MADISON - Scientists have long known that six thousand years ago, in what is now North African desert, hippos and crocodiles abounded, Neolithic fishermen thrived on the shores of numerous shallow lakes, and grasslands stretched to the horizon.
What they didn't know was why.
Now scientists are a step closer to solving this climatological riddle of the early Stone Age, and, importantly, their findings promise a helpful tuning of the sophisticated computer models used to predict future climate.
Writing in the Oct. 17 edition of the journal Science, University of Wisconsin-Madison researchers John E. Kutzbach and Zhengyu Liu describe a Neolithic Sahel that was watered to a significant extent by shifting tropical Atlantic monsoons, seasonal winds and rain that strongly influence climate over large regions of the Earth.
"The northernmost reach of the monsoon marks the limit of vegetation in the Sahel," said Kutzbach, a paleoclimatologist and director of the UW-Madison Center for Climatic Research. "There is a nice boundary where vegetation stops and sand begins."
Six thousand years ago, that boundary was 600 miles north, extending into a region that is now a drought-stricken swath of desert that spans much of North Africa. And for the past 15 years, Kutzbach and his colleagues have used some of the world's most intricate computer models of climate, developed at the National Center for Atmospheric Research, to tease out clues about why such a significant expanse of land experienced such a dramatic change in weather and climate.
"Here, the thing that makes the monsoon work is sun shining on continents, producing a temperature contrast between land and ocean," Kutzbach said. "Six thousand years ago, the monsoons were generally stronger because of changes in the Earth's orbit" that brought the planet closest to the sun during the Northern Hemisphere summer.
"The summers were warmer, and the warmed air rose allowing moisture-laden air from the tropical Atlantic to penetrate significantly deeper into North Africa and increasing rainfall by as much as 25 percent," said Kutzbach.
As a result, grasslands flourished in the Sahel, recycling moisture and driving an even greater increase in precipitation. But those two phenomena - the shift in the Earth's orbit and the increase in vegetation - were not enough to fully account for the extremely strong monsoons of the early Stone Age.
"There was something missing," said Kutzbach. "It prompted the question: 'Is there something going on in the ocean that we need to take into account?' "
By merging the ocean and the atmosphere in the supercomputer-driven climate model, Kutzbach and Liu found a slight warming of the oceans in the Northern Hemisphere.
"By warming the region of the northern tropical Atlantic through increased solar radiation from the orbital change, the tropical convergence zone near the equator shifted north and the result was a deeper wedge of moist air that acted as a conveyor belt to feed more moisture into Africa."
Rainfall increased by an additional 25 percent and brought the computer simulations into "reasonably close agreement with the picture we have of the ancient landscape," Kutzbach said.
The new study is important not only because it may finally lay to rest an important climatological and archeological question, but also because the results can be used to further sharpen the complicated models scientists are betting on to predict future changes in climate.
"Nature knows how the atmosphere, vegetation and the ocean interact," said Kutzbach. "We've been trying to mirror these interactions in computer models."
And looking to the past, he said, where physical evidence can be matched to what the models say, is an excellent way to test them.
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- Terry Devitt, (608) 262-8282, trdevitt@facstaff.wisc.edu
Journal
Science