Californians may feel like they’re enduring an epidemic of fire. The past decade has seen half of the state’s 10 largest wildfires and seven of its 10 most destructive fires, including last year’s Camp Fire, the state’s deadliest wildfire ever.

A new study, published this week in the journal Earth’s Future, finds that the state’s fire outbreak is real—and that it’s being driven by climate change. Since 1972, California’s annual burned area has increased more than fivefold, a trend clearly attributable to the warming climate, according to the paper.

Why are summertime forest fires so much more likely? Because climate change has already redefined the seasons in Northern California. Since the early 1970s, summers in Northern California have warmed by about 2.5 degrees Fahrenheit (1.8 degrees Celsius) on average. A few degrees may not sound like much, but heat has an exponential relationship with forest fire.

“Each degree of warming causes way more fire than the previous degree of warming did. And that’s a really big deal,” Park Williams, a climate scientist at Columbia University and an author of the paper, told me. Every additional increment in heat in the environment speeds up evaporation, dries out soil, and parches trees and vegetation, turning them into ready fuel for a blaze. For that reason, Williams said, hot summers essentially overpower anything else happening in Northern California. Even during a wet year, an intense heat wave can choke forests so that it is as though the rain never fell.

But this outbreak of climate-addled fires is limited to summertime fires in forests; it does not extend to other types of environment or other times of the year, the paper cautions. Williams and his colleagues found that the amount of burned non-forest area—such as Southern California’s shrub and grassland—has not significantly increased.

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