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Basal ice-shelf melting is the key driver of Antarctica's increasing sea-level contribution. In diminishing the buttressing force of the ice shelves that fringe the ice sheet, the melting increases the ice discharge into the ocean. Here we contrast the influence of basal melting in two different ice-shelf regions on the time-dependent response of an isothermal, inherently buttressed ice-sheet-shelf system. In the idealized numerical simulations, the basal-melt perturbations are applied close to the grounding line in the ice-shelf's (1) ice-stream region, where the ice shelf is fed by the fastest ice masses that stream through the upstream bed trough and (2) shear margins, where the ice flow is slower. The results show that melting below one or both of the shear margins can cause a decadal to centennial increase in ice discharge that is more than twice as large compared to a similar perturbation in the ice-stream region. We attribute this to the fact that melt-induced ice-shelf thinning in the central grounding-line region is attenuated very effectively by the fast flow of the central ice stream. In contrast, the much slower ice dynamics in the lateral shear margins of the ice shelf facilitate sustained ice-shelf thinning and thereby foster buttressing reduction. Regardless of the melt location, a higher melt concentration toward the grounding line generally goes along with a stronger response. Our results highlight the vulnerability of outlet glaciers to basal melting in stagnant, buttressing-relevant ice-shelf regions, a mechanism that may gain importance under future global warming. [ABSTRACT FROM AUTHOR]

We present the result of the third Marine Ice Sheet Model Intercomparison Project, MISMIP+. MISMIP+ is intended to be a benchmark for ice-flow models which include fast sliding marine ice streams and floating ice shelves and in particular a treatment of viscous stress that is sufficient to model buttressing, where upstream ice flow is restrained by a downstream ice shelf. A set of idealized experiments first tests that models are able to maintain a steady state with the grounding line located on a retrograde slope due to buttressing and then explore scenarios where a reduction in that buttressing causes ice stream acceleration, thinning, and grounding line retreat. The majority of participating models passed the first test and then produced similar responses to the loss of buttressing. We find that the most important distinction between models in this particular type of simulation is in the treatment of sliding at the bed, with other distinctions – notably the difference between the simpler and more complete treatments of englacial stress but also the differences between numerical methods – taking a secondary role. [ABSTRACT FROM AUTHOR]

Marine-based ice on Antarctica bears the potential to raise sea level worldwide by more than 20 m. It is prone to the marine ice sheet instability and increasing evidence from satellite observations and numerical simulations suggest that a self-sustaining discharge from West Antarctica has been initiated and might not be stoppable by reduction of greenhouse gas emissions. The associated sea level rise of more than 3 m would pose a serious challenge to highly populated areas including metropolises like Calcutta, Shanghai, New York City and Tokyo. Here we show that the West Antarctic Ice Sheet can be stabilized through mass deposition in coastal regions around Pine Island and Thwaites Glaciers. In our numerical simulations a minimum of 7400 Gt of additional snowfall stabilizes the flow if applied over a short period of 10 years onto the region. This is equivalent to a global sea-level drop of about 2 mm yr-1, if the mass is taken out of the ocean. Stabilization depends on the timing of the intervention. If the mass is deposited at a lower rate over a longer period an additional 500 Gt has to be added for each decade of prolongation. The power required to pump ocean water in the equivalent of 1 mm yr-1 to the medium height of the stabilization region in order to snow it on the ice sheet is approximately 90 GW and is available in the regional wind field. Although our findings suggest that the West Antarctic Ice Sheet can in principle be stabilized by mass deposition, we find that the precise conditions of the intervention are crucial and potential benefits need to be weight against environmental hazards, future risks and enormous technical challenges implied in such an operation. [ABSTRACT FROM AUTHOR]