Global warming and the early snowmelt it brings apparently help fuel wildfires that are far bigger, more frequent, and longer lasting than those of previous years, according to a report released today in the online edition of the journal Science.
And wildfires are expected to get worse, the study says, as Earth's average global temperature rises.
Every year, wildfires cost Americans billions in tax dollars and destroy hundreds of homes and many natural resources.
So far in 2006, more than 3.8 million acres have burned in the United States -- double the 10-year average for this time of year, according to the National Interagency Fire Center.
"Looking at the western United States, there's just been a tremendous increase in the frequency of large dangerous forest fires," says scientist Anthony Westerling, lead author of the study and a specialist in wildfire and climate at the Scripps Institution of Oceanography. "It's really been concentrated at sort of midelevations, around 7,000 feet in elevation."
The authors analyzed more than 1,100 large wildfires between 1970 and 2003, and discovered a dramatic increase in the number of wildfires, beginning in 1986.
"Wildfire frequency was nearly four times the average of 1970-1986, and total area burned by these fires was more than 6½ times its previous level," according to the report. The greatest increase in large wildfires -- 60 percent -- has been in the northern Rockies.
The problem is twofold, say scientists: Temperatures are rising, which makes fires more intense and harder to fight during fire season. In addition, warmer temperatures cause mountain snowpack to melt away weeks too early in most years now, leaving local drought that turns brush into the perfect dry fuel.
The scientists also found that the wildfire season is expanding. The period between the first and last wildfires has increased 78 days since 1987, according to the report.
And the fires are burning longer.
"The average time between discovery and control for a wildfire increased from 7.5 days in 1970-86 to 37.1 days in 1987-2003," the report says.
"The fires in Yellowstone Park in 1988 seemed to inaugurate this new era of major wildfires in the western United States," writes Steven Running, a researcher at the University of Montana School of Forestry.
The Yellowstone fires lasted more than three months, burned nearly 1.5 million acres and were only extinguished by a September snowfall, Running writes in a companion article to the Science study.
"The value in the snowpack is ... that it provides a water supply that is modestly dribbled out in spring and summertime," said co-author Dan Cayan, director of the Climate Research Division at Scripps.
But in the past 30 years, Cayan says that spring snowmelt in California has occurred as much as three weeks earlier than in past decades.
"Snow is very important," says Westerling. "If you melt the snow earlier in the year, or if you get less snow and more rain in the wintertime, you get a much longer dry season during the summer, and that means you get a lot more opportunities for fires to burn. You have a longer time when vegetation can dry out."
The study also looked at other factors affecting wildfires, such as fire management policies.