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 appearsin 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."
     Ruddiman said that early populations likely used a land-clearing method that involved burning forests, then planting crop seed among the dead stumps in the enriched soil. They would use a large plot until the yield began to decline, and then would burn off another area of forest for planting. They would continue this form of rotation farming, ever expanding the cleared areas as their populations grew. They possibly cleared five or more times more land than they actually farmed at any given time. It was only as populations grew much larger, and less land was available for farming or for laying fallow, that societies adopted more intensive farming techniques and slowly gained more food yield from less land.
     Five years ago, Ruddiman made headlines with a hypothesis that humans began altering global climate thousands of years ago, not just since the Industrial Revolution. That theory has since been criticized by some climate scientists who believe that early populations were too small to create enough carbon dioxide to alter climate. According to projections from some models of past land use, large-scale land clearing and resulting carbon emissions have only occurred during the industrial era, as a result of huge increases in population. But Ruddiman, and his co-author Erle Ellis, an ecologist at UMBC who specializes in land-use change, say these models are not accounting for the possibly large effects on climate likely caused by early farming methods. "We are proposing that much smaller earlier populations used much more land per person, and may have more greatly affected climate than current models reflect. We suggest in this paper that climate modelers might consider how land use has changed over time, and how this may have affected the climate," said Ellis.

Source: ScienceDaily (18 August 2009)