Human-caused climate change has nearly doubled the amount of land burned in western forest fires over the past three decades, but it isn’t solely responsible for the incendiary wildfires of recent years, says a new study from the University of Idaho.
Natural climate trends favoring drier summers across the West also are contributing to longer, more destructive wildfire seasons, according to the study, which was done in partnership with Columbia University.
“Over the past 30 years, we’ve documented about a ninefold increase in the number of acres burned in the western U.S.,” said John Abatzoglou, the study’s lead author and an associate professor of geography at UI. “We wanted to quantify the contribution of human-caused climate change to this dramatic increase.”
Politicians and fire chiefs have referred to the longer wildfire seasons as “the new normal” under climate change, but the actual situation is more complex, according to the study published Monday in the Proceedings of the National Academy of Sciences.
“Climate change is explaining about half of the increase,” Abatzoglou said. However, “even in the absence of human-caused climate change, we would have seen an increase in burned areas” from drier weather patterns that fall within normal climate variations, he said.
Abatzoglou and other researchers used models to track the effect of temperature and drier summers on forests’ readiness to burn. They studied forests in the Lower 48 states, from the Rockies to the Pacific Ocean.
Climate change has warmed the average temperature in those forests by an average of 2 to 2.5 degrees Fahrenheit since 1950.
“That may not sound like a lot, but it means the snow melts earlier, the forest fuels dry out earlier, and there are more days when the fuel is receptive to fire,” Abatzoglou said.
Since 2000, human-caused climate change has resulted in about nine additional days each year when forests are extremely dry and primed for wildfires, wrote Abatzoglou and the study’s co-author, Park Williams, a bioclimatologist at Columbia University.
By itself, climate change is responsible for an additional 16,000 square miles of western forests burning between 1984 and 2015, the study found. That’s a land mass about 190 times the size of Seattle, or the size of five national forests in North Idaho and Western Montana.
Adding the drier summers from natural climate patterns worsened the wildfire situation, Abatzoglou said. “They are compounding each other,” he said.
The study’s findings are significant for several reasons, he said. The research confirms that climate change already affects western forests and will be an influence on wildfires for decades to come.
In 2015, more than 10 million acres burned across the U.S., which is the largest acreage burned since the National Interagency Fire Center began tracking the areas burned in 1983. Federal firefighting costs topped $2.1 billion.
However, western forests could see some relief from destructive wildfires if natural climate variation trends back to wetter conditions, Abatzoglou said.
“Our study suggests that climate change could make wildfire years like 2015 more frequent,” he said. But, “I think it’s premature to say this is the new normal.”