Late last year, California experienced terrible—and in the case of the October Tubbs Fire, record-setting—wildfires. The fires were especially intense due to an unusually late start to the rainy season, which left vegetation dry as seasonal mountain winds kicked up like bellows in a forge.

This year, the situation has repeated. The Camp Fire in Northern California not only broke last year’s all-time record for structures burned, it also broke a much older record for the deadliest wildfire in the state’s history. And in Southern California, the Hill and Woolsey Fires have burned through homes on the north side of Los Angeles.

So what is going on with these extreme fires? Are they just chance or part of a trend? President Trump, via his Twitter account, has repeatedly blamed California for its fires and claimed that environmental policies for water use or forestry are somehow responsible. But these claims make no sense to anyone working in the state—or anyone who knows that forest fires aren’t put out by hose-carrying fire engines. In reality, many factors contribute to the current situation. And climate change is one of them.

Ready to burn

Mason Trinca for The Washington Post via Getty Images

Mason Trinca for The Washington Post via Getty Images

Patrick T. Fallon/Bloomberg via Getty Images

Patrick T. Fallon/Bloomberg via Getty Images

Patrick T. Fallon/Bloomberg via Getty Images

FREDERIC J. BROWN/AFP/Getty Images

David Crane/Digital First Media/Los Angeles Daily News via Getty Images

David Crane/Digital First Media/Los Angeles Daily News via Getty Images

Wildfires are all about dry fuels—the grass, brush, and trees that fires consume. When a forest is dry as a tinder box, a spark (whether lightning or electrical-line malfunction) or wayward flame can start a blaze. If a grass fire encounters mid-sized “ladder fuels,” it can burn hotter and climb into the forest canopy, transforming into a monstrous inferno.

This year, California’s autumn rains have been well below average; this followed a hot summer in which forest fuels reached record levels of dryness. Normal amounts of rain at the start of the wet season typically bring an end to the fire season, but, when that doesn’t happen, California’s weather brings in a unique risk.

At this time of year, high atmospheric pressure often sets up inland over higher-elevation land, with low pressure offshore. That means winds—known as the Diablos in Northern California and Santa Anas farther south—blow downslope over the Sierra Nevada mountains and out to sea. The winds can be extremely strong, but they’re also warm and dry. Just as moist air cools as it rises and eventually forms clouds that drop rain, sinking air heats up and its relative humidity drops.

On Sunday, for example, a weather station at the Santa Rosa airport north of San Francisco recorded the arrival of winds blowing seaward. In two hours, the temperature rose 30°F while the humidity dropped from 75 percent to 13 percent.

These winds supercharge wildfires and send them racing out of control. They also explain the patterns of drifting smoke in the last few days. The Camp Fire is well inland of the San Francisco Bay Area, but the winds have carried much of the smoke there en route to the sea, which is reflected by air quality data. The Hill and Woolsey Fires near Los Angeles, on the other hand, occurred (literally) on the coast. That smoke plume largely headed offshore rather than into the lungs of LA residents.

Humans vs. humans vs. nature

Precipitation is naturally variable in this area, but there are some relevant climate trends that can’t be ignored. Temperatures are obviously rising, which can translate to stronger evaporation drying out fuels. (That’s true across the American West.) California’s rainfall, meanwhile, seems to be becoming more concentrated in the middle part of the rainy season—meaning a declining trend in fall precipitation. That doesn’t mean the nightmare scenario of 2017 and 2018 is going to happen every year from now on, but it does mean it’s probably getting easier to roll the weather dice and end up with snake eyes.

Critically dry fuels that enable 🔥 do occur November in SoCal; but we project such conditions to become more frequent in the coming decades. Intersection of dry fuels + offshore winds (not explicitly included here) are key 4 future 🔥 potential in SoCalhttps://t.co/NjRx8ytUHo pic.twitter.com/bGVUUNmoxH — John Abatzoglou (@climate_guy) November 12, 2018

Climate change is not the only contributing human factor, though. As in other areas of the US, a 20th-century policy of putting out all wildfires as quickly as possible led to a build-up of brush and smaller trees. Instead of more frequent (but less intense) surface fires, this accumulated fuel leads to large forest fires. (Correcting this change in forest character is a primary aim of modern management efforts.)

Additionally, the continuing encroachment of urban and suburban development into wild lands causes problems in a couple of ways. First, development puts more homes and humans in harm's way when a fire does occur. And second, it puts more human activities in proximity to fuel, increasing the risk that we’ll accidentally start a blaze. That includes internal combustion vehicles and equipment as well as electrical transmission systems. (A transmission-line malfunction is currently being investigated as a possible source of the Camp Fire.)

All of this adds up to a wicked, dangerous, and expensive problem that won’t go away on its own. But at least with rain starting to appear in the forecast for parts of California, residents can start looking forward to an end to the 2018 fire season—hopefully before any more records fall.