Areas of U.S., Europe in danger of falling off a ‘snow loss cliff’
Snow is piling up across much of the United States this week, but new research shows this is the exception rather than the rule: Seasonal snow levels in the Northern Hemisphere have dwindled over the past 40 years due to climate change.
Even so, snow responds to a warming planet in different ways.
“A warmer atmosphere is also an atmosphere that can hold more water,” said Alex Gottlieb, a graduate student at Dartmouth College and lead author on the new study in the journal Nature. That can increase precipitation, spurring snow, or even extreme storms and blizzards that offset the effect of snowmelt amid warmer temperatures.
That has made it harder for scientists to calculate how snowpack has changed over time. But the new findings reveal that areas of the United States and Europe are nearing a tipping point where they could face a disastrous loss of snow for decades to come.
“Once you pass this threshold, which we refer to as the snow loss cliff … with even modest amounts of warming you can get these really accelerating losses,” Gottlieb said.
Cutting through
noisy data
When Gottlieb and his co-author Dartmouth Geography Professor Justin Mankin began looking into this, they were surprised by how inconclusive previous analyses had been.
A recent U.N. Intergovernmental Panel on Climate Change report had not assessed snow mass loss in the Northern Hemisphere, or globally.
The researchers said snow mass is hard to measure, making it difficult to link it to human-caused climate change.
There is a lot of variability, and because there are so many different methods researchers use to look at snowpack, estimates don’t necessarily agree with one another.
To get around that problem, the Dartmouth researchers compiled every data set of snowpack they could find in order to identify where they agreed with one another. They looked at snowpack levels from 1981 to 2020 in March in order to capture all the variation in the preceding winter’s weather – from one-off snow storms to midwinter melts.
Then they used modeling to look at what snow levels would be with or without greenhouse gas emissions.
“Pretty much regardless of which of these snow datasets you look at, even though they don’t necessarily agree with one another, we see a pattern of snow change that is only consistent with human emissions,” Gottlieb said.
‘Bad canary in
a coal mine’
The researchers found that the relationship between temperature rise and snow loss is not linear. There is very little snow loss until temperatures pass a certain threshold near the melting point – minus 17.6 degrees Fahrenheit – and then, snow levels fall off a “cliff,” and snow loss rapidly accelerates.
Mankin said this makes snow loss a “bad canary in the coal mine” for detecting human-caused impacts on the climate.
“Once you see snow loss,” Mankin said, “you are on or off that snow loss cliff.” That means communities that rely on snow for their water supply are not just facing a temporary shortage or emergency, but a “fundamental regime change.”
The researchers found that more than 2 billion people who rely on snow are at or near that tipping point – with projections for future warming accelerating the crisis.
Impact on Northern Hemisphere
The researchers did not see snow loss everywhere, which they believe is another reason this effect has not been previously quantified. Because of the non-linear relationship between temperature rise and snow loss, there are very cold places in North America, Asia and Europe where snow levels are fairly consistent, or there is even more snow because of the increase in precipitation.
The places that are seeing significant snow loss though are the very places that need snow most. Mankin found in a previous study that nearly 2 billion people in the Northern Hemisphere rely on snowmelt as a crucial source of water, and they tend to live in the warmer places where snow is more sensitive to temperature rise.
The western United States, the scientists write, is poised to experience sharp declines in snow runoff. In California this year, the first snow survey of the season found snow depth well below average, and the researchers project that snow loss will only accelerate as temperatures warm.
Although the researchers focused on the impact of snow loss on water supply, there are other implications – for local economies that rely on snow for recreation and tourism, and for ecosystems that have historically been protected by snow from blight and pests.
“I think what is really clear is that we are systematically underestimating the costs of global warming to people and ecosystems,” Mankin said, adding that policymakers need to act now – to cut planet-warming emissions, and to adapt to a world with less snow.