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.
“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.