LITTLE ICE AGE CLIMATE IN THE WIND RIVER RANGE, WYOMING, ESTIMATED FROM ICE-CORE DATA, TREE-RING RECORDS AND GLACIER MODELING

Recent studies in the Wind River Range, Wyoming provide two independent means of estimating climate changes that have occurred since approximately the Little Ice Age. Geochemical variations in ice and snow collected from the Upper Fremont Glacier preserve a record of changes in atmospheric composition dating back to the mid-1800’s, or about the end of the Little Ice Age. Analyses of oxygen-18 and oxygen-16 in ice and snow samples from the glacier and from more recent snowpacks suggest that the average winter air temperature in this region has increased by 1.5° to 3.5°C since 1960, and by approximately 5°C since the end of the Little Ice Age. Tree-ring records collected from the area extend back approximately 300 years further in time, to approximately 1550.  A chronology from a white bark pine tree near the alpine tree line adjacent to the Upper Fremont Glacier effectively records variability in summer temperature, while the record from a Douglas fir collected at lower elevation primarily records variability in winter precipitation. Analysis of the high-elevation tree-ring record supports the idea that there has been a rise in average summer air temperature starting in the mid-1880s but indicates a much more gradual change than does the glacial ice record.

The warming indicated by the glacial ice and tree ring records would have had a profound effect on the mass balance of the glaciers of the Wind River Range. To determine whether the estimated warming is consistent with geomorphic evidence of glacial changes since ~1850 - the Little Ice Age moraines, we simulate its effect on the Fremont Glacier using a 2-D snow & energy balance model combined with a 2-D ice flow model. Inputs to the model are primarily long-term monthly averages of temperature, precipitation and several secondary climate parameters, as simple functions of elevation. By making simple changes to those parameters, we can calculate what combinations of changes are needed to reproduce the glaciers of not only the Little Ice Age period but also the extensive Pinedale and Bull Lake glaciations. As a transient flow model, it also provides a means of determining glacier response time under different extents and we demonstrate how this can be important in paleoclimatic interpretation of the glacial features.