Illusory Stability of Marine-Terminating Glaciers at Bedrock Peaks

Abstract:

Observations show that many marine-terminating glaciers terminate at submarine bedrock peaks, and have persisted in such locations even while subject to strong variations in climate over periods of decades to millennia. Such observations are in conflict with conventional theories of marine-terminating glacier stability which indicate that termini on the precipice of reverse-sloping beds should be the most susceptible to rapid retreat. To explain this seeming contradiction we marshal a diverse range of evidence from satellite observations, paleo-glaciological indicators, models and glaciological theory. We show that the persistence of glaciers at bedrock peaks is: (1) due to the influence of lowered driving stresses from upstream reverse-sloping bedrock, and (2) often a transient state that is observationally indistinguishable from true glacier stability. Such transient persistence may continue for decades to millennia before giving way to previously committed rapid glacier retreat, even in the absence of further changes in climate. In Greenland and Antarctica, we develop a theory to identify many potential locations in the interior of glacier catchments where retreat may slow down or pause for long periods during otherwise irreversible retreat. Finally, we discuss the implications of these newfound dynamics for interpreting observations of glacier “stability”, and simulating future glacier retreat in models.