Over the last 30 years, the West has seen an uptick in the size and frequency of forest fires. Scientists have typically attributed the change to low snowpack and high summer temperatures. But researchers writing in the journal PNAS say the trend could have more to do with rain.
Researchers pulled up maps of forest wildfires from 1979 to 2016 and compared those maps against data on snow, rain, temperature and humidity.
“We compared the three hypotheses for why wildfire might be getting worse,” says Charles Luce, a research hydrologist with the U.S. Forest Service in Boise, Idaho and a coauthor on the paper.
One hypothesis is that the fire season is getting longer because snow is falling later and melting earlier. The second, he says, is that warming summer temperatures are drying out woody fuels faster.
“And then the third hypothesis is that rainfall patterns have changed,” says Luce. “And of those three, the strongest by quite a bit is the rainfall.”
Less rain -- and longer stretches between rainy days -- were most strongly associated with wildfire patterns, leading Luce and his colleagues to conclude that declining precipitation is a “major contributor” to the West’s increasingly large wildfires.
Rising summer temperatures are still important.
“There is no doubt that regional temperature increases, independent of feedbacks from precipitation, have increased atmospheric aridity and contributed to increased wildfire activity,” the authors write. But, they add, summer rain appears to be even more important, and lack of rainy days can exacerbate the problem of rising surface temperature.
“In addition to directly adding moisture to woody fuels and soil, rain days are accompanied by cooler temperatures, increased humidity, and clouds that reduce incoming radiation and consequently, near-surface heating,” they write.
Luce says the results could help better predict where fires will happen. And, he says, they show that higher elevations aren’t as protected by snowpack as previously thought.
“One of the expected effects of climate change over the next century is to see more dry days, or more days between rainfall events,” says Luce.
The authors write that if rain trends continue, “a continuing pattern of dry, warm summers” will probably result in “increasingly severe fire seasons.”
This story was produced by the Mountain West News Bureau, a collaboration between Wyoming Public Media, Boise State Public Radio in Idaho, Yellowstone Public Radio in Montana, KUER in Salt Lake City and KRCC and KUNC in Colorado.