A REGIONAL VIEW OF TEMPERATURE DEPRESSION IN THE COLORADO ROCKY MOUNTAINS DURING THE LAST GLACIAL MAXIMUM

Temperature-index modeling is used to determine the magnitude of temperature depression required to maintain steady-state mass balances of 39 reconstructed glaciers at their last glacial maximum (LGM) extents in the Sawatch Range, Mosquito Range, Elk Mountains, and Sangre de Cristo Mountains. Assuming no significant differences in precipitation compared to modern values, inferred temperature depressions during the LGM range from ~6.2 to 8.5 °C, with a mean of 7.3 ± 0.7 °C. Associated uncertainties in these values vary, but are typically on the order of ± 0.5 to 1 °C. Given these uncertainties, the differences in inferred LGM cooling may not be significant and could be due to the inability to account for potential changes in precipitation over the region. Alternatively, these differences could arise from regional asynchrony of glacial advances such that the LGM was time-transgressive and therefore the resulting estimates of temperature change are not contemporaneous. Uncertainties combined with sparse and clustered data also yield regional trends in LGM temperature depression that are equivocal. In contrast, clear regional west-to-east gradients of ~2.5 to 5 m km^-1 are evident in equilibrium-line altitudes (ELAs) reflecting general moisture transport during the LGM. Locally in the Mosquito Range and Sangre de Cristo Mountains, however, the gradient is reversed in that ELAs are lower on the eastern slopes presumably due to later winter/early spring upslope precipitation events. Nevertheless, if there was significant variation in LGM temperature depression over the region, this could imply local modulation of regional/global climate forcing and/or concomitant glacier response, or other complexities in the interactions among regional and local climates, and glacier dynamics.