Present, Future, and Past Tropical Andean ELAs: Assessing the Relative Importance of Precipitation and Temperature on Glacier Mass Balance Using a Positive Degree-Day Model

A positive degree-day glacier mass balance model is calibrated using modern meteorological and mass balance data from Zongo Glacier, Cordillera Real, Bolivia. Once, calibrated the model is applied in three ways. First, the effect of predicted future rising temperatures on glacial ELAs is examined. Although the forecast warming of the next century will cause retreat of all glaciers and elimination of some lower elevation glaciers, the model suggests that glaciers will remain on the high peaks of the range with future ELAs of about 5640 m a.s.l. Second, the model is used to suggest possible precipitation and temperature scenarios responsible for known ELA lowerings during the Late Glacial, and early and late Holocene. If paleo-precipitation was as high as estimated, then the model suggests only a 2.3 oC cooling during Late Glacial times. When the model is applied to the Western Cordillera, it indicates an ELA that is ~500 m below the actual ELA. This is believed to result from the fact that sublimation dominates the ablation regime in the arid Western Cordillera. Since sublimation is relatively insensitive to changes in temperature, use of a PDD model is ineffective in this high, arid, tropical environment.