Six thousand years ago, hippos lounged in lush land that is now the Sahara, and Lake Chad was as large as the Caspian Sea is today. But just how yesterday's savannas became today's sand dunes has stumped scientists for decades. Now new research, reported in today's issue of Science, suggests that wiggles in Earth's orbit may have dried up the monsoons that once watered the region.
Seasonal changes warm and cool the continents, but leave the oceans relatively undisturbed. The resulting temperature differences between land and sea drive monsoons--steady winds that change direction twice a year. Summer monsoons bring Northern Africa most of its rain during a 90-day stretch.
But climate modelers could never get the monsoons to bring enough water to support the level of vegetation that paleorecords show existed in the region 6000 years ago. Climatologists John Kutzbach and Zhengyu Lui of the University of Wisconsin, Madison, however, thought the precession of Earth's axis might be the missing ingredient. Earth spun closer to the sun during the Northern Hemisphere summers 6000 years ago, causing an estimated 5% more solar radiation to bathe Northern Africa than it receives today. The warmer climate overall, the researchers say, could have warmed the North Atlantic sea, drawing wet monsoon rains up from lower latitudes.
To test the hypothesis, Kutzbach and Lui ran an ocean model that responded to the increased radiation, then fed the revised ocean temperatures into an atmosphere model. The results, remarkably, predicted just the amount of rain--a 25% increase over previous models--and the places it fell to match the historical records. The Sahara began to dry up about 5000 years ago.
David Rind, a climate modeler at the NASA Goddard Institute for Space Studies in New York City, says the monsoon mechanism is sound, but warns that a more integrated climate model may give different results. "The atmosphere and the ocean are both complex dynamical systems," he says, noting that computers are still not up to the task of handling them both at the same time.