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Humans Are Igniting the Vast Majority of Wildfires in the U.S.

And as climate change leaves vast swaths of the country warmer and more arid, human-created blazes will only become more likely.

By Kate Wheeling


Firefighters attempt to save the Casa Loma fire station in the Santa Cruz Mountains near Loma Prieta, California, on September 27th, 2016. (Photo: Josh Edelson/Getty)

Wildfires are a natural and necessary part of many ecosystems; some trees, for example, only produce seeds after being scorched by fire. But the National Park Service has long known that humans are to blame for most wildfires in the United States. Exactly how human-created fires alter wildfire ecology, namely the length and geographic range of the wildfire season, has largely remained a mystery — until now.

In a new study, a team of researchers from across the U.S. looked at two decades’ worth of government wildfire records, and found that humans not only set the vast majority of the fires, but also started those fires in cooler and wetter conditions than they would typically occur.

Looking at 1.5 million records of wildfires with a known cause that took place between 1992 and 2012, the researchers found that humans started 84 percent of the fires over the 20-year period and tripled the length of the fire season. Lightning-ignited fires were most likely during the summer months, and during the spring, winter, and fall, human-induced blazes were 35 times more likely than lightning-started ones. People started plenty of summertime blazes as well; in fact, more human-ignited fires were started on July 4th than any other day of the year.

But the outsize role people play in sparking wildfires doesn’t preclude climate change from blame. “We know that, in the Western U.S. there are larger fires and a longer fire season because of climate change,” says Jennifer Balch, an assistant professor at the University of Colorado and lead author on the study. Indeed, the frequency of large fires sparked by lightning also increased over the study period, during which time hotter and drier conditions spread across the country. Anthropogenic climate change is expected to further increase the number of lightning strikes by 50 percent by the end of the century — but even then humans will remain the main driver of fires across much of the U.S., according to the study authors. Indeed, the study’s main takeaway is how human activities are interacting with a changing climate to exacerbate wildfires.

With that in mind, the next step is for the researchers to look at climate projections to find out “where people and climate are going to make fires worse,” Balch says. About 9 percent of land in the U.S. is classified as wildland-urban mix, where houses are built in relatively remote or untamed regions, and that area is projected to double by 2030, according to Balch, mostly in the Western U.S. “More and more people are going to be living in these flammable landscapes,” she says, which means more and more potential ignition sources and more and more vulnerable infrastructure.

Land management in those regions is going to become increasingly important. “There are good fires and there are bad fires,” Balch says. “What we need are more prescribed burns.”