An analysis of data going back to 1970 indicates the fires increased "suddenly and dramatically" in the 1980s and the wildfire season grew longer, according to scientists in Arizona and California.
"The increase in large wildfires appears to be another part of a chain of reactions to climate warming," said Dan Cayan, a co-author of the paper and director of the climate research division at Scripps Institution of Oceanography.
He said that while part of the increase may be attributed to natural fluctuations, evidence also links it to the effects of human-induced climate warming.
Scientists have become increasingly concerned in recent years about the amount of carbon dioxide released into the atmosphere by the burning of fossil fuels. Average worldwide temperatures have risen this century as a result of what many believe is a greenhouse effect from that pollution.
The researchers used the files of the U.S. Forest Service and National Park Service to analyze 1,166 fires of more than about 1,000 acres. Their findings are published Thursday in the online edition of the journal Science.
Beginning about 1987, there was a change from infrequent fires averaging about one week in duration to more frequent ones that often burned five weeks or more, they reported. The length of the wildfire season was extended by 78 days.
The researchers said the changes appear to be linked to annual spring and summer temperatures, with many more wildfires burning in hotter years than in cooler years.
They also found a connection between early arrivals of the spring snowmelt in the mountainous regions and the incidence of large forest fires. An earlier snowmelt, they said, can lead to an earlier and longer dry season, which provides greater opportunities for large fires.
"I see this as one of the first big indicators of climate change impacts in the continental United States," said research team member Thomas Swetnam, director of the Laboratory of Tree-Ring Research at the University of Arizona in Tucson. "We're showing warming and earlier springs tying in with large forest fire frequencies. 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 research was supported by the National Oceanic and Atmospheric Administration, the Forest Service and the California Energy Commission.