THE RAPID RETREAT OF GLACIERS IN THE NATIONAL PARKS OF THE CONTINENTAL US

The vast majority of the well-studied glaciers in the Continental US are located in the National Parks. The data history is due, in large part, to the long presence of the Parks and attention they attract, and the access to high mountain areas they provide. From aerial and ground-based images we reconstructed the glacial history over the past century for eight national parks through out the western US, excluding Alaska. Since about 1900, the glaciers have shrunk about 45% with the maximum shrinkage at Glacier National Park of 57%. The glaciers of the Olympics and Mount Rainier have shrunk the least, about 40%. The temporal pattern of change follows the global pattern, rapid decrease in the 1930s followed by an equilibrium or advance during the 1960s to 1970s, followed again by a retreat and acceleration over the last decade. The overall climatic cause of glacier retreat has not been changes in precipitation, but rather changes in air temperature. For glaciers below 3000 m we infer that the retreat results from decreased winter snowfall, as the state of precipitation changes from snow to rain due to warming winter temperatures, in addition to warming summer temperatures. For glaciers above 3000m they are retreating due to warming spring and summer air temperatures. From a land management perspective, glacier shrinkage imposes important changes on the character of high alpine hydrology, the ecosystems that depend on glacial runoff, and on the recreational use of these regions.