Africa's Fickle Climate

SAN FRANCISCO--The Sahara Desert's rapid advance in northern Africa is no fluke: Scientists have found evidence that the continent's rainfall and average temperatures suddenly plunged or rose dozens of times in the last 25,000 years. The findings, reported here today at the American Geophysical Union's fall meeting, suggest that future climate changes, such as the onset of global warming, could be just as capricious.

Traditionally, scientists have believed that climate alterations occur gradually--carbon dated records of plant life in Northern Africa, for example, had indicated that a few degrees of warming there might take thousands of years. Since ocean sediments can provide a more detailed measure of climate change, paleoclimatologist Peter deMenocal and his colleagues at Columbia University's Lamont-Doherty Earth Observatory took a set of core samples from about 10 miles off the coast of Senegal.

The sediment layers provided a chronological picture going back 25,000 years. The team measured the dust in each layer--higher amounts may represent a greater number of dust storms in a relatively dry period in North Africa. And they analyzed fossils of microorganisms; a change in the assortment of plankton, for instance, can represent ocean warming or cooling.

DeMenocal and his colleagues found dozens of periods when the climate appeared to have shifted rapidly within a century. For instance, about 7000 years ago the surface temperature of the Atlantic Ocean near their sample site rose nearly 4 degrees; this, the researchers believe, drew monsoon rains away from Africa and parched the Sahara. The team found that 17,000 years ago the Sahara became wet enough to have lakes practically overnight, before drying out again 10,000 years later. "It appears that the North African climate was very binary," DeMenocal says. "It was in one mode and it would suddenly flip to another."

The findings suggest that future global warming could happen far more rapidly than people might expect, says paleoclimatologist Thomas Crowley of Texas A & M University, College Station. "It's another important piece of information that says the system can change rather abruptly," he says.

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