Relative contribution of surface mass balance and ice flux changes to the accelerated thinning of the Mer de Glace (Alps) over 1979-2008

By subtracting surface topographies from 1979, 1994, 2000 and 2008, we measured ice thinning rates increasing from 1 m a-1 (1979-1994) to more than 4 m a-1 (2000-2008) on the tongue of the Mer de Glace. The relative contributions of changes in surface mass balance and ice fluxes to this acceleration in the thinning are estimated using field and remote sensing measurements. Between 1979-1994 and 2000-2008, surface mass balance diminished by 1.2 m w.e. a-1 mainly because of atmospheric warming. Mass balance changes induced by the growing debris-covered area and the evolving glacier hypsometry compensated each other. Meanwhile, the Mer de Glace slowed down and the ice fluxes through two cross-sections at 2200 m a.s.l. and 2050 m a.s.l. decreased by 60%. Between 1979-1994 and 2000-2008, two thirds of the increase in the thinning rates was caused by reduced ice fluxes and one third by rising surface ablation. However, those numbers need to be interpreted cautiously given our inability to respect mass conservation below our upper cross section. An important implication is that large errors would occur if one term of the continuity equation (e.g., surface mass balance) was deduced from the two others (e.g., elevation and ice flux changes).