What is the state of Northwest snowpack?

In fact, the statement said, heavier precipitation has offset the effect of rising temperatures by piling more snow on the ground.

The issue is important because mountain snow is a crucial storehouse of water across the Pacific Northwest, and its decline is often cited as telltale evidence that global warming is affecting Oregon.

For instance, a 2004 report to Gov. Ted Kulongoski warned that the spring snowpack in the Cascades fell about 50 percent since 1950. But scientists now say that figure is too high and the number in the report was a mistake: It should have said 35 percent.

Others argue wide swings in precipitation and temperature from year to year, and heavier snow in the late 1990s, make it hard to see any obvious decline in the snowpack.

The debate, although centered at UW, has swirled among scientists in Washington and Oregon through e-mails in which some accused others of ignoring the facts. It climaxed in a meeting this week at UW where the central players faced off.

Scientists involved are not questioning whether global warming is real -- they agree it is and carries serious consequences for the region. But the intense back-and-forth on snowpack offers a glimpse into the difficulty of teasing real proof of global warming from natural ups and downs in climate.

An international science panel concluded this month that warming is apparent on a global scale and almost certainly caused by the activities of humans. But it's tougher to state as clearly in the Northwest, where local shifts could mask global trends.

"There's evidence the snowpack is going down, but how much of that is natural variability and how much is global warming?" said Dennis Hartmann, head of the UW atmospheric sciences department . "It's probably some of both."

Scientists have increasingly found themselves in a sensitive role as more politicians seize on their usually carefully worded findings to argue for policies combating global warming.

"I'm just worried about the hyperbole and exaggeration related to global warming," said Clifford Mass, a UW professor of atmospheric sciences who leads a team developing climate prediction models. "We have to tell the politicians when they've gone too far."

The current debate arose after Seattle Mayor Greg Nickels, who is pushing nationally for cities to combat global warming, cited the same 50 percent decline of snowpack mentioned in the 2004 report to Kulongoski.

The report to Kulongoski was compiled by a task force of scientists and business leaders he appointed to develop a state strategy on climate change. It is being corrected to cite a 35 percent decline from 1950 to 2000, said Mark Abbott, dean of the College of Oceanic and Atmospheric Sciences at Oregon State University.

Much of the research on Northwest snowpack has come from Philip Mote, Washington state climatologist and a member of the Climate Impacts Group, a team of UW scientists. Mote is also a lead author of an international report on climate change released this month.

Little decline seen

But other UW scientists re-examining the figure the Seattle mayor used concluded there was little decline, in part because big snow years in the late 1990s pushed the snowpack trend up.

"The bubble surrounding what I have come to call the myth of the vanishing snowpack caused by global warming may be close to bursting," UW research meteorologist Mark Albright said by e-mail. He performed the new analysis.

The differences emerge in part because scientists are looking at data differently. Albright compared the average snowpack of the last 10 years to the average since 1943 and found only small declines when averaged across Oregon and Washington.

He also found an increase in snowpack since a drier period in the late 1970s. That's puzzling, because it's the same period when greenhouse gases are believed to be pushing global temperatures up.

"We're hearing all this talk about how it's getting worse," said Kelly Redmond, regional climatologist based at the Western Regional Climate Center in Reno, Nev. "If that's true, we should be seeing evidence of it accelerating."