully conducted, and the challenge would be to put together a more complex history of the area's climate.

“I don't think either record is somehow wrong,” said Holmes, who wrote a commentary accompanying the article in Science. “I think what they are representing are slightly different things.”

Holmes said one possibility was that the offshore dust might reflect a drop in water levels around Lake Chad, revealing more dust-producing soil.

However fast the drying occurred, it pushed people out of north-central Africa, deMenocal said, and that climatically forced migration might have led to the rise of the Pharaohs and Egyptian civilization.