Mountains See More Extreme Rain. It's a Major Worry

Scientists say it's a result of a warming world, with potentially dangerous consequences
By Newser Editors and Wire Services
Posted Jul 2, 2023 5:00 PM CDT
Mountains See More Extreme Rain. It's a Major Worry
Farmland in the Tulare Lake Basin is submerged in water in Corcoran, Calif.   (AP Photo/Jae C. Hong, File)

A warming world is transforming some major snowfalls into extreme rain over mountains instead, somehow worsening both dangerous flooding like the type that devastated Pakistan last year as well as long-term water shortages, a new study found. The AP reports that using rain and snow measurements since 1950 and computer simulations for future climate, scientists calculated that for every degree Fahrenheit the world warms, extreme rainfall at higher elevation increases by 8.3% (15% for every degree Celsius), according to a study in the journal Nature. Heavy rain in mountains causes a lot more problems than big snow, including flooding, landslides, and erosion, scientists said. And the rain isn't conveniently stored away like snowpack that can recharge reservoirs in spring and summer.

"It is not just a far-off problem that is projected to occur in the future, but the data is actually telling us that it's already happening and we see that in the data over the past few decades," said lead author Mohammed Ombadi, a Lawrence Berkeley National Laboratory hydrologist and climate scientist. As the world has warmed to the brink of the 1.5 degree Celsius (2.7 degrees Fahrenheit) internationally agreed threshold to stem the worst effects of climate change, this study shows "every degree (Celsius) matters because it comes with an additional 15% increase" in extreme rain over mountains, Ombadi said. That per-degree rainfall boost in the mountains is more than twice the increase the rest of the world gets from warming air holding more water.

The study looked at only the heaviest rains each year over six decades in the Northern Hemisphere, finding that as altitude rose, so did the turbocharging of rain. The biggest increase in rains were noticed at about 10,000 feet (3,000 meters). About one in four people on Earth live in an area close enough to the mountains or downhill that extreme rain and flooding would hit them, Ombadi said. It means more of the type of flooding off the mountains like the one that killed more than 1,700 people in Pakistan and put one-third of the country underwater.

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The study makes sense and "the implications are serious," said UCLA climate hydrologist Park Williams, who wasn't part of the research. Scientists expect more precipitation with warmer temperatures, but heavy snow's flooding impact is lessened because it takes time to melt and it's easier to monitor snowpack to see what's happening, he said. "But as the proportion of mountain precipitation falling as snow decreases, flood hazards may enhance especially rapidly," Williams said. So as warming causes rainier extremes, society is going to have to choose between cutting water use because of low water levels in reservoirs to absorb a possible large sudden mountain runoff event or build expensive new reservoirs, Williams said.
(More extreme weather stories.)

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