Warming causes more extreme rain, not snow, on the mountains. And that is a problem

TO world warming is turning some heavy snowfall into extreme rain over the mountains, making both dangerous and type flooding somewhat worse that devastated pakistan last year, as well as long-term water scarcity, a new study found.

Using measurements of rain and snow since 1950 and computer simulations for future climates, the scientists calculated that for every degree Fahrenheit the world warms, extreme precipitation at higher elevations increases by 8.3 percent (15 percent for every degree Celsius), according to a study on Wednesday nature magazine.

Heavy rains in the mountains cause many more problems than heavy snowfalls, including flooding, mudslides and erosion, the scientists said. And rain doesn’t store as conveniently as snowpack that can recharge reservoirs in spring and summer.

“It’s not just a problem far off that’s predicted to happen in the future, but the data actually tells us that it’s already happening and we see it in data from the past few decades,” said lead author Mohammed Ombadi, a hydrologist and climate scientist at Lawrence Berkeley National Laboratory.

As the world has warmed to the brink of Internationally agreed 1.5C threshold to curb the worst effects of climate change, this study shows that “every degree (Celsius) matters because it comes with an additional 15 percent increase” in extreme rainfall over mountains, Ombadi said. That increase in rainfall per degree in the mountains is more than twice the increase the rest of the world gets from heating air that holds more water.

The study looked at only the heaviest rainfall each year for six decades in the Northern Hemisphere and found that as altitude increased, so did rainfall. The greatest increase in rainfall was noted at about 10,000 feet (3,000 meters). That includes much of the American West, where Ombadi said “it’s very steep,” as well as parts of the Appalachian Mountains. Another big hot spot in Asia is the Himalayan, Tian Shan and Hindu Kush mountains, with the Alps also affected.

About one in four people on Earth live in an area close enough to mountains or downhill that extreme rainfall and flooding hit them, Ombadi said.

It means more of the kind of mountain flooding like the one that killed more than 1,700 people in Pakistan and left a third of the country under water, Ombadi said. But he pointed out that they have not studied the 2022 Pakistan floods precisely, so there may be some small differences.

The study makes sense and “the implications are serious,” said UCLA climate hydrologist Park Williams, who was not part of the research. Scientists expect more precipitation with warmer temperatures, but the flooding impact of heavy snowfall is reduced because it takes time to melt and it’s easier to monitor the snowpack to see what’s happening, he said.

A warming world is turning some heavy snowfall into extreme downpours over mountains, somewhat worsening both the dangerous floods like those that devastated Pakistan last year and long-term water shortages, a new study has found.

“But as the proportion of precipitation in the mountains decreases as the snowfall decreases, the dangers of flooding can increase especially rapidly,” Williams said.

In the western United States, it hits hard in two different ways, said study co-author Charuleka Varadharajan, a climate scientist and laboratory hydrologist.

“This type of extreme rain is going to make the flooding worse. And then you have to figure out where that water goes? she said. “We have that situation right now in the Sierras with the Tulare Lake flooding and such a serious problem related to that.”

Floods can also affect food production, Ombadi said. He pointed to the California Department of Agriculture estimates of $89 million in crop and livestock losses from torrential rains this year.

But in the long term, another problem is the water supply. When heavy snow falls in the west in the winter, that snow melts slowly in the spring and summer, filling reservoirs where it can be useful when needed later.

“It’s going to decrease their snowfall, their water supply in the future,” Varadharajan said. “There will be more short-term runoff causing more flooding and less snowpack recharging groundwater, and ultimately groundwater is what helps maintain stream flows.”

“These mountain systems are supplying most of the water in the west, so any decrease in water supply would be quite significant in terms of water management,” he said.

In times of drought, and much of the West is dealing with a megadrought of more than 20 years — Water managers like to keep water levels in reservoirs up, which they can do with heavy layers of snow because it melts slowly, Williams said. But they can’t do that in heavy rain.

So, as warming causes more rainy extremes, society will have to choose between reducing water use due to low water levels in reservoirs to absorb a potential sudden large runoff event in the mountains, or building expensive new reservoirs. Williams said.

Follow AP’s climate and environmental coverage at https://apnews.com/hub/climate-and-environment

Associated Press climate and environmental coverage is supported by several private foundations. See more about AP’s climate initiative here. The AP is solely responsible for all content.

Leave a Comment