June 13, 2010
Snowpack declines in the Rocky Mountains over the last 30 years are unusual compared to the historical record, according to research by U.S. Geological Survey scientists.
The recent warming and snowpack declines are projected to worsen through the 21st century, foreshadowing a strain on water supplies. Runoff from winter snowpack accounts for 60-80 percent of the annual water supply for more than 70 million people living in the western U.S., according to USGS.
The latest research was based on snowpack reconstructions from 66 tree-ring chronologies, looking back 500 to more than 1,000 years. A network of sites from north to south in the Rocky Mountains was chosen to characterize the range of natural snowpack variability over the long term.
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 snow years, and vice versa. Since the 1980s, however, there were simultaneous declines along the entire length of the Rocky Mountains, and unusually severe snowpack declines in the north.
Northern Rocky Mountains snowpack
“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.
“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,” explained Pederson. “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.”
“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,” said USGS scientist Julio Betancourt.
“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,” added Betancourt.
The La Niña episode this year is an example of large snowfalls in the north while severe drought occurred in the south. However, this year’s gains in the northern Rockies are only a small blip on a century-long snowpack decline.
In the West, the average position of winter storms 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, storms track to the north.
Snowpack declines and GHGs
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,” said Pederson.
For more information, check out The Unusual Nature of Recent Snowpack Declines in the North American Cordillera.