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Ice sheet models are the main tool to generate forecasts of ice sheet mass loss, a significant contributor to sea level rise; thus, knowing the likelihood of such projections is of critical societal importance. However, to capture the complete range of possible projections of mass loss, ice sheet models need efficient methods to quantify the forecast uncertainty. Uncertainties originate from the model structure, from the climate and ocean forcing used to run the model, and from model calibration. Here we quantify the latter, applying an error propagation framework to a realistic setting in West Antarctica. As in many other ice sheet modelling studies we use a control method to calibrate grid-scale flow parameters (parameters describing the basal drag and ice stiffness) with remotely sensed observations. Yet our framework augments the control method with a Hessian-based Bayesian approach that estimates the posterior covariance of the inverted parameters. This enables us to quantify the impact of the calibration uncertainty on forecasts of sea level rise contribution or volume above flotation (VAF) due to the choice of different regularization strengths (prior strengths), sliding laws, and velocity inputs. We find that by choosing different satellite ice velocity products our model leads to different estimates of VAF after 40 years. We use this difference in model output to quantify the variance that projections of VAF are expected to have after 40 years and identify prior strengths that can reproduce that variability. We demonstrate that if we use prior strengths suggested by L -curve analysis, as is typically done in ice sheet calibration studies, our uncertainty quantification is not able to reproduce that same variability. The regularization suggested by the L curves is too strong, and thus propagating the observational error through to VAF uncertainties under this choice of prior leads to errors that are smaller than those suggested by our two-member "sample" of observed velocity fields. [ABSTRACT FROM AUTHOR]

Mass loss due to dynamic changes in ice sheets is a significant contributor to sea level rise, and this contribution is expected to increase in the future. Numerical codes simulating the evolution of ice sheets can potentially quantify this future contribution. However, the uncertainty inherent in these models propagates into projections of sea level rise is and hence crucial to understand. Key variables of ice sheet models, such as basal drag or ice stiffness, are typically initialized using inversion methodologies to ensure that models match present observations. Such inversions often involve tens or hundreds of thousands of parameters, with unknown uncertainties and dependencies. The computationally intensive nature of inversions along with their high number of parameters mean traditional methods such as Monte Carlo are expensive for uncertainty quantification. Here we develop a framework to estimate the posterior uncertainty of inversions and project them onto sea level change projections over the decadal timescale. The framework treats parametric uncertainty as multivariate Gaussian and exploits the equivalence between the Hessian of the model and the inverse covariance of the parameter set. The former is computed efficiently via algorithmic differentiation, and the posterior covariance is propagated in time using a time-dependent model adjoint to produce projection error bars. This work represents an important step in quantifying the internal uncertainty of projections of ice sheet models. [ABSTRACT FROM AUTHOR]

Marine ice-cliff instability could accelerate ice loss from Antarctica, and according to some model predictions could potentially contribute >1 m of global mean sea level rise by 2100 at current emission rates. Regions with over-deepening basins >1 km in depth (e.g., the West Antarctic Ice Sheet) are particularly susceptible to this instability, as retreat could expose increasingly tall cliffs that could exceed ice stability thresholds. Here, we use a suite of high-fidelity glacier models to improve understanding of the modes through which ice cliffs can structurally fail and derive a conservative ice-cliff failure retreat rate parameterization for ice-sheet models. Our results highlight the respective roles of viscous deformation, shear-band formation, and brittle-tensile failure within marine ice-cliff instability. Calving rates increase non-linearly with cliff height, but runaway ice-cliff retreat can be inhibited by viscous flow and back force from iceberg mélange. Ice-cliff failure that leads to marine ice-cliff instability could accelerate Antarctic Ice Sheet retreat. Here, the authors use 3D glacier models to investigate ice-cliff failure, derive a retreat rate relationship, and quantify mélange back force necessary to suppress ice-cliff failure. [ABSTRACT FROM AUTHOR]

The response of tidewater glaciers to ocean warming remains a key uncertainty in sea level rise predictions. Here we use a 3‐D numerical model to examine the response of an idealized tidewater glacier to spatial variations in submarine melt rate. While melting toward the center of the terminus causes only a localized increase in mass loss, melting near the lateral margins triggers increased calving across the width of the glacier, causing the terminus to retreat at several times the width‐averaged melt rate. This occurs because melting near the margins has a greater disruptive impact on the compressive stress arch that transfers resistance from the side walls to the body of the glacier. We suggest that the rate of terminus advance or retreat may thus be governed by the difference between ice velocity and submarine melting in the slow‐flowing zones away from the glacier center. Plain Language Summary: The rapid retreat of tidewater glaciers (i.e., glaciers that drain directly into the ocean) has become an increasing source of concern in recent years. Increased melting of the submerged parts of these glaciers by warming ocean waters is thought to be an important driver of this retreat, but exactly how glaciers respond to this submarine melting remains unclear. Using a numerical model, we find that tidewater glaciers may be most sensitive to melting close to the valleysides, which reduces structural support for the central section of the glacier and so triggers an increase in iceberg calving. By better constraining the relationships between submarine melting, calving, and glacier retreat, our findings allow improved prediction of the ice loss expected from tidewater glaciers as the climate continues to warm. Key Points: We use a 3‐D numerical model to examine the response of an idealized tidewater glacier to spatial variations in submarine melt rateThe glacier is most sensitive to melting near the lateral margins, which triggers increased calving across the width of the terminusTerminus retreat may thus be paced by the difference between ice velocity and submarine melting across slow flowing marginal zones [ABSTRACT FROM AUTHOR]

Iceberg calving accounts for between 30 % and 60 % of net mass loss from the Greenland Ice Sheet, which has intensified and is now the single largest contributor to global sea level rise in the cryosphere. Changes to calving rates and the dynamics of calving glaciers represent a significant uncertainty in projections of future sea level rise. A growing body of observational evidence suggests that calving glaciers respond rapidly to regional environmental change, but predictive capacity is limited by the lack of suitable models capable of simulating calving mechanisms realistically. Here, we use a 3-D full-Stokes calving model to investigate the environmental sensitivity of Store Glacier, a large outlet glacier in West Greenland. We focus on two environmental processes: undercutting by submarine melting and buttressing by ice mélange, and our results indicate that Store Glacier is likely to be able to withstand moderate warming perturbations in which the former is increased by 50 % and the latter reduced by 50 %. However, severe perturbation with a doubling of submarine melt rates or a complete loss of ice mélange destabilises the calving front in our model runs. Furthermore, our analysis reveals that stress and fracture patterns at Store's terminus are complex and varied, primarily due to the influence of basal topography. Calving style and environmental sensitivity vary greatly, with propagation of surface crevasses significantly influencing iceberg production in the northern side, whereas basal crevasses dominate in the south. Any future retreat is likely to be initiated in the southern side by a combination of increased submarine melt rates in summer and reduced mélange strength in winter. The lateral variability, as well as the importance of rotational and bending forces at the terminus, underlines the importance of using the 3-D full-Stokes stress solution when modelling Greenland's calving glaciers. [ABSTRACT FROM AUTHOR]

The dynamic response of calving glaciers to warming climate is one of the greatest uncertainties in predictions of future sea level rise. A growing body of observational evidence suggests that the calving glaciers which drain the Greenland and Antarctic ice sheets are highly sensitive to environmental forcing, but the current generation of glacier models is unable to adequately resolve this critical interaction between ice sheets and climate. We present results from a new approach combining the continuum model Elmer/Ice and the discrete element/particle model HiDEM, applied to Store Glacier, a large calving glacier in West Greenland. We use Elmer/Ice to investigate the dynamic/stress response to both submarine melting and ice mélange buttressing of the calving front, and use HiDEM to investigate fracture propagation and calving for specific geometries. We implement a new approach to investigating ice mélange; by allowing the glacier to calve repeatedly into the fjord, we effectively 'build' a mélange from previously calved icebergs.Our results indicate that lateral support from valley walls plays a critical role in the stability of the calving front; a persistent compressive stress arch exists just behind the current terminus position, transmitting resistive stress from sidewalls to centreline. Furthermore, we find that minimal mélange buttressing applied near the lateral margins is sufficient to advance this compressive arch, allowing the terminus to advance in winter. With respect to submarine melting, we find that a modest melt undercut (60m) can promote calving at an otherwise stable terminus position. Our HiDEM simulations for Store Glacier closely match the calving style observed in satellite imagery; the ability to accurately resolve calving at the event scale is a major step forward in our understanding of calving processes. By comparing stress patterns in Elmer/Ice with fracture patterns in HiDEM, we seek to formulate improved calving laws which can be implemented in ice-sheet scale continuum models. [ABSTRACT FROM AUTHOR]

The structure of subglacial hydrological drainage systems beneath large Greenlandic tidewater glaciers influences ice velocity due to its effect on basal traction, and it may also influence calving by its control on melting at the ice-ocean interface. Understanding these systems is critical to being able to accurately predict the evolution of the Greenland Ice Sheet and the resulting sea-level rise, as the fifteen largest Greenland outlet glaciers are responsible for 77% of the additional mass loss from the ice sheet due to acceleration since 2000. In this study, we use numerical modelling and observations of Store Glacier in West Greenland to constrain the form of the subglacial drainage system and melt rates from the resulting plumes, both of which are poorly known.We investigate the evolution of Store Glacier's subglacial hydrology using the open-source, full-Stokes model Elmer/Ice in a novel 3D application, the GlaDS hydrological module, which includes a distributed sheet and the ability to form concentrated channels when the sheet locally reaches sufficient thickness to initiate melting in cavities. At first, we produce a baseline winter scenario in which the hydrological system contains only basally derived meltwater produced primarily from friction at the bed. We then investigate the hydrological system during summer, focussing specifically on 2012 and 2017, which provide examples of high and low inputs of surface meltwater, respectively. When the model is forced with constant average runoff from summer (Jun-Aug) 2012, outputs show a hydrological system with significant sheet activity extending 55km inland and channels with a cross-sectional area higher than 1 m2 forming up to 45 km from the margin. However, we also find the hydrological system to be active in winter, when significant sheet flux is evident up to 35 km inland, and channels form as far as 25 km inland. The discharge of meltwater into the fjord produces convective plumes throughout the year, and these drive average submarine ice-front melt rates of up to 0.15 m d-1 in summer and 0.12 m d-1 in winter, rising to 1.6 m d-1 and 1.07 m d-1, respectively, for average temporal maximum melt rates. When the model is forced with daily time-series of summer runoff, we find a higher intensity of plume melting, reaching 0.16 m d-1 on average (rising to 2.21 m d-1 for the average temporal maximum). Our study shows that plume-induced ice-front melting is substantial even in winter, and we hypothesise that the high spatial variability and intensity during summer may drive calving through plume-induced notches cutting into the ice front as well as by undercutting of the terminus as a whole. [ABSTRACT FROM AUTHOR]

Marine Ice Cliff Instability (MICI) has been proposed as a mechanism through which marine-based ice sheets could undergo rapid collapse and mass loss. MICI is based on the idea that the finite strength of ice places a limit on ice cliff height above sea level (freeboard); therefore, if buttressing ice shelves disintegrate and expose deep calving cliffs, widespread catastrophic ice-sheet disintegration could occur. Ice sheet models incorporating MICI processes predict rapid disintegration of the Amundsen Sea sector of the West Antarctic Ice Sheet under high CO2 scenarios (Pollard et al., 2015; DeConto and Pollard, 2016). These models, however, employ highly simplified parameterizations of key calving processes, and possible rates of future ice loss and sea level rise remain subject to large uncertainties. MICI invokes ice-front behaviors beyond the current observed range, complicating attempts to validate model results. DOMINOS (Disintegration of Marine Ice Sheets: Novel Optimized Simulations; part of the International Thwaites Glacier Collaboration) aims to develop detailed process-based models of MICI and its impact on the sensitivity of the West Antarctic Ice Sheet. Here we present preliminary results from the Helsinki Discrete Element Model (HiDEM), which aim to understand the mechanics of ice-cliff instability and determine the spatial and temporal scales of failure events. Collapse of deep ice cliffs (>90 m freeboard) occurs in response to temporally varying longitudinal and buoyant forces, and involves complex mixed-mode rheology including brittle failure, viscous deformation, and enhanced viscous flow along shear zones. Calved bergs form floating tongues of mélange, which may provide stabilizing backstress if jamming occurs against lateral or basal pinning points. In the absence of backstress, mechanical instability propagates upglacier, causing rapid, unstable retreat where basins deepen inland. [ABSTRACT FROM AUTHOR]