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We observe the evacuation of 11-year-old landfast sea ice in the Larsen B embayment on the East Antarctic Peninsula in January 2022, which was in part triggered by warm atmospheric conditions and strong offshore winds. This evacuation of sea ice was closely followed by major changes in the calving behaviour and dynamics of a subset of the ocean-terminating glaciers in the region. We show using satellite measurements that, following a decade of gradual slow-down, Hektoria, Green, and Crane glaciers sped up by approximately 20 %–50 % between February and the end of 2022, each increasing in speed by more than 100 ma-1. Circumstantially, this is attributable to their transition into tidewater glaciers following the loss of their ice shelves after the landfast sea ice evacuation. However, a question remains as to whether the landfast sea ice could have influenced the dynamics of these glaciers, or the stability of their ice shelves, through a buttressing effect akin to that of confined ice shelves on grounded ice streams. We show, with a series of diagnostic modelling experiments, that direct landfast sea ice buttressing had a negligible impact on the dynamics of the grounded ice streams. Furthermore, we suggest that the loss of landfast sea ice buttressing could have impacted the dynamics of the rheologically weak ice shelves, in turn diminishing their stability over time; however, the accompanying shifts in the distributions of resistive stress within the ice shelves would have been minor. This indicates that this loss of buttressing by landfast sea ice is likely to have been a secondary process in the ice shelf disaggregation compared to, for example, increased ocean swell or the drivers of the initial landfast sea ice disintegration. [ABSTRACT FROM AUTHOR]

The predicted Antarctic contribution to global‐mean sea‐level rise is one of the most uncertain among all major sources. Partly this is because of instability mechanisms of the ice flow over deep basins. Errors in bedrock topography can substantially impact the projected resilience of glaciers against such instabilities. Here we analyze the Pine Island Glacier topography to derive a statistical model representation. Our model allows for inhomogeneous and spatially dependent uncertainties and avoids unnecessary smoothing from spatial averaging or interpolation. A set of topography realizations is generated representing our best estimate of the topographic uncertainty in ice sheet model simulations. The bedrock uncertainty alone creates a 5%–25% uncertainty in the predicted sea level rise contribution at year 2100, depending on friction law and climate forcing. Pine Island Glacier simulations on this new set are consistent with simulations on the BedMachine reference topography but diverge from Bedmap2 simulations. Plain Language Summary: We investigate the impact of uncertainties in the elevation of the bedrock underneath the ice of a particularly vulnerable glacier in Antarctica. We propose a new approach to better estimate how much future projections depend on knowledge of bedrock elevation. The main focus of this study is to represent the current understanding of the bedrock elevation as closely as possible so that our simulations accurately reflect the extent of our knowledge of the future glacier behavior. In summary, we find that the mass of ice lost in simulations for year 2100, which contributes to the global mean sea level, can be affected by up to 25%. This highlights the value of closely‐spaced bedrock measurement and of careful consideration of related uncertainties in ice‐sheet simulations. Key Points: Uncertainty in topography estimates has a significant impact on predictions for all tested friction lawsSimulations with BedMachine and statistically generated topographies are more sensitive to upper‐end climate forcing than with Bedmap2Pine Island Glacier is likely to transition into a more unstable state late mid‐century for upper‐end climate forcing [ABSTRACT FROM AUTHOR]

The Antarctic Ice Sheet (AIS) remains the largest uncertainty in projections of future sea level rise. A likely climate‐driven vulnerability of the AIS is thinning of floating ice shelves resulting from surface‐melt‐driven hydrofracture or incursion of relatively warm water into subshelf ocean cavities. The resulting melting, weakening, and potential ice shelf collapse reduces shelf buttressing effects. Upstream ice flow accelerates, causing thinning, grounding‐line retreat, and potential ice sheet collapse. While high‐resolution projections have been performed for localized Antarctic regions, full‐continent simulations have typically been limited to low‐resolution models. Here we quantify the vulnerability of the entire present‐day AIS to regional ice shelf collapse on millennial timescales treating relevant ice flow dynamics at the necessary ∼1‐km resolution. Collapse of any of the ice shelves dynamically connected to the West Antarctic Ice Sheet (WAIS) is sufficient to trigger ice sheet collapse in marine‐grounded portions of the WAIS. Vulnerability elsewhere appears limited to localized responses. Plain Language Summary: The biggest uncertainty in near‐future sea level rise (SLR) comes from the Antarctic Ice Sheet. Antarctic ice flows in relatively fast‐moving ice streams. At the ocean, ice flows into enormous floating ice shelves that push back on their feeder ice streams, buttressing them and slowing their flow. Melting and loss of ice shelves due to climate changes can result in faster‐flowing, thinning, and retreating ice leading to accelerated rates of global SLR. To learn where Antarctica is vulnerable to ice shelf loss, we divided it into 14 sectors, applied extreme melting to each sector's floating ice shelves in turn, then ran our ice flow model 1,000 years into the future for each case. We found three levels of vulnerability. The greatest vulnerability came from attacking any of the three ice shelves connected to West Antarctica, where much of the ice sits on bedrock lying below sea level. Those dramatic responses contributed around 2 m of SLR. The second level came from four other sectors, each with a contribution between 0.5 and 1 m. The remaining sectors produced little to no contribution. We examined combinations of sectors, determining that sectors behave independently of each other for at least a century. Key Points: Sustained ice‐shelf loss in any of the Amundsen Sea, Ronne, or Ross sectors can lead to wholesale West Antarctic ungrounding and collapseEven with extreme forcing, loss is relatively modest for the initial century, increasing markedly after in West Antarctic collapse scenariosModeling suggests Antarctic drainage basins can be assumed to be dynamically independent for one to two centuries before they begin to interact [ABSTRACT FROM AUTHOR]

The retreating Pine Island Glacier (PIG), West Antarctica, presently contributes ~5-10% of global sea-level rise. PIG's retreat rate has increased in recent decades with associated thinning migrating upstream into tributaries feeding the main glacier trunk. To project future change requires modelling that includes robust parameterisation of basal traction, the resistance to ice flow at the bed. However, most ice-sheet models estimate basal traction from satellite-derived surface velocity, without a priori knowledge of the key processes from which it is derived, namely friction at the ice-bed interface and form drag, and the resistance to ice flow that arises as ice deforms to negotiate bed topography. Here, we present highresolution maps, acquired using ice-penetrating radar, of the bed topography across parts of PIG. Contrary to lower-resolution data currently used for ice-sheet models, these data show a contrasting topography across the ice-bed interface. We show that these diverse subglacial landscapes have an impact on ice flow, and present a challenge for modelling ice-sheet evolution and projecting global sea-level rise from ice-sheet loss. [ABSTRACT FROM AUTHOR]

Coupled ice sheet-ocean models capable of simulating moving grounding lines are just becoming available. Such models have a broad range of potential applications in studying the dynamics of marine ice sheets and tidewater glaciers, from process studies to future projections of ice mass loss and sea level rise. The Marine Ice Sheet-Ocean Model Intercomparison Project (MISOMIP) is a community effort aimed at designing and coordinating a series of model intercomparison projects (MIPs) for model evaluation in idealized setups, model verification based on observations, and future projections for key regions of the West Antarctic Ice Sheet (WAIS). Here we describe computational experiments constituting three interrelated MIPs for marine ice sheet models and regional ocean circulation models incorporating ice shelf cavities. These consist of ice sheet experiments under the Marine Ice Sheet MIP third phase (MISMIP+), ocean experiments under the Ice Shelf-Ocean MIP second phase (ISOMIP+) and coupled ice sheet-ocean experiments under the MISOMIP first phase (MISOMIP1). All three MIPs use a shared domain with idealized bedrock topography and forcing, allowing the coupled simulations (MISOMIP 1) to be compared directly to the individual component simulations (MISMIP+ and ISOMIP+). The experiments, which have qualitative similarities to Pine Island Glacier Ice Shelf and the adjacent region of the Amundsen Sea, are designed to explore the effects of changes in ocean conditions, specifically the temperature at depth, on basal melting and ice dynamics. In future work, differences between model results will form the basis for the evaluation of the participating models. [ABSTRACT FROM AUTHOR]

Abstract: A flowline ice sheet model is coupled to a box model for cavity circulation and configured for the Pine Island Glacier. An ensemble of 5000 simulations are carried out from 1900 to 2200 with varying inputs and parameters, forced by ocean temperatures predicted by a regional ocean model under the A1B ‘business as usual’ emissions scenario. Comparison is made against recent observations to provide a calibrated prediction in the form of a 95% confidence set. Predictions are for monotonic (apart from some small scale fluctuations in a minority of cases) retreat of the grounding line over the next 200yr with huge uncertainty in the rate of retreat. Full collapse of the main trunk of the PIG during the 22nd century remains a possibility. [Copyright &y& Elsevier]