A drying water source in the Western US

By Rachel Berkowitz

The Rocky Mountains have seen an unprecedented loss of snowpack in the past few decades. A study of tree rings suggests that the loss is being driven by increasing temperature rather than decreasing precipitation.

Records of the decline in snowpack exist for the late 20th century, but earlier documentation is rare. To look further back in time, Gregory Pederson of the US Geological Survey’s Northern Rocky Mountain Science Center turned to measurements of tree rings recorded at the World Data Center for Paleoclimatology’s International Tree-Ring Data Bank . Although tree rings have long been used to reconstruct climate histories, Pederson’s is the first systematic effort to produce multiscale snowpack reconstructions for the basins of three major rivers whose sources lie in the Rockies: the Columbia, Colorado, and Missouri.

Some mountain trees rely on winter snowpack for their main source of moisture: The bigger the snowpack, the bigger their rings. But for other trees, such as mountain hemlock (shown in the figure) and subalpine larch, a big snowpack hinders growth and yields small rings. Thus, rings from various species carry information about winter moisture and the onset of warm spring temperatures that melt snowpack. Together, the rings for a given year make for a good indicator of the size of that year’s snowpack, which is conventionally expressed as snow-water equivalent (SWE), the volume of water you’d get if you melted the snowpack.

Field SWE measurements carried out on 1 April of each year since 1950 provide a robust and direct estimate of snowpack. Using those relatively recent field measurements, Pederson calibrated the relationship between the extent of snowpack and the growth of trees as manifested by their rings. The relationship was then used to reconstruct past snowpacks from tree chronology data, which go back eight centuries.

“We looked at the thousand or so locations around the West and tried to understand statistically the connection between temperature and snow quantity on the ground on April first,” explains atmospheric scientist Philip Mote of Oregon State University in Corvallis, who worked on instrumental measurements of snowpack decline but was not involved in this study. He says that “for a specific basin, the [SWE] response could be substantially greater or less than the regional average.”

The snowpack reconstructions showed that when a low snowpack occurred in the northern Rockies, an above-average snowpack occurred in the southern Rockies, and vice versa. This see-saw pattern, or north–south dipole, is due largely to shifts in storm systems. The study appeared last year in Science.

But that pattern changed in the 1980s. Now, says Peterson, “snowpack decline is synchronous across the [Rocky Mountain region] cordillera.” Not only do the north and south snowpacks appear to have uncoupled, but the snowpack decline extends across the entire region. The magnitude of the recent decline has been matched only twice—in the 14th and 15th centuries, during periods of anomalous regional warmth. But the decline’s extent is unprecedented. According to Pederson, 30–60% of the recent decline can be attributed to human-induced warming, as it exceeds the extent of what could be caused naturally.

Pederson also compared his reconstructions with basin-wide averages of 20th-century observational records, including measurements of glacier size throughout the mountains. Records of storm systems that move north or south across the western US further verify that the north–south dipole had been maintained until recently. And when Meriwether Lewis and William Clark crossed the Rockies in June 1806, they noted persistent snow and low snowlines in the mountains that aren’t observed today: Camped at Horse Steak Meadow on Hungry Creek in Idaho, they wrote “Took to the hills which are very rough with a great many banks of snow.”

“The story gets painted again and again,” adds Pederson. “When you warm things up, the cryosphere melts. Trees can even see and tell the response.” But the synchronous decline was a surprise.

Follow-up studies aim to identify both the cause of snowpack change and its effects on forest fire activity, glacier dynamics, and water supply. Meanwhile, water managers are working hard to come up with future strategies for dealing with a longer dry season, declining supplies, and increasing demand for water needs.