05/18/2017 - 3:00pm
Bound to rise:
Constraints on the rate of sea level rise from the past and present behavior of ice sheets
Observations show that portions of ice sheets in contact with the ocean can respond rapidly to both atmospheric and oceanic warming. This has been dramatically illustrated during the last glacial period when the Laurentide Ice Sheet sporadically discharged vast armadas of icebergs through the Hudson Strait into the North Atlantic Ocean during so-called Heinrich Events. More recently, we have observed punctuated ice sheet decay during the unexpectedly catastrophic disintegration of ice shelves on the Antarctic Peninsula and during the (also unpredicted) retreat of marine terminating glaciers surrounding the Greenland Ice Sheet. Attempts to incorporate the processes responsible for these disparate phenomena into ice sheet models have led to diverging projections of sea level rise with the most dire projection hinting that large portions of the West Antarctic Ice Sheet could collapse on century time scales through a newly recognized effect called the `marine ice cliff instability’. The marine ice cliff instability is based on the idea that the finite strength of ice places a limit on the maximum ice cliff height possible at the ice sheet terminus; when this height is broached, catastrophic ice sheet disintegration can occur. Here, we examine the theoretical and observational basis for the marine ice cliff instability to show that the observations of Greenland, Svalbard and Alaskan glaciers are broadly consistent with the `upper bound’ on ice cliff height suggested by theory. We further show that this model can explain large-scale patterns of glacier advance and retreat and—subject to idealized forcing—even Heinrich Events. Applying the model to modern ice sheet configurations, we find that large-scale ice sheet stability depends sensitively on the rate of isostatic adjustment of the bed, which is controlled by the viscosity structure of the upper mantle. Seismic inference of the viscosity of the mantle beneath the sections of West Antarctic Ice Sheet most prone to collapse suggests a low mantle viscosity, implying a rapid uplift rate following retreat. This effect is a significant stabilizing effect that can not only arrest ice sheet retreat, but also lead to subsequent re-advance. This suggests that solid Earth structure may play an unexpectedly large role in century time scale projections of sea level rise and climate change.
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