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The researchers used data from other scientists who have been measuring oxygen and other gases trapped in ice core samples from Antarctica that are 850,000 years old.
The air samples from 5,000 to 8,000 years ago contained unmistakable levels of methane and carbon dioxide gases.
Methane levels rose from decomposing vegetation in terraced rice paddies, Vavrus said. The rising levels of carbon dioxide came from deforestation known to have taken place in Europe.
Three different climate models were used. The researchers removed the amount of greenhouse gases that humans sent into the atmosphere, based on the ice core samples. The models show more permanent snow and ice cover in Canada, Siberia, Greenland and the Rocky Mountains.
Vavrus said the research supports theories first put forward by William F. Ruddiman, a paleoclimatologist at the University of Virginia.
The old ice core BS again, eh? Thoroughly discredited on too many counts to enumerate.