MADISON - Scientists have long known that six thousand years ago, in what is nowNorth African desert, hippos and crocodiles abounded, Neolithic fishermenthrived on the shores of numerous shallow lakes, and grasslands stretched to thehorizon.
What they didn't know was why.
Now scientists are a step closer to solving this climatological riddle of theearly Stone Age, and, importantly, their findings promise a helpful tuning ofthe sophisticated computer models used to predict future climate.
Writing in the Oct. 17 edition of the journal Science, University ofWisconsin-Madison researchers John E. Kutzbach and Zhengyu Liu describe aNeolithic Sahel that was watered to a significant extent by shifting tropicalAtlantic monsoons, seasonal winds and rain that strongly influence climate overlarge regions of the Earth.
"The northernmost reach of the monsoon marks the limit of vegetation in theSahel," said Kutzbach, a paleoclimatologist and director of the UW-MadisonCenter for Climatic Research. "There is a nice boundary where vegetation stopsand sand begins."
Six thousand years ago, that boundary was 600 miles north, extending into aregion that is now a drought-stricken swath of desert that spans much of NorthAfrica. And for the past 15 years, Kutzbach and his colleagues have used some ofthe world's most intricate computer models of climate, developed at the NationalCenter for Atmospheric Research, to tease out clues about why such a significantexpanse 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. "Sixthousand years ago, the monsoons were generally stronger because of changes inthe Earth's orbit" that brought the planet closest to the sun during theNorthern Hemisphere summer.
"The summers were warmer, and the warmed air rose allowing moisture-laden airfrom the tropical Atlantic to penetrate significantly deeper into North Africaand increasing rainfall by as much as 25 percent," said Kutzbach.
As a result, grasslands flourished in the Sahel, recycling moisture and drivingan even greater increase in precipitation. But those two phenomena - the shiftin the Earth's orbit and the increase in vegetation - were not enough to fullyaccount for the extremely strong monsoons of the early Stone Age.
"There was something missing," said Kutzbach. "It prompted the question: 'Isthere something going on in the ocean that we need to take into account?' "
By merging the ocean and the atmosphere in the supercomputer-driven climatemodel, Kutzbach and Liu found a slight warming of the oceans in the NorthernHemisphere.
"By warming the region of the northern tropical Atlantic through increased solarradiation from the orbital change, the tropical convergence zone near theequator shifted north and the result was a deeper wedge of moist air that actedas a conveyor belt to feed more moisture into Africa."
Rainfall increased by an additional 25 percent and brought the computersimulations into "reasonably close agreement with the picture we have of theancient landscape," Kutzbach said.
The new study is important not only because it may finally lay to rest animportant climatological and archeological question, but also because theresults can be used to further sharpen the complicated models scientists arebetting on to predict future changes in climate.
"Nature knows how the atmosphere, vegetation and the ocean interact," saidKutzbach. "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 whatthe models say, is an excellent way to test them.
Materials provided by University of Wisconsin-Madison. Note: Content may be edited for style and length.
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