Climatic Change in the Colorado Rocky Mountains: Estimates Based on Modern Climate at Late Pleistocene Equilibrium Lines

A comparison of modern climates at late Pleistocene glacier equilibrium lines in the Colorado Rocky Mountains with the range of climates which occur at the equilibrium lines of modern glaciers worldwide allows an evaluation of the combinations of temperature and precipitation change which would have been necessary to sustain the late Pleistocene glaciers. Modern climatic conditions at late Pleistocene equilibrium lines at 12 sites in 7 ranges are approximated using instrumental and snow survey data. The comparison provides paired values of temperature and precipitation changes which would maintain the glaciers at their late Pleistocene maximum positions. If no change occurred in total precipitation or in seasonal distribution of precipitation between the late Pleistocene and the present, an approximately 8.5°C summer temperature depression would have been necessary to sustain the late Pleistocene glaciers. A 10 to 13°C late Pleistocene temperature depression, which has been suggested on the basis of other lines of evidence, would have been accompanied by a reduction of at least 44% in fall-through-spring precipitation compared to present-day values.