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."
Bigger and Badder
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."
Climate vs. Forest Management
The study also looked at other factors affecting wildfires, such as fire management policies.
"There's been a lot of discussion about the effects of a century of land management on fire risks," says Westerling. "Because when you suppress fires for a long time in some places you get an increased fuel load, more trees growing close together, and it can carry fire to a large size."
But, Westerling says, "that effect is completely overwhelmed by the temperature signal that we're seeing."
Westerling, who is also an economist, says the increased fire danger is bad news in places like California, where more people are building homes in fire-prone areas. He says that areas with a very small percentage of the state's population are driving the increase in property losses from wildfire.
"You're sprinkling homes in places that like to burn," he says. "And they're going to burn even more when you increase the temperature."
Estimates say that mountain forests in the United States store about 20 percent to 40 percent of the total amount of the country's carbon that is absorbed naturally each year. Stored carbon gets released into the atmosphere when a forest burns, further compounding the problem of global warming.
"The mountain ecosystems are an enormous carbon pool," Westerling says. "And if we start releasing that carbon because we are increasing temperatures and therefore burning more regularly, we're going to release a lot of carbon that's currently stored in the trees and soil. And that's going to be a positive feedback. It's going to exacerbate the whole problem with climate change."
The report predicts things will get worse in the coming decades - that warming and increased wildfire activity "is likely to magnify the threats to human communities and ecosystems, and significantly increase the management challenges in restoring forests and reducing greenhouse gas emissions."
Multiple climate models run by international teams of scientists have projected June to August temperature increases of 3.6 to 9 degrees Fahrenheit in western North America by as early as 2049. That increase, writes Running, "would be roughly three times the spring-summer temperature increase" that Westerling and colleagues have linked to current trends.