The Rocky Mountains have lost an unusual amount of springtime snowpack over the past 30 years or so. During that interval, the average springtime snowpack—the accumulated snow that piled up during the previous winter—is the lowest it has been since the early 20th century and has been comparably low for only three extended intervals during the past 800 years, according to a new study of tree-ring records. The recent decline in snowpack, experts say, presage a panoply of ill effects, from an increased number of wildfires to a reduced water supply from major rivers.
"If results hold up, they give us a better historical perspective on snowpack than we've had before," says Tim Barnett, a marine physicist at the Scripps Institution of Oceanography in San Diego, California.
Scientists have only limited amounts of snowpack data in the West because historical records are available only for the past century or so. To see further into the past, researchers have to analyze proxies such as tree rings. The wider the ring, the more the tree grew during that particular year. Effects vary by tree species: Some species grow more quickly when snowmelt is more abundant, but others, particularly those that grow at high altitude, have their growing seasons shortened by a thicker-than-normal, late-melting snowpack.
Now, Gregory Pederson, a paleoclimatologist with the U.S. Geological Survey (USGS) in Bozeman, Montana, and his colleagues have amassed 66 tree-ring records covering key portions of the Columbia, Colorado, and Missouri headwaters—the first such year-by-year insight into the entire region's snowpack for the past 500 to 1000 years, Pederson says. Recent portions of the tree-ring records accurately depict interannual and decade-to-decade trends in historical measurements of snowpack made each 1 April, the researchers note online today in Science.
In general, the researchers found, when snowpack was low in the northern Rockies, it was higher than average in the southern Rockies—a seesaw pattern due largely to shifts in the paths of storm systems caused by climate cycles such as the El Niño/Southern Oscillation and the Pacific Decadal Oscillation.
But since the 1980s, that seesaw has disappeared, and average springtime snowpack in the region has declined sharply, a diminution driven, the researchers contend, by a combination of short-term changes in climate cycles and the long-term warming due to rising atmospheric concentrations of greenhouse gases such as carbon dioxide. "Sometime in the 1980s, temperature [in the region] passed a threshold where snowpack declines became obvious and more widespread," says Gregory McCabe, a climatologist with USGS in Denver, who wasn't involved with the study.
"The recent changes in snowpack are unusual both in duration and in geographic extent, and they are outside the envelope of what can be considered natural changes," adds Philip Mote, an atmospheric scientist at Oregon State University, Corvallis. In the headwaters of the Columbia and Colorado rivers, only two periods in the past 800 years—from 1300 to 1330 and from 1511 to 1530—have seen springtime snowpack as low for an extended period as it was during the early and the late 20th century (from 1900 to 1942 and from 1980s to the present), Pederson and his colleagues found.
The impact goes far beyond disappearing ski slopes and scenic vistas. Between 60% and 80% of the water slaking the thirst of 70 million people and nourishing three of North America's largest rivers—Columbia, Colorado, and Missouri—comes from snowmelt.
If end-of-winter snowpack stays lower than average for the foreseeable future, water availability during the high-demand seasons of spring and summer will be reduced. Along some stretches of the Colorado River, water usage is already approaching the long-term average of the amount of available water, McCabe says. "They've pretty much reached the point of using all the water that's there."
Much of the West was settled during a period of higher-than-average water availability, McCabe notes—an unfortunate truth that many residents don't realize. "My question is, what will happen when climate shifts back to normal, or even to particularly dry conditions, at a time when both temperatures and water usage are much higher than they are today?" he adds. "The most striking part of this paper is how unusual the west-wide warming and spring snowpack declines are," says Jonathan Overpeck, a geoscientist at the University of Arizona, Tucson. "Unfortunately for the West, the recent highly unusual drop in spring snowpack is due mainly to a climatic warming that we're confident will continue absent efforts to curb greenhouse gas emissions," he says. "This will mean more wildfire and more reductions in water supply from many of our Western rivers. It's not a pretty picture."