Environment First Look

Study finds humans cause 84 percent of US wildfires

Either through arson or carelessness, humans have triggered five out of six wildfires in the US since 1992, new research finds.  

In an analysis of 20 years of wildfire records led by a team at University of Massachusetts Amherst and the University of Colorado-Boulder, researchers found human-started fires accounted for 84 percent of all wildfires, exhibiting 'a remarkable influence' on modern US wildfire regimes. Further, humans are expanding the fire niche into more locations and environments with historically low lightning-strike density.
Andrea Booher/FEMA
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Over the past two decades, humans triggered 84 percent of all wildfires and tripled the length of the fire season, making it a nearly year-round phenomenon under drier, warmer conditions, according to a new study.  

The findings come from a team of researchers at the University of Colorado and University of Massachusetts who examined wildfire data from 1992 to 2012 to conclude that humans had started 1.5 million fires during that period, expanding blazes into new and larger territories. The rest, researchers say, were sparked naturally by lightning.

And even as climate change makes conditions ripe for fostering blazes, the actions of humans in drier, warmer climates are mostly to blame for starting the fires.

“These findings do not discount the ongoing role of climate change, but instead suggest we should be most concerned about where it overlaps with human impact,” Jennifer Balch, the director of the University of Colorado Boulder’s Earth Lab and an assistant professor of geography, said in a statement. “Climate change is making our fields, forests and grasslands drier and hotter for longer periods, creating a greater window of opportunity for human-related ignitions to start wildfires.”

Most lightning fires occur in the summer months, marking a brief fire season for wildfires. But humans sparked an average of 40,000 fires during the spring, fall, and winter each year, extending the fire season far beyond its natural scope.

Of the human-caused fires in the study, 29 percent were caused by burning trash, while arson and misuse of equipment accounted for 21 and 11 percent of the blazes, respectively.

But the most common day for a fire to begin is the Fourth of July, when fireworks spark nearly 8,000 blazes per year.

The study’s findings have shifted how researchers view the cause of these devastating and wide-ranging blazes, which destroy natural habitats and can cause millions of dollars in damage.

“It’s generally pretty well known that people start a lot of fires; everything from campfires to burning yard waste to accidental fires in homes and other structures,” Bethany Bradley, an associate professor at the University of Massachusetts, Amherst, said in a statement. “But in the past, I used to think of ‘wildfire’ as a process that was primarily natural and driven by lightning. This analysis made me realize that human ignitions have an extraordinary impact on national fire regimes. From our analysis, we learned that human-started fires are amazingly common.”

And with that knowledge, it’s vital to rework national and local policy that focuses on curtailing human impact, particularly as the number of homes encroaching on natural areas doubles by 2030, researchers concluded.

As seasons and conditions shift with a warming planet, these blazes could become a growing concern.

"If a campfire grows out of control during a wet, cool period, then it probably isn't going to grow into a large wildfire," University of Utah fire scientist Philip Dennison, who wasn't part of the study, told the Associated Press. "Climate change loads the dice toward warmer, drier conditions that make it more likely that a fire will develop from human-caused ignitions."

This report contains material from the Associated Press.