The science behind Colorado’s historic avalanches

Are such unprecedented slides due to climate change, abnormal weather, or both?

 

This article was originally published by 5280 and is reproduced here with permission. 

Over the past two weeks, we’ve witnessed one of the most intense avalanche cycles in Colorado history. According to leading experts, recent avalanches — there have been more than 600 recorded in March alone — are running longer and larger than they have in decades, and in some cases, the slides are more intense than they’ve been in hundreds of years. The carnage has been well-documented across the state: cars have been buried on I-70 and elsewhere; homes have been destroyed; and already seven skiers have been killed by avalanches in the backcountry this year.

An avalanche covered part of a major highway in Summit County, Colorado.

To better understand the strength and scope of snow barreling down mountainsides in Colorado, we set out to learn what exactly has led to this intense avalanche cycle. Is it merely a lot of damn snow? Another catastrophic by-product of a warming climate? According to snow scientists and forecasters with whom we spoke, the answer is not nearly so simple. Yes, Colorado is experiencing a banner snow year. Yes, the climate is changing. But neither of those factors, they say, are solely responsible for the intensity of avalanches we’ve seen this month. Instead, it’s been a mixture of weather events over the past five months — a “perfect storm” of sorts — that created unstable snowpack across the state.

“It’s a combination of events,” says Karl Birkeland, director of the U.S. Forest Service National Avalanche Center. “You get your biggest avalanches not just when you get a big storm, but when you get a big storm that’s on top of a series of events that set up over your season.”

In Colorado’s case, according to Birkeland, that series of events involved early snowfall in October followed by dry weather in November, creating a weak base layer of snow. Colorado then saw consistent snowfall throughout the winter on top of that layer, but not too much in any given storm, which loaded snow in the mountains but didn’t “tip the bucket,” he says. These conditions ultimately created a snowpack that is extremely unsteady, and thus, when major storms arrived in early March, it triggered massive avalanches.

“You had a very deep snowpack with a very weak layer down at the bottom of it,” he says. “And then you just added a load really quickly and that was enough to push it over the edge.”

A historic-sized avalanche near Aspen, Colorado, on March 9, 2019. The house in the bottom of the image is protected by a defensive wedge but suffered some damage.

Moreover, wind throughout Colorado’s high country made vulnerable conditions even more likely to slide, according to Spencer Logan, lead avalanche scientist at the Colorado Avalanche Information Center (CAIC). Wind drifts snow around, he explains, and in some cases wind can move snow from one place to another at rates far greater than it is falling from the sky.

“If you imagine the snowpack being like an old man, it doesn’t like change,” Logan says. “Rapid change makes snow unstable. Wind is a really good way of creating that rapid change.”

Because of all of these factors, we are seeing avalanches “unlike anything anyone can remember,” Logan says. “Professionals who have been doing this for 40 years have never seen these kinds of avalanches.” 

When such extreme weather conditions like this develop, it’s natural to ask whether or not this is the most recent manifestation of climate change. In the case of avalanches, while global warming likely has an indirect influence, experts caution against conflating weather and climate too broadly.

“I don’t think we can directly tie this event to climate change. This is really tied to a particular sequence of events — a sequence of weather events over the course of a season,” Birkeland says. “I’d be reluctant to tie it to climate change necessarily. It’s more kind of the perfect storm in terms of these different things coming together to give you a snowpack that’s deep and has a weakness at the bottom with big storms on top of that.”

Dr. Kelly Elder, a research hydrologist at the U.S. Forest Service and an avalanche science instructor at Colorado Mountain College, agrees that the number and intensity of avalanches this month is largely do to abnormal weather. However, he says, one of the impacts of global warming is unpredictability.

“This is a weather issue, but climate change predictions suggest increased variability in weather,” he wrote in an email to 5280. “We should expect more seasons like this one, as well as more drought seasons where we are begging for more snow. Increased variability means greater extremes and greater uncertainty in predicting them. Climate change is real; how it will manifest remains to be seen.”

As a “bomb cyclone” ripped across the state on Wednesday, it seems the abnormal winter weather will continue in Colorado — at least for now. But how many more avalanches will break loose is hard to know: Logan says that his team at CAIC has recorded 609 slides in March so far, but that’s probably only a fraction — maybe 25 percent — of the total over that period.

As droves of skiers head for the hills again this weekend, it’s important that we all keep in mind the significant avalanche risk, both on the roads and on the slopes.

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