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Abstract The meltwater-driven disintegration of the Larsen B ice shelf has raised concerns that other ice shelves may face similar vulnerabilities as global temperatures rise. Climate projections show increased ice shelf vulnerability to surface melt in the coming century, yet the ability of large-scale climate models to simulate temperatures over ice shelves—a key factor in these projections—has rarely been assessed. We address this gap by using ERA5 reanalysis data to evaluate 31 CMIP6 models’ performance in simulating near-surface air temperatures over 46 Antarctic ice shelves from 1979 to 2014. We find that CMIP6 models exhibit annual and summer warm biases over most ice shelves. There is also inter-model variability of up to 13°C between model temperatures over the Amery and Riiser-Larsen shelves for both annual and summer periods. Significant regional differences are present: shelves in the Amundsen Sea Embayment show cold biases, while those in the Weddell Sea show warm biases. While topography corrections can reduce some biases, we find notable seasonal differences, including biases with opposite signs between annual and summer means. Our results underscore the importance of careful model selection by shelf and region to improve the reliability of future climate projections and assessments of Antarctic ice shelf vulnerability. 

Abstract Calving from tidewater glaciers and ice shelves is an important component of global mass balance and may contribute significantly to future sea-level rise. Current prognostic ice-sheet models cannot predict future calving losses because they lack a robust calving law. We argue that the key to finding a general calving law is to recognise that calving glaciers are stochastic dynamic systems that exhibit self-organisation. Collectively, calving events have statistical properties that reflect underlying fragmentation processes. These reflect distinct styles of calving and give rise to persistent patterns of advance and retreat, including fluctuations around pinning points and periods of instability and transition. These patterns motivate a stochastic calving function scaled to the stress within the ice, which we demonstrate in a set of model experiments with Elmer/Ice, for synthetic geometries representative of a Greenland outlet glacier and an Antarctic ice shelf. Self-organising behaviour emerges spontaneously from the model, including expected calving-size distributions and system convergence on quasi-stable states. The model simulates calving behaviour over a wide range of spatial and temporal scales and produces short calving cycles for a Greenland-type geometry and long cycles for an Antarctic shelf-type geometry. The long-standing calving law problem may yield to this kind of approach. 

Abstract Our ability to accurately quantify the total ice volume in glaciers and the loss of glacier volume, discharge and freshwater in response to climate change is limited by a paucity of ice thickness and bed topography observations. Consequently, glacial ice thickness is often inferred indirectly from more easily obtained surface measurements. Here, we present a simple inversion building on the assumption of perfect plasticity. In the traditional perfect-plastic approximation, the ice thickness (or bed) can be inferred from the surface elevation and yield strength. Here, we extend this to demonstrate that, provided glaciers are changing, we can simultaneously determine the yield strength and bed topography from observations of surface elevation alone. We demonstrate that the ice thicknesses and bed topographies we infer perform comparably to other inversions documented in the Ice Thickness Models Intercomparison eXperiment. Unlike other inversions, we do not require surface mass balance or glacier velocities, which can be inaccurate and difficult to obtain. Given the increasing availability of high-resolution surface elevation data, it may be possible to apply this method to glaciers worldwide to better constrain the ice thickness, bed topography and volume of glaciers globally. 

The collapse of ice shelves could expose tall ice cliffs at ice sheet margins. The marine ice cliff instability (MICI) is a hypothesis that predicts that, if these cliffs are tall enough, ice may fail structurally leading to self-sustained retreat. To date, projections that include MICI have been performed with a single model based on a simple parameterization. Here, we implement a physically motivated parameterization in three ice sheet models and simulate the response of the Amundsen Sea Embayment after a hypothetical collapse of floating ice. All models show that Thwaites Glacier would not retreat further in the 21st century. In another set of simulations, we force the grounding line to retreat into Thwaites’ deeper basin to expose a taller cliff. In these simulations, rapid thinning and velocity increase reduce the calving rate, stabilizing the cliff. These experiments show that Thwaites may be less vulnerable to MICI than previously thought, and model projections that include this process should be re-evaluated. 

Abstract The hypothesis that ice-sheet evolution is only controlled by the long-term non-Newtonian viscous behavior of ice has been challenged by observations indicating that effects like brittle failure, stick-slip sliding, tides and wave action may affect ice-sheet evolution on sub-daily timescales. Over these timescales, the quasi-static-creep approximation is no longer appropriate and elastic effects become important. Simulating elastic effects in ice-sheet models over relevant timescales, however, remains challenging. Here, we show that by including a visco-elastic rheology and reintroducing the oft neglected acceleration term back into the ice-sheet stress balance, we can create a visco-elastic system where the velocity is locally determined and information propagates at the elastic wave speed. Crucially, the elastic wave speed can be treated like an adjustable parameter and set to any value to reproduce a range of phenomena, provided the wave speed is large compared to the viscous velocity. We illustrate the system using three examples. The first two examples demonstrate that the system converges to the steady-state viscous and elastic limits. The third example examines ice-shelf rifting and iceberg calving. This final example hints at the utility of the visco-elastic formulation in treating both long-term evolution and short-term environmental effects. 

The largest uncertainty in future sea-level rise is loss of ice from the Greenland and Antarctic Ice Sheets. Ice shelves, freely floating platforms of ice that fringe the ice sheets, play a crucial role in restraining discharge of grounded ice into the ocean through buttressing. However, since the 1990s, several ice shelves have thinned, retreated, and collapsed. If this pattern continues, it could expose thick cliffs that become structurally unstable and collapse in a process called marine ice cliff instability (MICI). However, the feedbacks between calving, retreat, and other forcings are not well understood. Here we review observed modes of calving from ice shelves and marine-terminating glaciers, and their relation to environmental forces. We show that the primary driver of calving is long-term internal glaciological stress, but as ice shelves thin they may become more vulnerable to environmental forcing. This vulnerability—and the potential for MICI—comes from a combination of the distribution of preexisting flaws within the ice and regions where the stress is large enough to initiate fracture. Although significant progress has been made modeling these processes, theories must now be tested against a wide range of environmental and glaciological conditions in both modern and paleo conditions. ▪ Ice shelves, floating platforms of ice fed by ice sheets, shed mass in a near-instantaneous fashion through iceberg calving. ▪ Most ice shelves exhibit a stable cycle of calving front advance and retreat that is insensitive to small changes in environmental conditions. ▪ Some ice shelves have retreated or collapsed completely, and in the future this could expose thick cliffs that could become structurally unstable called ice cliff instability. ▪ The potential for ice shelf and ice cliff instability is controlled by the presence and evolution of flaws or fractures within the ice. 

Abstract Iceberg calving, the process where icebergs detach from glaciers, remains poorly understood. Moreover, few parameterizations of the calving process can easily be integrated into numerical models to accurately capture observations, resulting in large uncertainties in projected sea level rise. Recent efforts have focused on estimating crevasse depths assuming tensile failure occurs when crevasses fully penetrate the glacier thickness. However, these approaches often ignore the role of advecting crevasses on calculations of crevasse depth. Here, we examine a more general crevasse depth calving model that includes crevasse advection. We apply this model to idealized prograde and retrograde bed geometries as well as a prograde geometry with a sill. Neglecting crevasse advection results in steady glacier advance and ice tongue formation for all ice temperatures, sliding law coefficients and constant slope bed geometries considered. In contrast, crevasse advection suppresses ice tongue formation and increases calving rates, leading to glacier retreat. Furthermore, crevasse advection allows a grounded calving front to stabilize on top of sills. These results suggest that crevasse advection can radically alter calving rates and hint that future parameterizations of fracture and failure need to more carefully consider the lifecycle of crevasses and the role this plays in the calving process. 

Inaccurate representations of iceberg calving from ice shelves are a large source of uncertainty in mass-loss projections from the Antarctic ice sheet. Here, we address this limitation by implementing and testing a continuum damage-mechanics model in a continental scale ice-sheet model. The damage-mechanics formulation, based on a linear stability analysis and subsequent long-wavelength approximation of crevasses that evolve in a viscous medium, links damage evolution to climate forcing and the large-scale stresses within an ice shelf. We incorporate this model into the BISICLES ice-sheet model and test it by applying it to idealized (1) ice tongues, for which we present analytical solutions and (2) buttressed ice-shelf geometries. Our simulations show that the model reproduces the large disparity in lengths of ice shelves with geometries and melt rates broadly similar to those of four Antarctic ice shelves: Erebus Glacier Tongue (length ~ 13 km), the unembayed portion of Drygalski Ice Tongue (~ 65 km), the Amery Ice Shelf (~ 350 km) and the Ross Ice Shelf (~ 500 km). These results demonstrate that our simple continuum model holds promise for constraining realistic ice-shelf extents in large-scale ice-sheet models in a computationally tractable manner.