SEATTLE -- The decline in recent decades of the mountain snows that feed the West's major rivers is virtually unprecedented for most of the past millennium, according to new research published Thursday.
By measuring tree-ring growth from forests with trees more than 800 years old, scientists from the U.S. Geological Survey and the University of Washington are showing that snowpack reductions in the late 20th century have been unlike any other period dating back to at least the year 1200.
Their work also shows that half or less of the recent declines can be explained by natural climatic shifts.
The study was published online Thursday in the journal Science.
"I think the findings are pretty significant," said author Greg Pederson, with the U.S. Geological Survey.
"It means trees are telling the same stories as computer models and instrument records -- that human greenhouse gas emissions are contributing to the loss of snowpack. It's kind of hard to argue that warmer temperatures don't melt snow and ice."
Pederson and his colleagues say the findings are important because they suggest the mountain snows that produce the runoff that powers the Columbia, the Missouri and the Colorado river systems will continue to decline as global temperatures rise, even if precipitation increases.
Those river basins serve as the primary water source for 70 million people -- and 60 to 80 percent of that water originates as snowpack.
They also say years like this one, where snowpack is way above normal across the northern Rockies, are not inconsistent with that trend. While weather varies dramatically year to year, the trend in mountain snowpack since at least the early 1980s -- and perhaps since the 1950s -- has been a steady decline.