Correction appended, Oct. 17 2017 21:10 ET
The 14 large-scale fires still raging throughout California have claimed at least 41 lives, and scorched over 5,700 structures. Whipped by 70 mph gusts, the multiple blazes have spilled into residential neighborhoods and sprinted through about 220,000 acres—an area larger than New York City. These are among the deadliest, most destructive two weeks in California wildfire history, and the windswept flames are still going.
Over 11,000 firefighters continue to battle the blazes, assisted by helicopters, air tankers and fire engines. As of Monday night, some fronts were almost fully contained, but others raged almost unabated.
Wildfires are a natural and annual occurrence in California, typically appearing toward the end of the long, dry summers. In four out of five cases, people provide the igniting spark. At the same time, the environment has also become more flammable. As the state grows hotter and increasingly subject to prolonged droughts, and as more people live in high risk areas, the fires simply get bigger, more severe, and more costly to put out.
Experts say we should continue to expect more extreme fires in California. Here’s why:
1. Climate change has helped make California’s wildfires more intense
Preceding California’s 2017 wildfires was the hottest summer on the state’s record. A brutal heat wave stretched into September and in some cities the mercury topped 120 degrees.
In an unfortunate twist, the rainfall that had just months earlier brought relief to California’s historic, six-year drought may have also contributed more fuel for the blaze. The winter rains allowed grass and shrubs to grow, but under the withering summer, the new vegetation turned into highly combustible tinder.
The summer heat was part of a regional, significant warming trend. Southern California has warmed by three degrees over the last century, and the whole state is becoming warmer, according to the Environmental Protection Agency. The agency warned that the state’s higher temperatures, reduced snowpack and scarcer supply of water make for a more flammable environment, likely to increase the “severity, frequency and extent” of wildfires.
“The most straight-forward climate-change connection is through warming. As long as there is fuel to burn, drying the fuel out increases fire danger, and fuels dry out faster and more when it’s warmer,” Park Williams, a bioclimatologist at the Lamont-Doherty Earth Observatory of Columbia University, told TIME.
Williams was one of the authors of a study published last year by the U.S. National Academy of Sciences, which estimated that human-caused climate change has doubled the acreage that would have otherwise burned across western U.S. forests between 1984-2015.
The average wildfire has also grown larger, burning under 25 acres in 1982, to over 100 acres now, according to data from the National Interagency Fire Center.
Already, California’s 2017 fire season has scorched more acres than the annual average for the past five years, and as of this month, has burned almost twice as many acres as last year (1,046,995 acres burned as of October 15, compared to 564,835 acres last year).
2. The fires also tend to be deadlier thanks to urban expansion into high-risk fire zones
Four of the still-raging 2017 fires already rank among the 20 deadliest fires in California’s history. The Tubbs fire alone in Napa and Sonoma has claimed at least 18 lives, and many remain missing, according to Cal Fire.
California’s population increasingly lives in proximity to high-risk fire corridors, with residential neighborhoods spilling into previously rural or remote territory. As a result, wildfires that can devastate settled areas have become more frequent — so have tragic scenes of neighborhoods and trailer parks razed to the ground. The Forest Service calls these areas the wildland-urban interface (WUI).
According to a 2017 risk analysis by Verisk, 15% — or over two million — of California’s homes are concentrated in high or extremely high wildfire risk zones. Another 12% are located in moderate risk areas. This means more than a quarter of the state’s homes are at moderate to extremely high risk of being ravaged in an annual wildfire.
In these WUI areas, controlled burns of dry vegetation — an essential component of fire risk management — are no longer, or less frequently, conducted because of the potential hazards to residents. So when fires do erupt, a huge amount of vegetation goes up in smoke over a vast, combustible area.
3. Putting out the flames is now more costly
Thanks in part to fires in WUI areas, California’s blazes have become more costly to tame.
When firefighters focus on protecting homes, they are unable to prioritize containing a fire — or erecting a barrier to prevent the further spread of the blaze.
Putting out fires in neighborhoods also demands both more personnel and more expensive equipment, further driving up the cost.
The severity of the blazes in California have necessitated the expensive deployment of the country’s largest firefighting aircraft, a converted 747 “SuperTanker,” to help combat the ongoing flames. The aircraft costs up to $250,000 a day.
The American West’s average fire season now also stretches over a greater portion of the year, 2.5 months longer than it was in the 1970s, according to the Forest Service.
Federal firefighting costs have soared. The government spent $202.8 million to fight fires in 1986. By 2015, it was spending $2.1 billion.
4. A fiery future in store
It’s only the beginning of California’s 2017 fire season, and experts are predicting we may not have seen this decade’s, or even this year’s worst.
As climate change and residential expansion help fuel wildfires, the fires in turn can exacerbate the impact of global warming — when carbon-absorbing forests go up in smoke, they become another greenhouse gas source.
Weather models have long predicted that California will encounter more droughts, and more wildfires.
California Governor Jerry Brown said he expects the current fire disaster will cost “tens of billions” in damages, and the state should brace for more extreme fire events.
“That’s the way it is with a warming climate and dry weather and reducing moisture,” he said at a press conference last week. “These kind of catastrophes have happened, they’ll continue to happen, and we have to be prepared and do everything we can to mitigate.”
Correction: The original version of this story misstated the proportion of homes in California that are at moderate to extremely high risk of being impact by wildfires. It is more than one quarter, not more than one third.