Global warming may be largely to blame for the increasingly destructive wildfires in the Western United States in the last two decades, new research suggests.
Longer and fiercer wildfire seasons since 1986 are closely associated with warmer summer temperatures, earlier arrival of spring, and earlier snowmelts in the West, scientists reported yesterday in the online edition of the journal Science.
The new findings suggest that the most up-to-date forest management methods may be insufficient to slow the uptick in large forest fires. Most climate researchers believe that global teperatures will continue to warm as human activity increases the concentration of heat-trapping greenhouse gases, such as carbon dioxide, in the atmosphere.
``Local policies to manage forests are not going to be a magic bullet, they're not going to be successful in reversing this trend," said Anthony L. Westerling of the Scripps Institution of Oceanography , the study's lead author.
The study adds fuel to a debate on whether global warming causes extreme weather-related events across the United States, including last year's powerful hurricane season. Two papers published last year in the journals Nature and Science linked climate change to increases in hurricane intensity since 1970. Also, climate models suggest that both severe droughts and very heavy rain events in the United States will become more frequent as temperatures warm.
``Many of the changes in frequency and severity of extreme weather events that we're seeing now are exactly what we expect with increased greenhouse gas emissions," said climatologist Noah Diffenbaugh of Purdue University .
Westerling and his colleagues analyzed a comprehensive government database of forest fires larger than about 1,000 acres in the West since 1970. They found a dramatic increase in wildfires after 1986, with large fires four times more frequent than during the preceding years, and burning through 6 1/2 times more area. Also, the average wildfire season increased by 2 1/2 months.
Scientists had previously believed that increased wildfire activity resulted from changes in land use practices. In particular, tactics to suppress fires had allowed dead and dry vegetation to build up in Western forests, providing more fuel for fires.
But the new study shows that most of the increase in wildfires has occurred in the Northern Rocky Mountains, where few land-use changes have occurred. Also, the scientists found that 66 percent of the yearly variation in forest fires could be explained by temperature changes alone, with hotter years producing more fires.
The wildfires were also much more common in years with an early snowmelt, the researchers reported. When snow melts earlier, it allows more time for soil and vegetation to dry out, permitting fires to begin earlier in the season. On average, snowmelt in the West came about a week earlier after 1986, with spring and summer temperatures higher by about 1.6 degrees Fahrenheit.
Thomas W. Swetnam of the University of Arizona , a coauthor of the paper, said he was surprised that the study showed temperature had a greater influence than land-use changes on wildfire activity. Steven W. Running , a forestry professor at the University of Montana who was not involved in the study, said the research shows that climate change is already making its impact felt. As he talked, a forest fire burned less than a mile from his office, he said.
``Increasing wildfires is a big issue that's expensive to combat, it's dangerous, and everything we know about climate change in the future means it's going to get worse," he said. The cost of fighting wildfires can exceed $1 billion a year, he said.