Snowpack in the northern Rocky Mountains has shrunk at an unusually rapid pace during the past 30 years, according to a new study.

The decline is "almost unprecedented" over the past 800 years, say researchers who used tree rings to reconstruct a centuries-long record of snowpack throughout the entire Rocky Mountain range.

Their work, published yesterday in the journal Science, suggests that the plummeting snowpack could have serious consequences for more than 70 million people who depend on water from the runoff-fed Columbia, Colorado and Missouri rivers.

"The Northern Rockies have shown the greatest response to warming in terms of snowpack decline," said lead author Greg Pederson, a research scientist at the U.S. Geological Survey's Northern Rocky Mountain Research Center. "Temperature, especially now, seems to be undercutting snowpack."

That's a marked shift from the pattern that predominated from A.D. 1200 to the 1980s: When snowpack was low in the Northern Rockies, it was high in the Southern Rockies -- and vice versa.

But for the past 30 years or so, snowpack in both regions has shrunk. Pederson and his colleagues pin the blame on warmer springs driven by a combination of rising greenhouse gas levels in the atmosphere and natural climate variation.

"In the last 30 years, there's been this growing synchrony where the whole West is getting warmer," said Philip Mote, director of the Oregon Climate Change Research Institute at Oregon State University. "That part is not new, but they point out a few other occasions in the last millennium where there has been low snowpack in the West were also periods that were unusually warm. That's a pretty strong message: that historically, low snowpack and warm spring go hand in hand."

This time, no return to cooler period

Tim Barnett, a climatologist at Scripps Institution of Oceanography, said the new results appear to agree with his earlier work that used climate models to show humans' greenhouse gas emissions have contributed to declining snowpack in the western United States.

"I think we get some idea of what natural variability is in the snowpack," said Barnett, though he noted his expertise lies in climate models, not tree-ring studies. "The fact that things have sort of gone south here in the last 30 to 40 years [in the new study] pretty much jibes with what we've done."

Meanwhile, Pederson said he sees an important difference between modern conditions and the brief 14th- and 15th-century periods of warm temperatures and low snowpack along the Columbia and Missouri river headwaters.

"They were eventually followed by cooling," he said. "Now, alas, we don't expect to return to a cooler period."

That glimpse into the past was provided by 66 tree-ring data sets scientists used to stitch together an annual record of snowpack far older than modern observations, which began in the early 20th century.

They used some records from ponderosa pine and Douglas fir trees, which thrive at lower elevations and in years with heavy snowfall. But to track snowfall in higher elevations, the scientists examined rings from subalpine larch, mountain hemlock and subalpine fir trees -- species whose growth is retarded by snowfall.

Peering into the past

In those cases, "you're often looking at a tree you could easily put your arms around -- and it could be 500 to 800 years old," Pederson said.

The combined tree-ring record is valuable because it overcomes a vexing problem with many modern climate observations: They don't go far enough into the past to show scientists what conditions were like before human activities began transforming the environment.

"We built a system to observe our changing environment during the period it's been changing rapidly," said Mote.

Pederson says he's confident his tree-ring results are accurate in part because during a brief period of overlap during the 20th century, the snowpack depth derived from the tree rings and modern observations look like "photocopies" of one another.

Although the new study describes ongoing decline in snowpack throughout the Rockies, this year has bucked that long-term trend. Record snowpacks have been recorded in the northern West, according to the Agriculture Department's Natural Resources Conservation Service, which monitors snowpack.

Experts characterized it as a brief blip in a longer-term trend of decline. They attributed last year's unusually wet winter to the La Niña weather pattern that was in place from August to May.

"We're seeing 200 to 400 percent of normal for this time of year," said Michael Strobel, director of the service's National Water and Climate Center.

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