A warming world is turning what should have been heavy snowfall into extreme mountain rain, somewhat exacerbating dangerous floods like those that devastated Pakistan last year and long-term water shortages, a new study has revealed.
Using measurements of water and snow since the 1950s and computer simulations for future climates, the scientists calculated that for every degree Celsius the planet warms, there is 15% more intense rainfall at higher elevations (8.3% for every degree Fahrenheit), according to a study published Wednesday in the journal Nature.
Heavy rains in the mountains cause far more problems than heavy snowfalls, including flooding, mudslides and erosion, the scientists said. And rain doesn’t get stored conveniently like snow, which can replenish reservoirs in the spring and summer.
“It’s not just a problem far off that’s projected to happen in the future, but in fact the data tells us that it’s already happening and we see it in data from the past few decades,” said study lead author Mohamed Ombadi, hydrologist and climatologist at Lawrence Berkeley National Laboratory.
As the planet’s temperature has risen to the edge of the internationally agreed 1.5 Celsius (2.7 Fahrenheit) threshold to avoid the worst effects of global warming, this study shows that “every degree (Celsius) matters because it comes with an increase 15%” of extreme rainfall over the mountains, Ombadi said. That increase in precipitation per degree over mountains is more than double what the rest of the world gets from warmer air holding more water.
The study looked at only the heaviest rainfall each year over the past six decades in the Northern Hemisphere, finding that as altitude increases, so does rainfall overfeeding. The largest increases in rainfall were recorded at about 3,000 meters (10,000 feet). That includes much of the western United States, where Ombadi said they’re “very steep,” as well as parts of the Appalachian Mountains. Another focal point are the Himalayas and the Tian Shan and Hindu Kush mountains, in Asia, as are the Alps.
About 25% of the world’s population lives in an area close enough to mountains or on a slope to be affected by extreme rainfall or flooding, Ombadi said.
That means more floods from the mountains like those that have killed more than 1,700 people in Pakistan and submerged more than a third of the country, the author said. But he stressed that they have not specifically studied the 2022 floods in Pakistan, so there could be some small differences.
The study sounds logical and “the implications are serious,” said Park Williams, a climate hydrologist at the University of California, Los Angeles, who was not part of the research. Scientists anticipate more precipitation due to rising temperatures, but the impact of flooding from heavy snowfall is reduced because the snow melts slowly and it is easier to monitor its accumulation to see what is happening, he said.
“But as the proportion of precipitation that falls as snow in the mountains is reduced, the risks of flooding are increasing particularly rapidly,” Williams said.
This affects the western United States in different ways, said study co-author Hydrologist Charuleka Varadharajan.
“This kind of extreme rainfall will make the flooding worse. And then you have to figure out: where is that water going?” he said, highlighting some of the flooding hardship the western United States has already suffered so far this year. year after a series of atmospheric rivers and snowpack melt.
Floods can also harm food production, Ombadi noted. He referenced the $89 million in crop and livestock losses from torrential rains in 1997, according to estimates by the California Department of Agriculture.
But the water supply is another long-term problem. When the western United States receives heavy snowfall in the winter, that snow melts slowly during the spring and summer, filling reservoirs, allowing the water to be used later when needed.
“It’s going to reduce snowfall, water supply for the future,” Varadharajan said. “There’s going to be more runoff in the short term leading to more flooding and less snowpack recharging groundwater tables, which ultimately is what helps keep streams flowing.”
“These mountain systems supply most of the water in the (US) West, so any reduction in water supply would be quite significant in terms of water resource management,” he stressed.
In times of drought — and much of the western United States has dealt with a massive drought for more than 20 years — water managers prefer to keep levels elevated in reservoirs, which they can do when there is a lot of snow because it melts. slowly, Williams said. But they can’t do that in heavy rains.
So as warming causes heavier rainfall, society will be forced to choose between reducing water use due to low levels in reservoirs in order to absorb a potential massive runoff event from mountains, or building new and expensive dams, Williams added.
