Post-wildfire extreme rains to double in Western US, study says

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Extreme rainfall events in the wake of Western wildfires — on the rise due to climate change — may more than double by the end of the 21st century, posing a serious threat to human lives, a new study finds.

When precipitation drenches an area that has just experienced a blaze, the soil is unable to easily contain the moisture — resulting in significant destruction such as debris flows, mudslides and flash floods, scientists warned in a Science Advances article on Friday

If Americans continue to emit excessive amounts of heat-trapping greenhouse gases, then by the end of the century, extreme rainfall will be eight times more likely to occur within a year of a Pacific Northwest wildfire, the authors found. In California, the incidence of such consecutive extreme events will more than double.

“It’s very concerning, given the destruction that comes with these kinds of events,” lead author Danielle Touma, a postdoctoral fellow at the National Center for Atmospheric Research (NCAR), said in a statement. “Clearly we need to understand the risks better, as this creates a major threat to people and infrastructure.”

The national scientific research hub, located in Boulder, Colo., is footsteps from an unseasonal wildfire that shocked the region last weekend but caused no injuries.

Touma conducted much of the research for the study prior to her arrival at the NCAR, during a previous fellowship at the University of California, Santa Barbara.

While scientists have already recognized a link between climate change and increased wildfires in the West, as well as a separate association between climate change and extreme rainfall, the convergence of “extreme rainfall-after-fire events” was a surprise, she said.

To conduct their study, Touma and her colleagues said they used advanced computer models of past and future climate scenarios, as well as an index of weather variables that play a role in wildfire risk.

They incorporated several types of models, including the NCAR-based Community Earth System Model, which they say helped them project probable changes in climate conditions in the Western U.S.

Following a fire event, the risks of debris flows can persist for three to five years, while the risk of flash floods remains for five to eight years, due to the time necessary for vegetation to regrow, the authors observed.

Heavy rainfall on burned areas can be difficult to predict and have devastating impacts, the scientists warned.

In Montecito, Calif., debris flows following a 2018 fire left 23 people dead and caused widespread property damage, according to the study.

Meanwhile, heavy rains last year in Colorado’s Glenwood Canyon led to a massive mudslide in a freshly burned area — leaving more than 100 people stranded and shuttering a portion of the adjacent highway for weeks, the authors noted.

After evaluating their simulations, the scientists forecasted that by the end of the century, there will be a doubling of weather conditions that cause a risk of extreme wildfires throughout the West. The climate models also indicated a rise in extreme rainfall events.

From there, the authors evaluated the number of cases in which extreme rainfall is likely to overlap with a region that has recently experienced extreme wildfire.

In the West as a whole, they concluded that more than half of extreme wildfire events will be followed within a year by such rainfall.

They also found that more than 90 percent of extreme fire events in their three regions of focus — Colorado, California and the Pacific Northwest — will be followed by at least three extreme rainfall events within five years.

A key contributor to the convergence of extreme fire and rainfall events is climate change — and its propensity to override the seasonality of these weather events, according to the authors.

For example, they explained, more extreme rains are now occurring in the early fall in Colorado and the Pacific Northwest, closer to the May-September peak fire season.

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

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