MADISON -- Water would seem like a mirage today in the sweltering Sahara Desert, but climate researchers at the University of Wisconsin-Madison are finding the ancient Sahara was a wetter, greener place than ever imagined.
Writing in today's (Dec. 19) edition of the British journal Nature, UW-Madison climatologist John Kutzbach and colleagues report that the Sahara and Sahel regions of northern Africa were much wetter 12,000 to 5,000 years ago than earlier climate models predicted.
Decades ago, climate researchers first began collecting evidence of this strikingly different landscape in the Sahara. They found gigantic dried-up lake beds in the parched desert, and their shallow depressions held fossils of fish and snails. Archaeologists discovered old fishing villages on the fringes of these ancient lakes. And fossilized pollen was discovered from plants that today exist hundreds of miles away.
A slight shift in the earth's orbit forced those changes, causing stronger summer monsoons to sweep through the region. This naturally produced more vegetation and increased water content in the soil.
But the new study by Kutzbach and colleagues finds an even greater change than previous estimates. The vegetation and soil changes during that period actually enhanced the effects of the orbital shift, increasing the annual precipitation by as much as 10 percent. The increase occurred because the vegetation and soil encouraged water retention and recycling, rather than water loss by run-off, he said.
Combined with the orbital changes, Kutzbach estimates the southern Sahara received about 25 percent more rainfall than it does today.
"All of this points to the fact that you can1t ignore the role of vegetation in forcing its own changes in climate," Kutzbach said. "We're just beginning to recognize that changes in land cover are of major importance for climate and need to be included in climate models."
The work, supported by the National Science Foundation, was carried out on powerful supercomputers at the National Center for Atmospheric Research (NCAR) in Boulder, Colo. Co-investigators in the study are Gordon Bonan of NCAR, Jonathan Foley of UW-Madison, and Sandy Harrison of Lund University in Sweden.
This new attention to changes in the Earth1s biota is shedding more light on the magnitude of climate change, Kutzbach said. A recent UW-Madison study found that shifts between forest and tundra in the far north amplified climate changes. In another study, widespread losses in tree cover in the Arctic could have helped trigger the Ice Age 115,000 years ago.
Kutzbach said the better understanding of land cover changes will also help better predict future climate change. "One cannot simply look at global warming and the resulting impact on rainfall and temperatures," he said. "We must also consider what1s happening on the ground -- the changes in soil and vegetation -- that could add to the climate changes."
The Sahara of 12,000 to 5,000 years ago, known as the Holocene period, was vastly different from today, Kutzbach said. For example, Lake Chad, currently the desert1s largest lake, was massive in that period, covering a region roughly the size of the current Caspian Sea. And the grassland region that now borders the fringes of the Sahara once extended 300 miles further into the desert.
The research team has explored two of three major historical changes in the Earth1s terrain: tundra replaced by forest in the Arctic, and desert replaced by grasslands in the tropics. Kutzbach said they will turn attention in future studies to the mid-continent region of North America, which shifted from prairie to forest more than 6,000 years ago. Kutzbach said they will explore what climate changes that shift produced in Wisconsin and the Midwest.
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-- Brian Mattmiller, (608) 262-9772, (bsmattmi@facstaff.wisc.edu)