Yellowstone burned for three months in 1988, destroying more than 600,000 hectares of forest. The fire resisted the best efforts of 25,000 firefighters and only flickered out with the first snow in mid-September of that year. This catastrophic wildfire in the western U.S. may have heralded a new era of bigger and more frequent fires thanks to climate change, according to new research.

Anthony Westerling of the Scripps Institution of Oceanography and his colleagues compiled a database of all western wildfires from 1970 through 2003 that burned more than 1,000 acres: 1,166 in all. They found that large wildfires became four times more frequent after 1987 and burned more than six and a half times as much forest. In essence, the region shifted that year from infrequent large wildfires that lasted roughly a week to more frequent burns that lasted for an average of more than a month. In addition, the length of the yearly wildfire season expanded by 78 days.

The researchers compared this fire record to annual snowmelt and temperatures over the 34-year period to determine whether global climate, which has warmed by 1.5 degrees Fahrenheit, or care, such as forestry management, played a larger role in this shift. They found that more than half of the increase in frequent large fires occurred in the northern Rockies, an area where forest management has had no impact on the natural occurrence of wildfires. In addition, large fires most commonly happened in years when snowmelt came early. "When you have a warm spring and early summer, you get earlier snowmelt," Westerling explains. "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."

With firefighting consuming as much as $20 million a day, the annual cost of battling the more frequent blazes has reached $1.7 billion in recent years. And "ecological restoration and fuels management alone will not be sufficient to reverse current wildfire trends," the researchers argue in the paper presenting their findings published online yesterday in Science. That trend must be reversed or else western forests may be transformed from stores of the carbon that otherwise would add to global warming to a growing source of greenhouse gas emissions as they burn each summer. "Lots of people think climate change and the ecological responses are 50 to 100 years away," says co-author Thomas Swetnam of the University of Arizona. "But it's not 50 to 100 years away, it's happening now in forest ecosystems through fire."

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