On the heels of the planet’s hottest year on record, new research out of Dartmouth has found that seasonal snowpack across the Northern Hemisphere has shrunk significantly over the last 40 years due to global warming — potentially putting millions of people at risk of worsening water instability.
Between 1981 and 2020, dozens of river basins have seen a significant decline in snow water equivalent, or the amount of water contained in the snow, due to human-caused climate change, according to a study published Wednesday in the journal Nature.
The sharpest drops — between 10% and 20% per decade — were in the Southwestern and Northeastern United States, as well as Central and Eastern Europe.
That includes the Colorado River basin, a key source of water for California and the Southwest, which dwindled to dangerous lows during the most recent drought. The basin has seen spring snowpack declines of about 7% per decade over the last 40 years due to climate change, the study found — or roughly 25% to 30%.
“What we are looking at is a longer-term aggregate pattern, and the question is, is snow a reliable source of water availability for places like California going forward? And the answer, absolutely, is no, it is not,” said Justin Mankin, an associate professor of geography at Dartmouth College and one of the study’s authors.
Many of the snow-dependent watersheds now find themselves on the precipice of a threshold the researchers dubbed a “snow-loss cliff,” or a point at which marginal temperature increases imply larger and larger snow losses to come. The inflection point occurs when average winter temperatures in a watershed are warmer than 17 degrees.
“Once you get beyond that threshold, you start to lose an increasing percentage of your snow with each degree of warming,” said Alexander Gottlieb, the study’s lead author and a doctoral student at Dartmouth. “You sort of get this snowballing — for lack of a better word — loss.”
In all, roughly 2.1 billion people live in snow-dependent hydrologic basins that are either at, or just on the other side of, the temperature threshold, the researchers said.
The findings have worrisome implications for many places that depend on snowpack as a source of water, including California. The Golden State has long relied on the spring and summer snowmelt from the Sierra Nevada to provide nearly a third of its supply.
The Sacramento River basin has lost about 20% of its snowpack over the last 40 years due to climate change, while the San Joaquin River basin has lost about 14%, Gottlieb said. However, he noted there was slightly less statistical confidence around those findings than in the Colorado basin.
In fact, determining climate change’s influence on snow has been something of an elusive research question because there are so many variables, such as fluctuating temperature and precipitation patterns and the presence of El Niño or La Niña in a given year. Measurement methods, such as satellites and remote sensors, have also changed and improved over the decades.
The researchers worked to standardize the disparate data in order to determine where there is a coherent climate change signal. In all, they identified robust trends in 82 out of 169 river basins in the Northern Hemisphere, including 31 river basins where they are “extremely confident” that the trends that have occurred in snow are attributable to global warming.
“We were able to provide the most compelling evidence to date that there’s a really clear anthropogenic signal in all of our snowpack data,” Gottlieb said. “There’s almost no chance that we would be observing these long-term trends without human interference in the climate system.”
Most of those basins are located in lower latitudes that are climatologically warmer, where even a degree of warming can push them over the so-called cliff. Basins that are farther north are often cold enough to endure a degree or two of warming, and some actually saw expanded snowpack due to increased precipitation, including parts of Alaska, Canada and Central Asia, the researchers said.
But in California and the Southwest, the trend is only expected to get worse over time, with some areas projected to see 60% to 80% less snowpack by 2100, Gottlieb said.
“The train has left the station for regions such as the Southwestern and Northeastern United States,” he said. “By the end of the 21st century, we expect those places to be close to snow-free by the end of March.”
He referenced a 2021 study led by researchers at the Lawrence Berkeley National Laboratory that found that winters of low snow, or even no snow, could become a regular occurrence in California in as little as 35 years.
The study found a potential “cascade of implications” for the state beyond the loss of water supplies, including shifts in soil, plants and wildlife, along with the increased risk of flash floods, debris flows and wildfires.
Erica Siirila-Woodburn, who co-authored that Lawrence Berkeley study, said Gottlieb and Mankin’s study is “an important step towards understanding just why snow loss is occurring and how much snow loss we can expect in the future.”
Siirila-Woodburn said their findings are consistent with her own research and other snow loss projections, including the potential for precipitous declines in the Colorado River basin.
“Together, these projections are bad news for already water resource-stressed regions such as California that rely on snowpack for large fractions of its water supply,” she said. “In the ‘snow-loss cliff’ analogy, we are indeed hovering over that precipice.”
The Dartmouth researchers added that snow loss could have economic implications for areas that are reliant on winter recreation and ski tourism. That includes the Hudson, Susquehanna, Delaware, Connecticut and Merrimack watersheds, which feed winter recreation economies in Vermont, New York, New Hampshire and other Northeastern states.
“Ski resorts at lower elevations and latitudes have already been contending with year-on-year snow loss,” Mankin said. “This will just accelerate, making the business model inviable.”
It’s a trend Californians are observing in real time. Last week, officials with the Department of Water Resources conducted their first snow survey of the season and reported snow water content of just 25% of average for the date.
The state anticipates a 10% reduction in water supplies by 2040 and is looking to make up for that shortfall in a variety of ways, including improved hydrology forecasting, enhanced reservoir operations and new groundwater recharge projects to capture and store more water during wet years, according to Andrew Reising, water resources engineer at DWR.
“With declining snowpack projections, DWR will continue to capture as much water as possible while emphasizing that water conservation is now a way of life in California,” Reising said.
However, Mankin said there will still be years like 2023, when California saw record snow.
“A year where California has near-record or record snowpack is entirely consistent with a larger picture from global warming,” he said, noting that climate change is contributing to worsening swings between extreme precipitation and extreme dryness.
But long-term trends remain a persistent concern. Gottlieb noted that so much of the West’s infrastructure is built upon the notion of accumulating winter snow, and its gradual melting in the spring and summer.
“It’s really going to require a lot of changes in how we think about our water supply and water management,” he said. “That paradigm is going to have to change as that natural reservoir of storage for water in the mountains in the winter is going to become increasingly unreliable.”