Read: The simple reason that humans can’t control wildfires

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.

And it matters that heat is prompting this 800 percent explosion in forest fire—because among the many ways climate change might be messing with the environment, extra heat is among the simplest and most obvious. “Heat is the most clear result of human-caused climate change,” Williams said.

In other words, the climate models say that Northern Californian summers should be getting hotter as climate change takes hold. And that’s exactly what the data show—and exactly what’s driving an unprecedented outbreak of forest fires.

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.

Read: What it’s like to get caught in a wildfire

And while autumn wildfires such as the deadly Camp Fire dominate the news—and while there is some evidence that they may be getting larger—there is still not enough data to say that any increase is statistically significant. But the climate models do suggest that autumn fires across California will get more common as climate change continues to wrack the state.

“Revisit this in 20 more years, and we’ll almost definitely be saying, ‘Yeah, fall fires have the global-warming fingerprint on them.’ But right now we’re still emerging from the range of natural variability,” Williams said.

Don Hankins, a professor of geography at California State University at Chico, told me that he wanted to see more data before agreeing with the paper’s results. And he said that some large-scale changes to the landscape—such as the suspension of seasonal burns by indigenous people—may be producing the rise in fire.