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Paleoclimatology
Climate spawned mass migration from Africa
 I. Castaneda, University of Minnesota |
| Researchers recently pulled up a 400-meter-long core from the bottom of Africa’s Lake Malawi that shows severe megadroughts occurred between 135,000 and 75,000 years ago. A team member of the Malawi Scientific Drilling Project shows an example of a lake sediment core to a group of Malawian secondary school students. |
Many scientists think that modern humans ventured out and colonized the world from a single region in Africa, with some studies tracing our ancestors to a single individual who lived in Africa before 130,000 years ago (see Geotimes, September 2005). But why people left the continent has so far been a mystery. A new study of East Africa’s ancient climate now reveals that lush tropical Africa suffered from severe megadroughts between 135,000 and 75,000 years ago, which may have forced early humans to look for greener pastures. The findings also illuminate factors that influenced the region’s ancient climate.
A team led by Christopher A. Scholz of Syracuse University in New York drilled sediment cores extending down to about 400 meters below the bottom of Lake Malawi, one of the world’s deepest and oldest inland seas that spreads between Mozambique, Tanzania and Malawi. The cores harbor a large variety of clues about the region’s environmental history, ranging from pollen of plants that bordered the lake to the remains of various lacustrine species and phytoplankton. By analyzing this evidence and comparing it with similar records from lakes Tanganyika and Bosumtwi, both in West Africa, the team reconstructed a continuous record of tropical Africa’s climate back to about 135,000 years ago. They found that lake water levels dropped by as much as 600 meters, or about 95 percent of the lake’s volume, during two periods between 135,000 and 75,000 years ago. This shows that East Africa was beset by more severe droughts than any previously known for the region, Scholz says.
The timing of these droughts coincides with modern human migration from Africa and therefore “could have been the climatic factor that forced people out of Africa,” says co-author Peter Reinthal of the University of Arizona in Tucson.
“This team has retrieved an incredible time capsule of environmental change in Africa,” says John Smol of Queen’s University in Ontario. “Their study illustrates just how intricately linked humans and climate are.”
The findings, published in a pair of papers in Proceedings of the National Academy of Sciences on Oct. 16, also show that after about 75,000 years ago, the East African climate not only became wetter but also much more stable. This pattern may resolve a problem scientists have long puzzled over. “There have been two competing ideas about what has controlled tropical climate change in Africa,” Scholz says. One idea holds that local variations of Earth’s orbit around the sun have dominated climate. For example, during periods when more solar radiation hits the top of the atmosphere, tropical convection intensifies, resulting in enhanced precipitation, Scholz says, adding that the amplitude of these variations is greatest when Earth’s path is shaped more elliptically.
Another hypothesis is that climate change is tied to high-latitude climate processes. For example, major glacial advances in the high latitudes often coincide with periods of severe aridity in tropical Africa, Scholz says. “But during these megadroughts between 135,000 and 75,000 years ago, we don’t have corresponding intervals of glacial advance in high latitudes. Instead, it turns out that these big swings in lake levels coincide with big swings in the amount of solar radiation,” Scholz says. “That hints that Earth’s orbit dominated climate change during that time interval.”
After 75,000 years ago, however, the changes in the amount of solar radiation diminished because Earth’s orbit becomes much more circular. This suggests that there was a mode switch and high-latitude events began to dominate East African tropical climate, Scholz says. “It’s as if Earth’s orbit was the bass drum of a marching band, keeping everybody in line between 135,000 and 75,000 years ago. But after that, the big bass drummer wasn’t pounding quite as hard any more and climate started to pay attention to a different band, which happened to be the high-latitude system,” he says.
Links:
"A Changing Climate for Human Evolution," Geotimes, September 2005
