Stephen Hamway
The Bulletin

The annual snowpack in the Oregon Cascades has been remarkably resilient in the face of climate change over the past several decades.

However, a new study co-­authored by an Oregon State University faculty member concludes this resilience is largely due to luck, and luck eventually runs out.

“I’m fairly confident that the assist that we’ve had from nature for the last 35 years is very unlikely to continue for the next 35 years,” said Nick Siler, OSU assistant professor and co-author of the study.

Siler said his research began with a relatively simple question: Given the warming air temperatures observed across the Western United States over the last several decades, why haven’t we seen related declines in snow accumulation across the region?

The report, which was published in “Geophysical Research Letters,” concludes that climate variability, which has driven a decadeslong natural air circulation pattern across western North America, has largely kept climate change from affecting the snowpack as much as it might have been expected to. If not for that variability, Oregon’s snowpack could have decreased by around 38 percent over the past 35 years, Siler said.

While the report makes no predictions about when the circulation pattern could change, Siler said it’s likely only a matter of time.

“No trend can persist forever,” Siler said. “There’s always going to be a reversion to the mean.”

As has been the case more often than not over the past five years, Central Oregon’s snowpack is behind its historic averages through the middle of January. Scott Oviatt, snow survey supervisor for the Natural Resources Conservation Service, said the snowpack in the upper Deschutes River Basin stood at 69 percent of normal as of Friday morning.

Oviatt said Central Oregon has fared better than watersheds in the Willamette Valley, though it’s been drier than those in the eastern third of the state, which are significantly closer to normal. Overall, Oviatt said, a dry start to the water year in October and November put Central Oregon and the rest of the state behind schedule, and storms in December and January have yet to make up the difference.

“Until we cool down and start seeing snow impacts and snow accumulation, we’re still going to come out behind,” Oviatt said of the winter snowpack.

While Central Oregon has seen its fair share of dry winters in recent years, Siler cautioned against concluding that the warm trend is due to a change to the prevailing weather pattern, pointing to a similar stretch in the early 1990s, where several dry years were followed by record-setting snow totals.

The report notes that there have been slight declines to snowpacks across the West, including a 14 percent decline across Oregon since the winter of 1983-84, when the study began. However, Siler said these decreases at the majority of the sites where snow totals are measured are not statistically significant.

Over the same period, however, the study notes an increase on average of 1 degree Celsius across all sites, a statistically significant uptick. In theory, the warming should have corresponded with more significant declines in snow totals across the region, particularly in Oregon, which has a comparatively warm snowpack, meaning that a slight temperature increase could cause precipitation to fall as rain rather than snow.

To show how climate change could have affected snow totals across the West, Siler and the study’s other authors attempted to uncouple the impact of climate change from random variability caused by natural weather patterns.

Siler said the same type of variability that contributes to El Niño and La Niña weather patterns also contributes to longer-term cycles. To adjust for that, Siler and his colleagues focused on sea-surface temperatures in the northern Pacific Ocean.

The adjustment demonstrated that, if not for climate variability, many locations across the Western U.S. would have seen significantly less snow over the past 35 years. The Oregon Cascades saw one of the starkest declines, with potential declines to the region’s snowpack estimated between 18 and 54 percent over that period.

While there’s no predicting what may happen next, Siler said the cycle may disappear or reverse itself in the future. In a future expected to continue warming, natural variability may not provide a refuge.

“We’re still going to have good years; we’re still going to have bad years,” Siler said. “But it’s likely that the dice are going to be weighted toward bad years.”

After the trend was uncoupled from natural variability, Siler said the largest effect in Oregon could be seen in snow accumulation in October and November. Without variability at play, Siler said Oregon’s snowpack could begin growing 4 to 6 weeks later than its historic averages, making winters like this one much more common.

“We’re starting December already 15 to 20 percent behind,” Siler said.

—Reporter: 541-617-7818, shamway@bendbulletin.com

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