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The Antarctic Ice Sheet represents the largest source of uncertainty in future sea level rise projections, with a contribution to sea level by 2100 ranging from -5 to 43 cm of sea level equivalent under high carbon emission scenarios estimated by the recent Ice Sheet Model Intercomparison for CMIP6 (ISMIP6). ISMIP6 highlighted the different behaviors of the East and West Antarctic ice sheets, as well as the possible role of increased surface mass balance in offsetting the dynamic ice loss in response to changing oceanic conditions in ice shelf cavities. However, the detailed contribution of individual glaciers, as well as the partitioning of uncertainty associated with this ensemble, have not yet been investigated. Here, we analyze the ISMIP6 results for high carbon emission scenarios, focusing on key glaciers around the Antarctic Ice Sheet, and we quantify their projected dynamic mass loss, defined here as mass loss through increased ice discharge into the ocean in response to changing oceanic conditions. We highlight glaciers contributing the most to sea level rise, as well as their vulnerability to changes in oceanic conditions. We then investigate the different sources of uncertainty and their relative role in projections, for the entire continent and for key individual glaciers. We show that, in addition to Thwaites and Pine Island glaciers in West Antarctica, Totten and Moscow University glaciers in East Antarctica present comparable future dynamic mass loss and high sensitivity to ice shelf basal melt. The overall uncertainty in additional dynamic mass loss in response to changing oceanic conditions, compared to a scenario with constant oceanic conditions, is dominated by the choice of ice sheet model, accounting for 52 % of the total uncertainty of the Antarctic dynamic mass loss in 2100. Its relative role for the most dynamic glaciers varies between 14 % for MacAyeal and Whillans ice streams and 56 % for Pine Island Glacier at the end of the century. The uncertainty associated with the choice of climate model increases over time and reaches 13 % of the uncertainty by 2100 for the Antarctic Ice Sheet but varies between 4 % for Thwaites Glacier and 53 % for Whillans Ice Stream. The uncertainty associated with the ice–climate interaction, which captures different treatments of oceanic forcings such as the choice of melt parameterization, its calibration, and simulated ice shelf geometries, accounts for 22 % of the uncertainty at the ice sheet scale but reaches 36 % and 39 % for Institute Ice Stream and Thwaites Glacier, respectively, by 2100. Overall, this study helps inform future research by highlighting the sectors of the ice sheet most vulnerable to oceanic warming over the 21st century and by quantifying the main sources of uncertainty. [ABSTRACT FROM AUTHOR]

Thwaites Ice Shelf (TWIS), the floating extension of Thwaites Glacier, West Antarctica, is changing rapidly and may completely disintegrate in the near future. Any buttressing that the ice shelf provides to the upstream grounded Thwaites glacier will then be lost. Previously, it has been argued that this could lead to onset of dynamical instability and the rapid demise of the entire glacier. Here we provide the first systematic quantitative assessment of how strongly the upstream ice is buttressed by TWIS and how its collapse affects future projections. By modeling the stresses acting along the current grounding line, we show that they deviate insignificantly from the stresses after ice shelf collapse. Using three ice‐flow models, we furthermore model the transient evolution of Thwaites Glacier and find that a complete disintegration of the ice shelf will not substantially impact future mass loss over the next 50 years. Plain Language Summary: As ice is lost, global sea levels rise. Currently glaciers and ice sheets contribute about half of observed sea level rise (SLR). Some ice is lost through surface melting, but sea levels can also be impacted by ice that flows directly into the ocean. This loss of ice, from the grounded interior into the surrounding ocean, is particularly important for the Antarctic Ice Sheet. There the grounded sections of the ice sheet are sometimes surrounded by floating seaward extensions referred to as ice shelves. These ice shelves can restrict, that is, buttress, upstream flow. Here we estimate the buttressing provided to one of the largest and most dynamical glaciers of the Antarctic Ice Sheet, the Thwaites Glacier. The glacier is currently abutted by an ice shelf, which has become progressively smaller and weaker over the last few decades. It has been suggested that a disintegration of the ice shelf might lead to greatly enhanced ice flow. However, we find that the ice shelf does not significantly buttress upstream flow, and removing the ice shelf has little effect on SLR over the next 50 years. Key Points: Thwaites Ice Shelf provides limited buttressing to the grounded upstream Thwaites GlacierRemoval of the ice shelf changes the contribution of the West Antarctic Ice Sheet to sea level rise by at most a few millimeters over 50 yearsObservations and modeling suggest that Thwaites Glacier is currently unsteady and stable [ABSTRACT FROM AUTHOR]

The fastest projected rates of sea level rise appear in models which include "the marine ice cliff instability (MICI)," a hypothesized but mostly unobserved process defined by rapid fracture and wastage of terminal ice cliffs that outpaces viscous relaxation and ice‐shelf formation. Crane Glacier's response to the Larsen B Ice Shelf collapse has been invoked as evidence of MICI in the observational record. Using available remote sensing data, we analyze Crane's retreat, arrest, and regrowth over the last two decades. Much of Crane's terminus retreat occurred in floating, not grounded ice. Retreat accelerated by at least 54% during the 2 years following ice shelf collapse. This is inconsistent with contemporaneous regional forcing that promoted ice shelf growth during this period, but consistent with a geometrically controlled positive feedback. We infer a maximum possible cliff height of 111 m, which according to process models, could enable cliff calving assuming damaged ice. Plain Language Summary: The behavior of Antarctic glaciers will largely determine the pace and magnitude of future sea level rise. But the projections made by ice sheet models are uncertain, in part due to the uncertain response of Antarctica to the future loss of its floating ice shelves. It has been hypothesized that ice shelf breakup could trigger a self‐sustaining mechanism of ice loss whereby ice cliffs collapse under their own weight. This idea is controversial because it has not been unambiguously observed in modern glacier systems. We show that after the loss of its ice shelf, Crane Glacier experienced a 2‐year period of accelerating ice loss. While not an unambiguous validation of the instability hypothesis, what we do observe is consistent with a positive feedback and unforced retreat. Models of ice cliff failure that assume glacier ice has pre‐existing weaknesses are consistent with the behavior we observe at Crane, suggesting that if ice was sufficiently damaged at the Crane terminus, retreat by cliff failure was theoretically possible. Key Points: Retreat of Crane Glacier's terminus after the collapse of the Larsen B Ice Shelf accelerated between November 2002 and 20046.74 km of the 10.30 km retreat occurred in floating ice, with ice cliff failure possible during retreat assuming damaged iceCrane's retreat into a narrow fjord and sea‐ice growth re‐established buttressing stresses, slowed calving, and reversed terminus retreat [ABSTRACT FROM AUTHOR]