UW study examines decline of snowpack

Maybe the snow in the Cascade Mountains isn't in such immediate peril from global warming after all.

Despite previous studies suggesting a warmer climate is already taking a bite out of Washington's snowpack, there's no clear evidence that human-induced climate change has caused a drop in 20th century snow levels, according to a new study by University of Washington scientists.

In fact, the newest study also predicts the Cascade snows — vital to water supplies, crop irrigation and salmon — could enjoy a delay in the effects of global warming.

But the findings have already become part of a scientific debate with an unusually political tone. It's an ongoing disagreement that has UW researchers taking sides against each other and has attracted the attention of political groups.

And a leading scientist on the other side of the debate said the latest analysis speculates about the future and offers little new about the past.

"They're trying to forecast the next 20 years or so, and I don't think they can do it," said Alan Hamlet, a UW hydrologist who has written papers about historic Cascade Mountain snowpacks.

Past studies have frequently focused on steep declines in Cascade snowpack in the second half of the 20th century, with drops measuring 30 percent or more.

But Cliff Mass, a well-known UW meteorologist, said the new study, which he co-authored, shows it all depends on which years are examined. He and his co-authors argue snow levels were unusually high in the 1950s, creating a distorted picture of historic patterns.

Measurement of mountain snow levels were spotty before the 1950s, making it harder to get a complete picture. But Mass and his colleagues tried to estimate snowpack for earlier years based on measurement that did exist: the amount of water that flowed down streams as snow melted.

Using that method, they found a smaller drop in snowpack between the 1930s and today — 23 percent. That still may sound like a big drop, but the scientists argue that it could be statistically insignificant, so it's hard to say whether it's meaningful. They also say that many of the changes appear to be attributable to shifting weather patterns driven by the Pacific Ocean.

"We can't see the global-warming signature in terms of a decline in snowpack," said Mark Stoelinga, the study's lead author, and a professor in the UW's Atmospheric Sciences Department.

Mass and his colleagues also predict the oceans could help buffer Washington's snows from immediate impacts of climate change. A number of computer models show the northeast Pacific warming more slowly than most of the world's oceans, Mass said.

That could help keep temperatures in higher altitudes, which would mean the difference between rain and snow in the Cascades, from rising quickly over the next few decades, Mass said.

But, Mass doesn't say there's nothing to worry about. The Northwest is still on course for a big drop in snowpack — and the accompanying water-supply problems — by the end of the 21st century.

"We're in a place that is not going to warm up as quickly," Mass said at a recent conference by free-market think tank, the Washington Policy Center. But "eventually global warming will have a profound effect."

The study has not yet been peer-reviewed.

Ongoing dispute

Hamlet counters that the bigger historical picture — gradually declining snowpack over the 20th century — has already been put forward, most recently in a study published in 2008. In fact, he wrote it, along with State Climatologist Philip Mote, another UW scientist who has been a primary player in the ongoing dispute.

Mote was on vacation this week and couldn't be reached to review the latest study.

But Hamlet disagrees with Mass that the snowpack drop could be explained mostly by fluctuating ocean conditions. The Cascade snowpack trends in the second half of the century are consistent with rising temperatures in the western United States, which have been tied to global warming, he said.

Hamlet also criticizes some of the statistical analysis in the new study, saying it could exaggerate the role of decade-to-decade changes in ocean conditions while understating other potential influences, including global warming.

"I just don't think the science is there," Hamlet said.

Ocean conditions are hard to predict, Hamlet argues, making it impossible to predict snowpack levels over the next few decades. But in the long term it's safe to bet that rising temperatures are going to mean less snowpack.

Common ground

The dispute traces back to 2007, when UW meteorologist Mark Albright, an associate of Mass and co-author of the new study, challenged claims that Northwest snowpacks had fallen by half in the second half of the 20th century.

In the ensuing debate, Mote stripped Albright of his title as associate state climatologist. Mass then accused Mote of censorship.

Despite the acrimony, several prominent colleagues said the two side's findings really have a lot in common.

There's broad agreement that snowpack has fallen sharply since the 1950s, and the apparent rate of decline is lower when you go back to the early 1900s. Snowpack has held steady or even increased slightly since the 1970s.

But Mass insisted there are important differences. He pointed to Hamlet's claims that historic snowpack declines appear consistent with global warming.

"There is no evidence that it is influencing snowpack here in any significant way," Mass wrote in an e-mail.

Warren Cornwall: 206-464-2311 or wcornwall@seattletimes.com