In New Climate, California's Wildfires Are 500 Percent Larger
Each degree of warming causes way more fire than the previous degree of warming did. And thats a really big deal. On a hot July evening last year, a rancher tried to use a hammer and stake to plug a wasps 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 Californias 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 theyre enduring an epidemic of fire. The past decade has seen half of the states 10 largest wildfires and seven of its 10 most destructive fires, including last years Camp Fire, the states deadliest wildfire ever. A new study, published this week in the journal Earths Future, finds that the states fire outbreak is real and that its being driven by climate change. Since 1972, Californias 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 Firehuge 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 states overall increase in burned area. Read: The simple reason that humans cant 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 thats 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 firebecause 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 thats exactly what the data showand exactly whats 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 areasuch as Southern Californias shrub and grasslandhas not significantly increased. Read: What its like to get caught in a wildfire And while autumn wildfires such as the deadly Camp Fire dominate the newsand while there is some evidence that they may be getting largerthere 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 well almost definitely be saying, Yeah, fall fires have the global-warming fingerprint on them. But right now were 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 papers results. And he said that some large-scale changes to the landscapesuch as the suspension of seasonal burns by indigenous peoplemay 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 states 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. Thats because the fundamental relationship between excess heat and additional fire never changes in the studys 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 states forests forward to the end of the century under a worst-case carbon-pollution scenario . They couldnt do it. Its 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 couldnt happen: By then, there will be no more forest left to burn. Fires will have finished clearing all of Californias woods. The once-mighty Californian forest will have given way to scrub, grassland, and deserttypes of ecosystem that can rebound quickly after a wildfire, or that never burn at all. Its not a foregone conclusion that all of Californias timberlands will vanish, Williams said. It depends on how we reduce carbon pollution now and in the years to come. The future of the states forests, it seems, is up to us.