March 1, 2024

Alarming trends discovered in 40-year climate study

Recent winters have shown conflicting signs of climate change, with drastic snow shortages and heavy snowfall. A Dartmouth study clarifies this by showing a significant decline in Northern Hemisphere snowpack over the past 40 years due to human-caused climate change. This loss endangers water supplies and winter-dependent economies. The study’s conclusions, based on an analysis of temperature and precipitation data, indicate that many regions are approaching a “snow loss cliff”, requiring immediate adaptation in water management strategies.

The study reveals that areas of the Northern Hemisphere most dependent on snow for water are seeing the steepest declines.

Snow presents a contradictory clue for interpreting climate change. In numerous recent winters, including this one, decreasing snowfall in December has seemed to signal our future under global warming. This is evident in regions from Oregon to New Hampshire, where peaks have been more brown than white, and in the American Southwest, which is experiencing a significant snow drought.

On the other hand, record snowfalls like those in early 2023, which buried California mountain communities, refilled parched reservoirs and dropped 11 feet of snow in northern Arizona, challenge our conceptions of life on a warming planet.

Likewise, scientific data from ground-based observations, satellites, and climate models do not agree on whether global warming is consistently destroying the snowpack that accumulates in high-altitude mountains, complicating efforts to manage food shortages. water that would result in many population centers.

Loss of snowpack in North America

Effects of human-caused climate change on spring snowpack in North American watersheds by decade, 1981 to 2020, with labeled watersheds. The U.S. Southwest and Northeast have seen some of the steepest declines in the hemisphere, with more than 10% of spring snowpack lost per decade. These losses put millions of people in the American West at risk of a water crisis, while also threatening local economies in regions like the Northeast that depend on winter recreation. Credit: Justin Mankin and Alex Gottlieb/Dartmouth

Now, a new study from Dartmouth eliminates the uncertainty in these observations and provides evidence that seasonal snowpack across much of the Northern Hemisphere has decreased significantly over the past 40 years due to human-caused climate change. The steepest reductions in snowpack related to global warming – between 10% and 20% per decade – are recorded in the Southwest and Northeast of the United States, as well as in Central and Eastern Europe.

The water crisis and the economic impact

The researchers report in the journal Nature that the extent and speed of this loss potentially puts the hundreds of millions of people in North America, Europe and Asia who depend on snow for water on the precipice of a crisis that continued warming will amplify.

“We were more concerned about how warming is affecting the amount of water stored in snow. The loss of this reservoir is the most immediate and potent risk that climate change poses to society in terms of decreasing snowfall and accumulation,” said first author Alexander Gottlieb, a Ph.D. student in the graduate program in Ecology, Evolution, Environment and Society at Dartmouth.

“Our work identifies watersheds that have experienced historic snow loss and those that will be most vulnerable to rapid snowpack decline with additional warming,” Gottlieb said. “The train 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 we are not particularly well adapted when it comes to water scarcity.”

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

Loss of accumulated snow in the Northern Hemisphere

Effect of human-caused global warming on spring snowpack in Northern Hemisphere watersheds from 1981 to 2020, by percentage change by decade. Red indicates a decrease and blue indicates an increase. Snow accumulation in many far northern watersheds has increased as climate change has led to more precipitation falling as snow. But river basins at lower latitudes, which provide water and economic benefits to northern population centers, have suffered the greatest losses. Credit: Justin Mankin and Alex Gottlieb/Dartmouth

The Hudson, Susquehanna, Delaware, Connecticut and Merrimack watersheds in the northeastern US, where water shortages are not as severe, have seen some of the steepest drops in snowpack. But these heavy losses threaten economies in states like Vermont, New York and New Hampshire that depend on winter recreation, Mankin said — even machine-made snow has a temperature limit that many areas are quickly approaching.

“The recreational implications are emblematic of the way global warming disproportionately affects the most vulnerable communities,” said Mankin. “Ski resorts at lower altitudes and latitudes already face snow loss year after year. This will only accelerate it, making the business model unviable.”

“We will likely see greater consolidation of skiing into large, well-resourced resorts, at the expense of small and medium-sized ski areas that hold such crucial local economic and cultural values. It will be a loss that will affect communities,” he said.

Study Methodology and Results

In the study, Gottlieb and Mankin focused on how global warming’s influence on temperature and precipitation has driven snowpack changes in 169 watersheds across the Northern Hemisphere from 1981 to 2020. The loss of snowpack potentially means less spring meltwater to rivers, streams and soils downstream 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 snowpack, temperature, precipitation and runoff data for Northern Hemisphere watersheds.

This not only allowed them to identify where snow losses due to warming occurred, but also gave them the ability to examine the counteracting influence of climate change on temperature and precipitation, which decrease and increase snow thickness, respectively.

The researchers identified the uncertainties that the models and observations shared so they could identify what scientists had previously missed when assessing the effect of climate change on snow. A 2021 study by Gottlieb and Mankin similarly took advantage of uncertainties about how scientists measure snow depth and define snow drought to improve predictions of water availability.

Snow brings with it uncertainties that mask the effects of global warming, Mankin said. “People assume that snow is easy to measure, that it simply decreases with warming, and that its loss has the same impacts everywhere. None of that is 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 during the winter, and the risks of snow loss are not the same in New England as they are in the Southwest, or for a village in the Alps as they are in the high mountains of Asia. .”

Regional variations and the “snow loss cliff”

In fact, Gottlieb and Mankin found that 80% of the Northern Hemisphere’s snowpack – which is found in the far north and high-altitude regions – suffered minimal losses. In fact, snowpacks have expanded across vast areas of Alaska, Canada and Central Asia as climate change has increased the precipitation that falls as snow in these cold regions.

But it is the remaining 20% ​​of the snowpack that exists around many of the hemisphere’s major population centers – and provides water to – that has declined. Since 1981, documented declines in snow accumulation in 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 — and leaves population centers suddenly and chronically starved of new water supplies from meltwater.

Many snow-dependent watersheds now find themselves dangerously close to a temperature threshold that Gottlieb and Mankin call the “snow loss cliff.” This means that as average winter temperatures in a river basin increase beyond 17 degrees

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