Even before Southern California began its frantic fight against six major fires that have been raging through dry shrubland this week, 2017 was in the record books as the worst season ever for wildfires in California.

As of December 3, fire officials had tallied 8,747 fires that burned through more than 1 million acres and killed 43 people in the state. The total does not include two new fires that ignited Thursday in Riverside and San Diego counties.

This week’s fires have torched more than 158,000 acres and are inching perilously close to densely populated areas in Ventura and Los Angeles, shrouding them in dangerous air pollution. And the fires are still going, as you can see in this map of current wildfires in California below:

Luckily there have been no deaths so far in the Southern California fires. But Napa and Sonoma counties in the north are still reeling from the Tubbs fire, which ignited on October 8. It killed 22 people, damaged 5,643 structures, and burned 36,807 acres, making it the single most destructive fire in California history. (The New York Times has a fantastic graphic showing how quickly this fire spread.)

“It was like a blowtorch,” said Scott McLean, deputy chief at the California Department of Forestry and Fire Protection (Cal Fire). “We could’ve put every piece of equipment in its path and that fire would’ve gone over it, under it, through it. It wouldn’t have mattered.”

Entire neighborhoods turned to ash, and many were caught off guard as the fire roared through homes in the middle of the night, leaving little time for escape. “What really got to me was the amount of vehicles still parked in their driveways,” McLean said.

Though seasonal wildfires are a natural occurrence in the Golden State, humans are making them worse and more dangerous every step of the way.

And California’s fires are just the latest unfolding tragedy in what has already been an epic fire season across the United States as a whole, with more than 9.2 million acres burned and smoke choking many parts of the West.

Fires are more damaging because we keep building in harm’s way

The California fires stretch the definition of “natural disaster” because human activities have exacerbated their likelihood, their extent, and their damage. Deliberate decisions and unintended consequences of urban development over decades have turned many parts of the state into a tinderbox.

This year’s blazes particularly stand out because of how close they are to suburbs and major cities.

“When we get wildfires close to residential areas, that’s what makes them extraordinary events,” said Heath Hockenberry, fire weather program manager at the National Weather Service. It’s also getting increasingly hard to keep people at a safe distance from the embers.

Harrowing scenes of flames and smoke have emerged, like this video from Santa Rosa, 55 miles north of San Francisco:

Much of California is naturally hot, dry, and prone to fires for parts of the year. But the state’s population is growing, leading to a significant overlap between the areas of high fire risk and areas with a growing population density, as you can see in these maps from a 2014 study of population trends in California projecting out to 2050.

The study estimated that by 2050, 645,000 houses in California will be built in “very high” wildfire severity zones.

“We are definitely seeing [construction in fire-prone regions] happen more and more: 95 percent of the population of the state lives on 6 percent of the land,” said Lynne Tolmachoff, a spokesperson for Cal Fire.

Californians are drawn to views of mountains, forests, and grasslands and are building ever closer to these features that often have a propensity to burn. And places like Napa and Sonoma counties, picturesque regions that are now charred, have some of the fastest-growing property values and highest-priced homes in the United States.

This proximity is part of what’s driving the death toll and the embers haven’t discriminated between wealthy and poor Californians. “Where these fires occurred, I think the risk is generalized all around,” Tolmachoff said. “They went from the rural areas to very urban areas. ... It affected everyone pretty much evenly.”

Building in or near fire-prone forests has also led to fire prevention land management practices that paradoxically increase fire risk. For instance, policies for preventing wildfires have in some areas led to an accumulation of the dry vegetation that would ordinarily burn away in smaller natural blazes.

“The thing that gets missed in all of this is that fires are a natural part of many of these systems,” said Matthew Hurteau, an associate professor at the University of New Mexico studying climate impacts on forests. “We have suppressed fires for decades actively. That’s caused larger fires.”

We keep starting these fires

A study published earlier this year in the Proceedings of the National Academies of Science, or PNAS, found that 84 percent of wildfires are ignited by humans, whether through downed power lines, careless campfires, or arson.

“Human-started wildfires accounted for 84% of all wildfires, tripled the length of the fire season, dominated an area seven times greater than that affected by lightning fires, and were responsible for nearly half of all area burned,” the paper reported.

Transmission lines appear to be the culprit behind the wine country fires, but officials are still investigating other causes.

Video shows heavy flames, smoke next to California power lines https://t.co/0Lw5ilJog3 pic.twitter.com/4vbE9Pa98i — kcranews (@kcranews) October 10, 2017

The utility serving the region, Pacific Gas and Electric, has previously been billed for firefighting costs for fires stemming from its transmission lines and could have to pay billions of dollars in damages for some of the current blazes.

John Abatzoglou, a climatologist at the University of Idaho who studies wildfires and is an author of the PNAS study, noted that some of the fires in California ignited in multiple places around the same time, hinting at arson. “That is a possibility in play here,” he said. Whatever the cause, these fires don’t seem to be “natural” disasters, he said.

We keep changing the climate, which makes fires more likely

There are some unique weather conditions that are driving the exceptionally swift California fires, like strong winds and high temperatures. But long-term trends linked to global warming also exacerbated this year’s fire season across the United States.

“Fuel, wind, and long-term dry conditions: Those are the three facts that are really what’s causing this right now,” said the National Weather Service’s Hockenberry.

After years of drought that left behind ample dry vegetation, California saw intense rainfall last year and then a cool, wet winter. The increased precipitation led to a sudden growth spurt in combustible grasses, shrubs, and trees.

What followed during the summer was a period of intense, dry heat throughout the state, including the highest temperatures ever recorded in the Bay Area.

California just finished its hottest summer on record. It's no coincidence that this week's wildfires are blazing out of control. pic.twitter.com/7zLOc2l1Bc — Eric Holthaus (@EricHolthaus) October 11, 2017

“When it dried out, it dried out really hard, and it got really hot,” Hockenberry said.

It was the warmest April through September on record, Abatzoglou said. “Big fires typically happen a year after it being quite wet.”

Lastly, the dry autumn Santa Ana winds in the southern part of the state and Diablo winds in the north pushed flames through dry kindling.

While San Francisco is cooled by an ocean breeze year-round, the Diablo winds roll down the Sierra Nevada to the north and the east.

“Just like you pump up a bike tire, you’re compressing the air and heating it,” said Abatzoglou.

These winds were exceptionally strong this year and will likely continue the rest of the year. They typically blow through California at speeds between 35 and 40 mph, but meteorologists reported hurricane-strength gusts this year as high as 70 mph.

“Those northerly winds were fairly well forecasted,” Abatzoglou said. “We did see this coming, though people did not probably expect the breadth of fire activity.”

Though the winds are seasonal, and it’s difficult to attribute any individual wind event to climate change, humanity’s fingerprints are all over the fuel for forest fires that sparked earlier this year.

Abatzoglou co-authored a study last year that found that climate change due to human activity accounted for roughly 55 percent of the aridity in Western US forests between 1979 and 2015.

This led to a doubling of the area torched by forest fires than would have occurred in the absence of human-caused factors.

However, the California fires are burning through grasses and shrubs, not forests, and Abatzoglou was hesitant to make similar pronouncements about the current blazes.

“I would be cautious in saying climate change was a significant factor here,” he said. “This is very different from the fires we had [last month in forests] in much of the Western states.”

Jon Keeley, a research ecologist at the US Geological Survey, agreed. He noted that Southern California already has a hot and dry climate for much of the year, so rising temperatures don’t alter the fire risks and there is no discernible climate change signal in the current blazes.

The main factor behind the ongoing devastation around Los Angeles is that the fires ignited right as seasonal Santa Ana winds picked up. It’s a dangerous combination, but one that has a precedent in the region.

“The fire season is worse than usual, no question about that,” Keeley said. “It’s not anomalous in the history of California.”

Nonetheless, the California fires do align with what researchers expect to see across the United States as average temperatures rise.

“The length of the fire season is increasing in the Mountain West,” said the University of New Mexico’s Hurteau. “The mechanism for that is in part because [as] the atmosphere warms up, the air expands and can hold more moisture.”

This warming draws moisture out of plants, creating drier conditions earlier in the season. It also causes an earlier snowmelt in the spring, leading to more arid conditions in the summer.

“We could have a lot more fire on these landscapes,” Hurteau said.

Wildfires also contribute to global warming: Flames coursing through woodlands and grasses send greenhouse gases and particulates into the air.

“When the plant material in forests combusts, we’re putting a lot of emissions of different types into the atmosphere,” said Hurteau.

Some kinds of particles trap heat, while other particles have a cooling effect. Both pose a huge health hazard, and when they land on snowcapped mountains and glaciers, they accelerate melting.

The good news: We can take steps to reduce fire risks

Tactics like cutting fuel breaks — or strips of land where the vegetation has been cut back to block the spread of fires — between combustible vegetation and homes can help reduce risks. Better forecasts, early warning systems for fire risks, and mandatory evacuation can also keep people out of danger.

“It doesn’t solve the larger problem, but it does reduce the risk to property,” Hurteau said.

Firefighters are now bracing for more winds that may expand the range of these fires, especially after record heat baked Southern California last week, pushing back the prospect of a season-ending rainfall event to quench dry grasses and shrubs before they ignite.

“It’s sort of a little bit of a game of beat the clock,” Abatzoglou said. “What we typically see is the jet stream will start moving further south and it will start raining in California.”

“We’re hoping we get one of these juicy precipitation events pretty soon, because the longer we go without rain, the more tenuous the situation is,” he added.