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Climate change creating more devastating rainfall events in wildfire burn scars

Devastatingly large wildfires followed by intense precipitation events will grow in frequency as the effects of climate change become more widespread and entrenched, a study published Friday has found. 

The American West has long been susceptible to landslides and flooding that afflict areas recently scorched by wildfires. The fire destroys vegetation that stabilizes slopes and prevents large-scale erosion while exposing more of hillsides and riverbanks to the ravaging effects of intense rain. 

A new paper published in Science Advances by a team of scientists working at the National Center for Atmospheric Research in Boulder, Colorado, argues that if rising temperatures continue unabated, the number of times that an extreme fire event will be followed by an extreme precipitation event will double in California and increase by eight times as much by the end of the century. 

“It’s very concerning, given the destruction that comes with these kinds of events,” said lead author Danielle Touma. “Clearly we need to understand the risks better, as this creates a major threat to people and infrastructure.”

The research, which Touma mostly performed at University of California, Santa Barbara, before joining the national center builds off of prior research that shows extreme fires and rainfall events will increase as a result of higher surface temperatures and changes to ocean patterns as a result of man-made climate change. 

But Touma said that the increasing frequency of post-fire rain events was both surprising and alarming. 

“The gap between fire and rainfall season is becoming shorter,” Touma said. “One season of disasters is running into another.”

The researchers split the American West into three regions — California, Colorado and the Pacific Northwest — and ran climate models that showed if a large fire hits in any of the three regions, an extreme rainfall event will follow the fire in 90% of instances. 

Mudslides can be extremely damaging and in some cases fatal.

A 2018 mudslide in Montecito, California, in Santa Barbara County killed at least 23 people (a body of a missing person has never been found) with another approximately 163 people hospitalized as a result of the devastating mudflows. 

The incident came one month after a series of major wildfires that afflicted the region of Southern California in December 2017, including the Thomas Fire, which at 281,000 acres was at the time the largest wildfire in California history. That record has since been surpassed by a number of fires, some of which have exceeded 1 million acres. 

The mudflows in Santa Barbara County cost at least $177 million in property damage on top of the loss of life. 

In 2021, a torrential downpour struck Glenwood Canyon, Colorado, an area that had been burned by wildfire within the previous year and caused a major mudslide that stranded about 100 people in a mountain town and shut down Interstate 70, one of the country’s main thoroughfares, for weeks. 

The study says that these events are no accident and are occurring with greater frequency due to the effects of climate change. Typically a region that is scarred by wildfire needs about three to five years before new vegetation safely secures the topsoil to a point sufficient to ward of worry about mudflows. 

In some areas, eight years are needed to stave off the potential of flash floods. 

Touma and her colleagues at the national center used a climate modeling tool called the Community Earth System Model to plot the anticipated changes in frequency as a result of increasing temperatures. 

The models indicated there will be a doubling of the type of weather conditions that make the America West’s landscape susceptible to large-scale fires, with several regions experiencing particularly acute risk for large wildfires.

Touma’s team also predicted a pronounced uptick in extreme precipitation events. 

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They then looked to combine the results to gain an understanding of how wildfires and extreme rainfall events will correlate, finding that nearly all of the wildfires in the Pacific Northwest, for instance, will be followed by an extreme precipitation event within five years. 

Part of the reason for the increase in correlation is the changing seasonality as a result of climate change. Large rainfall events in Colorado and the Pacific Northwest will increasingly occur during the autumn, arriving much closer to the region’s typical fire season. 

In California, fire season is almost year-round. The Thomas Fire struck in December, and it and the extreme rainfall event the following month combined to create the disastrous mudslide.

If greenhouse gases continue to create a heat-trapping atmosphere around Earth, disasters such as the Montecito mudflows will continue to percolate with increasing frequency, according to the study.

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Paul Ingram/TucsonSentinel.com

The Bighorn Fire burning along the Catalina Mountains near Golder Ranch and Oracle Roads in 2020.