Firefighters are steadily beating back several massive autumn blazes in California, as heavy smoke from the wildfires blankets much of the state, creating unhealthy air quality.

The fierce Camp Fire near Chico has so far torched more than 150,000 acres, taken at least 77 lives, and destroyed 10,300 residences, making it the most destructive and deadliest wildfire in state history.

In Los Angeles and Ventura counties, the Woolsey Fire recently burned more than 96,000 acres and killed three people.

You can see the current fires underway in California in this map from the California Department of Forestry and Fire Protection (Cal Fire):

Nationwide, 8.5 million acres, an area larger than Maryland, have burned this year to date. But California, once again, is taking much of the heat.

It will take some time for the ash to settle and to get an accurate tally of all of the devastation. But given that the number of acres burned is already double what burned last year, 2018 sure to break several records.

The overall trend in California is troubling too. Six of California’s 10 most destructive wildfires on record have hit in just the past three years. The 2017 Thomas Fire in Santa Barbara and Ventura counties, which burned well into January this year, held the record for largest fire in state history for just eight months before the Mendocino Complex Fire came along in August.

It’s important to keep in mind that wildfires are a normal phenomenon across many parts of the US (and the world). They’re essential for restoring nutrients to the soil, clearing out decay, and helping plants like the lodgepole pine reproduce.

But fires in 2018 are 30 percent larger than the average over the past decade, making this year far worse than most in recent memory. And the growing massive, destructive, and deadly conflagrations we’re seeing now are hardly a force of nature at this point. At almost every step, human activity has exacerbated the risks, the damages, and the harms from fires. These risks are continuing to mount, which means the future holds more dangerous, frequent, and costly blazes for vast swaths of the United States.

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

The California fires stretch the definition of “natural disaster” because 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 large they are, how many structures they’ve burned, how many lives they’ve taken, and how close they’ve come to populated areas. They are also creating their own unique weather phenomena, like pyrocumulus clouds, and producing fire whirls with tornado-strength winds reaching 143 miles per hour near neighborhoods:

“When we get wildfires close to residential areas, that’s what makes them extraordinary events,” said Heath Hockenberry, the 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.

Many areas in California are naturally hot, dry, and prone to fires much 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,” Lynne Tolmachoff, a spokesperson for Cal Fire, told Vox last year.

Californians are drawn to views of mountains, forests, and grasslands. Their homes are cropping up ever closer to these features, which often have a propensity to burn. And picturesque places like Napa and Sonoma counties, which suffered immense fires in 2017, 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 ... 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.

The Carr Fire near Redding, for example, was ignited by a “mechanical failure of a vehicle,” according to fire officials. Transmission lines appear to be the culprit behind the 2017 wine country fires, but officials are still investigating other causes.

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 last year 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.

This year’s weather throughout the West eerily resembles last year’s: After years of drought that left behind ample dry vegetation, California saw intense rainfall last year and then a cool, wet winter in early 2017. The increased precipitation led to a sudden growth spurt in combustible grasses, shrubs, and trees.

California was again drenched with heavy rains earlier this year, leading to a similar pattern of vegetation.

What followed again during the summer was a period of intense, dry heat throughout the state, with a heat wave shattering temperature records in several cities.

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

According to Abatzoglou, “Big fires typically happen a year after it being quite wet.”

A big factor in last year’s fires was the dry autumn Santa Ana winds in the southern part of the state and Diablo winds in the north pushing flames through dry kindling.

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 sparking this year.

Abatzoglou was an author of 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, many of California’s fires are burning through grasses and shrubs, not forests. Signals of changes in the climate are harder to elucidate in these blazes.

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.

If Northern California had received anywhere near the typical amount of autumn precipitation this year (around 4-5 in. of rain near #CampFire point of origin), explosive fire behavior & stunning tragedy in #Paradise would almost certainly not have occurred. (1/n) #CAfire #CAwx pic.twitter.com/2LBKjSVBMF — Daniel Swain (@Weather_West) November 10, 2018

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.

All this adds up to a huge increase in the area burned by fires each year. A PLOS One study last year found that 11 states would suffer a surge in scorched area greater than 500 percent over the next 20 years.

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

Firefighters in the United States have learned from past fire attacks and developed robust methods to keep fires in check, which is why despite the massive fires we’re seeing, the death tolls have remained relatively low. At least nine people have died from wildfires this year, including firefighters engaging the flames.

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 (but not damming rivers to keep water from flowing into the Pacific Ocean, as President Trump suggested).

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

The tougher challenge will be to change behaviors and incentives so people don’t build or rebuild in fire-prone regions. Already, many homeowners harmed by last year’s fires are placing their roots back in the same fire-prone regions, either because of insurance payouts or because they have nowhere else to go.

For now, firefighters are bracing for more warm weather and winds in southern California that will spread choking smoke as they continue their battle with fires.