The snowpack across the northern Rocky Mountains has shrunk far more quickly in the past 50 years than in the previous 800, a new study shows.

Runoff from those layers of snow feed rivers that supply water to more than 70 million people, raising concerns that the declining snowpack will lead to water shortages in western North America, reported the study published online Thursday in Science Express.

"Snowpack depends on precipitation; it also depends on temperature," said Brian Luckman, a geography professor at the University of Western Ontario in London, Ont., who co-authored the study.

"What we think is going on is spring temperatures are warming up. Spring is coming sooner, so those big April-May dumps of snow which top up the snowpack at the end of the season are now rain."

That rain, in turn, runs off too quickly to feed waterways like Alberta's Bow River through the summer months.

Because climate records only go back about 50 years, the study led by Gregory Pederson at the U.S. Geological Survey's Northern Rocky Mountain Science Center reconstructed the climate over the past eight centuries from tree rings. The rings are wider or narrower depending on how well trees grew in a particular year, which in turn depends on the snowpack, Luckman said.

If there's a heavy snowpack the previous winter, larch and hemlock, which are found at high elevations, don't grow as much in the summer. But Douglas fir, which is found at lower elevations, grows better because more water is available.

Broken pattern


Trees grow below melting snowfields in the Rocky Mountains. The growth rings of the trees were used to reconstruct the snowpack over the past 800 years. (Greg Pederson/Science/AAAS)

In addition to the recent decline in snowpack, the study showed a break in a centuries-long pattern in which heavy snow in the northern Rockies was balanced by less snow in the southern Rockies in some years, and the opposite was true in other years. For the past 30 years, snowpack declines have happened simultaneously in both the north and south.

The findings suggest that the temperature has recently overtaken the amount of precipitation as the biggest influence on the snowpack.

Luckman said climate models predict that there will be a temporary increase in snow in the Rockies over the next decade or two, followed by a long-term decline.

"You're going to have less flow in the summer and obviously here in Alberta that summer flow is what drives irrigation," he said. "So the real message here is you're not likely to get more water for any long extended period of time. You're likely to get less and therefore it's important to put in place regulation about the use of water — who gets it and when."

Robert Sandford, chair of a group that connects policy makers with scientific research on water, said the study shows the declining snowpack will add to the gradual decline in stream flows that are already happening in some of Canada’s most important watercourses.

"What’s really critical about it is that it demonstrates that … those changes in temperature caused by people, in tandem with natural variability, could have a dramatic impact on how much water is available to all of us in western North America," he said. "Presently we're not taking water management as seriously as the science indicates that we ought."

He said provinces and states that have already allocated all their existing water should be developing new conservation efforts and strategies to ensure they can do with less water in the future.

Security issue

He added that the U.S. has declared the water supply a national security issue.

"Being a neighbour, that implied to us that these sorts of concern should be of great concern to us too."

Shawn Marshall, a glaciologist and climatologist at the University of Calgary, called the study "a strong statement" about how unusual the climate in the past few decades has been in the context of the last thousand. He said the study was convincing because of the number of trees and tree species it looked at over a large area.

But he said the study looked mainly at the U.S., at lower elevations than Canada’s snowpack.

"There is some hope that the snowpack here is holding its own at the highest elevations."

With files from Bryan Labby and Leslie Kramer