Rocky Mountains Losing Their Snow

More than 70 million people across the North American West depend on water from the Columbia, Missouri or Colorado Rivers. And 60 to 80 percent of that water originates as snowpack. But that snowpack has been declining in recent decades, a worrisome trend as Western cities continue to grow and water demand rises.

Researchers led by the U.S. Geological Survey, reporting this week in Science, wanted to see if these recent trends are truly unprecedented. So they created snowpack histories for three regions—the upper Colorado, the Northern Rockies and the greater Yellowstone area—by using 66 tree-ring chronologies. Trees record in their patterns of growth (i.e., tree rings) the amount of water available to them during the growing season. In the West, that water is largely controlled by the amount of water in the snowpack, and by concentrating their tree-ring data on trees from areas where the precipitation comes mostly in the form of snow and on trees known to be most sensitive to the snowpack, the scientists were able to create a good record of snowpack levels in the area going back to around 1200 A.D.

The record has plenty of variability—snowpack levels are dependent on many different variables, such as sea surface temperatures, that aren’t consistent from year to year. But around 1900, two of the three regions underwent a major decline in snowpack, and then all three dropped precipitously starting around 1980. “Over the past millennium, late-20th century snowpack reductions are almost unprecedented in magnitude across the northern Rocky Mountains,” the scientists write. The culprit? “Unprecedented springtime warming due to positive reinforcement of the anthropogenic warming by decadal variability.” Translation: climate change.

Last year when I was reporting my story on the Colorado River, Patricia Mulroy, who manages Las Vegas’s water, told me that we need a new attitude about water, especially in the West. “It’s not abundant, it’s not reliable, it’s not going to always be there,” she said. How many times do we need to be told before it sinks in?

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Sarah Zielinski is an award-winning science writer and editor. She is a contributing writer in science for Smithsonian.com and blogs at Wild Things, which appears on Science News.