Snow is a controversial clue for interpreting climate change. The decrease in snowfall in December in most of the recent winters, including this one, pointed to our future under global warming. This is evident in the mountain ranges from Oregon to New Hampshire, where the peaks are more brown than white, and in the American Southwest, where there is a severe snow drought.

On the other hand, record snowstorms like the one in early 2023 that buried California’s mountain communities, filled dried-up reservoirs, and dumped 10 feet of snow on northern Arizona contradict our understanding of life on a warming planet.

Similarly, scientific evidence from ground-based observations, satellites, and climate models disagrees on whether global warming is steadily destroying snowdrifts accumulating in high-altitude mountains, complicating efforts to combat water shortages that could hit many populated areas.

Now, a new study out of Dartmouth removes the uncertainty in these observations and provides evidence that seasonal snowpack across much of the Northern Hemisphere has actually decreased significantly over the past 40 years due to human-caused climate change. The sharpest declines in snowpack associated with global warming (between 10% and 20% per decade) are seen in the southwestern and northeastern United States, as well as in central and eastern Europe.

Water crisis and its economic consequences

Researchers report in the journal NatureThe scale and pace of this loss could push hundreds of millions of people in North America, Europe, and Asia who depend on snow for water to the brink of a continuing warming crisis.

“We were most concerned about how warming would affect the amount of water stored in snow. The loss of this reservoir is the most immediate and powerful risk that climate change poses to society in terms of reduced and accumulated snow cover,” says first author Ph.D. Alexander Gottlieb. student in the Ecology, Evolution, Environment and Society Master’s Program at Dartmouth.

“Our study identifies watersheds that have experienced historic snow loss and those most vulnerable to rapid snowpack declines with subsequent warming,” Gottlieb said. said. “The train left the station for areas such as the southwestern and northeastern United States. At the end of the 21st century, we expect these places to be virtually snow-free until the end of March. We are on this path, and we have not adapted very well to water scarcity.”

Water security is just one aspect of snow loss, said Justin Mankin, professor of geography and senior author of the paper.

The Hudson, Susquehanna, Delaware, Connecticut and Merrimack basins in the northeastern United States, where water shortages are not as severe, experienced some of the steepest declines in snowpack. But Mankin said these significant losses threaten the economies of states such as Vermont, New York and New Hampshire that depend on winter recreation; Even machine-made snow has a temperature threshold that many regions are quickly approaching.

“Recreational impacts are emblematic of how global warming disproportionately affects the most vulnerable communities,” Mankin said. “Ski resorts located at lower altitudes and latitudes are already struggling with annual snow loss. This will only accelerate the process and make the business model unsustainable.”

“We will see further consolidation of skiing into large, well-resourced resorts at the expense of small and medium-sized ski areas that have such significant local economic and cultural value. “This will be a loss that will ripple through communities,” he said.

Research methodology and results

In the study, Gottlieb and Mankin focused on how the effects of global warming on temperature and precipitation caused changes in snowpacks in 169 river basins in the Northern Hemisphere from 1981 to 2020. Loss of snowpack could mean less meltwater for rivers, streams and soils downstream in the spring when ecosystems and people need water.

Gottlieb and Mankin programmed a machine learning model to examine thousands of observations and climate modeling experiments that record data on snow cover, temperature, precipitation and runoff for Northern Hemisphere basins.

This not only allowed them to determine where snowpack losses due to warming were occurring, but also to examine the opposing effects of climate-induced changes in temperature and precipitation, reducing and increasing snowpack thickness, respectively.

Researchers have identified uncertainties shared by models and observations; so they were able to determine what scientists had previously missed when predicting the impact of climate change on snow. A 2021 study by Gottlieb and Mankin similarly used uncertainties in how scientists measure snow depth and define snow drought to improve estimates of water availability.

According to Mankin, snow brings with it uncertainty that masks the effects of global warming. “People assume that snow is easy to measure, that it decreases with warming, and that its loss has the same consequences everywhere. None of this is true,” Mankin said.

“Snow observations are complex at the regional scale and are most important for water safety assessments,” Mankin said. “Snow is very sensitive to fluctuations in temperature and precipitation during the winter months, and the risks of snow loss are different for a village in New England than in the Southwest or in high-elevation Asia for a village in the Alps.”

Regional differences and the “profit-losing gap”

In fact, Gottlieb and Mankin found that 80% of the Northern Hemisphere’s northernmost and higher-elevation snowpack experienced minimal loss. Snowdrifts have increased across large areas of Alaska, Canada, and Central Asia as climate change increases the amount of precipitation that falls as snow in these cold regions.

But 20% of the snowpack that surrounds and supplies water to most of the hemisphere’s major population centers has still shrunk. Since 1981, documented reductions in snow cover in these regions have been largely inconsistent due to observational uncertainty and natural climate fluctuations.

But Gottlieb and Mankin found that a steady pattern of annual decline in snowpack quickly emerged, and populations suddenly and chronically ran out of new water supplies from melting snow.

Many snow-dependent basins are now dangerously close to the temperature threshold that Gottlieb and Mankin call the “snow loss cliff.” This means that when the basin’s winter average temperature rises above 17 degrees Fahrenheit (minus 8 degrees Celsius), snow loss accelerates, even if local average temperatures rise slightly.

Losses will accelerate over the next few decades in many densely populated basins that depend on snow for their water supply, Mankin said.

“This means that water managers who rely on snowmelt cannot wait for all observations to agree on snow loss before preparing for permanent water supply changes. Then it is too late,” he said. “If the pond has fallen off the cliff, this is no longer a short-term emergency until the next big snowfall. Instead, they will adapt to constant changes in water availability.”