Nearly two dozen wildfires have burned almost 170,000 acres across California this week, destroying thousands of structures and killing 23 people so far, in what already amounts to one of the worst wildfire seasons in the state’s history.
The blazes are concentrated in Northern California’s wine country, where more than a dozen fires ignited late Sunday as powerful, dry fall winds ripped through the region. Thousands of residents have been forced from their homes, and hundreds are missing (see “Fighting Fires from the Sky, No Pilot Necessary”).
The cause of the fires remains under investigation, but some local media reports raised the possibility that downed power lines may have played a role. Regardless of what produced the initial sparks, however, there’s a good chance that human-induced climate change made it easier for those fires to spread.
The clearest way in which global warming increases wildfire risk—one supported by a growing body of peer-reviewed literature—is higher temperatures. Warmer air draws moisture from plants, trees, and soil, increasing what’s known as fuel aridity. This provides the dry fuel and conditions that feed wildfires. Other climatic factors can also contribute, including decreased rainfall and reduced or earlier-melting mountain snowpack.
Human factors can increase the dangers as well, including increased development along wilderness boundaries and fire suppression efforts that can build up fuel, making fires more deadly when they do ignite.
Noah Diffenbaugh, a professor of earth system science at Stanford, studies the links between single extreme events and climate change. Notably, in several earlier papers, he and his coauthors concluded that human-influenced global warming “very likely” contributed to California’s recent five-year drought.
The Northern California fires are an ongoing event that he hasn’t analyzed and can’t draw any firm conclusions from yet, he stresses. But he notes that the drought killed “millions of trees” (in fact, more than 100 million), building up huge amounts of fuel. Separately, while the 2016-2017 winter was an extremely wet season, it was followed by a dry and blistering summer that set temperature records around the state.
“So without having analyzed this specific event, we know that the pathways by which temperature has influenced wildfire risks historically are relevant for the conditions in which this wildfire is occurring,” he says.
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The winds fanning the Northern California flames this week are a common enough occurrence to have a name, the “Diablo winds.” They blow across the interior to the coast, heating up, speeding up, and drying out as they descend from higher elevations. The same winds were a major factor in the devastating 1991 Oakland Hills firestorm that killed 25 residents and destroyed nearly 3,500 homes.
Shifting atmospheric climate conditions can certainly affect wind patterns, but the data is mixed on whether climate change is heightening this kind of natural wind phenomenon.
Whether climate change is a contributing factor to any single fire in California is almost beside the point. By now we know it does contribute to fires and will exacerbate many more extreme events, as climate scientists have long predicted, steadily increasing costs, damage, and deaths.
Indeed, anthropogenic climate change has doubled the area scorched by forest fires during the last three decades across the American West, burning an additional 16,000 square miles, according to a study last year in Proceedings of the National Academy of Sciences.
And it’s only going to get worse as temperatures continue to climb.
“No matter how hard we try, the fires are going to keep getting bigger, and the reason is really clear,” said coauthor Park Williams, a bioclimatologist at Columbia University, in a statement. “We should be getting ready for bigger fire years than those familiar to previous generations.”