Since the late 1970s, global warming has been responsible for about half of the drying that leads to forest fires.
By Nathan Collins
(Photo: Justin Sullivan/Getty Images)
The last few years have been hotter and drier than most in the Western United States. That makes for prime conditions for forest fires. Now, researchers report in Proceedings of the National Academy of Sciences what many had suspected: Although it’s not the only ingredient, global climate change has roughly doubled the amount of land burned since 1984.
Of course, climate change alone isn’t responsible for the growing threat forest fires pose to humanity, and, according to one measure—the total number of acres burned in a year—fires have actually grown smaller worldwide, even as those fires seem to have had a greater impact on human beings. That’s partly because of misguided fire management policies and the fact that a growing number of people moving to fire-prone areas, as illustrated by this past summer’s Fort McMurray, Alberta, fire.
Still, climate change is likely to bring much hotter, drier conditions to the American West, and, with them, more forest fires. The question is, how much impact has climate change had on forest fires in the Western U.S.?
Global climate change has roughly doubled the amount of land burned since 1984.
To answer that question, John Abatzoglou, an associate professor of geography at the University of Idaho, and Park Williams, an assistant research professor at Columbia University’s Lamont-Doherty Earth Observatory, looked at eight measures of “fuel aridity” (a.k.a dryness) recorded at least monthly as far back as 1948 throughout the Western U.S. Abatzoglou and Williams next compared those records with data on the number of acres burned each year since 1984 and found a strong correlation—the West has been getting drier over time, they found, and, the drier the land was in a given year, the more of it burned.
The next step was to figure out how much of that drying could be due to climate change. To determine that, they compared observed aridity data to estimates made using the suite of climate models known as the Coupled Model Intercomparison Project Phase 5 (CMIP5). Although best known for its role in predicting future climate, CMIP5 can also be used to construct what might have happened without human contributions to climate change — in this case, how dry forests would have been.
With the aid of those models, Abatzoglou and Williams estimate that 55 percent of the increase in forest aridity since 1979 is due to anthropogenic climate change, although climate change had less affect on aridity. (There were no clear trends in aridity from 1948 to 1978, the researchers note.) Given the connection between aridity and forest fires, the researchers estimate that an additional 16,200 square miles—around the combined area of Massachusetts and Connecticut, the authors point out—burned because of climate change since 1984.
“We expect anthropogenic climate change and associated increases in fuel aridity to impose an increasingly dominant and detectable effect on western U.S. forest fire area in the coming decades while fuels remain abundant,” Abatzoglou and Williams write.