Four young skiers were killed in an avalanche outside Park City, Utah this past weekend, one of the deadliest in the state’s history. Days earlier, a 41-year-old experienced back country skier died in an avalanche in Vail, the eighth snow slide fatality in Colorado this season.
Overall it has been the deadliest week for avalanches in the US in more than a century. At least 15 people died between 31 January and 6 February, reported the Colorado Avalanche Information Centre (CAIC), on mountains from coast to coast in Utah, Montana, Colorado, California, Arkansas and New Hampshire.
This winter season, 21 people have died in avalanches in total in the US, according to figures published earlier this week. The US usually has an average of 27 avalanche deaths each year.
On the other side of the world, following a devastating debris flow from a Himalayan glacier on Sunday in northern India, 31 people were dead and dozens more missing.
Climate scientists say that a landslide and avalanche were the likely causes of the disaster. Experts told AP that they had warned the Indian government since 2014 that warming due to climate change was melting the Himalayan glaciers and facilitating avalanches, and that continued dam construction in the fragile ecosystem was dangerous.
While assessments of these tragic incidents continue to take place, avalanche experts say that, in general, climate change is having an impact on avalanche activity.
But that is where certainty ends. Exactly how different mountain ranges will respond to rising global temperatures is as varied as the peaks and the snow that falls on them.
Avalanches are caused when snowpack, an ever-shifting sandwich of thick, thin, hard and soft layers moulded by the wind and rain, collapses and flows down the mountain.
When it comes to forecasting avalanches there are always surprises, says Dr Jürg Schweizer, head of research at the WSL Institute for Snow and Avalanche Research (SLF) in Davos, Switzerland, the world leader in avalanche science.
“You have to be adaptive and open-minded. At the same time as avalanches are rare events, it’s sometimes difficult to acquire the experience to adequately deal with them," Dr Schweizer told The Independent.
“If you want to assess avalanche danger, it's about the snowpack and the weather. You have to look carefully at the present conditions and not rely too much on what you have seen before. It's important to keep good records that will help you to make good decisions.”
A special report in 2019 from the Intergovernmental Panel on Climate Change, the UN body for assessing climate science, found that as the coldest places on earth continue to warm rapidly, the natural hazards will change.
Glacier retreat and thawing of permafrost are expected to leave mountain slopes more unstable. And while snow avalanches at lower elevations are predicted to decline in number and runout distance (where debris flow comes to rest), avalanches involving wet snow will occur more frequently.
Floods caused by rain on top of snow will also occur earlier in spring and later in autumn, and be more frequent at higher elevations.
“Resulting landslides and floods, and cascading events, will also emerge where there is no record of previous events,” the report reads.
Dr Schweizer said that the Alps had seen major avalanche cycles over the last four years, in January of 2018, 2019, and 2021.
These coincided with the hottest six years on record all occurring since 2015 – with 2016, 2019 and 2020 being the top three, according to the World Meteorological Organisation.
Dr Schweizer said, however, that "avalanches don't care about the climate; they care about the weather".
“It’s not the average temperature that I think will be decisive but rather the precipitation events, and then the temperature during those precipitation events,” he added.
In January 2019 record levels of snow fell in some areas of the Swiss Alps which SLF described as an extraordinary event that happens only about every 30 years. In January 2018, SLF noted, more than three metres of snow fell in some areas, giving rise to an "exceptional avalanche situation". It was the first time since 1999 that the very highest danger level 5 was forecast for a 36-hour period.
In the US, mountain ecosystems, particularly in western states and the northern Rockies, are highly sensitive to climate change, according to the US Geological Survey. Warming in western Montana, for example, is nearly two times greater than the rise in global temperatures over the last century.
A survey in 2018 of seasoned avalanche experts in North America found that, despite their different locations, a consistent thread ran through the observations. All predicted more wet avalanches and more glide avalanches.
As the planet heats up there will be higher levels of water in the snow, making the snowpack more unstable. Wet avalanches have typically occurred in spring but with winters no longer as cold, they are likely to increase. They are also difficult to predict.
Wet avalanches are typically slow-moving, sludgy affairs which nevertheless can pack a punch of pressure as they move down the mountain. Glide avalanches, by comparison, sees the entire snow cover detach. While they are only likely to occur where the slope is steep enough and ground relatively smooth, glide avalanches are nearly impossible to forecast, says the Colorado Avalanche Centre.
One survey respondent in Alaska, with more than 40 years experience, said: “All my rules for weather and avalanche forecasting have changed in the last 10-15 years; weather patterns now are completely different … persistent weak layers and associated instability have increased due to wilder swings in weather."
The study concluded that while limited resources is always an issue, collecting and sharing quality data on avalanches is essential to “improving understanding of avalanches in a changing climate”.
A preliminary assessment of last Saturday's deadly avalanche in Utah found that some of the ski party had unintentionally triggered a “hard slab” – a perilous type of avalanche which tends to break with people above them and are difficult to escape. (People trigger 90 per cent of avalanche disasters and the majority of deaths in the US since the 1950s are those taking part in backcountry, or off-piste, mountain pursuits.) The problem that led to the avalanche was a “persistent weak layer”, the initial report noted.
Craig Gordon, a forecaster with the Forest Service of the Utah Avalanche Centre, has worked in avalanche control in the state since the mid-Eighties, and also as a helicopter ski guide. He has seen conditions in the mountains change over his three-decade career.
“[In Utah], winters seem to be starting a bit later and this year, in particular, the snowpack is thin and shallow. That creates a type of snow structure which is inherently dangerous," he told The Independent. "Weak layers near the ground tend to form earlier in the season and without consistent storm [systems] to add insulating layers, these become persistent weaknesses.
"Every time we load those up with snow – and it doesn't have to be a raging snowmageddon – it reacts with the characteristics of the avalanches we see right now."
In previous years when Utah has had a consistent storm track, the snowpack had been evenly layered and had some degree of predictability and stability. This hasn't been the case this season.
"At this time of year there should be about 100in of settled snow on the ground. Instead we're experiencing a snowpack characterised by thin snow which is very susceptible to avalanching," Mr Gordon said.
He said that the current avalanche conditions are dicey and risk catching out even the most experienced.
"These avalanches can break to the ground and take out the entire season's snowpack," he said. "In some cases, they are eight-feet deep and hundreds of feet wide. You can trigger avalanches from a distance, even from the bottom of the slope.
"It's a very unpredictable type of avalanche and even experienced people are getting tricked. The way you manage it is through avoidance, even if you've been travelling in these mountains for a respectable amount of time."
This article has been updated
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