Increasing ice flux from glaciers retreating over deepening bed
topography has been implicated in the recent acceleration of mass loss
from the Greenland and Antarctic ice sheets. We show in observations
that some glaciers have remained at peaks in bed topography without
retreating despite enduring significant changes in climate. Observations
also indicate that some glaciers which persist at bed peaks undergo
sudden retreat years or decades after the onset of local ocean or
atmospheric warming. Using model simulations, we show that glacier
persistence may lead to two very different futures: one where glaciers
persist at bed peaks indefinitely, and another where glaciers retreat
from the bed peak suddenly without a concurrent climate forcing.
However, it is difficult to distinguish which of these two futures will
occur from current observations. We conclude that inferring glacier
stability from observations of persistence obscures our true commitment
to future sea-level rise under climate change.