Massive burning of forests for agriculture thousands of years ago may have increased atmospheric carbon dioxide enough to alter global climate and usher in a warming trend that continues today, according to a new study that appears online Aug. 17 in the journal Quaternary Science Reviews.
Researchers at the University of Virginia and the University of Maryland-Baltimore County say that today’s 6 billion people use about 90 percent less land per person for growing food than was used by far smaller populations early in the development of civilization. Those early societies likely relied on slash-and-burn techniques to clear large tracts of land for relatively small levels of food production.

“They used more land for farming because they had little incentive to maximize yield from less land, and because there was plenty of forest to burn,” said William Ruddiman, the lead author and a professor emeritus of environmental sciences at the University of Virginia. “They may have inadvertently altered the climate.”

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