Warmer climate means more fires — any questions?

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More wildfires ahead as world warms up. Photo courtesy U.S. Forest Service.

Study ties ancient wildfire surge in the Rockies to warmer climate phase

Staff Report

Offering yet more evidence that the West’s recent spate of megafires is linked with a warming climate, University of Wyoming researchers showed that a warm spell about 1,000 years ago also spurred more blazes. The study suggests that large fires will continue to increase as a result of a warming climate.

“What our research shows is that even modest regional warming trends, like we are currently experiencing, can cause exceptionally large areas in the Rockies to be burned by wildfires,” says John Calder, a Ph.D. candidate in UW’s Program in Ecology and the Department of Geology and Geophysics.

Calder and fellow researchers examined charcoal deposits in 12 lakes in and near the Mount Zirkel Wilderness of northern Colorado to trace wildfire history during the 300-year Medieval Warm Period, when temperatures rose by about 1 degree Fahrenheit. That was the only time when fires burned substantially more area than in the 20th century.

Temperature increases over the past few decades have been comparable to those of the MWP, resulting in some of the largest wildfires in U.S. history. Since the mid-1980s, beginning with the large fires in Yellowstone National Park in 1988, there has been an increase in the frequency of large wildfires in the American West.

If the warming trend continues as projected, the fires of recent years could be just the start of more extensive and devastating blazes, the researchers say.

“When we look back in time, we only see evidence of large areas burning one time in the last 2,000 years,” Calder says. “This suggests that large wildfires of the magnitude we have recently seen used to be very infrequent.”

The researchers estimate that 83 percent of the 385-square-mile study area burned at the beginning of the MWP, when the climate warmed 0.9 degrees. That represented an increase of more than 250 percent, compared to the 20th century.

By comparison, the average increase in temperature in the Rocky Mountain region since 2000 has been about 1.25 degrees higher than during the 20th century.

“Corresponding to those higher temperatures, 12 percent of our study area burned in the large Zirkel Complex fire in 2002,” Calder says. “Our data indicate that, in the Medieval Warm Period, fires were either much larger, or large fires similar to the Zirkel Complex fire burned in that same wilderness area once every decade or two when the temperatures warmed by 0.9 degrees Fahrenheit.”

The Medieval fires would be unusual almost anywhere in the Rockies.

“Using Yellowstone fire history as a baseline for comparison, our minimum estimate of 50 percent of (Mount Zirkel) sites burned within a century at the beginning of the MWP exceeds any century-scale estimate of Yellowstone burning for the past 750 years,” the scientists wrote. Over the century that led up to and included the massive 1988 fires, only about 30 percent of Yellowstone burned.

“The large increase in the number of sites burned by fires (during the MWP) highlights the risk that large portions of individual landscapes may burn as climates continue to warm today,” the researchers concluded.

The findings are published in the Proceedings of the National Academy of Sciences. The paper, “Medieval warming initiated exceptionally large wildfire outbreaks in the Rocky Mountains,” was co-written by UW researchers Calder, Dusty Parker, Cody Stopka and Bryan Shuman, along with Gonzalo Jimenez-Moreno of the University of Granada in Spain.

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