The reduction in snowpack in the Rocky Mountains over the last 30 years is unusual compared to the past few centuries, according to a new study that includes University of Arizona researchers.

Previous studies 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 U.S. 

U.S. Geological Survey scientists, with partners at the UA, Washington, Wyoming and Western Ontario, led the study.

"We were interested in seeing how snowpack varied in three watersheds," said co-author Connie A. Woodhouse, UA associate professor of geography and regional development. "One in the northern Rockies that is the headwaters of the Columbia, one in Yellowstone that is the headwaters of the Missouri and one for Colorado that includes the headwaters of the Colorado." 

Secretary of the Interior Ken Salazar said, "This scientific work is critical to understanding how climate change is affecting Western water supplies. 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."

The researchers 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.