Glaciers in the American West have been studied since their scientific discovery in 1871. Yet no comprehensive inventory has been published until now. Such inventories are important for documenting the state of glacier extent, providing baseline data for alpine hydrology and ecology, and for tracking glacier change in the future. Our inventory is derived from the US Geological Survey 1:24,000 maps dating to the mid 20th
century. At that time, about 5036 glaciers and perennial snowfields (G&PS) covered an area of 672 km2
, of which 1275 (554 km2
) were glaciers. Two populations of G&PS are identified based on air temperature and precipitation, the larger is found in a maritime climate of the Pacific Northwest, characterized by warm winter air temperatures, high winter precipitation (~2100 mm) and lower average elevations (~2044 m). The other population is continental in climate, characterized by cooler winter air temperatures, relatively low winter precipitation (~880 mm), and located at higher elevations (~3426 m). The G&PS in the Pacific Northwest, especially the Olympic Mountains, are particularly vulnerable to rapid shrinkage because warming winter air temperatures will change the phase of winter precipitation from snow to rain, reducing mass input, in addition to warmer summer temperatures increasing snow and ice melt.
Over the past century three phases of glacier change are apparent: rapid loss during the first few decades; a near equilibrium, if not small advance, during the Mid-Century cool period; and rapid loss since then. Since the mid-20th Century glacier area across the West has receded by about 39%. Locally, glacier area in the Lewis Range, MT (Glacier National Park) has also receded about 39%, whereas in the Sierra Nevada it has receded about 50%. In contrast, glacier area in the Olympic Mountains has receded about 35%. At century time scales the cause of glacier shrinkage is warming air temperatures. At decadal time scales glacier variation is controlled by air temperature and precipitation. The glacier future was examined using a distributed glacier model coupled to a global climate model. For the Olympics Mountains, results indicate that glacier area will shrink dramatically during the current century due to elevated air temperatures. By 2100 they are predicted to largely vanish.