Person:

Mitrovica, Jerry

We investigate the stability of marine ice sheets by coupling a gravitationally self-consistent sea level model valid for a self-gravitating, viscoelastically deforming Earth to a 1-D marine ice sheet-shelf model. The evolution of the coupled model is explored for a suite of simulations in which we vary the bed slope and the forcing that initiates retreat. We find that the sea level fall at the grounding line associated with a retreating ice sheet acts to slow the retreat; in simulations with shallow reversed bed slopes and/or small external forcing, the drop in sea level can be sufficient to halt the retreat. The rate of sea level change at the grounding line has an elastic component due to ongoing changes in ice sheet geometry, and a viscous component due to past ice and ocean load changes. When the ice sheet model is forced from steady state, on short timescales (<∼500 years), viscous effects may be ignored and grounding-line migration at a given time will depend on the local bedrock topography and on contemporaneous sea level changes driven by ongoing ice sheet mass flux. On longer timescales, an accurate assessment of the present stability of a marine ice sheet requires knowledge of its past evolution.

Climate change could potentially destabilize marine ice sheets, which would affect projections of future sea-level rise1, 2, 3, 4. Specifically, an instability mechanism5, 6, 7, 8 has been predicted for marine ice sheets such as the West Antarctic ice sheet that rest on reversed bed slopes, whereby ice-sheet thinning or rising sea level leads to irreversible retreat of the grounding line. However, existing analyses of this instability mechanism have not accounted for deformational and gravitational effects that lead to a sea-level fall at the margin of a rapidly shrinking ice sheet9, 10, 11. Here we present a suite of predictions of gravitationally self-consistent sea-level change following grounding-line migration. Our predictions vary the initial ice-sheet size and also consider the contribution to sea-level change from various subregions of the simulated ice sheet. Using these results, we revisit a canonical analysis of marine-ice-sheet stability5 and demonstrate that gravity and deformation-induced sea-level changes local to the grounding line contribute a stabilizing influence on ice sheets grounded on reversed bed slopes. We conclude that accurate treatments of sea-level change should be incorporated into analyses of past and future marine-ice-sheet dynamics.

A long-standing hypothesis for global ice-sheet synchronization on orbital timescales invokes sea-level rise from increased loss of Northern Hemisphere (NH) ice sheets in response to insolation and greenhouse gas forcing causing grounding line retreat of marine-based sectors of the Antarctic Ice Sheet (AIS)(1-3). Recent evidence indicates that the AIS also experienced substantial millennial-scale variability during and after the last deglaciation(4-7), further suggesting a possible sea-level forcing. Global sea-level change from ice-sheet mass loss is strongly nonuniform8, however, suggesting that the response of AIS grounding lines to NH sea-level forcing is likely more complicated than previously considered(1,2,6). Here we show, using a coupled ice sheet - global sea-level model, that a large or rapid NH sea-level forcing during deglaciation reduces or exceeds the sea-level fall at AIS grounding lines driven by the gravitational and deformational effects of AIS mass loss, enhancing grounding line retreat and associated AIS mass loss. In contrast, during NH glaciation, the sea level forcing acts to enhance grounding line advance. We find that including these effects causes NH sea-level forcing to increase AIS volume during the Last Glacial Maximum (LGM, ~26-20 ka) and triggers an earlier retreat and millennial scale variability through the last deglaciation, consistent with geologic reconstructions of LGM AIS extent and subsequent ice-sheet retreat and relative sea-level change in Antarctica(3-7,9).

We present gravitationally self-consistent predictions of sea level change that would follow the disappearance of either the West Antarctic Ice Sheet (WAIS) or marine sectors of the East Antarctic Ice Sheet (EAIS). Our predictions are based on a state-of-the-art pseudo-spectral sea level algorithm that incorporates deformational, gravitational and rotational effects on sea level, as well as the migration of shorelines due to both local sea-level variations and changes in the extent of marine-based ice cover. If we define the effective eustatic value (EEV) as the geographically uniform rise in sea level once all marine-based sectors have been filled with water, then we find that some locations can experience a sea level rise that is similar to 40 per cent higher than the EEV. This enhancement is due to the migration of water away from the zone of melting in response to the loss of gravitational attraction towards the ice sheet (load self-attraction), the expulsion of water from marine areas as these regions rebound due to the unloading, and the feedback into sea level of a contemporaneous perturbation in Earth rotation. In the WAIS case, this peak enhancement is twice the value predicted in a previous projection that did not include expulsion of water from exposed marine-sectors of the West Antarctic or rotational feedback. The peak enhancements occur over the coasts of the United States and in the Indian Ocean in the WAIS melt scenario, and over the south Atlantic and northwest Pacific in the EAIS scenario. We conclude that accurate projections of the sea level hazard associated with ongoing global warming should be based on a theory that includes the complete suite of physical processes described above.

Sea-level histories during the two most recent deglacial-interglacial intervals experienced significant differences1-3 despite both periods having similar changes in global mean temperature4,5 and forcing from greenhouse gases6. Although the last interglaciation (LIG) experienced stronger boreal summer insolation forcing than during the present interglaciation7, understanding why LIG global mean sea level may have been 6-9 m higher than present has proven particularly challenging2. During glacial as well as interglacial periods, extensive areas of polar ice sheets were grounded below sea level, with grounding lines and fringing ice shelves extending onto continental shelves8, suggesting that oceanic forcing by subsurface warming may also have contributed to ice-sheet loss9-12 analogous to ongoing changes by the Antarctic13,14 and Greenland15 ice sheets. Such forcing would have been especially effective during glacial periods when the Atlantic Meridional Overturning Circulation (AMOC) experienced large variations on millennial timescales16, with a reduction of the AMOC causing subsurface warming throughout much of the Atlantic basin9,12,17. Here we show that greater subsurface warming induced by the longer duration of reduced AMOC during the penultimate deglaciation can explain the more-rapid sea-level rise than during the last deglaciation. This greater forcing also contributed to excess loss from the Greenland and Antarctic ice sheets during the LIG, causing global mean sea level to rise at least 4 m above modern. When accounting for the combined influences of penultimate and last-interglacial deglaciation on glacial isostatic adjustment, this excess loss of polar ice during the LIG can explain much of the relative sea level recorded by fossil coral reefs and speleothems at intermediate- and far-field sites.