Fires, then floods: risk of deadly climate combination increasing

Global warming is greatly increasing the risk that extreme wildfires in the American West will be followed by heavy rains, according to a new study, which highlights the need to better prepare for hazards like mudslides and flash floods, which can continue to wreak havoc long after the blazes have been severe are made.

Fires devastate forests, destroy homes, and kill people and animals, but they also destroy vegetation and make the soil less permeable. This makes it easier for even short torrential rainstorms to cause flooding and uncontrolled flows of mud and debris. Rainfall from wildfires can also contaminate drinking water, choking rivers and pools with sediment from eroded hillsides.

Scientists believe human-caused climate change is leading to more hot and dry conditions leading to catastrophic fires. Warmer air can hold more moisture, which means precipitation will also increase.

So far, however, climate scientists studying the western United States have not attempted to pin down how often these two opposing extremes might occur in the same place within a short period of time, said Danielle Touma, a postdoctoral researcher at the National Center for Atmospheric Research in Boulder, Colorado Lead author of the new study.

Three months to six months after a fire, before the soil and vegetation have had time to recover, “are the times when these events can be really risky,” said Dr. touma The study was published in the journal Science Advances on Friday.

Residents of Western countries have experienced many of these double whammy weather disasters and their harrowing consequences in recent years.

The new study uses computer models to project how the frequency of such combined events might change in the West in a high global warming scenario for decades to come.

Climate scientists believe it is less likely than it used to be that greenhouse gas emissions from human activity alone will lead to such high levels of warming. The study’s authors said they expect a smaller but still significant increase in rainfall after wildfires under less pessimistic global warming pathways.

The study finds that by the end of the century, more than half of the extremely high risk wildfire days in parts of the Pacific Northwest, Idaho, Nevada and Utah could be followed by heavy downpours within a year. The proportion is smaller for California and Colorado, the study found, although it’s still well above the 1980-2005 average. And the increase is significant both within six months after severe fire days and within one year.

Western Colorado and most of the Pacific Northwest are expected to see an increase in the likelihood of heavy rains within three months of dangerous fire conditions. In California, the wildfire season and wet season tend to be more separated throughout the year.

“Even by mid-century, some places are seeing a doubling or tripling” of the risk, said Daniel L. Swain, a climate scientist at the University of California, Los Angeles and another author of the study. “That’s not that far in the future, and that’s not much more additional warming than we’ve already seen.”

dr Swain said he and his colleagues were impressed that their computer models showed such a consistent increase in risk across the west, despite the region’s varied climate. California experiences dry summers and wet winters, while Colorado experiences both flooding and wildfires during the warm season.

It doesn’t take a lot of rain to trigger a debris flow on a recently burned slope, said Jason W. Kean, a hydrologist with the United States Geological Survey in Golden, Colorado, who wasn’t involved in the study. In some areas, a drop of just a fraction of an inch in 15 minutes could be enough, he said.

But as more wildfires occur in places where they weren’t previously a major problem, scientists are working to understand how thresholds might differ in these wetter climates, said Dr. kean “It’s a struggle for us to come out on top,” he said.

dr Touma performed most of the analysis for the new study while she was a postdoctoral researcher at the University of California, Santa Barbara, not far from Montecito, which was devastated by post-fire mudslides in 2018. Authorities there had asked residents to evacuate certain areas, but many chose not to.

“Just a month earlier, there was a lot of evacuation fatigue from the fire,” said Dr. touma

Westerners are generally very aware of the risks of flooding and mudslides in fire areas, said Samantha Stevenson, an environmental scientist at the University of California, Santa Barbara, who also collaborated on the study. But “the extent to which they are increasing as a result of climate change, and the rate at which that increase is happening, is something we might want to become more aware of,” she said.

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