ESTIMATES OF CLIMATE AT THE LAST GLACIAL MAXIMUM LGM IN THE COLORADO ROCKIES: CONVERGENCE OF RESULTS FROM DIFFERENT PALEOGLACIOLOGICAL APPROACHES

Change in alpine glacier extent was among the first and most compelling lines of evidence for Quaternary climate change. However, extraction of quantitative data on climate change from former ice extent has proven difficult due to the multiplicity of climatic factors affecting glacier mass balance and extent. The primary controls on mid-latitude mass balance – accumulation season precipitation and ablation season temperature – are difficult to disentangle based on the geologic record, and consequently, climate-change estimates commonly involve paired values of temperature and precipitation change that would have sufficed to maintain Pleistocene glaciers.

Studies of LGM climate in the Colorado Rockies have employed several paleoglaciological methods, including distributed energy and mass balance modeling, degree-day modeling, and modeling by analogy to climates at equilibrium lines of modern glaciers. Despite the differences in approach and assumptions of different methods and workers, results generally converge on pairings of summer temperature change and winter accumulation that could have sustained glaciers at their LGM extents. This poster compares results of nine past and current studies.

Glaciological estimates of LGM summer temperature depression in absence of any change from modern precipitation range from 6.4-9.5^oC, with most estimates falling between 7.2 and 8.5^oC. With precipitation halved from modern values, modeled temperature depression necessary to sustain LGM glaciers ranges from 8.2 to nearly 14^oC, although most results cluster between 8.2 and 9.2^oC and only one study indicates a depression of >10.4^oC. With a doubling of modern precipitation, modeled LGM temperature depression ranges from 3.7 to 6.8^oC, with most values at the high end of this range (6.0-6.5^oC). Although the relationship between precipitation change and temperature depression varies somewhat study-to-study, the studies examined generally support the contention that glaciation in the Colorado Rockies is controlled primarily by temperature change and is relatively insensitive to precipitation change.