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New Clue to Sahara's Origins

Could a tiny change in the angle of the Earth's orbital axis trigger a cascade of events that turned an ancient Eden into the Sahara desert? Yes, says a report in the 15 July issue of Geophysical Research Letters.

Studies of fossilized pollen have shown grasses and shrubbery covered what is now the Sahara desert until some unknown environmental catastrophe dried up all the water, leaving nothing but sand. The exact timing is uncertain, but one interpretation of the pollen data suggests that a relatively mild arid episode between 6000 and 7000 years ago was followed by a severe 400-year drought starting 4000 years ago. Such a disaster might have driven entire civilizations out of the desert, leading them to found new societies on the banks of the Nile, the Tigris, and the Euphrates rivers. But the cause of the postulated droughts remained a mystery.

Now, climatologist Martin Claussen and co-workers at the Potsdam Institute for Climate Impact Research in Germany are proposing that the changing tilt of the Earth triggered the rapid drying of the Sahara. Like a spinning top slowly wobbling on its tip, the Earth's tilt has decreased from 24.14 degrees to 23.45 degrees in the last 9000 years, resulting in cooler summers in the Northern Hemisphere. When Claussen introduced cooler Northern summers into a computer simulation of the Earth's atmosphere, ocean, and vegetation, the monsoon storms that provide water to the Sahara grew weaker, killing off some of the native plants. The initial reduction in vegetation further reduced rainfall, says Claussen, starting a vicious cycle of desertification that began to accelerate about 4000 years ago. Less than 400 years later, Claussen's team found, the drought caused by the vegetation-feedback mechanism could have wiped out almost all plant life in the desert.

"This is a very exciting result," says climatologist John Kutzbach of the University of Wisconsin, Madison. This is the first time anyone has demonstrated that a change in the Earth's tilt can cause a sudden vegetative feedback, he says. "It opens up a whole new class of research problems involving the biosphere," such as predicting feedback effects in global warming.