New study says climate change behind drop in Northern Hemisphere snowpack

Paper says river basins that start in Montana could have steep drop in yearly runoff

By: - January 31, 2024 4:42 pm
The Nevada Mountains northwest of Helena seen from Great Divide Ski Area on Jan. 20, 2024. (Photo by Blair Miller, Daily Montanan)

The Nevada Mountains northwest of Helena seen from Great Divide Ski Area on Jan. 20, 2024. (Photo by Blair Miller, Daily Montanan)

New scientific research published earlier this month shows that human-caused climate change is putting the most densely populated areas of the Northern Hemisphere, including the American West, at risk of losing vast portions of their water supply because of decreasing snowpack.

Published in the journal Nature on Jan. 10, the report led by two Dartmouth College researchers found climate change-driven snowpack trends in half of the 169 river basins in the Northern Hemisphere, 31 of which they said they could “confidently attribute to human influence.”

“Together, our findings portend serious water-availability challenges in basins where snowmelt runoff constitutes a major component of the water supply portfolio,” the researchers wrote in their conclusion. “Improving our understanding of where and how climate change has and will affect snow water resources is vital to informing the difficult water resource management decisions that a less snowy future will require.”

The researchers built a model that analyzed observations and models of snowpack, temperature, precipitation, and runoff data from 1981 through 2020, then used the uncertainties from the models and observations to try to cut through temperature and precipitation variations and account for regional differences to see how a warmer planet would affect snowpack and runoff in the future.

Those models, they wrote, allowed them to produce more than 12,000 estimates of the effects of human-caused climate change on March snowpack across the Northern Hemisphere.

This map shows snowpack changes from 1981 to 2020 based off the research. Dark red basins lost the most snowpack, while blue basins gained snowpack. (Image by Justin Mankin and Alexander Gottlieb)
This map shows snowpack changes from 1981 to 2020 based off the research. Dark red basins lost the most snowpack, while blue basins gained snowpack. (Image by Justin Mankin and Alexander Gottlieb)

The researchers said that better understanding the unknowns in analyzing snowpack, and how human-cause climate change is affecting the snowpack in general, can lead to better climate models and also better policymaking for governments and water managers who will have to deal with the effects of the reduced runoff in the future.

“It means that water managers who rely on snowmelt can’t wait for all the observations to agree on snow loss before they prepare for permanent changes to water supplies. By then, it’s too late,” Dartmouth’s Justin Makin, the senior author of the paper and an associate professor of geography, said in a statement. “Once a basin has fallen off that cliff, it’s no longer about managing a short-term emergency until the next big snow. Instead, they will be adapting to permanent changes to water availability.”

One of the keys in being able to account for uncertainties and still make predictions, according to the paper, was identifying -8 degrees Celsius, or 17.6 degrees Fahrenheit, as the average winter temperature where snowpack can start to decrease rapidly with minimal air temperature increases.

The authors found that below that temperature threshold, warmer temperatures have little effect on snowpack. But for each 1 degree Celsius that temperatures go above the threshold, accelerated losses in snowpack result.

Those regions below the temperature threshold are generally along or north of 60 degrees latitude, in the northern reaches of Alaska, Canada and Asia. But they account for about 80% of March snowpack in the Northern Hemisphere, according to the paper.

However, the researchers wrote, the remaining 20% of the snowpack primarily sits just above the temperature threshold, making it prone to what the researchers called a “snow-loss cliff,” where each degree of warming, on average, during the winter, will have a broader effect on the snowpack and water retention.

“Such a relationship suggests that further warming and thus additional time spent beyond this −8 °C threshold will homogenize snow trends towards more consistent declines, portending widespread and accelerating snow losses for many basins over the coming decades,” the paper says.

That 20% area of the hemisphere that is just on the edge of the “cliff” also happens to include about 80% of the people that live in the Northern Hemisphere, according to the paper. Most of Montana, save for the coldest areas, averages winter temperatures above 17.6 degrees Fahrenheit.

“As such, further warming is likely to have rapidly emerging impacts on snow water resources in the mid-latitude basins where people reside and place competing demands on fresh water,” the researchers wrote.

River basin snow-water equivalent as a percent of the 1991-2020 median across the West as of Jan. 30, 2024. (Image via USDA/NRCS)

The researchers said their models both uncovered human-caused snow-water equivalent declines in some areas where observations had shown increases during the past 40 years, like the Columbia River basin, but also signs that steep decreases could be in store for other parts of the West, including Montana.

The paper said the Upper Mississippi River basin, which starts in Montana, could see a 30% decline in spring runoff, and the Columbia River basin could see a 33% decline. Those two basins are home to 90 million people, according to the paper. The researchers found the Colorado River basin, home to 14 million people, could see a runoff decline of 42%.

The declining snowpack and runoff would in turn affect several facets of the economy, from winter recreation, which brings in millions to Montana and other Western states each year, to agriculture, water recreation, and land management.

“We’ll 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,” Mankin said in a statement from Dartmouth. “This will just accelerate, making the business model inviable.”

Climate change and snowpack in Montana

The 2017 Montana Climate Assessment found that snowpack in Montana had been declining in the mountains since the 1930s, but that the decline sped up since the 1980s. It found that climate change would further reduce snowpack at middle and lower elevations, and that snowpack would melt earlier at all elevations and reduce water availability later in the summer.

The report found that groundwater demand would likely go up as temperatures increase and surface-water resources waned and said with high confidence that a warming climate would “strongly influence Montana’s snowpack, streamflow dynamics, and groundwater resources, with far-reaching consequences for social and ecological systems.”

Four years later, the 2021 Climate Change and Human Health in Montana report said researchers found climate change would lead to earlier snowmelt, more intense precipitation, increases in flooding, and also drought during the summer, which affects the water supply. The report also noted how Montana is home to three of North America’s major river basin headwaters, the Missouri, Columbia and Saskatchewan.

Montana river basin snow-water equivalent as a percent of the 1991-2020 median as of Jan. 30, 2024. (Image via USDA/NRCS)
Montana river basin snow-water equivalent as a percent of the 1991-2020 median as of Jan. 30, 2024. (Image via USDA/NRCS)

“As such, snow levels in our region affect water availability far beyond the state’s border. Our mountain snowpack comes largely from Pacific storms. This winter precipitation is the primary water supply serving our state’s waterways, ecosystems, municipalities, farms and ranches, and recreational tourism industries.”

Building off the 2017 and 2021 climate reports, the state in December released its newly updated Drought Management Plan, which notes that climate change would exacerbate drought in Montana when it occurs despite some longer periods of drought here historically.

“This means droughts may have more rapid onset and greater intensity, and patterns of snowpack accumulation and runoff will shift, increasing the likelihood of drought in late summer and early fall,” the report says.

The plan makes 36 drought management recommendations, which include better understanding snowpack and streamflows and how they can inform drought reporting, a study on cloud seeding, and expanding snowpack, water and drought monitoring.

The new research on longterm snowpack and climate change, along with the state’s new drought plan, come during a winter in Montana that has provided one of the worst snowpacks statewide in 30 years and increased drought intensity.

Snowpack still weak for 2024, drought intensifies

The statewide median snowpack was the lowest on record as of Wednesday, at 6.2 inches of snow-water equivalent, when compared to 1991-2020. The median snowpack for those years on Jan. 31 is 10.4 inches.

The previous record low statewide median snowpack for Jan. 31 was 6.6 inches in 2001.

The black line shows the median statewide snowpack this winter compared to winters from 1991 through 2020. (Image via USDA/NRCS)
The black line shows the median statewide snowpack this winter compared to winters from 1991 through 2020. (Image via USDA/NRCS)

The Big Horn basin sat at 75% of its median snow-water equivalent as of Wednesday, but every other basin in the state was below 70% of median. The Upper Clark Fork (47% of median), Upper Missouri (45% of median) and Sun-Teton-Marias (35% of median) basins were all under 50% of median levels. The rest of the state’s river basins sit between 50% and 70% of median levels.

Most of western Montana is now also experiencing moderate drought, and the areas along the Continental Divide have moved into the severe drought category as of last week. Eighty percent of the state is abnormally dry or worse, according to the U.S. Drought Monitor, when only 42% of Montana was in late October.

While the lower snowpack this winter and increasing drought were not unexpected because of the El Niño predictions for this winter that have mostly borne out, forecasters warned earlier this month that a lack of snow through the rest of the winter could mean an early and minimal runoff, low streamflows and more drought this summer.

That could spell another season of quick decision making for officials in deciding on dam outflows, stream and river closures and ongoing fish population studies all while the Montana Supreme Court decides whether the state needs to account for climate change to comply with the state constitution.

And while what happens with the snowpack and runoff over the next several months involves short-term weather trends, the Dartmouth researchers said their study is about taking those observations, putting them in long-term models, and officials utilizing the data to make those kinds of decisions before it’s too late.

“This highly nonlinear marginal sensitivity to snow we identify clarifies why such warning in the observations so far has been elusive, and also why waiting until the impacts manifest could be too late to effectively manage their risks,” the authors wrote. “Such warning, we show, will probably only come from the observations once warming is sufficient to push regions into this highly nonlinear snow-loss regime.”

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Blair Miller
Blair Miller

Blair Miller is a reporter based in Helena who primarily covers government, climate and courts. He's been a journalist for more than 12 years, previously based in Denver, Albuquerque and mid-Missouri.

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