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Southern California to see steep hike in fire danger due to warming, study finds

Study projects large increase in number of high-risk fire days and a longer fire season

The Silverado Fire burns in Irvine, Calif., in October 2020. (Jae C. Hong/AP)
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Days when large damaging wildfires are possible could nearly double in Southern California by the end of the century if climate change continues unchecked, according to a study released Thursday in the journal Communications Earth & Environment.

The study projects an overall lengthening of the fire season, with more large fires during historically cooler and wetter months in late fall and early spring. That’s particularly problematic in autumn, when Santa Ana winds strengthen and vegetation tends to be driest before the winter rainy season arrives.

For a scenario of high greenhouse gas emissions, the region could see 71 high-risk fire days per year by 2100, compared to the 36 it averaged during the 1970-1999 period, which the study uses as a historical baseline.

By 2100, Southern California is projected to be nearly 9 degrees warmer than it was near the end of the 20th century for an emissions scenario known as “RCP 8.5,” which assumes a heavy use of coal in the coming decades. With coal use declining in many countries, a number of climate scientists say this scenario has become less probable.

But even under a more moderate emissions scenario, the region warms by 5 degrees, and high-risk fire days increase to 58.

We may avoid the very worst climate scenario. But the next-worst is still pretty awful.

Fast-spreading wildfires become more likely as temperatures rise because a warmer atmosphere can draw more moisture from plants and soils, as measured by a metric called the vapor pressure deficit.

“As temperatures go up and humidity goes down, there’s a deficit of water vapor in the air,” said Glen MacDonald, a professor of geography at the University of California at Los Angeles and a co-author of the study. “That has been shown by other studies to really be tied into the potential for large fires — it helps promote drying of the fuels.”

Previous research found that climate change is the main driver behind a sharp rise in vapor pressure deficit in the Western United States over the last several decades.

Nighttime-fire intensity in the U.S. West has increased by 28 percent over the past two decades

With vegetation reaching tinder-dry levels more often during the year, fires ignite easily and burn more intensely.

“We’re predicting how many days are going to be ripe for large fire — that’s kind of like loading the dice,” MacDonald said. “There are going to be more of those days when, if you get that ignition and it’s windy, that fire is going to take off.”

Over the last several decades, Northern California has seen a clear upswing in burned area as temperatures rise, but the same trend hasn’t yet been observed in Southern California, perhaps because the region lacks the dense forests of the northern half of the state and is heavily populated — cut-through with freeways and housing developments.

Fires are lasting longer into the night, and researchers may have found out why

It has, however, experienced devastating wildfires in recent years, including the Thomas Fire in December 2017 and the Woolsey Fire in November 2018 — both late-season fires fueled by Santa Ana winds after unusually warm and dry spells.

Mitigating fossil fuel emissions will be key to avoiding an increasingly fiery and destructive future, because the risk diverges considerably after mid-century between the high and lower emissions scenarios.

Chunyu Dong, the study’s lead author and an associate professor at the Center for Water Resources at Sun Yat-sen University in China, wrote in an email that the authors used an improved and more refined modeling technique to simulate the future changes in daily large fire occurrence and in the length of the large fire season, and that after about 2050 “the destructive power of the high emissions [scenario] will be progressively released.”

This winter, California has seen a spike in wildfire activity after nearly six weeks without significant rain, a record-breaking February heat wave and repeated dry winds.

On Wednesday, the Airport Fire ignited near Bishop, Calif. in the Owens Valley and quickly grew to 2,800 acres by Thursday, driven by fierce winds.

MacDonald, the study co-author, directs the nearby White Mountain Research Center, where staff had to be evacuated as firefighters worked to contain the blaze.

“This is a story that’s happening in real time as we speak here,” he said. “This is the climate trajectory.”