Fires Then Floods: High Risks Of Deadly Climate Mix

A new study finds that global warming dramatically increases the risk of intense wildfires in the American West followed by more torrential rain, highlighting the need for better preparedness for hazards, such as mudslides and flash floods, that can wreak havoc long after a fire breaks out. Severe fires broke out.

Fires destroy forests, destroy homes, and kill people and animals, but they also destroy vegetation and make soil less permeable. This makes it easy for even short periods of heavy rain to cause flooding and a runaway flow of mud and debris. Rain after wildfires can also contaminate drinking water by smothering rivers and basins with sediment from eroded hillsides.

Scientists believe that human-caused climate change is causing more hot, dry conditions that lead to catastrophic fires. Warmer air can contain more moisture, which means precipitation gets more intense as well.

Until now, climate researchers studying the western United States have not attempted to determine how often these two extremes might occur in the same place over a short period of time, said Danielle Toma, a postdoctoral fellow at the National Atmospheric Center. Research in Boulder, Colorado, and lead author of the new study.

“Those are the times when these events can be really risky,” said Dr. Toma, three months to half a year after the fire broke out, before the soil and vegetation had time to recover. The study was published Friday in the journal Science Advances.

Residents of Western countries have witnessed many of these painful climatic disasters, and their horrific consequences, in recent years.

The new study uses computer models to predict how the combined frequency of such events across the West might change under a warming scenario for the next decades.

Climate scientists believe that greenhouse gas emissions from human activity are less likely to lead to such higher levels of warming on their own than they did before. The study authors said they expected smaller but still significant increases in precipitation after the bushfires under less pessimistic trajectories of global warming.

The study finds that by the end of the century, more than half of the days with very high wildfire risk in parts of the Pacific Northwest, Idaho, Nevada and Utah could follow heavy rainfall within a year. The study found that the proportion is smaller in California and Colorado, although it is still well above the average between 1980 and 2005. The increase is noticeable within six months of severe fire days and within a year.

Western Colorado and most of the Pacific Northwest are also expected to see a jump in the chance of heavy rain within three months of severe fire conditions. In California, wildfire season and rainy season tend to be the hottest of the year.

Swain, a climate scientist at the University of California, Los Angeles, and another author of the study, “Even by mid-century, some places are seeing double or triple” risks. “That’s not 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 surprised that their computer models showed such a persistent increase in risk across the West, even though the region’s climate is very diverse. California has dry summers and wet winters, while in Colorado, flooding and wildfires peak during the warm season.

It doesn’t take much rain to create a debris flow on a recently-burned slope, said Jason W. Kane, a USGS hydrologist in Golden, Colorado, who was not involved in the study. In some areas, he said, falling just a fraction of an inch in 15 minutes might be enough.

With more wildfires occurring in places where it wasn’t a big deal before, scientists are working to understand how thresholds might differ in those wetter climates, Dr. Keen said. “It’s a struggle for us to stay ahead,” he said.

Dr. Thoma did most of the analysis of the new study when she was a postdoctoral researcher at the University of California, Santa Barbara, near Montecito, which was devastated by the mudslides that followed the fire in 2018. There authorities have urged residents to evacuate certain areas, but many have chosen not to. So.

“There was a lot of fire fatigue just a month ago,” said Dr. Toma.

Samantha Stephenson, an ecologist at the University of California, Santa Barbara, who also worked on the study, said Western residents are generally aware of the dangers of flooding and mudflows in burning areas. But “the degree to which it is increasing as a result of climate change, and the speed of that increase, is something we should probably try to be more aware of,” she said.

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