Global warming hitting Northwest hard, researchers warn

To find the most compelling evidence to date that global warming could shrink damp Cascade snows by half in coming decades, Seattle scientists first took a step back in time.

They picked through a half-century of snow data from Arizona to British Columbia to better grasp how an atmospheric stew of greenhouse gases may shape our region for years to come.

Their conclusion: Their earlier warnings about future water shortages in the Northwest were more accurate — perhaps even understated.

"If you think the water fights we have now are intense ... you ain't seen nothing yet," University of Washington professor Ed Miles said yesterday during the annual meeting of the American Association for the Advancement of Science.

Miles said the moisture in snow that nourishes the West and gives life to its network of rivers has been steadily declining since at least World War II.

And the hardest-hit region has been the Cascades, where battles to provide enough water for fish, agriculture and power have been worsening for years.

Across the West, winter snows pile up in the mountains, forming the foundation for water supplies that eventually spill as spring runoff into rivers. Since the Dust Bowl of the early 20th century, federal agriculture officials each April have taken snow-moisture samples in the mountains to gauge when and how they should release water for fish, hydropower and farms through dams and storage reservoirs.

Yesterday, Miles, who leads UW's Climate Impacts Group — one of the country's leading climate-change research bodies — told a panel of experts that included the British government's top science adviser that his team reviewed temperature records, precipitation records and data from 50 years of snow-water surveys at 800 stations across the West.

The result: A minute rise in global temperatures just since 1950 already has reduced mountain snows across 75 percent of the West.

In the interior Rocky Mountain states, where mountains are taller and winter temperatures are lower, moisture content changed mildly and usually only at the snow line.

But in coastal regions, such as the Cascades and parts of northern California, where winter temperatures are balmier, warming during the same period actually reduced moisture in spring snows by more than 30 percent.

"The results were striking; I was shocked by the magnitude of the (snow-moisture) declines," said UW climate scientist Phil Mote, who conducted much of the survey work and completed his analysis within the last month.

"In some places, particularly in Oregon, we saw declines of 100 percent. It had gotten so warm there was no snow left in April at all."

His review of records showed that at several locations in Oregon, snow surveyors long since had abandoned their snowpack stations — presumably, Mote said, because they regularly were bone dry by April.

The findings bolster climate modeling the UW scientists updated last year that now suggests the water content of Cascade snows would drop by 59 percent in the coming half-century under even the most conservative warming scenarios prepared by international experts.

"When we look at the next 50 years, the trend remains consistent," said Ruby Leung, a climate scientist at the federal Pacific Northwest National Labs in Richland, who has worked with Miles' group in recent years.

Or, as Miles put it: "Whether we do it by observation using real data or with models, we reach the same conclusion."

In the past 100 years, the consensus among scientists is that the Earth has warmed 1.1 degree Fahrenheit, with accelerated warming during the past two decades, according to the National Academy of Sciences, which advises the federal government on scientific matters.

In the Northwest, that warming is closer to 1.5 degrees. A few prominent scientists still take issue with those findings, arguing that pre-1970 temperature-collection was less sophisticated and subject to human error.

The buildup of carbon dioxide and other gases in the atmosphere causes heat to reflect down on the earth's surface.

Miles said his team's models assume that the release of carbon dioxide into the atmosphere would continue to increase globally at the rate of 1 percent a year — what the Intergovernmental Panel on Climate Change considers its "business as usual" model. Increasing greenhouse-gas emissions at that rate translates to temperature increases of between 2 and 10 degrees Fahrenheit.

But "even if we make rapid advances in reducing emissions right now, it's not going to reduce CO2 in the next 20 or 30 years," said Leung.

The consequences for the West appeared, at first, counterintuitive. Warming produced increased rains in the desert areas of Arizona, New Mexico, southern Colorado and southern Utah, which compensated for the loss of snowpack. The interior West's extensive system of water-storage reservoirs allowed for the capture of that water to be flushed through river systems.

But in the Northwest, where battles to provide enough water for fish, agriculture and power systems already highlight a shortfall in the ability to store excess water, the consequences could prove more dire.

The likelihood of precipitation coming as rainfall rather than snow is higher, and with storage reservoirs full, that would mean more early-winter flooding. Meanwhile, less snow accumulating in the mountains would mean spring runoff will likely come a month to six weeks earlier.

In other words, "the parts of the West best situated to handle water storage are the least vulnerable to temperature changes," Mote said. "Those with the least capacity to store water — the Cascades and Northern California — are most vulnerable. So it's sort of a double-whammy."

Craig Welch: 206-464-2093 or cwelch@seattletimes.com