Update, October 11: Fires have broken out in Southern California, forcing thousands of households to evacuate. In Northern California, the fire risk is still active, but PG&E has restored power to about half of the roughly 700,000 households affected by the planned electricity outages.
California’s largest utility has turned off the power for 500,000 accounts, with 250,000 more scheduled, in an attempt to avoid setting off another catastrophic burn like 2018’s Camp Fire, which killed 85 people and displaced thousands more. About 2.5 million people in the state are likely to be affected by the power outages.
Many of California’s most devastating burns over the past few years, from the Camp Fire to 2017’s Thomas Fire, were started by sparks from power lines. PG&E, the utility that provides electricity to some 16 million people across the state, is preemptively shutting down transmission through power lines that crisscross parts of central and northern California, hoping to reduce the chances that an errant spark will start another inferno.
The decision unfolded over a background of ever-increasing fire risk in the state, as climate change pushes California’s flammable ecosystems into ever more fire-prone conditions and management decisions leave communities vulnerable.
Climate change is contributing to California’s fires
California’s most destructive fires often occur in the autumn, when vegetation has been thoroughly dried out by long, hot summers and when the annual offshore winds start whipping up.
Frequent fires are part of California’s natural state. Many of its ecosystems, from the chaparral of Southern California to the northern pine forests, evolved to burn frequently. But since the 1980s, the size and ferocity of the fires that sweep across the state have trended upward: Fifteen of the 20 largest fires in California history have occurred since 2000. And since the 1970s, the amount of area burned in the state has increased by a factor of five.
Climate change’s fingerprint is evident in many of the fires, scientists say, primarily because hotter air means drier plants, which burn more readily.
Over the past century, California has warmed by about 3 degrees Fahrenheit, more than the global average of about one degree Fahrenheit. Hotter air draws water out of plants and soils more efficiently than cool, leaving the trees, shrubs, and rolling grasslands of the state dry and primed to burn.
Crucially, that effect increases exponentially with every degree of warming, explains Daniel Swain, a climate scientist at the University of California, Los Angeles. That means that today’s hotter, climate-changed air is much more effective at drying vegetation to a crackle than it was 100 years ago.
“All else being equal, in a warmer world, vegetation is going to be drier, even in a place like California where vegetation is usually dry by autumn. You can still make it drier,” says Swain—and that’s exactly what scientists have watched develop over the past few decades.
Summertime air temperatures in California have warmed by over 3.5 degrees Fahrenheit since the late 1800s, and that summer warming is particularly impactful, new research shows. The area burned across California during the summertime is about eight times higher today than it was only in the 1970s.
The major fires that have devastated the state in the past few years, though, have occurred in the fall, at the end of long, hot summers that sucked the wetness out of trees, shrubs, and other burnable material, but before the winter rains have kicked in.
Overall, the fire season—the time before the winter rains dampen the vegetation—has lengthened by 75 days over the past decades, according to CalFire.
Some of that happens at the early end of the season, in the spring. There’s less and less snow accumulating in the high mountains of California as the climate warms, and that snow is melting away earlier. As spring comes sooner, the dry season extends, leaving vegetation vulnerable to fire sooner in the season.
In some years—often the ones with the most devastating fires—the dry season extends deep into autumn, too.
“Usually—or, I don't want to even say usually anymore because things are changing so fast—we get some rains around Halloween that wet things down,” says Faith Kearns, a scientist at University of California Institute for Water Resources in Oakland.
Every day those rains don’t come is a day when fires can spark and spread. And in recent years, those rains haven’t kicked in until November, or even December.
But unlike earlier springs, those delays in the autumn rains don’t yet appear to be part of a longer-term trend in most of the state. The biggest climate-induced changes to the precipitation cycle so far seem to be about variability: when it rains, it rains more intensely, but when it’s dry, the droughts are worse. And in the future, the dry season is predicted to stretch longer into the fall.
The winds make things more difficult
Climate change may have already affected the characteristic autumn winds that have so often contributed to spreading fires across large swaths of the state. In the fall and winter, east-to-west (“offshore”) winds often flow across the state, with warm, dry air cascading down the western side of big mountain ranges like the Sierras. As the air flows downwards, it can get channeled into canyons or valleys, speeding as it falls. Gusts can reach 70 or 80 miles per hour. If those speeding winds pass over a flame, they can spread it far and wide, fast—which is exactly what happened during 2018’s Camp Fire, 2017’s Thomas Fire, and many more.
(Learn about how some scientists are studying giant fire clouds to understand how nuclear winter might play out).
There’s some evidence that climate change may actually make some of those wind patterns, like southern California’s Santa Ana winds, less frequent in the future. But that doesn’t necessarily presage a respite. The intensity is likely to stay strong in the winter, and in a drier, hotter future, the outcome could be fires that start later in the season but burn for longer.
Scientists are working hard to understand exactly how rain, snow, and winds will change—but the warming and drying patterns are clear.
“It just gets harder to predict,” says Kearns. “We used to have a much more reliable rainy season and fire season, and a lot of variables are just shifting at the moment.”