On a hot July evening last year, a rancher tried to use a hammer and stake to plug a wasp’s nest. The hammer slipped, a spark flew, and a patch of dry grass ignited, according to the Los Angeles Times. Within minutes, the brush fire fed on bone-dry conditions and became too big to control.
It soon merged with another blaze and became the Mendocino Complex Fire, the largest wildfire in California’s history. It burned almost half a million acres, or roughly 720 square miles, before it was finally extinguished four months later. It killed one firefighter and injured four.
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.
The trend is dominated by fires like the Mendocino Complex Fire—huge blazes that start in the summer and feed mostly on timberland. Over the past five decades, these summertime forest fires have increased in size by roughly 800 percent. This effect is so large that it is driving the state’s overall increase in burned area.
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.
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.
Williams agreed that climate change is not the only potential driver of increased fire in the state. Over the past century, Americans have gotten better at suppressing fires, meaning that easily burnable fuels may be accumulating in the state’s forests. But he said that even if fires are burning through that excess fuel, the effects of climate change are much clearer in this study, during this time frame. That’s because the fundamental relationship between excess heat and additional fire never changes in the study’s data; the correlation is “just as strong for the last 20 years as for the first 20 years,” he said. That suggests that across the five decades, the forests have remained the same. Only the air temperature has changed.
There may be a day when the forests do change. Williams recently asked some of his students to simulate the survival of the state’s forests forward to the end of the century under a worst-case carbon-pollution scenario. They couldn’t do it. “It’s basically impossible,” he said. The state gets so hot that “in the 2070s, you have individual years where a quarter to a half of all the forest area in California is burning.”
But that couldn’t happen: By then, there will be no more forest left to burn. Fires will have finished clearing all of California’s woods. The once-mighty Californian forest will have given way to scrub, grassland, and desert—types of ecosystem that can rebound quickly after a wildfire, or that never burn at all.
It’s not a foregone conclusion that all of California’s timberlands will vanish, Williams said. It depends on how we reduce carbon pollution now and in the years to come. The future of the state’s forests, it seems, is up to us.