Looking at a database of 1,166 forest wildfires from 1970 to 2003 in the western United States, researchers compared the number and potency of wildfires to spring and summer temperatures and the timing of snowmelts.
Wildfire season and potency increased "suddenly and dramatically" in the late 1980s, the scientists say.
"The increase in large wildfires appears to be another part of a chain of reactions to climate warming," said study co-author Dan Cayan, director of Scripps Institution of Oceanography's Climate Research Division. "The recent ramp-up is likely, in part, caused by natural fluctuations, but evidence is mounting that anthropogenic effects have been contributing to warmer winters and springs in recent decades."
Cayan and colleagues noted that in the mid-1980s there was a jump of four times the average number of wildfires in the West, compared with the early 1980s and 1970s. The total area burned was six and a half times greater in the mid-1980s than the earlier years examined.
The wildfire season has also extended by 78 days in the more recent period of 1987 to 2003, compared with 1970 through 1986.
The researchers also found that 56 percent of the wildfires and 72 percent of the total burned area occurred during the years when the snow melted early. When the snowmelt season occurred later than average, only 11 percent of wildfires occurred.
"At higher elevations, what really drives the fire season is the temperature. When you have a warm spring and early summer, you get earlier snowmelt," said study co-author Anthony Westerling of Scripps Institution of Oceanography at the University of California at San Diego. "With the snowmelt coming out a month earlier, areas then get drier earlier overall, and there is a longer season in which a fire can be started — there's more opportunity for ignition."
Changing the forest
The increased frequency of large wildfires could reduce the density of trees and change forest landscapes.
The country's Western forests, which traditionally act as storage "sinks" for sequestering 20 to 40 percent of all U.S. carbon output, are now transforming into a source of atmospheric carbon dioxide as they burn up, the authors write.
"I see this as one of the first big indicators of climate change impacts in the continental United States," said study co-author Thomas Swetnam, director of the Laboratory of Tree-Ring Research at the University of Arizona in Tucson. " Lots of people think climate change and the ecological responses are 50 to 100 years away. But it's not 50 to 100 years away — it's happening now in forest ecosystems through fire."
The study is detailed in this week's online version of the journal Science.