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Against the backdrop of an 800-foot high mountain wall scarred by avalanches, Arapahoe Basin mountain operations director Tim Finnigan says he can’t remember a cycle of winter storms and avalanches in Colorado as extreme as this.

In the past week, masses of snow sliding off mountains shut down ski resorts, damaged gas lines and buried cars on busy highways. Along Interstate 70—a key east-west corridor through the Rocky Mountains—massive clouds of pulverized snow moving at speeds of up to 200 mph pushed pickup trucks into the median and left the road covered with piles of compressed frozen snow as hard as concrete.

Normally, avalanche experts with the Colorado Department of Transportation reduce the risk by releasing controlled avalanches while roads are closed, but the extreme conditions in early March took even seasoned Colorado veterans by surprise, with large avalanches unexpectedly hitting roads while they were open. On March 8, the Colorado Avalanche Information Center said 346 avalanches had been reported in the previous seven days. Two people died in the avalanches.

“CDOT will have to rethink their avalanche work along I-70 after this,” Finnigan said as he slurped soup at the Arapahoe Basin lodge after a day of blasting mountain slopes. He has worked in the snow safety field for about 30 years, sometimes triggering controlled avalanches with hand-thrown charges or by cutting into brittle slopes with his skis.

Snow scientists say extreme avalanches are among the accelerating impacts of climate change in mountain regions. Global warming can affect avalanches in several ways:

• More moisture in a warmer atmosphere can fuel more extreme snowstorms, which means bigger avalanches.
• Warmer temperatures can make snow layers collapse and slide.
• More rain-on-snow events also destabilize snow layers.

“I always thought it would reveal itself, even on a scale the general public can recognize,” said Chris Wilbur, an avalanche consultant based in southwestern Colorado.

He recently surveyed 240 avalanche experts and snow scientists to learn how global warming is changing the risks associated with dangerous snow slides.

“The findings were really consistent, with an increase in observed avalanches, and even more predicted, especially wet snow and wet slab avalanches, because that’s related to warming temperatures,” he said.

Their observations are supported by research, which is being used to improve safety. An analysis of tree rings published 2018 in the Proceedings of the National Academy of Sciences, for example, suggested links between global warming and the frequency and size of avalanches along a busy mountain road in India. Snow scientist Juan Antonio Ballesteros-Cánovas used those findings help design a safer path for new roads leading to a critical regional tunnel.

Mountain Regions Scramble to Prepare

The extreme avalanches in Colorado have disrupted transportation corridors across the Rockies, affecting food and energy supplies, as well as some access to Colorado’s $5 billion ski industry.

Switzerland has experienced similar impacts in recent years and is already adapting avalanche mitigation plans based on global warming.

Scientists and engineers in the mountainous country are expecting more extreme snowstorms, so they are building higher avalanche barriers to prevent masses of snow from sliding off the peaks. Transportation departments there an elsewhere in the Alps are planning and building expensive new tunnels and other barriers to protect roads from snow slides.

Swiss experts are also updating avalanche hazard maps, because they expect that global warming will put new areas at risk. Detailed satellite measurements of extreme avalanches in the winter of 2017-2018 will help assess future risks, said Perry Barthelt of the Swiss Federal Institute for Forest, Snow and Landscape Research.

Colorado’s recent avalanches were fueled by unusually powerful atmospheric rivers streaming off the Pacific Ocean, which has been warming as the planet’s temperatures rise and is also in the midst of a warm El Niño.

A moist atmosphere fuels wetter storms in summer and winter—the National Oceanic and Atmospheric Administration just announced that the December 2018 to February 2019 period was the wettest winter on record for the contiguous U.S. In cold areas, that means more snow, which leads to more avalanches.

The warmer the air gets, the more moisture it holds, said Barthelt, who said there has been a measurable increase in large avalanches that entrain not only snow, but water, mud, sand and rocks.

In recent weeks, avalanches have also killed skiers at a ski area in New Mexico; damaged homes in Ketchum, Idaho; and closed roads in California and Utah. In January, a series of avalanches swept through the European Alps, damaging mountain hotels and trapping thousands of peoples in ski resorts. Some of the European slides hit areas that were thought to be safe.

‘No Question Things Are Changing’

Scott Toepfer, who studied and forecast avalanches with the Colorado Avalanche Information Center for several decades, said he’s worried that global warming will fuel super snowstorms that could produce and larger and more unpredictable avalanches.

“If these outlier monster storms come in, you’re not going to see the typical avalanche pattern. There’s no question in my mind that things are changing,” Toepfer said. That will make it harder to forecast the risks, he said.

Winter and spring are warming faster than other seasons, and that could mean more risk for slow-moving, but grinding wet snow avalanches, which are “an under-researched category in the avalanche world,” he said.

One study done in 2008 suggested that global warming could significantly increase the avalanche danger at the ski areas around Aspen.

Colorado-based snow scientist Brian Lazar found that, even under low- to mid-range greenhouse gas emissions scenarios, disruptive wet snow avalanches at the top of the mountain would start 16 to 27 days earlier than historical averages by 2100, and up to 41 to 45 days earlier under a high emissions scenario. More recent research in Switzerland also found an increase in wet snow avalanches linked with increasing temperatures.

“Some of the patterns that we’ve been seeing and have relied on for decades as avalanche forecasters have been changing. In colder interior climates, like here in Colorado, there have been more rain-on-snow events and more avalanche activity in mid-winter,” Lazar said. “We may see things we haven’t observed in the historical record.”

Forecasting Avalanche Risk Gets Harder

One thing is certain—global warming will make the task for forecasting avalanches to ensure public safety more difficult than ever, and it’s already tricky when dealing with a dynamic, metamorphic substance like snow.

“What’s going to happen, if there are warming temperatures, the forecasting of the avalanches is going to be different,” said Barthelt. Old forecasting equations based on predictable mountain temperature patterns won’t apply anymore, he explained.

Warmer temperatures and more rain falling on snow will also increase the risk of mountain avalanches transporting large amounts of water, mud and debris. Such slush-flow events are now more common in Norway and Russia.

“You get water-saturated flows that carry debris, and they will probably move very fast. That’s something that we’re worried about,” Barthelt said.

“Anything that changes the thermodynamics is something we have to think about,” he said. “Snow is a very interesting material because it exists near its melting point. It’s a reactive agent in all of these processes.”