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15 agosto 2019

‘Snow droughts’ are coming for the American West

By the middle of the century, two or more bad snow years in a row in mountains from Colorado to California will likely be the norm.

Photo :  Skiers and snowboarders hike a windy ridge above Squaw Valley ski resort in Truckee, California. Within a few decades “snow droughts” could come much more frequently than they do today, limiting water availability across the Western United States.

Photograph by Skip Brown, Nat Geo Image Collection

 

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Source : National Geographic

On April 1 each year, researchers ski and snowshoe out into the high mountains of the western Unite States to jab stakes into the bright, crystalline snow, checking the thickness of the blanket. But in 2012, many researchers could barely travel on snow to their test sites—and when they got there, there was almost no snow to measure.

2013 was almost as bad. 2014, the same. And in 2015, on the April 1 assessment, many sites across the Sierra Nevada mountains were bald and snowless—the worst snow drought, scientists found, in at least the last 500 years.

One bad snow year can wreak havoc on water systems across the western U.S. More than two is territory scientists are just beginning to understand.

Now, scientists have found that back-to-back bad snow years are likely to become much more frequent in the not-too-distant future, according to work published in August in Geophysical Research Letters.

Nowadays there’s about a 7 percent chance that snowy areas in the western U.S. will get two really bad snow years in a row—years with snowpack lower than a quarter of the long-term average. But within a few decades, if climate change continues apace, those bookending “snow droughts” could occur about 40 percent of the time.

“That has impacts on so many different things we care about like water, wildlife, forest health, winter recreation, and more,” says Adrienne Marshall, the lead author of the study and a scientist at the University of Idaho.

Snow is a secret savior

Snow does a lot for the West. Bright snow keeps mountains cool and reflective, glinting away the sun’s incoming rays and keeping the whole region cool. Snowpack melts slowly through the spring and early summer, storing invaluable water high in the mountains until it’s needed later in the year. Snowmelt in the spring and summer also feeds forests, keeping soils moist and protected against wildfires. And wildlife depends on snow, from wolverines who dig into the icy stuff, to fish that bask in chilly meltwater cascading down mountainsides. (Read about how wolverines in Canada are threatened by climate change).

But the very nature of snow in the region is changing. Warming temperatures are shrinking both the volume of snow that falls across much of the West, from Nevada to the Northwest, and the length of the season. (Read about what happens when the snows fail).

“Snowpack is this bridge between winter and summer seasons,” said Alan Rhoades, a snowpack expert at the Lawrence Berkeley National Laboratory. “It helps to massage that lack of precipitation you get from peak winter to early summer,” a crucial time for many water managers, farmers, animals, and more.

Many of these stakeholders can skate through one bad winter, but more than that can be devastating. In California, the reservoir water storage system can generally store enough water for about two to three years so any snow droughts that last longer than a year push managers into a tenuous position, Rhoades explained. For example, by the second extra-dry winter of that 2012 to 2016 drought, water resources in the state had been whittled down so much that the governor declared a state-wide drought emergency, which lasted until the state got drenched in 2017.

The science team recognized that it was crucial to understand whether these back-to-back years were going to become more frequent. They used a suite of climate models to look at the patterns of snowfall across the western U.S. in the past, between 1970 and 1999, to see how often “snow droughts” held steady for two years or more. Then they used those climate models to look forward to 2050 to 2078—just a generation or so into the future.

To look into the future, they pushed the models using forecasts of greenhouse gas concentrations that assume humans continue on the emissions track we’re currently on.

The likelihood of those two-year-long snow droughts increased by a factor of six, from about 7 percent of years to 42 percent. The frequency of a four-year-long drought, one like the California drought that parched the state a few years ago, increased from under about 0.25 percent of years in the past to about 25 percent in the future. That means that what we think of as an incredibly rare 500-year event could soon become much more normal.

“This gets to the heart of it,” says Sarah Kapnick, a climate scientist at NOAA’s Geophysical Fluid Dynamics Laboratory. “Will this be something that’s more frequent in the future? And the answer is yes.”

“This kind of thing happening more frequently in the future could have huge social and economic impacts,” says Rhoades.

During the California drought, the state enacted mandatory water use reductions; agricultural producers across the state suffered greatly, and in many parts of the state, access to clean, safe drinking water became tenuous. And since California alone accounts for about 13 percent of the entire agricultural revenue in the U.S., any future shifts in water accessibility are deeply concerning.

Shortening the snow season

The team also looked at how the timing of snow and the length of the season might constrict in the future. By the second half of the century, they found, the peak of the winter snowpack is likely to happen earlier in the season. In the past, peak snowpack has been recorded in April in most high elevation mountain sites. By a few decades from now, that peak shifts to sometime between February and March.

That could wreak havoc on the winter sports industry. Ski resorts in the Rocky Mountains get about a quarter of all their visitors in March, when spring break brings floods of snow-seekers to the mountains. Less snow during that critical time—or less reliable snow, even—is on track to destabilize the multi-billion-dollar industry.

“In 2014 and 2015 in California, during these back-to-back years of low snow—the ski industry visitation declined by about 35 percent,” said Elizabeth Burakowski, a climate scientist and snow sports industry expert at the University of New Hampshire. “That’s a big chunk of their annual revenue.”

So some in the industry are pushing to leap headfirst into conversations about how climate change will impact them—and to take steps to mitigate it.

“The industry can take, and has survived, a drought every once in a while. That’s the nature of the business. But if you have two droughts every 10 years instead of just one, the business model is looking a whole lot weaker,” says Auden Schendler, the vice president of sustainability for the Aspen Snowmass ski areas in Colorado.

So it’s time for anyone who loves winter to get into the climate conversation.

“What does it mean to be a mountain person without winter? This is about who we are, what ties us to our history, our culture. So we have to protect it,” he says.