The Sierra Nevada of California, in the United States, is being punished this week by a series of storms that are heading towards Colorado and triggering avalanche warnings in the Rocky Mountains.
While scientists are cautious about attributing climate change to any single weather event without in-depth study, research suggests that global warming is increasing the overall risk of avalanches at higher elevations, as storms dump large amounts of snow that can overwhelm and collapse mountain slopes.
California authorities have not identified the precise cause of last Tuesday’s avalanche that killed at least eight skiers in a remote region near Lake Tahoe. However, they point to a combination of heavy snow on top of an unstable snowpack as the conditions that led to the avalanche.
Climate scientists are identifying a paradox about snowfall, avalanche risk and climate warming: Drier, warmer winters will occur in the western U.S. — but more snow is expected to fall at higher elevations.
“We expect that in the higher elevations of the mountain range, for example, there will actually be more snowfall,” said Ned Bair, a researcher at the University of California, Santa Barbara and former research president of the American Avalanche Association. “What really matters in avalanches is the intensity of the atmospheric rivers.”
Atmospheric rivers are a phenomenon that occurs when a high-altitude current of moisture flows from tropical oceanic regions, potentially leading to much more intense precipitation wherever it passes.
Atmospheric rivers in the Pacific Ocean are becoming wetter and warmer, and can lead to heavy snowfall at higher mountain elevations, even as the number of snowy days decreases, according to a peer-reviewed study published in 2023 in the journal Climate Dynamics.
Mountain peaks in the central and southern Sierra Nevada may see occasionally extreme snow accumulations in January and February due to wetter atmospheric rivers, the study said.
The Lake Tahoe region is expected to experience an atmospheric river early next week, according to Heather Richards, a meteorologist with the National Weather Service in Reno, Nevada. The snowstorms that hit the Sierra this week were the result of a series of cold air masses moving in from the Pacific Northwest, she said.
Bair also researched the relationship between climate change and avalanches. He analyzed the frequency of avalanches in the Sierra while studying the fate of endangered bighorn sheep, which inhabit higher elevations.
By combining future climate models (or scientific predictions of the effects of a warming world) and existing avalanche prediction models used in the region, Bair and his colleagues found that avalanche frequency could stay the same or increase at higher elevations, according to a paper he presented at the 2024 International Snow Science Workshop and the annual meeting of the American Geophysical Union.
Another study published in 2017 found that mountain snowfall caused by atmospheric rivers was responsible for nearly a third of the 123 avalanche deaths in the western United States between 1998 and 2014, according to peer-reviewed research in the Journal of Hydrometeorology.
The Sierra Nevada region is already experiencing the effects of climate change, given recent warmer and milder winters, altered seasonality and variable weather patterns, according to a 2021 U.S. Forest Service technical report on the effects of climate change in the region.
That report also noted that warmer temperatures were leading to earlier, warmer rains on the existing snowpack and heightened risk of avalanches at higher elevations.
“Storms will be warmer, with more precipitation as rain and less as snow, and snow will melt earlier. Storm patterns are expected to be more erratic, runoff peaks are expected to occur earlier in the year, and rainfall intensities are expected to be greater,” the report said.
In Colorado, emergency authorities were warning skiers and hikers that the risk of avalanches was at level four, on a scale that goes up to five, as of 5pm this Friday (20).
“The biggest storm of the season is bringing strong winds and feet of snow to an exceptionally shallow and very light snowpack,” the Colorado Avalanche Information Center said in an advisory issued Wednesday.
“You can easily trigger large and dangerous avalanches on most steep slopes,” the statement said. “It is not recommended to travel in or below avalanche terrain.”
