Shrinking snowpack a threat for West

Human-caused global warming has been shrinking the snowpack across the mountain ranges of the West for five decades, suggesting the region's long battle for water will only worsen, according to a computer analysis released Thursday.

As temperatures have increased, more winter precipitation has fallen as rain instead of snow, and the snow is melting sooner, according to the report in the journal Science.

The result is that rivers are flowing faster in the spring, raising the risk of flooding, and slower in the summer, increasing the risk of drought.

"These trends will only intensify over the next few decades," said Richard Seager, a scientist at Lamont-Doherty Earth Observatory at Columbia University.

While the decline of the Western snowpack over the past few decades has been documented before, the new study is the most definitive in assigning the blame to human-induced climate change.

The changes will be felt differently in different parts of the West, scientists said.

In Colorado, lower temperatures probably will protect the snowpack and reservoirs are large enough to store several years of water supply, said Brad Udall, a Western water expert at the University of Colorado.

In California, reservoirs already operate on a delicate balance. They are kept well below capacity during the winter as protection against flooding. After the rainy season, they are filled with the spring snowmelt, storing water to be released during the dry summer months.

Heavier winter rains and earlier snowmelt probably will overwhelm reservoirs, forcing an early release of water, leaving too little for the summer.

"The handwriting is on the wall," said lead author Tim Barnett, a marine geophysicist at the Scripps Institution of Oceanography at the University of California, San Diego. "Mother Nature is going to stop being our water banker."

Between 1950 and 1999, the total amount of precipitation that fell in the Rockies, the Cascades, the Sierra Nevada and smaller mountain ranges across the West did not vary significantly.

But the portion arriving as snow steadily declined, falling by an average of 4.3 percent per decade in the nine areas included in the study.

Average daily minimum temperatures between January and March climbed an average of 0.34 degrees Celsius per decade. And three rivers — the Columbia, Sacramento and Colorado — ran higher earlier in the year. The date at which half their yearly flow had occurred was pushed up by an average of 0.8 days each decade.

To trace the cause of the trend, the researchers used computer climate models to simulate a world with greenhouse gases held at preindustrial levels.

They factored in fluctuations in solar radiation and changing concentrations of volcanic dust, which reflects the sun's heat back into space, over the second half of the 20th century.

Based on their simulations, along with historical data on snowpack, temperature and river flow, the researchers concluded there was a less than 1 percent chance that those 50 years were a natural aberration.

One computer model showed that rising concentrations of greenhouse gases accounted for 60 percent of the changes. A second analysis using another climate model calculated a contribution of 35 percent.

The study also included researchers from Lawrence Livermore National Laboratory, the University of Washington and the National Institute for Environmental Studies in Japan.

Material from The Washington Post and The Associated Press is included in this report.