A CENTURY OF GLACIER CHANGE IN THE AMERICAN WEST, USA

Since 1900 glaciers in the American West have shrunk by an average of 45%, with regional variations from 24% – 66%. The glaciers follow a similar temporal variation with rapid shrinkage from the beginning of the century to the 1940s. In the 1950’s the glaciers generally stabilized and either reached equilibrium or advanced until the 1970s. The glaciers resumed retreat in the 1980s, slowing over the turn of the new millennium then accelerating retreat in the past decade. Differences from this generalized trend include both timing and magnitude of change. While it is clear that the century-scale driver is warming air temperature, decadal variations are controlled by changes in both temperature and precipitation. These latter factors can be regional in influence changing the regional glacier response in comparison to the generalized trend. Also, glacier size and physical setting are important. Small glaciers (< 1 km²) show extreme variability in area change while larger glaciers show much less variation. This points to the influence of local topography enhancing or diminishing regional climate variations. Glaciers in the Pacific Northwest have retreated much less than those in other regions underscoring the maritime influence of nearby alpine landscapes. In addition, glaciers on the volcanoes of the Cascade Range show much smaller change due to their high elevation accumulation zones compared to lower elevation glaciers elsewhere in the range.