California winter rains used to ward off wildfires — but not anymore, new study finds

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Storms have dumped torrential rains and snow across California this winter, triggering deadly floods, mudslides and other catastrophes. But all that water is also banishing drought conditions in the state, and bringing scrubby hills to life with vegetation.

Could those wet conditions signal a reprieve from the recent rash of catastrophic wildfires, too?

Don’t bank on it, an international team of researchers warned in a study published Monday in the Proceedings of the National Academy of Sciences.

Scientists found that wet winter weather, historically a predictor of more modest California fire seasons, is no longer linked to less damaging fires. The link between more rain and less fire fell apart thanks to modern fire management and accelerating climate change, the study said.

“Fire not being influenced by moisture anymore? That is surprising,” said study co-author Alan Taylor, a Pennsylvania State University geography professor, according to a University of Arizona news release. “It’s going to be a problem for people, for firefighters, for society.”

Intense, catastrophic fires have ravaged just about every part of the state in the last several years, including the devastating Camp Fire in Northern California last year, which killed more than 80 people to become the deadliest in state history, the Sacramento Bee reported. Meanwhile, Southern California’s Woolsey Fire burned in the Los Angeles area around the same time, reducing to rubble the homes of Miley Cyrus, Neil Young and other celebrities.

“The last three years may be a harbinger of things to come,” Taylor said in the news release. “Between 1600 and 1903 there was not a single case of a high-precipitation year coupled with a high-fire year as occurred in 2017.”

The strength and position of the North Pacific jet stream (winds from the west that control California’s winter rainfall) helped predict the risk of wildfires in the Sierra Nevada during the following summers as far back as 1600, and as recently as 1904, according to a news release from the National Oceanic and Atmospheric Administration on the findings.

But that started changing in the early 20th Century, the study said.

From 1904 on, there was a weaker link between precipitation from December to February and how bad summer fires were, just as federal policies began suppressing wildfires in the West — and by 1977, the once-strong connection between winter wetness and less severe fires had vanished altogether, researchers said.

“I didn’t expect there to be no relationship between jet stream dynamics and fire in the 20th century,” said study co-author Valerie Trouet, a University of Arizona professor and tree ring researcher, according to the university news release. “I expected it to be maybe weaker than before, but not to completely disappear.”

The link between jet stream position and precipitation is still intact, according to researchers.

“When the jet stream is positioned over California, it’s like a fire hose — it brings storms and moisture straight over California,” Trouet said. “What we see post-1900 is that the position of the jet stream is still an important driver of moisture to California — it brings moisture to California when it’s in the right position — but there’s a disconnect with fire.”

Trouet specifically pointed to the relatively wet California winter in 2016 and 2017, which was nonetheless followed by the deadly Tubbs and Thomas fires, according to the university press release.

“It’s not either climate change or historical fire management — it’s really a combination of the two that’s creating a perfect storm for catastrophic fires in California,” Trouet said.

Researchers wrote in the study that “recent California fires during wet extremes may be early evidence” that climate change-driven temperature increases, drier summers and less snowpack have already canceled out the link between winter rainfall and less severe fires.

Researchers said they relied on paleoclimate data and climate model simulations to complete the study — including “natural archives of climate and fires stored in tree rings that go back in time for centuries,” according to the university news release.

NOAA, a federal agency, warned in its news release on the study that “if warming continues, as is the scientific consensus, then significant wet season rain and snow may not ensure a quiet fire season afterward.”