New maps show where snowfall is disappearing

Snowfall is declining globally as temperatures rise due to human-caused climate change, new analysis and maps from a NOAA climate scientist show.

But less snow falling from the sky is not as harmless as having to shovel less; threatens to reinforce warming and disrupt food and water 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 than snow, all else being equal.

In the short term, climate change may cause more extreme winter storms and some years of higher snowfall (as data for the northeastern U.S. shows), but as global temperatures rise, there will be fewer of those years. and eventually we could see amounts of snow falling off a cliff.

“Over time, the laws of thermodynamics mean that as we continue to warm, more and more snow will turn to rain,” said 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 hide some trends, but overall the laws of thermodynamics will prevail.”

Snow also won’t decrease linearly, or at a 1-to-1 rate, with rising temperatures, said Justin Mankin, a climate scientist and associate professor of geography at Dartmouth College. Instead, there is rather an inflection point, which would mean that once a certain temperature threshold is reached, “we should expect losses to accelerate.”

“This means we can expect that many of the places that haven’t shown massive declines in snowfall will perhaps start to show snowfall with a little more warming,” Mankin told CNN.

There has already been a 2.7% decline in annual global snowfall since 1973, according to Brettschneider’s analysis of data from the European Union’s Copernicus Climate Change Service. The downward trend is particularly notable in the mid-latitudes of the Northern Hemisphere: the middle area north of the tropics and south of the Arctic, where the United States and much of the world’s population reside.

The sun is more direct there compared to higher latitudes, especially during spring and fall when it is still snowing. The white of the snow acts like a car’s sunshade, deflecting sunlight and its heat back into space. Without it, the soil absorbs more sunlight, warming the atmosphere.

Less snow falling from the sky also means less snow accumulating in the snowpack, a deep, persistent layer of snow that builds up during the winter. It is crucial to the water supply because it acts as a natural reservoir, storing water as snow during wet times and then releasing it as snowmelt when water is harder to come by, Jessica Lundquist, a professor of environmental engineering at the University of California, told CNN. University of Washington.

The threat to water supplies from declining snowfall is more pronounced in climates subject to more extreme precipitation cycles, such as the Mediterranean climate found in California and other parts of the western United States, Lundquist said.

“California is the model: it doesn’t rain in the summer in California, so the runoff from snowmelt, the snow that waits and runs off later in the season, is absolutely essential for all ecosystems, all agriculture, all cities. or anyone who wants water during the dry season,” Lundquist told CNN.

Snowpack provides water for more than 50% of the water supply in the arid West, according to a 2017 study. The same study predicted that snow levels in the West would continue to decline by more than a third by 2100 in a scenario of high increase in pollution that warms the planet.

As the map shows, the decline in snowfall over the past 50 years has been particularly pronounced in parts of the western United States. This trend is in line with other studies that have shown a decrease in snow cover in more than 90% of the western sites where it is measured.

The increase in snowfall in the Northeast seen on the maps illustrates the complicated nature of changing precipitation patterns with climate change, scientists told CNN.

“While the overall snowfall trend was positive, the snowfall days per year trend is negative,” Brettschneider told CNN. This means more snow was falling on fewer days, which could be a sign of more extreme snowfall predicted along with climate change.

“The likelihood of extreme snowfall actually increases with global warming and that’s because as we warm our atmosphere, the ability of the atmosphere to be a reservoir of moisture increases,” Mankin told CNN. “So you get this compensatory response where extreme snowfall can actually increase with 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 are much less reliable before the 1970s, but starting to use the data in that decade also meant that the analysis included some exceptionally snowy years in that part of the United States. If the analysis had started earlier, it might also show a decline, he said.

Manage water with less snow

Understanding the implications of less snowfall on the global water supply is much more complicated than simply saying that less snow means less available water, Mankin said. It depends largely on location and a variety of other snow dynamic factors.

The important thing to follow in determining water availability is not the amount of snow, but the amount of water in the snow, Mankin said, which can vary greatly. A light, fluffy snow will have a low water content, but a dense, heavy snow will have a high water content.

Additionally, the same extreme precipitation events that cause more snow can also mean more rain, which “could offset those snow losses,” Mankin said.

But the scope of the lack of snow problem remains enormous.

A 2015 study by Mankin found that 2 billion people who rely on melting snow for water are at risk of snowfall declines of up to 67%. This includes parts of South Asia, which depend on melting Himalayas; the Mediterranean, including Spain, Italy and Greece; and parts of North Africa like Morocco, which depend on melting ice from the Atlas Mountains.

But Mankin said the study didn’t capture hyperlocal water management, including potential strategies that could mitigate or even replace water lost from missing snow.

“Snow loss becomes a huge management challenge,” Mankin said. “This is not necessarily an insurmountable challenge everywhere, but it is a considerable management challenge, particularly in places like the American West that rely heavily on snowmelt runoff.”

Mankin and Lundquist said more research is being done to better understand the nuanced relationship between snow and water supply, 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 multi-scale constellation of solutions and money that can only be conceived after the scope of the problem is understood and identified,” Mankin said.

“To the extent that any of these places are managing water for the status quo, global warming is eliminating that status quo,” Mankin said. “To the extent that our infrastructure and our management practices are hard-coded to a historical climate, that is irrelevant to the climate that is developing.”

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