Beneath this spring's ponderous snowpack in the Northern Rockies is hiding a long-term trend of a very different character.
For more than 30 years now, scientists warn, the annual snow accumulation on these mountains that supplies water to 70 million people has been shrinking more than at any other time in the past 800 years.
A new study by watershed specialist Gregory T. Pederson of the U.S. Geological Survey in Bozeman, Montana, and colleagues identifies a sharp departure in the 1980s from a centuries-long pattern of natural variability that emerges from a network of 66 tree-ring chronologies that reconstruct annual snowpack from north to south along the range from British Columbia to New Mexico.
“Over most of the 20th century, and especially since the 1980s, the northern Rockies have borne the brunt of the snowpack losses,” said Pederson, the lead author of the study published this week in the journal Science. “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."
Emphasizing the difference between short-term weather and long-term climate, the U.S. Geological survey described this winter's gains — bearing the wet-north, dry-south imprint of a La Niña episode — as "only a small blip on a century-long snowpack decline."
The profile of snowpack variations since the 13th Century most often showed a see-saw pattern as winter storm tracks changed in response to natural climate shifts such as El Niño-La Niña, but in the last three decades the "dipole" pattern has broken down, the scientists found.
"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 Julio Betancourt, a USGS scientist and co-author of the study. "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 interplay of natural variability and global warming may have set off a new era of stressed water supplies across the western states in the Colorado, Columbia and Missouri River drainages, the study reports.
Said Pederson: "What we have seen in the last few decades may signal a fundamental shift form precipitation to temperature as the dominant influence on western snowpack."
IMAGE: These mountain hemlock in Garibaldi Provincial Park, BC, are bearing the weight of a heavy snowpack. CREDIT: Jeremy S. Littell, UW Climate Impacts Group.