A new study has found that a pattern of ocean temperatures and atmospheric circulation has offset most of the impact of global warming on mountain snowpack in the western U.S. since the 1980s.
The study from Oregon State University, the University of Washington and Lawrence Livermore National Laboratory was published Jan. 11 in Geophysical Research Letters.
"The western U.S. has received a big assist from natural variability over the past 35 years,” said lead author Nick Siler at Oregon State University, who began thinking about the project as a doctoral student in atmospheric sciences at the UW. "That’s been great for us so far, but it’s bad news for the future.”
Western snowpack brings white-capped mountaintops and happy skiers, but it also plays a crucial role in our water supplies by storing freshwater as snow that will melt in the drier summer months in places like Washington, Oregon and California.
The research was prompted by conversations that began at the UW.
"There were a lot of discussions within the department of the surprising stability of the western U.S. snowpack, because it went against the predictions,” said co-author Cristian Proistosescu , a postdoctoral researcher at the UW’s Joint Institute for the Study of the Atmosphere and Ocean.
The authors used snow measurements that began in 1983 at 329 automated SNOTEL stations across the central and western United States, mostly at high-elevation sites. During the subsequent 35-year observational period, only four sites experienced a statistically significant decline in April 1 snowpack, while the others showed no significant trend.
The authors compared the snow measurements with oceanic and atmospheric climate data to see which most affected the snow accumulation. They then used a climate model for the whole 35-year observation period to determine what was affecting the overall trend. Results show that without the contribution from natural variability in the nearby Pacific Ocean, the western U.S. would have experienced a much larger decline in winter snowpack, especially in the Cascades and Sierra Nevada, due to an average winter warming of about 1 degree Celsius over the western U.S.
In Washington’s portion of the Cascade Range, for example, the authors found that rising temperatures due to human emissions would, on their own, have caused average snowpack on April 1 to decline by about 23 percent since the early 1980s, with a range of 2 to 44 percent decrease. However, this was offset by an increase in snowpack resulting from natural variability. The two factors together explain why snowpack has not declined significantly over the past 35 winters.
The natural variability is an atmospheric circulation pattern associated with stronger winds that bring more moisture from the Pacific Ocean. The pattern is driven by natural, long-term variations in Pacific Ocean temperatures. The northeast Pacific has not warmed as much as the land in recent decades, and stronger westerly winds have thus delivered more moisture to the coastal mountain ranges.
"One of the more important broader questions in climate science right now is: What is the Pacific Ocean doing, and why is it doing it? The top-level story is that the northeast Pacific hasn’t really exhibited a lot of warming,” Proistosescu said.
By contrast, during the 2015 year of " the blob ,” when the northeast Pacific had unusually warm surface temperatures, the West saw markedly less snow. This, the authors say, may be what the future looks like.
Siler said he expects a different scenario to play out over the next few decades, as the current phase of natural variability subsides, likely giving way to a circulation pattern that is less favorable for snowpack accumulation. The northeastern Pacific is likely to warm eventually, portending an accelerated decline in winter mountain snowpack over the next few decades.
"Natural variability has masked the impact of global warming on snowpack for as long as I’ve been alive," said Siler, who was born in 1983. "But in the next few decades, I think we’re more likely to see natural variability amplify, rather than offset, the loss of snowpack due to global warming.”
The other co-author is Stephen Po-Chedley , a former UW graduate student who is now a research fellow at Lawrence Livermore National Laboratory in Berkeley.