The central estimate of the Intergovernmental Panel on Climate Change is that the magnitude of anthropogenic warming since 1850 is equal to 100% of the observed warming. However, the IPCC is notably much more timid in attributing glacier mass loss to anthropogenic warming over the same period. Disagreements have arisen in previous research, primarily stemming from ambiguity in the dynamic disequilibrium of preindustrial glaciers and its lingering effects. Accounting for variability in glacier disequilibrium entering the industrial era, Roe et al., (2021) used simple glacier models and synthetic climate scenarios to estimate  a mass-loss attribution of ˜100% [90-130%, likely range] over the full industrial era. Our work further assesses this claim for a case study of glaciers in North America and the Alps using: i) realistic ice dynamics, ii) observed glacier geometries, iii) ensembles of last-millennium reconstructions (LMR) and GCM simulations, and iv) a comprehensive sensitivity and uncertainty analysis. In addition to CMIP6 past1000 simulations, we use recently developed LMR paleoclimate reconstructions, specifically adapted for melt-season temperatures. By using millennial-scale climate time series, we avoid the need for an accurate initial condition. We simulate glacier mass-balance and length fluctuations over the last millennium for a variety of potential climate histories to produce an uncertainty envelope for each glacier’s preindustrial state. For our case-study of glaciers, we find that all: i) exhibited slow growth over the last millennium, ii) have lost mass over the industrial era, and that iii) the magnitude of industrial-era mass loss for each glacier greatly exceeds natural variability over the last millennium. Given that 100% of industrial-era temperature change is attributable to anthropogenic activity, these results imply that mass loss for these glaciers can be confidently attributed to anthropogenic warming since the beginning of the industrial era (1850 vs. the IPCC’s 1990). Work is ongoing to expand the analysis scope to a larger network of well-observed glaciers, with potential for a global assessment in the future.

How to cite: Otto, D., Roe, G., and Christian, J.: Assessing the attribution of alpine glacier mass loss to anthropogenic warming over the last millennium using ensemble paleoclimate reconstructions and GCM simulations, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-10630, https://doi.org/10.5194/egusphere-egu23-10630, 2023.

Many marine-terminating outlet glaciers have retreated rapidly in recent decades, but these changes have not been formally attributed to anthropogenic climate change. A key challenge for such an attribution assessment is that if glacier termini are sufficiently perturbed from bathymetric highs, ice-dynamic feedbacks can cause rapid retreat even without further climate forcing. In the presence of internal climate variability, attribution thus depends on understanding whether (or how frequently) these rapid retreats could be triggered by climatic noise alone.

We present simulations with idealized glaciers to analyze glacier variability in the presence of topographic thresholds, and to demonstrate a framework for attribution. We find that when termini are positioned near bed peaks in a noisy climate, rapid retreat is a stochastic phenomenon. We therefore assess the likelihood of rapid retreat, using ensembles of many simulations with independent realizations of random climate variability. Synthetic experiments show that century-scale climate trends substantially increase the likelihood of retreat. The strength of this effect is related to the timescales over which ice dynamics integrate forcing, implying that the time of onset of anthropogenic forcing is a key factor to constrain for attribution studies. We close by discussing broader considerations for framing attribution studies on marine-terminating glacier retreat, and ongoing work towards applying this framework to glaciers in Greenland.

How to cite: Christian, J. E., Robel, A., Catania, G., Verjans, V., and Rashed, Z.: A probabilistic framework for quantifying the role of anthropogenic climateforcing in marine-terminating glacier retreats, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-11044, https://doi.org/10.5194/egusphere-egu23-11044, 2023.

Mass loss from the West Antarctic Ice Sheet, driven by interactions with the ocean causing melting of ice shelves, is currently Antarctica’s largest contribution to sea level rise. It is not well known how ice shelf melting may evolve in the future, and to what degree this response can be tempered by climate change mitigation. Here we present the most comprehensive future projections of the Amundsen Sea region to date: nearly 4000 years of ice-ocean simulations considering different fossil fuel scenarios and pathways of internal climate variability. All scenarios exhibit significant and widespread future warming of the Amundsen Sea ocean and increased melting of its ice shelves. Even under the most ambitious scenario, where global warming is limited to 1.5°C, the Amundsen Sea warms three times faster than in the historical period. The warming is driven by an increase in onshore transport of warm Circumpolar Deep Water, causing the present-day oscillations between warm and cold periods to converge towards a state of permanent warmth. Until the 2070s, all scenarios are statistically indistinguishable in terms of Amundsen Sea warming; after that, it is only the extreme RCP 8.5 scenario which diverges from the others. Furthermore, the additional ice shelf melting in RCP 8.5 is concentrated in regions which are less glaciologically important for sea level rise. These results suggest that climate mitigation has limited power to prevent West Antarctic ice loss, and that a substantial baseline of future sea level rise is already committed. 

How to cite: Naughten, K., Holland, P., and De Rydt, J.: Substantial future ice shelf melting projected in West Antarctica regardless of fossil fuel scenario, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-2651, https://doi.org/10.5194/egusphere-egu23-2651, 2023.

The seasonal cycle of the Antarctic sea ice extent is largely controlled by the evolution of the insolation received at the top of the atmosphere. However, sea ice processes and feedbacks with the ocean and the atmosphere can modulate this seasonal cycle. Here, the atmospheric feedbacks are quantified in a series of idealized sensitivity experiments performed with an eddy-permitting (1/4°) NEMO-LIM3 Southern Ocean configuration, including a representation of ice shelf cavities, in which the model was either driven by an atmospheric reanalysis or coupled to the COSMO-CLM² regional atmospheric model. In these experiments, sea ice thermodynamics and dynamics as well as the exchanges with the ocean and atmosphere are strongly perturbed. This perturbation is achieved by modifying snow and ice thermal conductivities, the vertical mixing in the ocean top layers, the effect of freshwater uptake/release upon sea ice growth/melt, ice dynamics and surface albedo. We show that the changes in surface heat fluxes are very different between the configurations driven by the reanalysis and those coupled to the atmosphere. Atmospheric feedbacks enhance the response of the modelled winter ice extent to any of the perturbed processes, and the enhancement is strongest when the albedo is modified. The response of sea ice volume and extent to changes in entrainment of subsurface warm waters to the ocean surface is also greatly amplified by the coupling with the atmosphere. By contrast, the atmospheric feedbacks can damp the impact of the perturbations affecting the heat conductivity through sea ice.

How to cite: Goosse, H., Allende Contador, S., Bitz, C. M., Blanchard-Wrigglesworth, E., Eayrs, C., Fichefet, T., Himmich, K., Huot, P.-V., Klein, F., Marchi, S., Massonnet, F., Mezzina, B., Pelletier, C., Roach, L., Vancoppenolle, M., and van Lipzig, N. P. M.: Importance of atmospheric feedbacks in simulating the seasonal cycle of the Antarctic sea ice and its response to perturbations., EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-1531, https://doi.org/10.5194/egusphere-egu23-1531, 2023.

The parameterization of the heat conduction through sea ice and snow remains simple in state-of-the-art models. Specifically, it relies on prescribed conductivity parameters constant in time and space, therefore neglecting the substantial heterogeneity of these mediums down to the unresolved subgrid scale. This assumption clashes with robust observational evidence, which indicates that snow and ice conductivities can vary greatly depending on the environmental conditions and the history of the sea ice. The winter observations collected during the MOSAiC expedition are unique tools for advancing the quantitative understanding of heat conduction in sea ice and improving the realism of the thermodynamic parameterizations in models. Our investigation utilizes gridded helicopter-borne thermal infrared imaging, laser scanner (ALS) elevation observations, and meteorological measurements to assess the model bias and diagnose the importance of unresolved processes and topographic heterogeneity on heat conduction. We evidence different heat conduction regimes depending on the ice thickness, type (i.e., ridged or level ice), and snow patchiness. In light of these results, we will discuss strategies for an effective parametrization of these unresolved processes in sea ice models, and their harmonization with the preexisting model infrastructure. Furthermore, I will comment on the potential of emerging data-driven analysis techniques and machine learning in facilitating the formulation of parameterization at different stages of the development process.

How to cite: Zampieri, L., Hutter, N., and Holland, M.: Improving the representation of the sea ice and snow heat conduction in models through the lens of the MOSAiC dataset, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-8813, https://doi.org/10.5194/egusphere-egu23-8813, 2023.

We present a new brittle rheology and an accompanying numerical framework for large-scale sea-ice modelling. We have based this rheology on a Bingham-Maxwell constitutive model and the Maxwell-Elasto-Brittle (MEB) rheology for sea ice. The key strength of the MEB rheology is its ability to represent the scaling properties of simulated sea-ice deformation in space and time. The new rheology we propose here, which we refer to as the brittle Bingham-Maxwell rheology (BBM), represents a further evolution of the MEB rheology. We developed BBM to address two main shortcomings of the MEB rheology and numerical implementation: excessive thickening of the ice in model runs longer than about one winter and a relatively high computational cost. The BBM addresses these shortcomings by demanding that the ice deforms under convergence in a purely elastic manner when internal stresses lie below a given compressive threshold. It also improves numerical performance by introducing an explicit scheme to solve the ice momentum equation. We show that using an implementation of BBM in the neXtSIM sea-ice model, the model gives reasonable long-term evolution of the Arctic sea-ice cover. It also gives very good deformation fields and statistics compared to satellite observations.

How to cite: Ólason, E., Boutin, G., Korosov, A., Rampal, P., Williams, T., Kimmritz, M., Dansereau, V., and Samaké, A.: A new brittle rheology and numerical framework for large-scale sea-ice models, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-11613, https://doi.org/10.5194/egusphere-egu23-11613, 2023.

Sea ice plays a key role setting up a climate state of the Polar Oceans through moderating interactions between the ocean and atmosphere. As it is seen from satellite data, on the synoptic and sub-seasonal time-scales sea ice partly moves as a solid body – large areas of sea ice cover drift as single polygons – and partly deforms as a plastic material, shearing along the deformation lines – linear kinematic features (leads). Leads are important for the heat fluxes and also for navigational safety.

In this study we focus on winter sea ice. Currently sea ice is thinning and more deformable; thinner ice is easier to crack. We compare the effect of different rheologies on sea ice and have developed a very high resolution (1 km) Arctic model, which allows for examining lead formation. The model shows a step change in behaviour compared to the previous high-resolution configuration (3 km). Specifically, we compare the EVP and EAP sea ice rheologies; these show substantial differences in the number and orientation of leads. EAP produces diamond patterns which has so far been difficult to create in models. The stand-alone sea ice model simulations will be coupled to ocean to examine eddy interaction.

We acknowledge funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement No 821926 (IMMERSE project) and from the LTS-S CLASS Programme (grant NE/R015953/1). The work reflects only the authors’ view; the European Commission and their executive agency are not responsible for any use that may be made of the information the work contains. This work also used the ARCHER-II UK National Supercomputing Service and JASMIN, the UK collaborative data analysis facility.

How to cite: Rynders, S., Aksenov, Y., and Coward, A. C.: Arctic sea ice dynamics at 1km resolution with SI3, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-15574, https://doi.org/10.5194/egusphere-egu23-15574, 2023.

Linear Kinematic Features (LKFs) are found everywhere in the Arctic sea-ice cover. They are strongly localized deformations often associated with the formation of leads and pressure ridges. Viscous-plastic sea-ice models start to produce LKFs at high spatial grid resolution, typically with a grid spacing below 5km.  Besides grid spacing, other aspects of a numerical implementation, such as discretization details, may affect the number and definition of simulated LKFs. To explore these effects, simulations with different sea-ice models such as MPAS, CICE, ICON, FESOM and MITgcm are compared in an idealized configuration.

We found that the nonconforming finite-element  CD-grid discretization produces more LKFs than the CD-grid approximation based on a sub-grid discretization. Furthermore the nonconforming finite-element approach simulates the same number of LKFs as conventional Arakawa A-grid, B-grid, and C- grid methods, but on grids with less degrees of freedom ( a  coarser mesh). This is due to the fact that CD-grid approaches have a higher number of degrees of freedom to discretize the velocity field. Due to its enhanced resolving properties, CD-grid methods are an attractive alternative to conventional discretizations. 

How to cite: Mehlmann, C., Capodaglio, G., and Danilov, S.: Simulating deformation structure in viscous-plastic sea-ice models with CD-grid approaches, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-11737, https://doi.org/10.5194/egusphere-egu23-11737, 2023.

Earth system models (ESM) strive to describe more and more details. This is often accomplished with the use of more complex descriptions or higher resolution. The limitation of this approach is often the computer system at hand. In many cases, ESM’s are written with a focus on the physical system development and less on how to structure the code according to infrastructure on the computer. This presentation focuses on the sea ice dynamics, and particularly on the solver for the Elastic-Viscous-Plastic (EVP) equations. The EVP approach introduces artificial elastic waves that are iteratively dampened. Hundreds of iterations are necessary to reach a solution. In the traditional implementation, each iteration requires communication between the processors using MPI calls.

An analysis of the existing solver’s performance was first carried out based on the sea ice model CICE. Three performance challenges were identified with the current implementation: Two challenges relate to the parallelization itself, namely 1) General imbalance issues due to the nature of the challenge and 2) MPI synchronization after each sub-cycling. The third issue relates to the data structures chosen and their corresponding memory access patterns. This study aim at removing all three limiting factors by adjusting the memory access patterns and by adjusting the parallelization approach so that we can avoid the costly MPI synchronization after each sub-cycle, which enables the use of parallel instructions, which are available in modern hardware. The adjusted implementation runs significantly faster.

EVP is just one component out of many others in sea ice/ESM model (and other modelling systems). The refactoring includes a novel integration that shows how the EVP solver can be integrated into various model systems via the MPMD pattern and hence also runs on heterogeneous systems.

How to cite: Rasmussen, T. A. S., Poulsen, J. W., Ribergaard, M. H., Rethmeier, S., Hunke, E. C., and Craig, A. P.: Refactorization of the EVP solver, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-15209, https://doi.org/10.5194/egusphere-egu23-15209, 2023.

Currently climate models represent sea ice by a continuums approach. The continuums assumption implies that statistical averages can be taken over a large number of floes.  But the application of continuum rheological models at or below the scale of individual floes is only appropriate if the mode of failure of a single floe is the same as the mode of failure of an aggregate of floes. Continuum models have been developed for a grid resolution of 100 km. In last years computing power has increased and sea-ice models are often run at high mesh resolutions where a grid cell may no longer contain a representative sample of sea-ice floes.

We are addressing these shortcomings of current continuum sea-ice models by developing a hybrid model. The idea of the hybrid approach is to nest a Discrete Element Model into a continuum sea-ice model in order to predict sea ice on fine spatial scales in a region of interest. To facilitate the coupling between the continuum and Discrete Element Model a Particle-in-Cell scheme is used which ensures mass conservation in the hybrid approach. We analyze the coupling of the continuums and particle mechanics and discuss the advantages and disadvantages of this approach.

How to cite: Kahl, S. and Mehlmann, C.: On the development of a hybrid sea-ice model by combining particle and continuums mechanics, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-7098, https://doi.org/10.5194/egusphere-egu23-7098, 2023.