American forests in the West have burned bigger and more frequently since 1987, and climate change is a big reason.
That is the conclusion of a research paper published Thursday by Sciencexpress.org, the online version of the journal Science. It is the first to document a link between global warming and increased fire intensity in Western U.S. forests.
The study doesn't address what causes fires or firefighting effectiveness. But it finds that climate change creates longer, drier seasons and better conditions for catastrophic fires.
Anthony Westerling, a climatologist at the Scripps Institution of Oceanography at the University of California-San Diego, and his co-authors found a fourfold increase in large wildfires between 1987 and 2003 compared with the preceding 16 years.
The increase corresponds to an average 1.5-degree Fahrenheit temperature rise across the region, which includes the Rocky Mountains, the Southwest, the Sierra Nevada and the Northwest.
During the warmer period, fires burned 6 1/2 times more land, and the fire season grew by an average of 78 days. The average fire duration also increased, from 7.8 days to 37 days.
"The real message of the paper is not as much about forest management," said Steven Running, a University of Montana ecology professor who was one of the study's peer reviewers. "It's that this is yet another dimension of global warming's impact. To me, it's the equivalent of the hurricanes on the Gulf Coast. This is our hurricane."
As Running spoke in a telephone interview, he watched firefighting aircraft battle a wildland blaze on the edge of Missoula, only a mile from his office.
Study co-author Westerling, who started a new job this week at the University of California-Merced's Sierra Nevada Research Institute, documented the link between fires and climate change after assembling a database of more than 1,100 Western wildfires, plus climate data including precipitation and snowmelt.
The increased fire activity is partly a result of reduced winter rains, earlier snowmelt and a resulting early start to the dry season. These are all linked to warmer temperatures, he said.
"I think it's really clear that it's climate," said Westerling. "When you look at the timing of these larger fire years in the Sierra Nevada, they're very strongly correlated with the timing of spring and temperature changes."
His co-authors in the study are Hugo Hidalgo and Dan Cayan of the Scripps Institution, and Tom Swetnam of the University of Arizona.
There is broad agreement among scientists that the earth's climate is warming, and that people's habits are partly to blame. Burning fossil fuels such as gasoline and coal releases carbon dioxide that had been trapped in the ground. This causes more heat to be trapped in the atmosphere.
Forest managers in different regions should respond to climate change differently, Westerling said, because people have influenced forests in different ways.
The Sierra Nevada historically had sparse forests and smaller, more frequent fires. During the Gold Rush of the 1800s, many Sierra forests were cleared to supply wood for mines, railroads and fast-growing cities.
A century of determined fire suppression followed, allowing flammable brush and small trees to accumulate on the forest floor. Westerling said forest thinning can be helpful in avoiding big fires in the Sierra.
But in the Rockies, where forests retain more of their natural structure, thinning is less effective.
"There are lots of areas in the Southwest or parts of the Sierra Nevada where fuels management is still a very important policy to pursue," he said.
Matt Mathes, a spokesman for the U.S. Forest Service, said the study's findings are troubling.
The Forest Service uses fire and cutting on about 100,000 acres statewide every year to reduce fire risk. This includes a controlled burn the agency started on June 23 in the Marble Mountain Wilderness of the Klamath National Forest. It will eventually cover 1,200 acres. That's only a sliver of the 20 million acres the agency manages in California.
"One of the things we're seeing is the ecosystem is getting increasingly out of whack," said Mathes. "Over the past decade or so, our fire scientists are telling us that our fires are getting bigger and hotter. They're showing greater resistance to control and a faster rate of spread."
Forests are increasingly being viewed as a way to solve global warming, because as trees grow, they take carbon dioxide out of the atmosphere.
But in a companion commentary also published today, Running writes that the Intergovernmental Panel on Climate Change, the leading authority on global warming, predicts a temperature increase for western North America from 3.6 to 9 degrees Fahrenheit between 2040 and 2069.
This could mean the problem documented by Westerling could worsen, Running said, turning forests into a cause of global warming, rather than a solution.
That's because there would be fewer trees to absorb carbon dioxide as forest fires intensify, releasing more carbon dioxide into the air.
"This is a response that has the kind of feed-forward acceleration we don't want to see," he said.
Distributed by Scripps Howard News Service.