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A water source in the western US running dry?

Trees growing at high elevations below melting snow fields.
[Photo: Greg Pederson, 2009, © Science/AAAS]

The layers of snow, or the snowpack, covering the northern Rocky Mountains has been growing and shrinking, depending on the climate, for centuries. And when that snowpack melts, the runoff feeds into the Colorado, Columbia and Missouri Riversóthe primary water sources for more than 70 million people.

Now, researchers have studied how that snowpack has been changing for the past 800 years, and they say that it's shrunk the most in the past 50 years alone. Since so many Americans depend on the snowpack for clean water, this discovery raises many questions about how global warming might affect peoples' water supply and water security in the future.

Researcher samples ancient Limber Pine (Pinus flexilis) at the southern end of the Wind Rivers, WY.
[Photo: Greg Pederson, 2006, © Science/AAAS]

Gregory Pederson and colleagues used information they gathered from tree rings to reconstruct the growth and the shrinkage of the northern Rocky Mountain snowpack. In light of their findings, they suggest that the snowpack has decreased in size more during the late 20th century than during any other period of time since 1200 AD.

The researchers also found that the snowpacks in the northern and southern Rockies no longer behave like a see-sawóbalancing the level of snowpack between the two regions so that one has heavy snowpack while the other has light snowpackólike they used to.

It seems that temperature, rather than precipitation, is now the strongest influence on snowpack in the Rocky Mountains, the researchers say. And if that is true, their findings could have serious implications for future water supplies across the warming, western United States. This research appears in the 09 June 2011 issue of Science Express.