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Glaciers along the Amundsen Sea coastline in West Antarctica are dynamically adjusting to a change in ice-shelf mass balance that triggered their retreat and speed-up prior to the satellite era. In recent decades, the ice shelves have continued to thin, albeit at a decelerating rate, whilst ice discharge across the grounding lines has been observed to have increased by up to 100 % since the early 1990s. Here, the ongoing evolution of ice-shelf mass balance components is assessed in a high-resolution coupled ice–ocean model that includes the Pine Island, Thwaites, Crosson, and Dotson ice shelves. For a range of idealized ocean-forcing scenarios, the combined evolution of ice-shelf geometry and basal-melt rates is simulated over a 200-year period. For all ice-shelf cavities, a reconfiguration of the 3D ocean circulation in response to changes in cavity geometry is found to cause significant and sustained changes in basal-melt rate, ranging from a 75 % decrease up to a 75 % increase near the grounding lines, irrespective of the far-field forcing. These previously unexplored feedbacks between changes in ice-shelf geometry, ocean circulation, and basal melting have a demonstrable impact on the net ice-shelf mass balance, including grounding-line discharge, at multi-decadal timescales. They should be considered in future projections of Antarctic mass loss alongside changes in ice-shelf melt due to anthropogenic trends in the ocean temperature and salinity. [ABSTRACT FROM AUTHOR]

Mass loss from the Antarctic Ice Sheet is the main source of uncertainty in projections of future sea-level rise, with important implications for coastal regions worldwide. Central to ongoing and future changes is the marine ice sheet instability: once a critical threshold, or tipping point, is crossed, ice internal dynamics can drive a self-sustaining retreat committing a glacier to irreversible, rapid and substantial ice loss. This process might have already been triggered in the Amundsen Sea region, where Pine Island and Thwaites glaciers dominate the current mass loss from Antarctica, but modelling and observational techniques have not been able to establish this rigorously, leading to divergent views on the future mass loss of the West Antarctic Ice Sheet. Here, we aim at closing this knowledge gap by conducting a systematic investigation of the stability regime of Pine Island Glacier. To this end we show that early warning indicators in model simulations robustly detect the onset of the marine ice sheet instability. We are thereby able to identify three distinct tipping points in response to increases in ocean-induced melt. The third and final event, triggered by an ocean warming of approximately 1.2 ∘ C from the steady-state model configuration, leads to a retreat of the entire glacier that could initiate a collapse of the West Antarctic Ice Sheet. [ABSTRACT FROM AUTHOR]

Pine Island Glacier in West Antarctica is among the fastest changing glaciers worldwide. Over the last 2 decades, the glacier has lost in excess of a trillion tons of ice, or the equivalent of 3 mm of sea level rise. The ongoing changes are thought to have been triggered by ocean-induced thinning of its floating ice shelf, grounding line retreat, and the associated reduction in buttressing forces. However, other drivers of change, such as large-scale calving and changes in ice rheology and basal slipperiness, could play a vital, yet unquantified, role in controlling the ongoing and future evolution of the glacier. In addition, recent studies have shown that mechanical properties of the bed are key to explaining the observed speed-up. Here we used a combination of the latest remote sensing datasets between 1996 and 2016, data assimilation tools, and numerical perturbation experiments to quantify the relative importance of all processes in driving the recent changes in Pine Island Glacier dynamics. We show that (1) calving and ice shelf thinning have caused a comparable reduction in ice shelf buttressing over the past 2 decades; that (2) simulated changes in ice flow over a viscously deforming bed are only compatible with observations if large and widespread changes in ice viscosity and/or basal slipperiness are taken into account; and that (3) a spatially varying, predominantly plastic bed rheology can closely reproduce observed changes in flow without marked variations in ice-internal and basal properties. Our results demonstrate that, in addition to its evolving ice thickness, calving processes and a heterogeneous bed rheology play a key role in the contemporary evolution of Pine Island Glacier. [ABSTRACT FROM AUTHOR]

Whereas the termini of Hektoria and Green glaciers extend into a wide bay, the tongues of the other studied glaciers are narrower and laterally confined by mountain ridges. GLL - Location of the grounding line; CST - glacier front in 2011, 2013, 2016. c and d: Sections of Sentinel-1 image of 2017-04-01 showing the location of the velocity transects on Hektoria Glacier (c) and Crane Glacier (d). The Larsen outlet glaciers have cold snow/ice bodies in which a substantial portion of the meltwater released at the surface would freeze, in particular after cold periods[7]. [Extracted from the article]

Glaciers in the Amundsen Sea are rapidly thinning and retreating as a result of ocean-induced changes in ice-shelf buttressing. Here we use area-wide projections of ice shelf thickness and velocities to investigate changes in buttressing over the next 150 years under two limiting scenarios, i.e. sustained warm and cold ocean conditions. Our results are based on simulations using an asynchronous coupling between a high-resolution regional configuration of the 3D ocean model MITgcm, and the state-of-the-art SSA ice flow model Úa, with a coupling timestep of 6 months. Results provide the first Amundsen Sea-wide projections of glacier dynamics that fully account for the important feedbacks between ice-shelf geometry and ocean-induced melting. Limited changes in buttressing occur for existing ice shelves in a cold ocean scenario. However, a regionally diverse response is found for ice shelves subjected to warm ocean forcing, with increasing areas of ice providing reduced buttressing as the grounding line retreats. [ABSTRACT FROM AUTHOR]