Here’s the release from the United States Geological Survey:
A USGS study released today suggests that snowpack declines in the Rocky Mountains over the last 30 years are unusual compared to the past few centuries. Prior studies by the USGS and other institutions attribute the decline to unusual springtime warming, more precipitation falling now as rain rather than snow and earlier snowmelt.
The warming and snowpack decline are projected to worsen through the 21st century, foreshadowing a strain on water supplies. Runoff from winter snowpack – layers of snow that accumulate at high altitude – accounts for 60 to 80 percent of the annual water supply for more than 70 million people living in the western United States.
“This scientific work is critical to understanding how climate change is affecting western water supplies,” Secretary of the Interior Ken Salazar said. “It helps land managers adapt to changing conditions on the ground, assists water managers with planning for the future, and gives all of us a better understanding of the real impacts that carbon pollution is having on our resources and our way of life.”
USGS scientists, with partners at the Universities of Arizona, Washington, Wyoming, and Western Ontario, led the study that evaluated the recent declines using snowpack reconstructions from 66 tree-ring chronologies, looking back 500 to more than 1,000 years. The network of sites was chosen strategically to characterize the range of natural snowpack variability over the long term, and from north to south in the Rocky Mountains.
With a few exceptions (the mid-14th and early 15th centuries), the snowpack reconstructions show that the northern Rocky Mountains experience large snowpacks when the southern Rockies experience meager ones, and vice versa. Since the 1980s, however, there were simultaneous declines along the entire length of the Rocky Mountains, and unusually severe declines in the north.
“Over most of the 20th century, and especially since the 1980s, the northern Rockies have borne the brunt of the snowpack losses,” said USGS scientist Gregory Pederson, the lead author of the study. “Most of the land and snow in the northern Rockies sits at lower and warmer elevations than the southern Rockies, making the snowpack more sensitive to seemingly small increases in temperature. Also, winter storm tracks were displaced to the south in the early 20th century and post-1980s. Forest fires were larger, more frequent and harder to fight, while Glacier National Park lost 125 of its 150 glaciers.”
USGS scientist and co-author Julio Betancourt explains that “The difference in snowpack along the north and south changed in the 1980s, as the unprecedented warming in the springtime began to overwhelm the precipitation effect, causing snowpack to decline simultaneously in the north and south. Throughout the West, springtime tends to be warmer during El Niño than La Niña years, but the warming prior to the 1980s was usually not enough to offset the strong influence of precipitation on snowpack.”
The La Niña episode this year is an example with lots of snow in the north while severe drought afflicts the south. But, in the north, this year’s gains are only a small blip on a century-long snowpack decline.
In the West, the average position of the winter storm tracks tend to fluctuate north and south around a latitudinal line connecting Denver, Salt Lake City and Sacramento. In El Niño years, winter storms track south of that line, while in La Niña years, they track to the north.
This study supports research by others estimating that between 30-60 percent of the declines in the late 20th century are likely due to greenhouse gas emissions. The remaining part of the trend can be attributed to natural decadal variability in the ocean and atmosphere, which is making springtime temperatures that much warmer.
“What we have seen in the last few decades may signal a fundamental shift from precipitation to temperature as the dominant influence on western snowpack.” Pederson said.
The study, The unusual nature of recent snowpack declines in the North American Cordillera, is online at Science magazine http://www.sciencemag.org/lookup/doi/10.1126/science.1201570.
More coverage from Chris Woodka writing for The Pueblo Chieftain. From the article:
USGS scientists, with partners at the universities of Arizona, Washington, Wyoming, and Western Ontario, led the study that evaluated the recent declines using snowpack reconstructions from 66 tree-ring chronologies, looking back 500-1,000 years. With a few exceptions (the mid-14th and early 15th centuries), snowpack reconstructions show that the northern Rocky Mountains experience large snowpacks when the southern Rockies experience meager ones, and vice versa. Since the 1980s, however, there were simultaneous declines along the entire length of the Rocky Mountains, and unusually severe declines in the north.
More coverage from Felicity Barringer writing for The New York Times weblog Green. From the post:
On the one hand, the underlying narrative was depressingly familiar, synchronizing with other studies or predictions involving California’s Sierra, the Himalayas and other spots around the world. What drew my attention is the reminder that snowmelt in the Rockies feeds three huge river systems — the Colorado, the Columbia and the Missouri — on which 70 million Americans depend for water.
Not that everyone living along — or flooded out of the communities along — the Missouri River or its swollen tributaries would consider these findings intuitive. But the U.S.G.S. scientists who led the study point out that climate in the Rocky Mountains has for centuries been an either-or proposition.
It works like this. Draw a hypothetical line in your mind from Denver to Salt Lake City to Sacramento. If there is heavy snow to the north of this line, chances are there will be a drought to the south of it. And vice versa. Or, as an Interior Department press release said, “With a few exceptions (the mid-14th and early 15th centuries), the snowpack reconstructions show that the northern Rocky Mountains experience large snowpacks when the southern Rockies experience meager ones, and vice versa.”
More coverage from United Press International. From the article:
The study backs research that the USGS said estimates that as much as 60 percent of the snowpack declines in the late 20th century are because of greenhouse gas emissions, which are linked to higher global temperature averages…Runoff from winter snowpack makes up nearly 60 percent of the annual water supply for people in the western United States.
More coverage from Craig Welch writing for The Seattle Times via the Bend Bulletin. From the article:
Pederson and his colleagues say the findings are important because they suggest the mountain snows that produce the runoff that powers the Columbia, the Missouri and the Colorado river systems will continue to decline as global temperatures rise, even if precipitation increases.
More coverage from Margaret Munro writing for The Vancouver Sun. From the article:
The study, published Thursday in the journal Science, says the changes are affecting the Colorado, Columbia and Missouri Rivers, which together supply water to 70 million Americans. And they are also altering river flows in the Canadian prairies and central British Columbia, said co-author Brian Luckman, at the University of Western Ontario.
“Snowpack is essential for water supply to many of these areas,” Luckman said, noting that the Rockies feed rivers flowing through central B.C. and the Bow, Athabasca and Oldman rivers in Alberta. “Between 60 to 80 per cent of the water in those rivers is snowmelt from the mountains.”
Mountain snow records don’t go back far beyond 1950, so the team, led by Gregory Pederson at the U.S. Geological Survey, looked at tree rings at 66 sites from B.C. to Colorado to get a read on snowpack levels over the past 800 years.
They found that the snowpack shrank more during the late 20th century than during any other period since 1200 AD, with more severe declines at the northern end of the range studied.
Luckman says the tree rings track the depth of the snowpack because some species such as the alpine larch tend not to grow well in heavy snowpack years because it takes so long for water to start flowing on mountaintops. Trees like Ponderosa Pines at lower elevations thrive in years of heavy snowpack, resulting in thicker tree rings.
More coverage from Joey Bunch writing for The Denver Post. From the article:
The dire long-term forecast cites warmer springs, earlier snowmelts and shifting winter storm patterns, all possible byproducts of global warming caused by greenhouse-gas emissions, according to the USGS. While that could prove disastrous for Colorado’s ski and rafting industries, it’s too early to say if it means less water overall, said Marc Waage, the manager of water-resources planning for Denver Water, which supplies 1.3 million people in the metro region. “I don’t think from this study you can conclude that the overall amount of water is going to decline,” he said, “because we could be compensating snowpack with rain.”
More USGS coverage here.