Big dumps of powder snow are a precious gift in the best of times around the West, where 40 or 50 feet can fall during a winter, forming frozen mountain water towers that slowly melt and sustain the region through hot, dry summers with life-giving water.

When the snow falls faster than the mountains can hold it, though, big storms can also be deadly. During the first week of February, avalanches killed 14 people across the United States, the highest weekly avalanche death toll in more than 100 years. Halfway through the season, 31 people have died across the nation this winter—more than the annual average of 27 deaths.

This week the danger zone shifted from the Rocky Mountains to the Pacific Northwest, where an atmospheric river has prompted avalanche experts to hoist red flag danger warnings for life-threatening snow slides in mountain regions from Seattle to Juneau. The series of wet storms slopping ashore could deliver a lethal mix of rain and snow in a region where global warming is expected to increase certain types of avalanche risks. 

Some of the climate factors contributing to this year’s dangerous conditions could be exacerbated by changes consistent with climate projections for the Mountain West, including extreme temperature swings early in the season that make snow layers less cohesive, and  longer dry spells punctuated by more intense storms. Midwinter rains can also build slick ice layers and more dust layers can also destabilize the snowpack.

Snow has been in short supply around the West recently. Climate research suggests that global warming is intensifying a regional 20-year pattern of snow droughtsshortening winter,shrinking snowpacks and rising snowlines. That threatens water supplies and ecosystems, and also frustrates tens of thousands of people who frequent the snow-covered high country on skis, snowshoes and snowmobiles. 

When the storms do come, those outdoor enthusiasts head for the hills all at once, and this year, cabin fever brought on by coronavirus containment measures is driving even more people outside. Some social psychologists have even begun asking whether it’s possible that global warming plus a global pandemic equals a recipe for deadly avalanche accidents.

There’s no easy answer to that question, because both avalanches and people are complex and essentially still unpredictable, but there are clues. After the Feb. 6 avalanche deaths in Utah, one of the surviving skiers, traumatized after trying to rescue his companions, said that the stress of the pandemic may have clouded his judgment on that deadly day.

The house-bound cabin fever of the pandemic with severe limits on what we can do “makes us crave activity and backcountry skiing, and being outdoors and distanced may feel safe compared to other activities,” said Sue Kraus, a social psychologist at Fort Lewis College in Durango, Colorado, who has also worked on avalanche research.

“Our definition of safe has changed drastically in the last year,” she said.

What Are Avalanches?

Avalanches are rapid downward flows of big masses of snow over a steep slope, and happen in mountain regions all over the world. They can break away from adjacent and underlying areas of snow, sliding on layers of ice or dust, slick grass or even off a roof. 

Cold powder snow avalanches speed down mountains at up to 100 mph, with an airblast that can destroy structures and knock trucks of roads. Wet snow avalanches can creep at a snail’s pace, uprooting giant trees and dragging along house-sized boulders. Sometimes they release as giant slabs of wind-compressed snow, brittle as styrofoam, rumbling downhill like a crumbling wall, but if the snow is unsettled, it streaks down the mountain in billowing clouds of tiny ice crystals.

In all their forms, avalanches have always been one of the West’s deadliest natural hazards. Just a little more than 100 years ago, entire mining camps in the Rockies and the Sierra Nevada, as well as farm outposts in the European Alps, were wiped out by avalanches.

Mass deaths are not uncommon. In 1965, a large avalanche slammed unexpectedly into a cable car station and hotel near the summit of the Zugspitze, Germany’s highest peak, killing 10 people. And as recently as 1999, avalanches killed 31 people in Galtür, a mountain ski resort town in Tirol, Austria. 

Avalanches also affect ecosystems by killing animals like mountain goats and bighorn sheep, and by knocking down forests and sometimes blocking or rerouting streams.

Understanding how global warming will change avalanches can protect communities and people from new and unexpected threats. Thousands of mountain road segments, as well as water and power infrastructure are at risk. Global warming will have different effects in different regions. Warm and moist snow in coastal and low elevation areas will react differently than the dry, cold powder of the high Rockies.

Explosively Unleashing Frozen Climate Energy

Another way of looking at avalanches is to think of them as frozen packets of energy from different parts of the climate system that are all being intensified by global warming—tropical heat, moist atmospheric rivers and Arctic winds all stored in the form of snow on a mountainside. Avalanches often start with a boom or a crack, as all that energy suddenly releases kinetically. And the more explosives you pack into a bomb, the bigger the bang.

It’s almost impossible to say any one particular avalanche was caused by global warming, but it’s also not accurate to say that global warming is not a factor, since all of today’s weather is happening in a climate that’s already been fundamentally changed by global warming. 

Many avalanches simply release under the pull of gravity or a slight surface perturbation, like the temperature increase when clouds lift over a steep snow-covered slope. By its nature, snow is one of the substances most sensitive to climate, so it stands to reason that global warming will affect avalanches, said Perry Bartelt, a researcher with the Swiss Federal Institute for Forest, Snow and Landscape Research in Davos. 

“A 1 to 2 degree Celsius (1.8 to 3.6 degrees Fahrenheit) rise in temperature will change the dynamics of how an avalanche will flow,” he said, explaining that the varied and complex effects are under scrutiny at a recently-founded research center for climate and extremes in Switzerland. “We want to know what this means for us and for avalanches,” he said.

It’s hard enough to find a global warming signal on something as big and devastating as hurricanes, which kill hundreds every year, and looking for it in avalanches is like trying to find a lost ski pole in the Vail parking lot on a busy Saturday. So for now, it’s still impossible to say exactly how global warming will affect avalanche hazards, Bartelt said.

What We Do Know About Avalanches and Climate Change

There are clues all over the world that avalanche patterns are changing on our warming planet, along with changes to other related extremes, including temperature swings and more intense rainstorms and snowstorms. 

study published in early February recreated a record of avalanches in northwestern Montana going back to the 1600s by looking at tree rings. The true long-term picture is partly skewed because not that many old trees survive. But the study’s findings showed the greatest number of avalanche scars since the 1980s, and especially since 2000. 

Erich Peitzsch, a snow and avalanche scientist with the United States Geological Survey who led the Montana study, said the next step is to link that avalanche record with detailed climate data from the same period to track how avalanche activity responded to changes in moisture and temperature.

A similar 2018 hazard study in the Swiss Alps showed that the number of slides in one particular path increased from four in the 1900 to 1950 period to 17 from 1950 to 2000. But there is no smoking gun, said Jürg Schweizer, director of the Swiss Federal Avalanche Institute. What is obvious from the climate models is that there will be less snow in the long run with global warming. At the same time, the storms that do come could be more intense, since a warmer atmosphere can hold more moisture.

“Less snow doesn’t mean less avalanches. You will still have those intense major storms,” he said. One of the crucial things, he added, is the temperature during the snowfall. 

In the big picture, he noted that some of the biggest avalanche cycles on record in Switzerland happened just in the last four years, coinciding with some of the warmest years ever in the high country of the Swiss Republic.

In addition to air temperature, winds and snowfall, avalanche risks are also determined by changes within layers of snow. A large contrast between temperatures on the surface of snowpack and at the bottom, near the ground, leads to the formation of smooth crystals that lack cohesion. As a result, it’s also possible that, in certain circumstances, a warming climate will lead to a more stable snowpack that is less prone to slide.

Teasing Out Climate Signals

To tease out climate signals, Bartelt said his research team has programmed data from nearly one million observed avalanches into climate models to calculate how the snow would have responded in different conditions linked with global warming, like overall warmer temperatures, a higher snowline or snow with more moisture.

Just in the last three years, he said, he’s seen events atypical of the long historical record in Switzerland, with spring-like avalanches during midwinter warm spells and a tendency to more wet snow avalanches in the spring.

“This has to do with climate change,” he said. “It gets cold, it gets very warm, then very cold again. Thermal forcing is changing the property of the snowpack and that changes expectations.” 

Those differences are important because different measures are needed to protect against different types of avalanches. Preventing powder avalanches could require more snow fences near the ridgelines where the snow builds up, or more preventive avalanche releases to keep it from launching in the first place. A trend toward more and bigger oozing wet snow slides might require the relocation of power lines, or sturdier snow sheds over key roads. 

“I think we just have to stay alert, and not assume that things will be the same as before,” Schweizer said. 

The big avalanches of concern for engineers and planners are affected by release volume, snowpack structure and temperature, all of which are affected by climate change, added Colorado-based avalanche consultant Chris Wilbur. 

He said the Colorado Avalanche Information Center correlated the state’s last extreme avalanche cycle in 2019 to a 20 percent increase in the snow’s water content.

“This implies that extreme precipitation events will result in extreme avalanches, and many climate scientists agree that more extreme precipitation events should be expected,” he said. 

“This causes more concern at higher elevations and higher latitudes, where warming will not shift much precipitation from snow to rain.”

Urban Avalanche Threat in Alaska

Avalanches also constitute an active threat in Juneau, Alaska, where entire neighborhoods, as well as the important subarctic harbor, are vulnerable, said snow and avalanche scientist Gabriel Wolken, manager of the Climate & Cryosphere Hazards Program with the Alaska Department of Natural Resources.

Projected warming, increased precipitation intensity and rain-on-snow events “could significantly impact the extent, behavior, and predictability of snow avalanches … which are the most deadly natural hazard in the state,” he said. “As climate warming continues, there is an expectation of an increase in Alaska’s vulnerability to avalanche hazards.” 

Juneau has the highest avalanche danger of any urban area in North America, with some existing neighborhoods at the base of the nearby mountains threatened by more than a dozen existing slide paths off Mt. Juneau and Mt. Gastineau, and new development sprawling into potential new danger zones from stronger avalanches fueled by global warming.

Like in other parts of the world, development in Juneau is outpacing avalanche research and mitigation efforts, he said. Global warming could shift the risks and the mapping of danger zones is not keeping up, he said.

“We see that in developed parts of the world, development can outpace avalanche research and mitigation efforts,” he said. “There have been some very large avalanches in the past, and the current avalanche hazards maps don’t fully capture the full extent of the danger.”

Storms fueled with more moisture from global warming could drop more snow in the high mountain avalanche starting zones, Wolken said, with the resulting wetter and heavier avalanches extending farther than expected, cutting off emergency transport routes and even threatening boats in the Juneau harbor. 

“Naturally, much of the drive behind avalanche research and mitigation has been in response to fatalities and the impacts to infrastructure in the developed world,” he said. 

In Alaska, new avalanche risks may be emerging for Indigenous communities that have long avoided the threat.

“Not many of them are in avalanche-prone areas,” Wolken said. But that may be changing with the possibility of larger and less predictable slides, he added, singling out the community of Klukwan, northeast of Haines, as a potential hotspot, “with the potential for avalanches caused by rain on snow events, things they have not really seen before.”

Does the Coronavirus Pandemic Contribute to Avalanche Deaths?

As avalanche-related deaths have surged across the country, experts have begun probing possible links between climate, the pandemic and snow slides.

Wolken said that there is an intuitive link “between trips to the backcountry for socially distanced activities” during the pandemic and “an increase in avalanche fatalities, but that is quite anecdotal at the moment.”

Kraus, the Colorado social psychologist, said the effect of the pandemic’s social restrictions are complex. 

Some research suggests that people depressed by the pandemic may be more realistic in assessing risks, but may, at the same time, be more willing to take those risks, especially if they are fatalistic about climate change, she said.

And the long-running snow droughts in the West could also push people toward scarcity behavior, “to make quicker, less thoughtful decisions” with regard to avalanche risks, she added.

Rosemary Randall, a psychologist in the United Kingdom researching climate anxiety, said it’s possible that the Covid lockdowns have exacerbated other mental and emotional stress that can lead to riskier behavior, including lack of sleep, and work or home demands. 

But she said people suffering from depression related to Covid-19 are not the most likely “to see skiing as a solution to their difficult feelings. “Anxious and depressed people tend to stay home. Risk is usually the last thing they seek out.”

This story originally appeared in Inside Climate News and is republished here as part of Covering Climate Now, a global journalism collaboration strengthening coverage of the climate story.