US study finds mountain's snowpack may not yet be affected by climate change

Scientists report no clear evidence that human-induced climate change has caused a drop in 20th century snow levels

guardian.co.uk,
Wednesday August 06 2008 15:33 BST

Maybe the snow in the Washington state's 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 snowpack.
Past studies have frequently focused on steep declines in Cascade snowpack in the second half of the 20th century, with drops measuring 30% 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%. 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.
But, Mass doesn't say there's nothing to worry about. The north-west US 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 Centre. But "eventually global warming will have a profound effect".
The study has not yet been peer-reviewed.
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 this year.
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 couldn't be reached to review the latest study.