Last Glacial Maximum Climate in the Central Rocky Mountains, USA

While the changing extent of alpine glaciers is compelling evidence of climate change, extraction of quantitative data on past climate from changes in ice extent is difficult. It is particularly difficult to separate the effects of temperature and precipitation changes. Studies in the central Rocky Mountains have used several glaciological methods, including distributed energy/mass balance modeling, degree-day modeling, and modeling by analogy to climates at equilibrium lines of modern glaciers, to determine paired values of temperature and precipitation change that would have sufficed to maintain glaciers at their Last Glacial Maximum (LGM) extents. In the Colorado Rockies different glaciological methods produce generally similar estimates of LGM temperature/precipitation change combinations, indicating that in the absence of precipitation change, temperature depression ranging from 6.4 to 8.5°C would have been necessary to sustain LGM glaciers. With precipitation halved from modern values, modeled LGM temperature depression ranges from 8.0 to 10.4°C, with twice modern precipitation, from 3.7 to 6.8°C. A distributed energy/mass balance model (Plummer and Phillips, 2003) for the LGM Middle Boulder Creek Glacier in the Colorado Front Range indicates temperature depressions of 6.8°C, 8.0°C, and 5.0°C, for no change, halving, and doubling of precipitation respectively. Comparison to studies applying the same model (Laabs et. al, 2006; Refsnider et al., 2008) indicates that LGM glaciation in the Wasatch Range of Utah required either about twice as much precipitation relative to present as did LGM glaciation in Front Range, or a 2 to 2.5°C greater temperature depression. LGM glaciers in the Uinta Mountains of Utah, geographically between the Wasatch and the Front Range, generally required intermediate temperature/precipitation changes. This pattern may reflect the effect of pluvial Lake Bonneville as a local moisture source for LGM glaciers or may reflect a broader pattern of regional differences in climate change.