A new study has found that changes in the atmospheric circulation since the 1980s have offset most of the impact of global warming on winter snowpack in the mountains of the western United States.
Researchers say the observed circulation changes are driven by natural fluctuations in sea-surface temperatures in the Pacific Ocean, and are unlikely to last much longer, portending an accelerated decline in winter mountain snowpack over the next few decades.
The study appears in the journal Geophysical Research Letters.
“The western U.S. has received a big assist from natural variability over the past 35 years,” said Nick Siler, an Oregon State University atmospheric scientist and lead author on the study. “That’s been great for us so far, but it’s bad news for the future.”
Changes in snowpack reflect both natural variability and human forcing, primarily from carbon dioxide emissions. To isolate the forced component of snowpack trends, Siler and his colleagues identified the natural circulation patterns that control year-to-year fluctuations in winter snowpack, and then removed their contribution to snowpack trends since the early 1980s, when monthly snowpack observations began.
The authors found that without the contribution from natural variability, the western U.S. would have experienced a much larger decline in winter snowpack, especially in the Cascades and Sierra Nevada.
In the Oregon Cascades, for example, the authors found that rising temperatures due to human emissions on their own would have caused average snowpack on April 1 to decline by 18 to 54 percent since the 1980s. However, this forced response has been mostly offset by a positive contribution to snowpack trends from natural variability. Historical trends represent the sum of these competing contributions, explaining why snowpack has not significantly declined over the past 35 winters.
The natural contribution reflects a shift in the atmospheric circulation associated with strong winds that bring more moisture into the region from the Pacific Ocean. 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.
“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.”
Siler is in OSU’s College of Earth, Ocean, and Atmospheric Sciences. Co-authors on the study include Cristian Proistosescu, University of Washington, and Stephen Po-Chedley, Lawrence Livermore National Laboratory.