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Study: Wildfire increase due to climate change

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Warming temperatures, not fire suppression or drought, played the biggest role in causing a major increase in the number, scale and duration of forest fires across the West in the past two decades, a new study shows.

Billed as one of the first major indicators of the effects of global warming in the continental United States, the forest fire increase is strongly associated with higher spring and summer temperatures since 1986 and an earlier spring snowmelt since then, said the study from four researchers including the head of the University of Arizona's Laboratory of Tree Ring Research.

Westwide, forest fires have struck nearly four times more often in the years since 1986 than in the period 1970 to 1986, said the report, which was published today in Science Express, the online version of the journal Science. The total area burned since 1986 was more than six and one-half times larger than in the earlier period, according to the study. The wildfire season's average length has stretched by 78 days, or 64 percent, in the years since 1986 compared to the 1970-86 span. The time to control wildfires has also increased dramatically - to 37.1 days since 1986 from 7.5 days in the earlier period.

While the report says it's unclear right now whether the climate changes and wildfire increases are due to greenhouse-gas induced global warming or natural climate variations, it says that the wildfire problem is likely to get worse in the coming decades, because virtually all climate models predict warmer springs and summers in the near future. The combination of warming and increased drought - if that occurs - would also aggravate the problem of greenhouse-gas induced warming, the study says, because more wildfires will reduce the density of trees - which take in carbon dioxide.

That suggests that the western U.S.'s forests could become a source of increased carbon dioxide in the atmosphere, the study aid.

But the effects of climate change's effects have been strongest in the Northern Rockies, and haven't been as great in the Southwest, the study said. That's because fire suppression in the northern states hasn't had nearly as major an effect as in the Southwest because the spruce fir and lodge pole pine forests of the Rockies never had had a history of frequent fires. But the Southwest's Ponderosa pines had burned regularly before the Forest Service and other federal agencies started to routinely put out fires a century ago.

But the Northern Rockies accounted for 60 percent of the increase in large fires, compared to 11 percent for the Southwest and 18 percent for the Sierra Nevada, southern Cascades and the Coast Ranges of northern California and southern Oregon. The shares of the increase were much less, ranging from one percent to five percent, in other regions of the West.

The tree ring lab's director, Thomas Swetnam, said he was a little surprised by the study's findings, since he had previously believed that the buildup of small trees and other fuels in the forests due to fire suppression was the biggest cause of the fire increase.

"My thoughts were that the increase in the area burned and the number of large fires we've seen would be tied primarily to increasing fuel loads," Swetnam said this week. "Now, I'm reconsidering that thinking. Climate change is likely to be a primary driver."

The study's lead researcher was Anthony Westerling, a project scientist at the Scripps Institution of Oceanography in La Jolla, Calif.

Overall, with climate change occurring and with more people moving into forested areas, Swetnam says he feels "pretty apocalyptic" about the prospect of more forest fires as he looks toward the future.

"It's like a perfect storm, all coming together," he said. "I think we will start to see more efforts to try to mitigate these effects." But he said he is a bit discouraged at the lack of seriousness most people give to the issue of climate change.

"It will be the dominant environmental issue in the next decade or so," Swetnam said. "I don't think most people have really waken up to that fact."