New research suggests that the Pacific Northwest's watershed managers could face more of a climate challenge than they bargained for.
A paper published last week in the journal Science hypothesizes that climate change is behind a steady, 60-year slowdown of the westerly winds blowing off the North Pacific. The winds play a key role in carrying precipitation into the Cascade and northern Rocky mountain ranges.
This finding may alter earlier projections on how a warming planet will affect the region's water supply, although some climate scientists not involved in the study remain skeptical of the paper's claims.
Other climate researchers have warned that global warming will cause the Pacific Northwest's mountain snowpack to melt earlier, reducing the supply of late-spring runoff relied on by many of the region's farmers, cities and hydroelectric plants (ClimateWire, Nov. 5).
But global climate models cited by the United Nations' Intergovernmental Panel on Climate Change generally predict a slight increase in winter precipitation for the Pacific Northwest, while precipitation extremes -- dry spells and downpours -- are expected to intensify.
The new research, led by U.S. Forest Service research hydrologist Charlie Luce, suggests that the region is already facing an overall decline in mountain precipitation.
"What this says is that some of what we have been seeing in terms of earlier snowmelt is simply that there's less snow to melt," he said.
"This really applies just to mountains," Luce added, "but mountains are, at least in the Northwest, where we get most of our water supply."
Case of the missing mountain water
The research was motivated by what the paper calls "the missing mountain water." In a 2009 study, Luce noted that annual stream flows in the Pacific Northwest's mountains have declined since the 1950s.
But during that same period, measurement gauges throughout the region showed no significant decrease in precipitation. When Luce and his co-authors looked at where these precipitation gauges were located, most of them were in low elevations, leading them to believe that the apparent decline was only happening at high altitudes.
After running an analysis concluding that the stream-flow decline couldn't be chalked up to increased evapotranspiration, the researchers then had a second look at the historical trends involving the east-to-west winds, called westerlies, that push water vapor into the mountains.
They found westerlies have slowed at a steady rate for the past 60 years. According to Luce, their average speed during the 1950s was about 9.5 meters per second, but today, the average speed is about 8.5 meters per second.
"If the winds are slow, two things happen," he explained. "One, you're not bringing in as much moisture to precipitate, but also the water droplets are smaller and just form as clouds, and then pass over the [mountain] range and re-evaporate on the lee side."
Some of this change can be blamed on natural climate variability patterns like El Niño and the Pacific decadal oscillation. But the observed slowdown since the 1950s was larger than what these patterns could account for, the researchers said, so they suspect that climate change is also at play.
During the Pacific Northwest winter, the ocean is usually warmer than the land, causing a pressure gradient that, in turn, drives westerlies up mountain slopes. But because climate change is warming the land faster than the ocean, Luce explained, the pressure gradient is reduced, taking the "oomph" out of the region's winter winds.
Other climatologists skeptical
Global climate models used by the IPCC can only analyze large chunks of the landscape and are still struggling to simulate the topographic complexity of mountain ranges, Luce explained. That makes it impossible for them to pick up on this small but important phenomenon.
"Maybe the models, in the broad scheme, are projecting increases," said John Abatzoglou, one of the paper's co-authors and an assistant professor at the University of Idaho's Department of Geography. "The effect of declining wind speeds might result in those projections sort of sliding a bit towards either less of an increase or more of a decrease."
Richard Seager, a professor at Columbia University's Lamont-Doherty Earth Observatory, questioned the paper's conclusions, saying he wasn't convinced that increased evaporation or evapotranspiration were not responsible for the decreases in mountain stream flow.
While Seager allowed that global climate models have difficulty accounting for mountain ranges, "it's hard to believe that they would get the wrong sign of the precipitation change," he said.
In his own research, Seager has found that climate change will also cause southerly winds to increase in strength along the U.S. West Coast, bringing up warmer, moister air and therefore causing an overall increase in precipitation.
"This decline in the strength of the westerlies is just one factor that can be considered," Seager said. "Clearly in the models, it's these other things that are overwhelming the decreasing speed of the westerlies and winning out."
Potential implications for reservoir plans
Both Abatzoglou and Luce emphasized that more research is needed to confirm the overall impact of weaker westerlies on the Pacific Northwest's water supply.
But if this phenomenon turns out to be significant, it could make a difference in how watershed managers prepare for climate change. A 60-year decrease in high-elevation precipitation could account for the recent increase in mountain forest fires, Luce said.
Also, in many Pacific Northwest communities, Luce said, "we've got a combination of snowpack and dams as our storage for the irrigation, and as the snowpack starts melting sooner, conceptually, then we need to replace that snow storage with storage behind dams, and there's quite a few initiatives going on in the Pacific Northwest to do just that."
But if future water shortages are not, in fact, due to earlier snowmelt, but rather because there is simply less snow melting in the first place, "there may not be the supply to fill new dams," Luce said.