Climate study: Rising risk of wildfires and extreme rain overlapping dangerously by mid-century

By Nicholas Gerbis
Published: Friday, April 1, 2022 - 2:50pm

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Flash flood
Daniel Swain
Flash flooding due to post-Thomas Fire rainfall in Montecito, California, in January 2018.

Climate change could dramatically increase the risk of extreme rainfall after wildfires, potentially triggering more flash floods and mudslides across much of the western U.S., according to a new study in the journal Science Advances.

"It's increasing the magnitude of rainstorms, and it's also increasing the hot, dry, windy conditions that can lead to wildfires," said co-author Samantha Stevenson, an assistant professor in the UC Santa Barbara Bren School of Environmental Science & Management. "That risk is already higher than it was before, because of climate change, and that risk is only going to increase in the future." 

Based on a high-carbon-emission, "business-as-usual" scenario, computer models predicted a dramatic increase in the risk of extreme rainfall after wildfires by mid-century.

By century's end, the frequency of that overlap could double in California and grow 700% in the Pacific Northwest.

Worsening droughts and lengthening fire seasons only worsen those odds.

"That increased risk is going to be normal. And so we need to start planning for a future where most of the wildfires are pretty likely to experience some form of extreme rain that could cause really damaging impacts in the future," said Stevenson.

After a fire, rain can cause debris flows for up to five years and flash floods for up to eight. To account for such varying time windows, the authors looked at a range of time periods. 

"The risk is still elevated even a few years after the fact. So maybe you're not at as much risk of a super-devastating debris flow, but there's still going to be the potential for flash flooding to things like that even a few years out," said Stevenson.