Wildfires that ravage California and other major forested areas around the world are speeding the pace of global warming as they pump more and more greenhouse gases into the atmosphere.

The smoke from Southern California wildfires, which burned over 500,000 acres in a two-week period, drifts out over the Pacific Ocean in this NASA file satellite image from October 2007. In a vicious cycle made worse by humans, scientists now believe fires spur climate change, which in turn makes blazes bigger, more frequent and more damaging to the environment. (REUTERS/NASA/Handout) And the planet's rising temperatures that spur droughts and hotter summers, in turn, are sparking even more widespread fires.

Those warnings come from an international team of fire specialists from six nations, who declare that the global science agency that governs international actions aimed at reversing climate change must include the effects of forest fires in predicting how fast temperatures will rise in coming years..

The report on wildfires and climate is being published today in the journal Science.

One of fire's most significant contributors to global warming are the thousands of acres deliberately torched each year - particularly in the tropics - to clear forested land for farms, according to three leading authors of the study who spoke Thursday in a teleconference organized by the National Science Foundation.

That kind of deforestation accounts for about one-fifth of all the human-caused greenhouse gas emissions every year, the scientists agreed - and the proportion could become larger quickly as more and more land is cleared by burning, said Thomas Swetnam of the University of Arizona in Tucson.

"It's very clear that fire is a primary catalyst of global climate change," Swetnam said. "In the light of increased global warming, we'll be seeing more and longer droughts, and with more hot and dry years, we'll be seeing still more and larger fires."

"Fire affects the world in subtle and often misunderstood ways," said David Bowman of Australia's University of Tasmania. "It can change the climate quickly with large emissions of carbon dioxide and other pollutants."

One of those pollutants, the scientists agreed, is the black carbon soot that falls windblown over large areas of the Earth from major forest fires or deliberate "deforestation" blazes. That soot on the ground absorbs the sun's radiant energy - thus heating swaths of ground and adding to the burden of global warming, the scientists said.

Jennifer K. Balch of the National Center for Ecological Analysis and Synthesis at UC Santa Barbara noted that a consortium of scientists known as the IPCC - the Intergovernmental Panel on Climate Change - has in the past limited the role of wildfires in its assessments of climate change.

The panel's periodic reports form the basis of treaties and government actions to curb global warming, and Balch said the new report in Science should persuade the panel to include a greater role for the impact of wildfires in its next assessment of global warming due in three years.

"There are fires now where we don't normally see fires," she said, "and there are bigger and more frequent fires throughout the Western United States and the tropics."

In California alone, fires last year covered 380,310 acres in the state's wildlands protected by the California Department of Forestry, but the five-year average was far lower, totaling fewer than 260,800 acres, according to the department's figures.

Crawford Tuttle, chief deputy director of the department, told The Chronicle that climate change has directly affected the severity, the season, and the numbers of wildfires the state has encountered for at least the past 10 years - "and maybe even longer."

"We've encountered hotter and dryer conditions and higher levels of fire danger throughout the state year after year," Tuttle said, "and the large fires, in turn, have added large quantities of greenhouse gases into the atmosphere."

California is facing its third year of severe drought, he said, "and the hotter and dryer it gets earlier and earlier each year adds to the wildfire danger."

Nor do things look good for this season, Tuttle said. He noted there were two small fires on U.S. forest land in the Sierra north of Interstate 80 this week, and although they were quickly contained, they indicated that, once again, snow cover is melting early, increasing the danger of California wildfires and increases in greenhouse gas emissions.