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room at a small ski resort outside Bend, Fox and a collection of about 50 water experts from the Northwest settle in to listen to Mote describe some of his group's latest data on western snowpacks. Perhaps fitting- ly, outside the temperature has warmed up on this mid-January day to about 50C. Ici- cles encircling the roof drip steadily. Mote describes work published last year in Geophysical Research Letters, in which he took a detailed look at the trend in snow- pack accumulations throughout the Pacific Northwest over the last half of the 20th cen- tury. Mote reviewed federal records of snow water equivalents (SWE)--the amount of water in a given depth of snow---on 1 April, typically the peak of the season's snowpack. Of the 230 sites where SWEs were meas- ured back to the 1950s, Mote found that nearly all showed negative trends, even as precipitition increased in most places. The hardest hit: areas in the Cascade Mountains in Oregon and Washington, which saw as much as 60% declines in total snow accu- mulation. The most likely explanation, Mote says, was the region's temperature rise. When he plotted the snowpack declines z against the elevation of the snow-tracking ~ sites, he foUnd that the biggest decreases oc- z 'C curred at the lowest elevations, suggesting ~ (; that the moderate warming throughout the ~ region was raising the freezing level. ~ That's just the beginning. In work pre- ~ sented last month at the American Meteoro- ~ logical Society meeting in Seattle, Washing- ~ ton, Mote teamed up with UW Seattle col- ~ league Alan Hamlet and University of Col- ~ orado, Boulder, hydro climatologist Martyn ~ Clark to expand his initial analysis to look at ~ historical snowpack levels throughout the ~ West (see righthand figure). The news was 15 better, but not much. Snowpacks decreased ~ at 85% of the nearly 600 snow-measurement 5i !; sites throughout the West. The biggest de- 6 creases hit the Northwest, where the moun- ;;; tains are smaller and the temperatures ~ warmer, thanks to their proximity to the Pa- ~ cific Ocean. Declines in the northern Rock- z ~ ies were mostly in the range of 15% to 30%. ~ In these inland areas, Clark points out, win- ~ ter temperatures are typically far lower than ~ in the Pacific Northwest, so a rise of a few ~ degrees still does not push the mercury I above freezing. "In the interior regions all 0: the winter precipitation falls as snow," Clark ~ says. And some regions in the Southwest ~ even witnessed large SWE increases, thanks - primarily to a rise in precipitation. 1!i p Other clues also suggest that the West's ~ snowpack is changing. The biggest: Snow is fl melting earlier in the spring. "There has ~ been a fairly broad tendency in snowmelt i basins to exhibit advances in runoff timing," u says Daniel Cayan, a climate researcher at the Scripps Institution of Oceanography in La Jolla, California. Last month, Cayan, postdoctoral assistant Iris Stewart, and Michael Dettinger, a hydroclimatologist with the US. Geological Survey (USGS) in San Diego, reported in Climatic Change that the peak of the annual spring runoff in streams throughout California's Sierra Nevada now comes as much as 3 weeks ear- lier than it did in 1948 (see lower figure). Again, the effect was most pronounced in streams adjacent to lower elevation snow that is more sensitive to temperature increas- es. ''This is very consistent with the evidence Phil [Mote] and company have seen with the snowpack," Cayan says. In a paper now under review at the Journal of Climate, Clark and colleagues at the Univer- sity of Colorado recently found much the same shift for streams in the Northwest. ''There is defmitely something happening," Clark says. That evidence is. further bol- stered, Cayan points out, by records that track the first springtime blooms of flowers such as honey- suckle and lilac, which show a simi- lar I - to 2-week advance. ''This is a totally independent measure and one that is quite strongly related to temperatures in the springtime," Cayan says. Not everyone is yet ready to believe that these trends will continue. George Taylor, the state of Oregon climatologist and a cli- mate researcher at Oregon State University in Corvallis, for example, argues that broad trends in temperature and snow accumula- tion over the past century are most likely due to natural multidecade swings as the cli- mate oscillates between periods of relative warm and cold temperatures. "There was significant wanning in the 1920s, '30s, and 000790 '40s, cooling in the '50s and '60s, and warming again from the 1970s through '90s," Taylor says. "In my opinion, the ef- fects of human-induced global warming are small compared to the multidecadal cycles." Greg Johnson, a climatologist with NRCS in Portland, also points out that Mote typically starts his analysis of snowpack trends at the beginning of the 19508, which saw some of the largest" snow accumulations over the past century. "If you use those num- bers, you will show large decreases," he says. Decadal swings in climate caused by lnaease . . . . In retreat. A modest temperature rise since the 1950s has reduced spring snowpacks throughout the West (top) and shifted the peak snowmelt earlier in the year (left). El Nino and the Pacific Decadal Oscillation, he adds, further muddy the numbers. "I'm not saying it's a nonissue, just that we need to keep watching it closely," John- son says. "The point is, if you look at the historical record, we've seen some warming and drops in low-elevation snowpack. The question is what can we tie it to. But from a planning standpoint, I think people have to be concerned about this." Mote agrees that the trend data may be skewed to some degree by the high-snow years of the early 1950s. However, he says, before the 1950s there were so few snow measurement sites that earlier data are sus- pect. Furthermore, he says, the snow loss is still best explained by the region's modest wanning. "The thing that really stands out is that the largest losses are at the lowest eleva- www.sciencemag.org SCIENCE VOL 303 20 FEBRUARY 2004 1125