More re drying of the Sahara

Scientific American

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.

New York Times

Lake Yoa, in northeastern Chad, has remained a lake through the millennia 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 sediment in a core drilled from the bottom of this lake, an international team of scientists has reconstructed the region’s climate as the savannah changed to Sahara.

In Friday’s issue of the journal Science, the researchers, led by Stefan Kröpelin, 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 surrounding terrain shifted from trees to shrubs to grasses to sand — “where today you don’t find a single piece of grass,” Dr. Kröpelin 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 of Africa, 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.