SAHARA SAVANNA?: The sediment deposits in northern Chad's Lake Yoa, pictured here, reveal how the once-green Sahara became a dusty desert.
STEFAN KROPELIN

In a finding that may help scientists better predict the pace of climate change, research published in Science shows how the Sahara Desert, a region as big as the U.S. that stretches from the Atlantic Ocean to the Red Sea across northern Africa, went from bountiful to bone-dry over a period of several thousand years.

Scientists peered into the Sahara's verdant past by analyzing sediment samples drilled out of the bottom of one of the desert's last living lakes. The samples revealed long-held secrets of how desert-friendly species replaced tropical plants and animals as monsoon rains retreated farther south into the continent.

Finding such a detailed archive in a place as desolate as the Sahara was quite unexpected. "It's the only record of its kind," says study co-author James Russell, an assistant professor of geological sciences at Brown University.

The researchers found bits of pollen, algae, insects, sand and minerals in the samples from Lake Yoa in northern Chad, which they determined had accumulated into thin layers over the past 6,000 years.

"It's like looking at tree rings," says lead study author Stefan Kröpelin, a geoarcheologist at the Institute of Prehistoric Archaeology at the University of Cologne in Germany. "These layers give us detailed seasonal information, and it is even possible to determine which year a species was introduced."

The initial greening of the Sahara occurred about 10,500 years ago and it stayed lush for several millennia. (Cave paintings of crocodiles and giraffes left by prehistoric humans attest to the once savannalike climate.)

Like slides in a presentation, the sediment layers reveal how these humid conditions changed and reduced Yoa to an isolated oasis. Tropical plants and evergreen shrubs, still plentiful about 5,500 years ago, began to decline as the area dried out over the next 1,000 years. The desiccation continued, and, by 700 B.C., mostly desert flora like the hardy acacia tree dotted the now-parched landscape.

This gradual shift in the Sahara's overall climate contradicts a common theory that the region dried rapidly over a few hundred years, and provides clues about a potential re-greening triggered by global warming, Kröpelin says.

The data also confirms that a drop in rainfall was the major reason the area turned into a dusty badland. Based on the sediment samples, the researchers determined that Yoa's waters suddenly grew quite salty about 4,000 years ago. They speculate the salinity spiked because the streams that had previously drained salt out of the lake vanished as rainfall lessened. This abrupt event shows how a relatively small occurrence—such as a slight slide in rainfall—may have a tremendous impact.

"It's like turning a dial for the rain, but flipping a switch for the lake," says Richard Alley, a professor of geosciences at Pennsylvania State University who was not involved in the study.

This crossing of such a threshold, which could result in rapid changes in a region's climate through a series of local events, has stoked fears of spreading deserts in the Sahel, a semiarid region just south of the Sahara. Scientists worry that a runaway "positive feedback" loop, in which one event reinforces or strengthens the next, is already taking place, with fewer plants leading to less rain, leading to still fewer plants, and so on.

Kröpelin says that the new findings will help climatologists fine-tune their computer models—which he says were wrong about what happened to the Sahara—to more accurately predict the effect of global warming.