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A new study, published in the journal Science, using tree rings to reconstruct snowpack in river basins during the last several centuries shows that past variations are clearly attributable to natural factors affecting temperature and precipitation.

The work confirms work by others indicating that between 30 percent and 60 percent of the declines are likely from warming caused by greenhouse gas emissions from human activities with the rest arising from natural climate phenomena.

“Our results indicated that it’s due both to human-caused warming and natural long-term fluctuations. The Northern Rockies in particular have been hit by climate change and natural variability in temperature,” says Lisa Graumlich of University of Washington and co-author of the study.

Rresearchers considered snowpack in Canada and the U.S. that feeds three major river basins, the Columbia, Missouri, and Colorado, that among other things, provide water to more than 70 million people.

Snowpack losses across the West since the 1980s “may signal a fundamental shift from precipitation to temperature as the dominant influence on snowpack in the North America Cordillera, with significant consequences for regional water supplies,” according to the study.

Cordillera is applied to a continent’s principal mountain ranges.

If temperature is playing a larger role in snowpack, this could be more of an impact for the northern Rockies and the headwaters of the Columbia River compared to the other regions, says Jeremy Littell, research scientist and a co-author of the paper.

That’s because the area is low in elevation and temperatures already hover closer to freezing in winter and spring compared to the rest of the Rockies where it is colder, meaning there is an earlier transition to precipitation falling as rain instead of snow in the future.

The snowpack decline in the Rockies from 1950 to 2000 varies by region and elevation, from a 10 percent decline in the Central Rockies to 40 percent decline in the Oregon Cascades, Littell says.

Averaged over the Cascades Mountain range, the snowpack decline is about 30 percent.

In the future in Washington state, it’s estimated that April 1 snowpack may decline on the order of 40 percent by the 2040s relative to the period 1916-2006.

Littell developed the use of trees rings, which reveal a tree’s growth each year, as a way to document snowpack in the Pacific Northwest and Northern Rockies.

At the highest elevations, heavy snows in this region decrease the length of time trees can grow each year, while low snowpack years make for a longer growing season.

The study area includes three regions: the Northern Rockies, Greater Yellowstone and Upper Colorado with headwaters respectively of the Columbia, Missouri and Colorado rivers.

Comparing their tree-ring data with actual snowpack measurements—or snow water equivalent measurements—made April 1 by water resource managers since the late 1930s, gave the researchers the means to then calculate snowpack from just the tree-ring data going back centuries before humans started making measurements.

Natural variability in snowpack in the past has included decades-long shifts where winter storms were concentrated alternately over the northern Rockies for a period of time and then over the southern Rockies for a period of time.

This shifting appears to have broken down after the 1980s, partly because of human-caused warming.

More news from University of Washington: http://www.washington.edu/