Study of 800-year-old tree rings backs global warming
The decline in recent decades of the mountain snows that feed the West's major rivers is virtually unprecedented for most of the past millennium, according to new research published today.
Seattle Times environment reporter
Surging Columbia River: Don't let this year fool you
This year, the Columbia River is swollen with massive runoff from mountain snowpacks, making for wild and sometimes perilous conditions for barge traffic. But new research shows that the long-term trend in the Columbia Basin and throughout the West is just the opposite: Snowpacks in most years are far below historic highs, thanks in part to climate change.
They looked at the rings of thousands of ancient trees in the mountains above the most important rivers in the West.
What they found may influence how water gets used from Arizona to Canada — and particularly in the Columbia River basin.
Despite odd years like this one, researchers have long reported declines in the mountain snows that power Western rivers. But on Thursday a group of scientists said they now also know this: Those declines are virtually unprecedented throughout most of the last millennium.
Scientists from the U.S. Geological Survey (USGS) and University of Washington measured tree-ring growth from forests that included 800-year-old trees. They learned that snowpack reductions in the late 20th and early 21st centuries were unlike any other period dating to at least the year 1200, according to new research published in the journal Science.
The precise amount of those declines, particularly in the Cascades, has been the subject of fierce debate even within UW climate circles. But no more than half of the declines can be explained by natural shifts, the study shows.
"I think the findings are pretty significant," said lead author Greg Pederson, with the USGS. "It means trees are telling the same stories as computer models and instrument records — that human greenhouse-gas emissions are contributing to the loss of snowpack."
Pederson said the research suggests the runoff that fuels the Columbia, Missouri and Colorado River systems will continue to decline as global temperatures rise — even if precipitation increases. Those rivers supply water to 70 million people, and 60 percent to 80 percent originates as snowpack.
The hardest hit will likely be the Columbia River system, where balmier mountain winters mean very small temperature increases can cause massive melting. That makes the snow that fuels the Northwest's signature river more susceptible to warming than the high, cold snows of the Colorado basin.
The findings also highlight a fundamental flaw in how the U.S. views Western rivers, researchers said. Assumptions about how to allocate water have largely been based on early 20th century hydrology and flow patterns that may not be sustainable.
"Nature allowed us to expect things that we probably shouldn't expect in the long term," said study co-author Jeremy Littell, a professor with the University of Washington's Climate Impacts Group. "We made a lot of investments and built up infrastructure based on expectations that don't appear tenable."
Given all the variables that influence the pace of climate change, it's difficult to project how and when snowpack reductions may translate into significant declines in river systems. Along the Colorado, for example, the Bureau of Reclamation and the seven states that rely on its water are in the middle of a multiyear study to answer that very question.
In fact, the nine authors of Thursday's paper were motivated in part by the need for research that could help water managers plan better.
Samples tell story
Every April since the Dust Bowl days in the 1930s, federal officials have taken snow-moisture samples in the mountains. Since some trees grow less and others grow more in high snow years, the researchers knew that by looking at tree-growth rings they could reconstruct past snowpack levels. So they sampled thousands of trees at 66 sites in key runoff areas of the West. Some were dead, and some were live. From the live trees they withdrew harmless pencil-thin core samples. They compared historic snowpack levels to temperature histories and charted the results.
"For us, it had this 'Oh my God' moment,' " said co-author Lisa Graumlich, dean of UW's College of the Environment. "We hadn't appreciated — until the day Greg [Pederson] made those maps — that the last few decades were truly unusual compared to the last 800 years."
Here's why: Throughout most of that time, much like this year, wet northern winters in the West typically came amid dry Southern ones, and vice versa. This year, for example, while snowpack in parts of the north are double or more above normal, the Southwest is scratch-dry and seeing record spring wildfires.
Throughout history, as seen through tree-ring data, even as weather fluctuated year to year — sometimes following decades-long El Niño and La Niña patterns — this climatic seesaw almost always held.
Only in the 1350s and 1400s, during two unusually warm periods, did snowpack decline throughout the West — until the late 20th century. And "the decline now is bigger, longer and more widespread," said Littell.
That suggested to the researchers that temperature, far more than precipitation, is now influencing snowpack levels. Temperatures, on average, have increased throughout the West. And mountain snowpack across the landscape since at least the early 1980s — perhaps since the 1950s — has been in a declining trend, too.
Even years like this one are not inconsistent with that trend, the scientists said, since weather can vary dramatically year to year.
For snowpack, "This year is basically just a blip on what otherwise has been a pretty consistent long-term downward trend," Pederson said.
Craig Welch: 206-464-2093 or email@example.com
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