The persistent and dramatic decline in the snowpack of many mountains in the West is caused primarily by human-induced global warming and is not the result of natural variability in weather patterns, researchers reported yesterday.
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 yesterday 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. The researchers' computer models showed that climate change is clearly the explanation that best fits the data.
"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 Scripps Institution of Oceanography at 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."
Many in the West and the Southwest depend on the snowpack's springtime melt for power, irrigation and drinking water. When the snow fields melt earlier and more suddenly, dams are able to capture less of the water and must release more of it to 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."
Although parts of the West have been hit by record snowfalls this winter, the data collected by the team showed that since 1950, the water content of the snowpack as of April 1 each year has decreased in eight of the nine mountain regions studied, by amounts ranging from 10 percent in the Colorado Rockies to 40 percent in the Oregon Cascades. Only the southern Sierra Nevada range did not show a drop.
The study is part of what has become a drumbeat of dire assessments based on reports of quickening climate change caused by the buildup of carbon dioxide from vehicles, power plants, industry and deforestation. Last week, the American Geophysical Union, a leading scientific group in the field, issued a warning 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.
Although the decline of the Western snowpack over the past few decades has been documented before, yesterday'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, sun spots, volcanoes and climate change could do to the snowpack. The climate-change model best matched the actual trends of the period from 1950 to 1999.
The chance that the model is incorrect, he said, is somewhere between 1 in 100 and 1 in 1,000.
"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. His team included researchers from the Lawrence Livermore National Laboratory, the University of Washington and the National Institute for Environmental Studies in Japan.
Researchers have also predicted that the Southwest is likely to get less winter rainfall as a result of the buildup of greenhouse gases. Because the region gets much of its water from the Colorado River -- one of the rivers affected by the reduced snowpack -- the already-dry area could be losing water from both of its main sources, Barnett said.
Jonathan Overpeck, a climate scientist at the University of Arizona in Tucson, said the new study "closes the circle" in terms of understanding what is happening to the climate of the West.
"Almost all of the models we've seen in recent years show the area becoming warmer and more arid due to climate change, but the question was always whether we could believe them," he said. "Now someone has done the statistical analysis to connect the dots so they can say with real confidence that this is happening because of greenhouse gases."
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