Snowfall is rather scarce this winter in much of Spain, however, in central Europe snow has covered large areas with records even higher than the average of previous years. Precipitation at specific times and places, experts recall, is subject to meteorological variations that are to a certain extent normal. In order to relate these phenomena - and their effect on the maintenance of the seasonal ice cover - with the more than proven climate change, it is necessary to carry out studies of larger areas and periods. And this is precisely what the authors of a study whose results are presented in the journal Nature (January 10) have carried out.
The conclusions of this work provide evidence that seasonal snow cover in most of the Northern Hemisphere has significantly reduced over the past 40 years due to climate change driven by human activities.
The most pronounced reductions in snow cover related to global warming (between 10% and 20% per decade) occur in the southwestern and northeastern United States, as well as in central and eastern Europe.
The researchers detail in their results presentation paper that the scope and speed of this loss potentially puts hundreds of millions of people in North America, Europe and Asia who depend on snow for water on the brink of a crisis that the Global warming threatens to amplify.
"What concerned us most was how warming is affecting the amount of water stored in snow. The loss of that reservoir is the most immediate and potent risk that climate change poses to society in terms of reduced snowfall and its accumulation," said first author Alexander Gottlieb, of the Ecology, Evolution, Environment, and Society graduate program at Dartmouth.
"Our work identifies basins that have experienced historic snow loss and those that will be most vulnerable to a rapid decline in snowpack with further warming," Gottlieb said. "The train has left the station for regions such as the southwest and northeast of the United States. By the end of the 21st century, we expect these places to be almost snow-free by the end of March. We are on that path and "It's not particularly good adapted when it comes to water scarcity.
Water security is just one dimension of snow loss, said Justin Mankin, associate professor of geography and lead author of the paper.
The Hudson, Susquehanna, Delaware, Connecticut and Merrimack watersheds in the northeastern United States, where water shortages are not as severe, experienced some of the steepest declines in snowpack. But these large losses threaten the economies of states like Vermont, New York and New Hampshire, which depend on winter recreation, Mankin said; Even machine-made snow has a temperature threshold that many areas are rapidly approaching.
"The recreational implications are emblematic of the ways in which global warming disproportionately affects the most vulnerable communities," Mankin said. "Ski resorts at lower elevations and latitudes have already had to deal with snow loss year after year. This will only accelerate, making the business model unviable."
"We will likely see further consolidation of skiing into large, well-resourced resorts at the expense of small and medium-sized ski areas that have such crucial local economic and cultural values. It will be a loss that ripples through communities." he said.
Effects of human-caused climate change on spring snow accumulations in North American watersheds by decade from 1981 to 2020 with labeled river basins. The southwestern and northeastern United States experienced one of the steepest declines in the hemisphere, with more than 10% of spring snow cover lost per decade. These losses put millions of people in the Western United States at risk of a water crisis, while threatening local economies in regions like the Northeast that rely on winter recreation.
In the study, Gottlieb and Mankin focused on how global warming's influence on temperature and precipitation drove changes in snow cover in 169 river basins across the Northern Hemisphere between 1981 and 2020. The loss of snow cover means potentially less spring meltwater for rivers, streams and downstream soils when ecosystems and people demand water.
Gottlieb and Mankin programmed a machine learning model to examine thousands of observations and climate model experiments that captured data on snow cover, temperature, precipitation, and runoff from Northern Hemisphere watersheds.
This not only allowed them to identify where snowpack losses occurred due to warming, but also allowed them to examine the opposing influence of climate-driven changes in temperature and precipitation, which decrease and increase snowpack thickness. of snow, respectively.
The researchers identified uncertainties shared by models and observations to refine what scientists had missed when measuring the effect of climate change on snow. A 2021 study by Gottlieb and Mankin similarly took advantage of uncertainties in how scientists measure snow depth and define snow drought to improve predictions of water availability.
Snow carries uncertainties that have masked the effects of global warming, explained Justin Mankin. "People assume that snow is easy to measure, that it simply decreases with warming, and that its loss means the same impacts everywhere. None of these are the case," Mankin said.
"Snow observations are complicated at the regional scales most relevant to assessing water security," Mankin said. "Snow is very sensitive to variations in temperature and precipitation within the winter, and the risks of snow loss are not the same in New England as in the Southwest, or for a village in the Alps as in the high mountains of Asia. ".
Gottlieb and Mankin, in fact, found that 80% of the Northern Hemisphere snowpack (found in the far north and at higher elevations) experienced minimal losses. In fact, snowpacks expanded across vast swaths of Alaska, Canada and Central Asia as climate change increased the precipitation that falls as snow in these frigid regions.
But it is the remaining 20% of the snow cover that exists around many of the hemisphere's major population centers and provides them with water that has declined. Since 1981, documented decreases in snow cover for these regions have been largely inconsistent due to uncertainty in observations and natural climate variations.
But Gottlieb and Mankin found that a steady pattern of annual declines in snowpack quickly emerges, leaving population centers with sudden and chronic shortages of new water supplies from melting snow.
Many snow-dependent watersheds are now dangerously close to a temperature threshold that Gottlieb and Mankin call the "snow loss cliff." This means that as average winter temperatures in a basin rise beyond 17 degrees Fahrenheit (minus 8 degrees Celsius), snow loss accelerates even with only modest increases in local average temperatures.
Many highly populated watersheds that rely on snow for water supplies will suffer increasing losses in the coming decades, Mankin said.
"This means that water managers who rely on snowmelt cannot wait for all the observations on snow loss to agree before preparing for permanent changes in water supply. By then, it will be too late," he said. "Once a watershed falls off that cliff, it's no longer about managing a short-term emergency until the next big snowfall. Instead, they will adapt to permanent changes in water availability."
