Equilibrium-line altitude (ELA)

Abstract

The equilibrium-line altitude (ELA) marks the area or zone on a glacier where accumulation is balanced by ablation over a 1-year period. The ELA is sensitive to several meteorological factors, such as variations in winter precipitation, summer temperature, and wind transport of dry snow. When the annual net mass balance is negative, the ELA rises, and when the annual net mass balance is positive, the ELA falls. Fluctuations in the ELA provide an important indicator of glacier response to climate change that allows reconstructions of palaeoclimate (accumulation-season precipitation, ablation-season temperature, and prevailing snow-bearing wind directions). Palaeoclimatic reconstructions based on former glacier extent commonly include estimates of equilibrium-line altitudes (ELAs) and depression of ELAs from present values. Several methods have been developed to estimate steady-state ELAs of former glaciers as a tool to reconstruct palaeoclimates in glaciated regions. A survey of literature related to ELA depressions during the last glacial maximum, Younger Dryas, and the “Little Ice Age” shows lowerings from modern values in the order of 1,000 ± 300 m, 500 ± 200 m, and 100 ± 50 m, respectively. High-resolution reconstructions of ELA give valuable palaeoclimatic information from the terrestrial realm allowing comparison with other high-resolution archives, e.g., ice core and marine sediment records.