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Climate change may mean more extreme rain after wildfire in western US

Under severe warming scenarios, the risk is growing that areas of the western US will experience extreme rainfall within a year or so of being hit by a wildfire

Environment 1 April 2022

Golden Gate Bridge

San Francisco’s Golden Gate Bridge during an extreme storm in August 2020 that ignited wildfires

Gary Qian/Getty Images

The risk of extreme rainfall in areas that have recently experienced wildfires may increase significantly by the end of the century in the western United States, if greenhouse gas emissions continue to rise unabated.

“In many places in the western US, we experience a lot of natural disasters,” says Samantha Stevenson at the University of California, Santa Barbara. “Some of the most important ones are wildfires, many of which have burned through California and other western states recently. We also have rainstorms that can lead to devastating floods. Climate change has been known to amplify both these things.”

Following wildfires, there is a higher risk of landslides and flash floods for several years in the burned area because it takes time for the ground cover and vegetation that was once there to regrow. Heavy rainfall can trigger these events.

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Stevenson and her colleagues decided to study how often these extreme rainfall events will occur following a wildfire over the coming decades. The team ran simulations of the climate in the western US, under the most extreme warming scenario – in which greenhouse gases continue to be emitted uncapped.

In an extreme warming scenario, the team found that by the end of this century, extreme rainfall events in California will be twice as likely to occur in the year following a wildfire than they were in the late 20th century. Such events will be eight times more likely to occur in the Pacific Northwest. For over 90 per cent of extreme wildfire events that will happen in this century in Colorado, California and the Pacific Northwest, the team’s model predicts that extreme rainfall events will occur at least three times within five years of the fire.

The increased prevalence of these extreme rainfall events after wildfires is probably explained by the fact that both phenomena are becoming more frequent due to climate change, says Stevenson. This is reducing the gap between fire and rainfall seasons.

“A lot of these compound events we are already seeing,” says Stevenson. For example, in July 2021, landslides in Glenwood Canyon, Colorado – which had suffered from wildfires the previous year – severely damaged infrastructure. “We must be aware of the need to protect those landscapes following wildfire for a while,” she says.

Journal reference: Science Advances, DOI: 10.1126/sciadv.abm0320

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