California’s Sierra Nevada is being pummeled this week by a series of storms that is headed to Colorado and prompting avalanche warnings across the Rocky Mountains.
While scientists are careful not to blame climate change for any single weather event without close study, research suggests that a warming climate is increasing the overall risk of avalanches at higher elevations, as storms dump large amounts of snow that can overload and tumble down a mountain slope.
Authorities in California have not pinpointed the precise cause of the Tuesday avalanche that killed at least eight backcountry skiers in a remote region near Lake Tahoe. However, they are pointing to a combination of heavy snow on top of an unstable snow pack as conditions that led to the avalanche.
Climate scientists are identifying a paradox about snowfall, avalanche risk and a warming climate, namely that drier, warmer winters will occur in the West — but more snow is expected to fall at higher elevations.
“We do expect that in the highest elevations in the Sierra, for example, there to actually be more snowfall,” said Ned Bair, a researcher at the University of California, Santa Barbara, and the former research chairman of the American Avalanche Association. “What really matters with the avalanches is the intensity of the atmospheric rivers.”
Atmospheric rivers are a phenomena that occur when a high-altitude current of moisture flows from the tropical ocean regions, potentially leading to much heavier precipitation where they pass overhead.
Atmospheric rivers from the Pacific Ocean are becoming wetter and warmer, and can lead to heavy snowfall in higher mountain elevations even as the number of snowy days decreases, according to a 2023 peer-reviewed study published in the journal Climate Dynamics.
Mountain peaks in the central and southern Sierra Nevada might see occasionally extreme snow accumulations in January and February from wetter atmospheric rivers, the study said.
The Lake Tahoe region is expecting an atmospheric river by early next week, according to Heather Richards, a forecast meteorologist at the National Weather Service office in Reno, Nev. The snowstorms that hit the Sierras this week were the result of a series of cold air masses from the Pacific Northwest, she said.
Dr. Bair has also researched the relationship between climate change and avalanches. He looked at the frequency of avalanches in the Sierras while he was 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-forecast models used in the region, Dr. Bair and his colleagues found that avalanche frequency could remain the same or increase at higher altitudes, according to a paper he presented in 2024 at the International Snow Science Workshop and the American Geophysical Union annual meeting.
A separate study published in 2017 found that mountain snowfall driven by atmospheric rivers was responsible for nearly one-third of the 123 avalanche fatalities in the Western United States between 1998 and 2014, according to a peer-reviewed study in the Journal of Hydrometeorology.
The Sierra Nevada region is already experiencing the effects of climate change, given recent warmer, milder winters; altered seasonality; and variable weather patterns, according to a 2021 technical report on the effects of climate change in the Sierras from the U.S. Forest Service.
That report also noted that warmer temperatures were leading to earlier, warmer rains on existing snowpack and the heightened risk of avalanches at higher altitudes.
“Storms will be warmer, with more precipitation as rain and less as snow, and snow will melt earlier. Storm patterns are projected to be more erratic, runoff peaks are expected to occur earlier in the year, and rainfall intensities are expected to be higher,” the report said.
In Colorado, emergency officials were warning skiers and hikers that the risk of avalanches was at a four on a scale of five through 5 p.m. Friday.
“The biggest storm of the season is bringing strong winds and feet of snow to an unusually shallow and very weak snowpack,” the Colorado Avalanche Information Center said in a warning issued Wednesday.
“You can easily trigger large and dangerous avalanches on most steep slopes,” the statement said. “Travel in or below avalanche terrain is not recommended.”
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