California is burning.

The dangerous fires that have broken out across the state show no signs of stopping, driven by record powerful seasonal winds that are forcing hundreds of thousands of residents from their homes as the flames roar across hilltops and through vineyards. The biggest of them, the Kinkaid Fire in Northern California, is not under control yet and is expected to grow as winds pick up later in the week.

The most disastrous fires in California often occur in the fall. The long, dry summers transform vegetation into the perfect fuel for the annual winds that whip across the landscape.

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 stamp is evident in many of the fires, scientists say, primarily because hotter air means drier plants, which burn more readily.

Fuelling the fires

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.

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“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. 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 early, in the spring. There’s less and less snow accumulating in the high mountains of California as the climate warms, and any snow that does fall 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, 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.

A problem with wind

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 the speeding winds pass over a flame, they can spread it far and wide, and fast—which is exactly what happened during 2018’s Camp Fire, 2017’s Thomas Fire, and many more.

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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 it doesn’t necessarily mean that there will be 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.”