Humans are responsible for the vast majority of wildfires in the U.S.

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Wildfire

Wildfire

Increasingly in the United States, wildfires are caused by humans, not lightning.Forest Service Northern Region

In the past 20 years, thousands of wildfires have raged across the United States—and most of them are our fault.

In a study published Monday in the Proceedings of the National Academy of Sciences, researchers found that a full 84 percent of wildfires between 1992 and 2012 were caused by humans. Not only that, but humans also lengthened the seasons during which fires occurred each year, and were responsible for about 44 percent of the total area of land burned in the United States during that time period.

Taken at face value, the fact that humans are the cause of so many wildfires is almost counterintuitive.

“It was really surprising for me,” says Bethany Bradley, one of the lead authors of the paper. “They’re called wildfires right? That terms has always implied for me that this is a natural process.”

But the reality at present is that if you hear about a wildfire, it was, in all likelihood, set by human hands—not a natural cause like a lightning strike. That’s especially true on the East Coast. The region tends to be more damp than other parts of the country, so starting a large wildfire there is more difficult.

“In the East, if you get one lightning strike, the odds are that you’re not going to start a fire," Bradley says. Instead, it might take several strikes in one area to ignite a fire, where a stray lightning bolt in a dry area might touch off a fire immediately.

As a result, on the densely populated East Coast, fires are far more likely to be set by humans than in the West Coast, intermountain west, or Florida, which gets many lightning strikes.

Bradley and co-authors used a huge and publicly available dataset compiled by Karen Short at the U.S. Forest Service, which gathers together all reports of wildfires from federal, state, and local sources.

That includes any wildfires that firefighters have been called to combat, and excludes prescribed burns used to help keep fire-dependent ecosystems happy and healthy.

Among the categories of human-caused fires, Bradley says that debris burning reigns supreme, making up 29 percent of all human-caused fires in the database. Arson, a broad category for any deliberately set fire, sits at 21 percent.

Many of the resulting blazes have been devastating. The East Tennessee wildfires that decimated the Gatlinburg area last December were allegedly caused by two teenagers in an incidence of arson that ended with thousands of homes and acres of woods burned.

Equipment use clocks in at causing 11 percent of human-caused fires, with campfires and children each holding steady at 5 percent each. The remaining causes only weighed in at less than a percent each.

But it’s not just the numbers of fires that humans have caused that is so astounding. We’ve also managed to dramatically extend the fire season. Lightning-caused fires typically occur in a tight geographical place (out West) during July and August. But human actions don’t limit themselves to a season, and many human-caused fires occur in the spring and fall, widening the amount of time during which wildland firefighters must remain vigilant.

Firefighting costs, the authors say, are already at over $2 billion annually. And that doesn’t account for the economic damage caused by the fires burning through property.

One of the more interesting findings of the paper was that in at least one instance, American culture definitely helped shape the timing of fires. July 4 was by far the most common day for humans to set forests ablaze.

The researchers are already hoping to build on their research, looking more deeply into the sizes of fires and how those relate to their ignition sources, and also into the geographical distribution of where fires start—whether they are more likely to start in the wilderness, or on the boundaries of where humans and wilderness interact.