Sensitivity of mass balance and equilibrium-line altitude to climate change in the French Alps

Assessment of the sensitivity of surface mass balance and equilibrium-line altitude (ELA) to climate change is crucial for simulating the future evolution of glaciers. Such an assessment has been carried out using an extensive dataset comprising numerous measurements of snow accumulation and snow and ice ablation made on four French glaciers over the past 16 years. Winter mass balance shows a complicated pattern with respect to altitude, with no clear linear relationship. Although the ratios of winter mass balance to valley precipitation differ considerably from site to site, they are relatively constant over time. Relationships between snow/ice ablation and temperature are stable, with no link with altitude. The mean snow and ice positive degree-day (PDD) factors are 0.003 and 0.0061 m w.e. degrees C-1 d(-1). This analysis shows that, at a given site, ablation depends mainly on the amount of snow precipitation and on cumulative PDDs. The sensitivity of annual ablation to temperature change increases almost linearly from 0.25 m w.e. degrees C-1 at 3500 m to 1.55 m w.e. degrees C-1 at 1650 m. ELA sensitivity to temperature change was found to range from 50 to 85 m degrees C-1.