On estimating length fluctuations of glaciers caused by changes in climatic forcing

The reaction of alpine glaciers to shifts in the equilibrium line altitude (ELA) iscalculated by using a two-dimensional numerical model to solve the full equations for thevelocity and stress fields (full-system model) in the absence of basal motion. Rates ofadvance and retreat of the snout of typically sized alpine glaciers are found to beinsensitive to the details of the flow at the snout, even when the glaciers are far fromsteady state. A comparison of results obtained with a full-system model and a shallow iceapproximation (SIA) model yields no significant differences in advance or retreat rates.This assumption has been implicitly made in numerous previous climatic studies and ishere shown to be well justified. Using a realistic mass balance altitude feedback, onlyslight model-dependent changes in steady state lengths are found. The relative importanceof mass balance and glacier dynamics for the transient response of alpine glaciers tochanges in the ELA is given a precise meaning by determining the model-dependentadditional shifts in ELA needed for the SIA and the full-system models to produceidentical changes in length. For alpine glaciers, these additional shifts in ELA are on theorder of 10 m, which is within the error range of ELA estimates. It follows that at leastin the absence of significant basal motion, there is no need to include the effects ofhorizontal stresses when calculating the reaction of alpine glaciers to climatic changes.Attention should focus on accurate determination of the mass balance distribution andmodel tuning to give realistic ice thickness distributions