Thomas Swetnam, director of UA's Laboratory of Tree-Ring Research, said that they estimated that "deforestation due to burning by humans is contributing about one-fifth of the human-caused greenhouse effect – and that percentage could become larger." [right, smoke plumes from southern California wildfires billowing out over the Pacific ocean. The red outlines indicate active fires. These wildfires spread over a two-week period in October 2007, burning more than 500,000 acres, according to the California Department of Forestry and Fire Protection. Credit, MODIS Rapid Response Project at NASA/GSFC]
"It's very clear that fire is a primary catalyst of global climate change," he said. "The paper is a call to arms to earth scientists to investigate and better evaluate the role of fire in the Earth system."
The team also reports that, currently, all fires combined annually release an amount of carbon dioxide into the atmosphere equal to 50 percent of that coming from the combustion of fossil fuels.
Ref: "Fire in the Earth System," Science 24 April 2009: Vol. 324. no. 5926, pp. 481 - 484, DOI: 10.1126/science.1163886
[the quotes above are taken from the UA news release]