Rising temperatures throughout the West have stoked an increase in large wildfires over the past 34 years as spring comes earlier, mountain snows melt sooner and forests dry to tinder, scientists reported Thursday.
More than land-use changes or forest- management practices, the researchers concluded, the changing climate was the most important factor driving a quadrupling in the average number of large wildfires in the Western U.S. since 1970.
All told, the average fire season has grown more than two months longer, while fires have become more frequent, longer-burning and harder to extinguish.
They destroy 6.5 times more land than in the 1970s, the researchers found.
Last year was the worst wildfire season on record, with more than 8.53 million acres burned nationwide by the end of December. This year, more than 60,000 wildfires have charred almost 3.9 million acres – twice the number of fires during the same period last year, according to the National Interagency Fire Center in Boise, Idaho.
“It all fits together,” said climate researcher Anthony Westerling, who led the research while at the Scripps Institution of Oceanography in La Jolla, Calif. “The (fire) seasons do start earlier and run longer. It is consistent with a changing climate.”
In the first detailed study of its kind, scientists at Scripps and the University of Arizona analyzed 34 years of wildfire activity, temperature records, snow- melt trends, stream flows and other climate-related data.
The research, published online Thursday by the journal Science, was funded by grants from the National Oceanic and Atmospheric Administration, the U.S. Forest Service and the California Energy Commission.
Hurricanes of the West
Rising average temperatures are worsening the regional pattern of annual wildfires.
The scientists found that climate change magnified a regional pattern of natural disaster that every year costs more than $1 billion in federal firefighting expenses and untold property damage.
“I think this is the equivalent for the West of what hurricanes are (elsewhere in the country),” said fire ecologist Steven Running at the University of Montana in Missoula, who was not connected with the research. “This is an illustration of a natural disaster that is accelerating in intensity as a result, I feel, of global warming. This really links fire activity in the West to global warming.”
The study doesn’t prove that the climate change is human-induced, Westerling said. But it does show that wildfires are likely to increase if the warming trend continues – and the researchers were confident they had documented a broad climate trend at work and not a fluke of weather variability.
“I see this as one of the first big indicators of climate-change impacts in the continental United States,” said Thomas Swetnam, an expert on fire history and director of the laboratory of tree- ring research at the University of Arizona in Tucson, who was part of the research team.
The researchers studied more than 1,100 large wildfires between 1970 and 2003.
Average spring and summer temperatures in the West were more than 1.5 degrees higher between 1987 and 2003 than during the previous 17 years. In fact, the seasonal temperatures were the highest since record-keeping started in 1895, the researchers said.
They reported that almost seven times as much forested federal land burned between 1987 and 2003 as during the previous 17 years.
During the same period, the length of the wildfire season increased by 78 days. The average time between when a fire was discovered and it could be extinguished also lengthened – from 7.8 days to 37.1 days.
The impact of rising temperatures on wildfires seemed most profound in the forests of the northern Rockies, which accounted for 60 percent of the blazes from 1987 to 2003, and least pronounced in arid Southern California, the researchers said.
“The most stark conclusion here is that while they do say land use and management has played an important role, the broad-scale increase in wildfire frequency has been driven primarily by recent changes in climate,” said wild land fire analyst Tom Wordell at the National Interagency Fire Center. “It does not paint a pretty picture for future fire activity, given the climate model predictions.”
The Baltimore Sun contributed to this report.
