Snowfall is declining globally as temperatures rise due to human-induced climate change, a NOAA climate scientist’s new analysis and maps show.
But less snow falling from the sky is not as harmless as just shoveling less snow; it threatens to intensify warming and disrupt food and water supplies for billions of people.
Climate scientists say the future of snowfall is pretty clear: a warmer world, driven by human pollution, means precipitation is more likely to fall as rain rather than snow, all else being equal.
It’s possible in the short term that climate change will bring more extreme winter storms and some years of increased snowfall – as the data for the northeastern US shows – but as global temperatures rise there will be fewer of those years, and eventually we might see amounts of snowfall falling off a cliff.
“Ultimately, the laws of thermodynamics mean that as you keep warming, you’re going to turn more and more of that snow into rain,” says Brian Brettschneider, a climate scientist at the National Weather Service in Alaska and the brains behind the data. analysis in this story. “You can get away with it for a while and it may hide certain trends, but overall the laws of thermodynamics will win.”
Snow also won’t decrease linearly, or at a 1-to-1 rate, as temperatures rise, says Justin Mankin, a climate scientist and associate professor of geography at Dartmouth College. Instead, there is more of a tipping point, which would mean that once a certain temperature threshold is reached, “we can expect losses to increase.”
“It means we can expect that many of the places that haven’t seen a huge decline in snowfall might start to show one with just a little bit more warming,” Mankin told CNN.
According to Brettschneider’s analysis of data from the European Union’s Copernicus Climate Change Service, there has already been a 2.7% decline in annual global snowfall since 1973. The downward trend is especially striking in the mid-latitudes of the Northern Hemisphere – the mid-range north of the tropics and south of the Arctic, where the US and much of the world’s population live.
The sun is more direct there than at higher latitudes, especially during spring and autumn, when it still snows. The white of snow acts like a car’s sunshade, bending sunlight and its heat back into space. Without this, more sunlight is absorbed by the ground, warming the atmosphere.
Less snow falling from the sky also means less snow accumulating in a snowpack – a deep, persistent layer of snow that accumulates in winter. It is critical to the water supply because it acts as a natural reservoir, storing water in the form of snow during wet times and then releasing it in the form of melting snow when water is harder to come by, Jessica Lundquist told , professor of environmental engineering at the University of Washington, told CNN.
The threat to water supplies from declining snowpack is most pronounced in climates subject to more extreme precipitation cycles, such as the Mediterranean climate in California and other parts of the American West, Lundquist said.
“California is the textbook example – in California it doesn’t rain in the summer, and so the melting snow, the snow that waits and runs off later in the season, is absolutely essential for all ecosystems, all agriculture, all of the cities or everyone who wants water during the dry season,” Lundquist told CNN.
Snowpack supplies water to more than 50% of the arid West’s water supply, a 2017 study found. The same study predicted that snowpack in the West would continue to decline by more than a third by 2100 due to sharply increasing pollution scenario for global warming.
As the map shows, the decline in snowfall over the past fifty years has been particularly pronounced in parts of the western US. This trend is consistent with other studies that have shown a decrease in snow cover in more than 90% of western locations where it is measured.
The increase in snowfall in the Northeast seen on the maps illustrates the complicated nature of changing precipitation patterns due to climate change, scientists told CNN.
“Although the overall snowfall trend was positive, the days per year with snowfall are negative,” Brettschneider told CNN. This means that more snow fell in fewer days, which could be a sign of the predicted more extreme snowfall combined with climate change.
“The likelihood of extreme snowfall is actually increasing with global warming, and that’s because as we warm our atmosphere, the ability of the atmosphere to be a reservoir for moisture increases,” Mankin told CNN. “So you get a compensatory response where extreme snowfall may actually increase as a result of global warming.”
The increase in snow in the Northeast is also partly due to the time period these maps look at, Brettschneider told CNN. Snowfall data is much less reliable before the 1970s, but starting the data in the 1970s also meant that the analysis included some exceptionally snowy years for that part of the US. If the analysis had started earlier, it could also show a decline, he said.
Managing water with less snow
Understanding the consequences of less snowfall for global water supplies is much more complicated than simply saying that less snowfall means less available water, Mankin said. It depends a lot on location and a variety of other dynamic snow factors.
The most important thing to monitor water availability is not the amount of snow, but the amount of water in the snow, Mankin said, which can vary widely. A light, fluffy snow has a low water content, but a dense, heavy snow has a high water content.
Additionally, the same extreme precipitation events that cause more snow could also mean more rain, which “could potentially offset those snow losses,” Mankin said.
But the scale of the missing snow problem is still enormous.
A 2015 study by Mankin found that 2 billion people who rely on snowmelt for water are at risk of snowfall declines of up to 67%. This includes parts of South Asia, which depend on snowmelt in the Himalayas; the Mediterranean, including Spain, Italy and Greece; and parts of North Africa such as Morocco, which rely on snowmelt from the Atlas Mountains.
But Mankin said the study did not take into account hyper-local water management, including possible strategies that could mitigate or even replace water loss from missing snow.
“The snow loss is going to be a huge management challenge,” Mankin said. “This is not necessarily an insurmountable challenge everywhere, but it is a significant management challenge, especially in places like the American West that rely heavily on snowmelt runoff.”
Mankin and Lundquist both said more research is underway to better understand the nuanced relationship between snow and water resources, especially at a hyperlocal scale, which will help water managers better plan for a more volatile relationship with snow.
“There is no silver bullet here – it will be a constellation of solutions and money at different scales, which can only be devised after the scale of the problem has been understood and identified,” Mankin said.
“To the extent that any of these places are managing water for the status quo, global warming is taking away that status quo,” Mankin said. “To the extent that our infrastructure and our management practices are hard-coded for a historical climate, those things are not relevant to the climate that is unfolding.”
For more CNN news and newsletters, create an account at CNN.com