The persistent and dramatic decline in the snowpack of the mountains of the West is caused primarily by human-induced global warming and not the result of natural variability of weather patterns in the region, researchers reported today.
Using data collected over the past 50 years, the scientists confirmed that the mountains are getting more rain and less snow, that the snowpack is breaking up faster, and that more rivers are running dry by summer.
The study, published online today by the journal Science, looked at possible causes of the changes -- including natural variability in temperatures and precipitation, volcanic activity around the globe and climate change driven by the release of greenhouse gases. Their computer models showed that the explanation that best fit the data was clearly climate change.
"We've known for decades that the hydrology of the West is changing, but for much of that time people said it was because of Mother Nature and that she would return to the old patterns in the future," said lead author Tim Barnett of the University of California at San Diego. "But we have found very clearly that global warming has done it, that it is the mechanism that explains the change, and that things will be getting worse."
The mountain snowpack is crucial to many in the West and Southwest who depend on its springtime melt for power, irrigation and drinking water. When the snow fields melt earlier and more suddenly, downstream dams are able to capture less of the water and must release more of it in springtime torrents that flow on to the ocean.
"Our results are not good news for those living in the western United States," the researchers wrote, adding that the changes may make "modifications to the water infrastructure of the western U.S. a virtual necessity."
The paper is part of what has become a steady drumbeat of dire predictions based on reports of quickening climate change. Last week, the American Geophysical Union, a leading professional group in the field, issued a statement that "Earth's climate is now clearly out of balance and is warming."
"Many components of the climate system -- including the temperatures of the atmosphere, land and ocean, the extent of sea ice and mountain glaciers, the sea level, the distribution of precipitation, and the length of seasons -- are now changing at rates and in patterns that are not natural and are best explained by the increased atmospheric abundances of greenhouse gases and aerosols generated by human activity during the 20th century," the organization said in its strongest statement to date on the subject.
While the decline of the western snowpack over the past few decades has been documented before, today's study is the most definitive in assigning the blame to human-induced climate change.
Barnett said his team used computer models to assess what natural climate variability, sunspots, volcanoes and climate change could do to the snowpack. The climate change model best matched the actual trends from 1950 to 1999.
The chance that the model is incorrect, he said, is somewhere between 1 in 100 and 1 in 1,000.
"We're very confident that warming from greenhouse gases is the cause," he said.
The result of the warming, he said, is that the flow of water downstream will become concentrated in a shorter time and will fill up reservoirs more quickly. As a result, more water will have to be released when dams reach their safety levels and will continue on to the sea rather than being available for drinking, irrigation and industry.
"Given the amount of carbon in the air and the trends for future releases, we have to expect that conditions will get progressively worse for some time, no matter what we do now," he said.
Researchers have also predicted that the Southwest will most likely get less rainfall as a result of changed atmospheric conditions caused by the buildup of greenhouse gases. Since that area gets much of its water from the Colorado River -- which is one of the rivers affected by the reduced snowpack -- the area could be in for a substantially drier future, he said.