Glacier and fjord geometry dominates its response to environmental forcing
Greenland's contribution to future sea-level rise remains uncertain and a wide range of upper and lower bounds has been proposed. These predictions depend strongly on how mass loss, which is focused at the termini of marine-terminating outlet glaciers, can penetrate inland to the ice sheet interior. Previous studies have shown that, at regional scales, Greenland Ice Sheet mass loss is correlated with atmospheric and oceanic warming. However, velocity, terminus position and elevation within individual outlet glacier catchments exhibits unexplained heterogeneity, hindering our ability to project ice sheet response to future environmental forcing. Here we examine a set of 15 glaciers in West Greenland where significant heterogeneity in mass loss exists using satellite and in-situ observations to uncover the local controls on the observed variability. We show that terminus retreat, and the upglacier extent of thinning initiated by this retreat, and thus mass loss, is limited by glacier geometry. Further, we find that 94% of the total dynamic loss occurs between the terminus and the location where the downglacier advective speed of a kinematic wave of thinning is at least 4 times larger than its diffusive speed. This empirical threshold enables the identification of glaciers that are not currently thinning but are most susceptible to future thinning in the coming decades.