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city's population by 50 percent. "We suggest that these droughts will become more common as the climate changes, and that they're likely to precipitate major social and economic change, both in Mongolia and in any other semi arid regions," Hessl said.
The strong uptick in moisture in the tree ring data jives with records from lake sediments in the area. Those sediments provide a cruder record but suggest a wet period more or less around the same time. "It's a super strong moisture signal," Stahle, a tree ring expert from the University of Arkansas who was not involved with the study, said in an interview. The researchers hypothesize that the wet period not Nike Roshe Running Shoes Leopard only goosed tree growth but also helped grasses grow more vigorously. Those grasses in turn provided forage for the horses and cattle that sustained Genghis Khan's march across the Mongolian steppe. But Stahle doesn't see such strong evidence for the next step of reasoning that productive grasslands and the livestock they nurtured allowed Genghis Khan to become powerful enough to launch his campaigns of conquest. Because of the lack of reliable written records from that era, it's hard to confirm or refute that portion of the researchers' theory. "It's just a working hypothesis at this time," Pederson said.
to the 600s, and others to the 1100s. Pederson didn't find any evidence of fire, but he was surprised to see a series of unusually thick growth rings representing the years 1215 1226. That Nike Roshe Run White Mesh
to stop on the way back and take some samples. When Pederson finally got around to analyzing them, he found they were much older than expected. One went all the way back Nike Roshe Gray And White
told the scientists that those years were especially wet, allowing the trees to grow more over that period. While he's not a historian, Pederson said, "I knew enough to realize this was right at the time the Mongols began to build their empire. "Pederson, Hessl, and their colleagues hadn't taken enough samples to be certain their analysis was right, but in 2012 they returned to the site to take more samples. Analysis back in the lab confirmed their original conclusion. The physical findings appear to be well grounded.
"This is just the first step of a big, complex research project. "Ultimately, figuring out what motivated Genghis Khan is a fascinating historical exercise but the implication for the future is perhaps more important. And what the scientists found at the modern end of the tree ring record is what Stahle calls "sobering. " The years 1215 1226 may have been unusually wet, but the drought that afflicted the region from 2002 2009 rivals the worst seen over the past thousand years. Its only competition is a terrible drought that came just before Genghis Khan's rise. But that happened long before humans began adding heat trapping carbon dioxide to the atmosphere,Hessl said the 21st century drought was driven in part by climate change. She said it triggered a mass migration to Mongolia's capital, Ulan Bator, driving up the Nike Roshe Shoe Carnival
March Madness, NASA Style: Cast Your Vote NowGAO: Climate Change T.
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