What was done
The authors analyzed snow water equivalent (SWE) data obtained from four different measuring systems -- snow courses, snow telemetry, aerial markers and airborne gamma radiation -- at more than 2000 sites in the eleven westernmost states of the conterminous USA over the period 1910-1998.

What was learned
The long-term SWE trend of the entire studied region was negative, but with some significant within-region differences.  In the northern Rocky Mountains and Cascades of the Pacific Northwest, for example, the trend was decidedly negative, with SWE decreasing at a rate of 0.1 to 0.2 inches per year.  In the intermountain region and southern Rockies, however, there was no change in SWE with time.

The authors say their results "reinforce more tenuous conclusions made by previous authors," citing Chagnon et al. (1993) and McCabe and Legates (1995), who studied snow course data from 1951-1985 and 1948-1987, respectively, at 275 and 311 sites.  They too found a decreasing trend in SWE at most sites in the Pacific Northwest but more ambiguity in the southern Rockies.

What it means
Nearly all climate models suggest the planet's hydrologic cycle will be enhanced in a warming world and precipitation will increase.  This prediction is especially applicable to the Pacific Northwest of the United States.  Kusnierczyk and Ettl (2002), for example, report that climate models predict "increasingly warm and wet winters" for this region, as do Leung and Wigmosta (1999).  Over the period of this study, however, when there was well-documented worldwide warming, precipitation that fell and accumulated as snow in the western USA did not respond as predicted.  In fact, over the Pacific Northwest, it did just the opposite.  Admittedly, this region comprises but a small part of the earth; but the study of New et al. (2001) suggests much the same for the bulk of the planet, i.e., a slight decrease in precipitation since 1915.  Hence, real-world data in both the USA's Pacific Northwest and most of the rest of the world appear to contradict one of the most basic of all climate model predictions.

References
Changnon, D., McKee, T.B. and Doesken, N.J.  1993.  Annual snowpack patterns across the Rockies: Long-term trends and associated 500-mb synoptic patterns.  Monthly Weather Review 121: 633-647.

Kusnierczyk, E.R. and Ettl, G.J.  2002.  Growth response of ponderosa pine (Pinus ponderosa) to climate in the eastern Cascade Mountain, Washington, U.S.A.: Implications for climatic change.  Ecoscience 9: 544-551.

Leung, L.R. and Wigmosta, M.S.  1999.  Potential climate change impacts on mountain watersheds in the Pacific Northwest.  Journal of the American Water Resources Association 35: 1463-1471.

McCabe, A.J. and Legates, S.R.  1995.  Relationships between 700hPa height anomalies and 1 April snowpack accumulations in the western USA.  International Journal of Climatology 14: 517-530.

New, M., Todd, M., Hulme, M. and Jones, P.  2001.  Precipitation measurements and trends in the twentieth century.  International Journal of Climatology 21: 1899-1922.