This story was updated Oct. 14 with new numbers from fire officials.

As deadly wildfires sweep through California’s wine country, fire management experts are blaming their violent speed on the powerful Diablo winds. But the conditions that made the fires so destructive arose from this summer’s record-breaking heat—a kind that experts say will continue to fuel fires across much of the West as the planet warms.

The state has seen more than 11,200 fires flare up so far this year, putting it on track to outpace the number that hit the state in 2015, which was the country’s worst fire season on record, based on acres burned. 

The hot, dry Diablo winds that blow across the San Francisco Bay Area in the spring and fall were especially fierce late Sunday and Monday, when they whipped up the blazes in the middle of the night across Napa and Sonoma Counties. The fires—which ignited almost simultaneously and spread so quickly that some fused together into larger blazes—killed at least 37 people as they spread through neighborhoods and damaged or destroyed more than 5,700 homes and other structures, including well-known wineries. 

“Gusts were blowing up to 70 miles an hour,” said Richard Halsey, executive director of the Chaparral Institute, a conservation group that studies California’s fire-prone ecosystems. “There was strong onset, and extremely dry conditions. They sprung up in the middle of the night. A lot of people were asleep, and the fires burned quickly.”

In Santa Rosa, video from state emergency officials shows street after street of homes reduced to ash (below). The entire city of Calistoga was ordered to evacuate on Wednesday afternoon, and more communities were threatened by the flames, with high winds expected through the weekend. The smoke was also a health threat. The Bay Area’s air quality agency issued an alert saying the fires were “causing unprecedented levels of air pollution” that would likely continue for days. 

What actually ignited the fires is under investigation. Many fire experts and agencies have said downed power lines may have sparked the blazes, which then grew so quickly and powerfully that they jumped ridge-lines and sent embers a half-mile or more ahead of their own path.

On Sunday, the National Weather Service issued a prescient warning that the winds could be dangerous. Extremely dry conditions provided the fuel.

“Even yesterday, without wind, these fires were driven by the critically dry fuel bed,” Ken Pimlott, director of the California Department of Forestry and Fire Protection, said in a briefing on Wednesday. “We are still impacted by five years of drought. The significant rain that we had last winter, those effects are gone, and we are literally looking at explosive vegetation.” 

Wet Winter, Hot Summer Create Explosive Mix

The rainy winter and hot, dry summer combined to create this explosive mix.

“The long-term drought is over, but there’s still the legacy of that drought in terms of stressed forests. And on top of that we had a wet winter, which meant that the grasses and brush grew like crazy this spring,” said Daniel Swain, a climate scientist at UCLA.

Then, the state experienced its hottest summer on record, with a record heat wave in late August and September that dried vegetation to a crisp. “California summers are always warm and dry, but this one had a particularly warming and drying effect,” Swain said. “And we know there’s a long-term trend toward warmer and hotter summers.”

Already this year, 8.5 million acres have burned across the United States, not including the ongoing California fires. More than 200,000 acres have burned in the wine country fires, and several thousand more are ablaze in Southern California where the Santa Ana winds are fanning flames.

According to an analysis of NASA satellite data by the World Resources Institute’s Global Forest Watch Fires, California has experienced 11,273 fire alerts—where satellites detect the heat signatures of fires—as of September. That number was 10,177 in September of 2015.

“It’s following very closely with what happened in 2015,” said Susan Minnemeyer, a mapping and data manager at WRI. “While 2017 is not a record year across the U.S., in California it is.”

Across the nation, wildfires burned more acres in 2015 than any year in modern record keeping, surpassing 10 million acres for the first time. Despite the high number of fires across the West, it’s unlikely that 2017 will catch up to that figure. (This year, however, has broken at least one wildfire-related record: The government has spent more than $2 billion fighting fires, making it the most expensive ever.)

Flames Jumping House-to-House

Projections that drought and increased temperatures will continue to fuel fires has sparked a debate over how best to address wildfires, especially as they continue to encroach into urban areas in what’s known as the urban-wildland interface.

“This is what was so extraordinary about this event,” Swain said. “Essentially it was a forest fire, a wildfire, that moved into an urban area. At some point it was jumping from house to house, not tree to tree.”

State law requires property owners to keep vegetation away from their homes in certain fire-prone areas. These areas, known as “defensible spaces,” are intended to make structures easier to protect from blazes.

“They talk about defensible space—and that’s important,” Halsey said. “But more important is why does a home ignite in the first place? It’s because these embers are coming, driven by wind, and they ignite patio furniture and get into the attic through vents.”

“The fact of the matter is homes burned and things were destroyed because we live in a fire-prone environment,” Halsey added. “And we don’t build structures that are resistant to fires.”