MODELING THE 20th CENTURY RETREAT OF THE AGASSIZ GLACIER, GLACIER NATIONAL PARK, MONTANA

The Agassiz Glacier has experienced dramatic retreat since abandoning its Little Ice Age moraine, illustrating the sensitivity of mountain glaciers to climate changes of the 20^th century. To better understand how the Agassiz Glacier might respond to continuing climate change, we developed a 2-D model of the glacier, and tested the model’s ability to accurately simulate changes in a glacier in response to climate changes of the past 100 years.

The modeling approach computes net-annual mass balance of a glacial valley based on average monthly temperature, precipitation, and radiation balance along with numerous secondary parameters including the distribution of snow by avalanches. The calculated mass balance is combined with a 2-D glacier-flow model to simulate areal ice extent under specified climate conditions. Meteorological inputs to the model were derived from the PRISM model (http://www.prism.oregonstate.edu/) characterizing the mean climate for 1971-2000 of the area occupied by the Agassiz Glacier. These parameters were then modified to simulate the changes in temperature and precipitation according to a 100-year historical record from nearby Kalispell, Montana. Although modeled mass balance and ice extent under the 1971-2000 average nearly match observations of the Agassiz Glacier over this interval, the simulated glacier underestimates ice extent during the first half of the 20^th century when driven by the record from Kalispell. This discrepancy is likely due to differences in the magnitude of climate change at high altitude compared to low altitude; high altitude climate changes may have been of significantly greater magnitude. This inference is supported by a comparison of high altitude (SNOTEL) and low-altitude records for the past twenty-five years.

Model simulations of known changes of ice extent through the past 100 years provide a guide for inferring high altitude temperature change. By combining these results with improvements to high altitude records of climate change, we can set more precise limits on the sensitivity of mountain glaciers to decadal-scale climate change. This effort should aid in improving predictions of the response of the Agassiz Glacier to the changes in temperature and precipitation projected for upcoming decades.