Reply on CC1

doi:10.5194/tc-2021-238-cc2

Abstract

Due to global warming and particularly high regional ocean warming, both Thwaites and Pine Island glaciers in the Amundsen region of the Antarctic Ice Sheet could lose their buttressing ice shelves over time. We analyze the possible consequences using the Parallel Ice Sheet Model (PISM), applying a simple cliff-calving parameterization and an ice-mélange-buttressing model. We find that the instantaneous loss of ice-shelf buttressing, due to enforced ice-shelf melting, initiates grounding line retreat and triggers the marine ice sheet instability (MISI). As a consequence, the grounding line progresses into the interior of the West Antarctic Ice Sheet and leads to a sea level contribution of 0.6 m within 100 a. By subjecting the exposed ice cliffs to cliff calving using our simplified parameterization, we also analyze the marine ice cliff instability (MICI). In our simulations it can double or even triple the sea level contribution depending on the only loosely constraint parameter which determines the maximum cliff-calving rate. The speed of MICI depends on this upper bound on the calving rate which is given by the ice mélange buttressing the glacier. However, stabilization of MICI may occur for geometric reasons. Since the embayment geometry changes as MICI advances into the interior of the ice sheet, the upper bound on calving rates is reduced and the progress of MICI is slowed down. Although we cannot claim that our simulations bear relevant quantitative estimates of the effect of ice-mélange buttressing on MICI, the mechanism has the potential to stop the instability. Further research is needed to evaluate its role for the past and future evolution of the Antarctic Ice Sheet.

Highlights

Ice loss from the Greenland and Antarctic ice sheets is contributing increasingly to global sea level rise (Rignot et al, 2014; Shepherd et al, 2018b; WCRP Global Sea Level Budget Group, 2018)
We performed Parallel Ice Sheet Model (PISM) simulations of the West Antarctic Ice Sheet (WAIS) to investigate the potential speeds of the two marine instabilities, marine ice sheet instability (MISI) and 25 marine ice cliff instability (MICI)
We found that MISI, whether forced by the ’floatkill’ parameterization or by high subshelf melt rates, has the potential to contribute 0.6 m of sea level rise within 100 a

Summary

Introduction

Ice loss from the Greenland and Antarctic ice sheets is contributing increasingly to global sea level rise (Rignot et al, 2014; Shepherd et al, 2018b; WCRP Global Sea Level Budget Group, 2018). Ice sheets gain mass through accumulation of snowfall. Whether they contribute to sea level changes depends on how much this mass gain is offset or overcompensated by mass 20 losses due to surface and basal melting as well as iceberg calving. Both ice sheets in Greenland and Antarctica are currently losing ice (Enderlin et al, 2014; Shepherd et al, 2018b; Mouginot et al, 2019; Larour et al, 2019; Bell and Seroussi, 2020).

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Conclusion