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Fires, then floods: the risk of deadly climatic combinations increases


Global warming dramatically increases the risk that extreme wildfires in the American West will be followed by heavy rains, according to a new study, underlining the need to better prepare for hazards, such as mudslides and flash floods, which can wreak havoc long after the flames. serious fires are extinguished.

Fires ravage forests, destroy homes and kill people and animals, but they also destroy vegetation and make soils less permeable. So even short bursts of heavy rain can more easily cause flooding and uncontrollable flows of mud and debris. Rains after forest fires can also contaminate drinking water by choking rivers and ponds 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 hold more moisture, which means precipitation also becomes more intense.

Until now, however, climatologists studying the western United States had not tried to determine how often these two opposite extremes could occur in the same place in a short period of time, said Danielle Touma, postdoctoral fellow at the National Center for Atmospheric. Research in Boulder, Colorado, and lead author of the new study.

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

Residents of western states have seen many of these one- or two-punch weather disasters, and their harrowing aftermath, in recent years.

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

Climate scientists believe it is less likely than before that greenhouse gas emissions from human activity alone will cause such high levels of warming. The study authors said they expected smaller but still significant increases in rainfall following wildfires along less pessimistic pathways for global warming.

The study finds that by the end of the century, more than half of the days with extremely high wildfire danger in parts of the Pacific Northwest, Idaho, Nevada and Utah could be followed by heavy downpours in a year. The study found that the fraction is lower for California and Colorado, although it was still considerably higher than the average between 1980 and 2005. And the increase is significant both within six months of the fires serious and within the year.

Western Colorado and most of the Pacific Northwest are also expected to see an increased risk of heavy rain within three months of dangerous fire conditions. In California, the wildfire season and the rainy season tend to be more separated throughout the year.

“Even in the middle of the century, some places are seeing a doubling or tripling of risk,” said Daniel L. Swain, a climatologist at the University of California, Los Angeles and another author of the study. “It’s not that far into the future, and it’s not much more additional warming than what we’ve already seen.”

Dr Swain said he and his colleagues were struck that their computer models showed such a consistent increase in risk across the West, even though the region’s climate is so varied. California has dry summers and wet winters, while Colorado floods and wildfires peak in the warm season.

It doesn’t take much 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 was not involved in the study. In some areas, as little as a fraction of an inch falling in 15 minutes could be enough, he said.

But as more and more wildfires occur in places where they hadn’t been a big problem before, scientists are scrambling to understand how thresholds may differ in these wetter climates, said the Dr Kean. “It’s a scramble for us to stay ahead of the game,” he said.

Dr Touma did most of the analysis for the new study when she was a postdoctoral fellow at the University of California, Santa Barbara, not far from Montecito, which was devastated by landslides after a fire in 2018. Authorities had urged residents in some areas to evacuate, but many chose not to.

“There was a lot of evacuation fatigue because of the fire a month before,” Dr Touma said.

Western residents are generally very aware of the risk of flooding and mudslides in burned areas, said Samantha Stevenson, an environmental scientist at the University of California, Santa Barbara, who also worked on the study. But “the degree to which they are increasing due to climate change, and the speed of that increase, is something that maybe we should try to be more aware of,” she said.

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