The basal friction parameterization is often mentioned as key source of uncertainty when using ice sheet models to project future evolution of the ice sheet. Previous work suggests that parameterizations with an exponential relationship between friction and basal velocity (power laws) predict lower sea level rise than ‘Coulomb-style’ friction laws. For Coulomb laws, the basal friction asymptotes for high velocities and is effectively independent of velocity for fast-flowing ice. We use the Community Ice Sheet Model (CISM) for two kinds of simulations: one with present-day climate forcing, including sub-shelf ocean temperatures kept constant and one with 1-degree ocean warming in the Ross Sea, both matching the current rate of ice thickness changes. In the constant scenario, Thwaites Glacier and Pine Island Glacier collapse, creating a huge, laterally bounded ice shelf. In the scenario with Ross Sea warming, a large part of the Ross Ice Shelf disappears, allowing Siple Coast glaciers to flow freely into the ocean. For Thwaites and Pine Island Glaciers, there are competing processes causing increases or decreases in ice flux across the grounding line, ice shelf thickness, buttressing, and ice velocities, once the glaciers are collapsing. These processes work in the opposite direction of the differences caused by choosing different basal friction parameterizations. Therefore, in our model runs, the choice of basal friction parameterization has little effect on the collapse of Thwaites and Pine Island glacier. In the unbuttressed Siple Coast case, we confirm earlier results: Coulomb friction leads to more ice mass loss and sea level rise. We conclude that unbuttressed glaciers are more sensitive to the choice of basal friction parameterizations than are heavily buttressed glaciers, and that the presence of a large buttressing ice shelf decreases the sensitivity of glacier dynamics to different basal friction parameterizations. 

How to cite: van den Akker, T., Lipscomb, W. H., Leguy, G. R., van de Berg, W. J., and van de Wal, R. S. W.: Modelled ice sheet sensitivity to basal friction parameterizations is controlled by the amount of buttressing, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-15275, https://doi.org/10.5194/egusphere-egu24-15275, 2024.

Subglaical hydrology significantly influences the basal sliding that controls how fast ice sheets transport ice from land to oceans. The absence of hydrologic systems in ice sheet models is therefore a notable source of uncertainty in projected ice-mass loss and its subsequent impact on sea-level rise. Specifically, the uncertainty associated with the representation of effective pressure (the difference between subglacial water pressure and ice overburden pressure) in basal sliding lacks comprehensive investigation in Antarctic sea-level rise projections. Here we use Elmer/Ice ice-sheet model setups to examine how different approaches to determining effective pressure in the regularised Coulomb sliding law impact the projected ice mass loss pre-2300 under both continental and basin scales. Our results reveal basin-specific responses to the representation of effective pressure in basal sliding, significantly influencing projected ice-mass loss and the timing of the passing of tipping points. We find that the ongoing interactions between ice dynamics and the hydrologic system render the grounding line much more mobile than in models with no such interaction. Notably, for the entire Antarctic Ice Sheet, grounding line flux is more than doubled by 2300 when employing a smoothly decreasing effective pressure near the grounding line, compared to constant pressure. Remarkably, Thwaites Glacier shows a tenfold increase in its grounding line flux by 2300. These findings underscore the critical need to better understand the interactions between ice dynamic evolution and the subglacial hydrologic system. Explicitly modelling the hydrologic system in a coupled ice sheet-subglacial hydrology models is crucial to make more robust predictions of Antarctica's future ice-mass loss, thereby reducing uncertainty in sea-level rise projections. 

How to cite: Zhao, C., Gladstone, R., Zwinger, T., Gillet-Chaulet, F., Wang, Y., Caillet, J., Mathiot, P., Saraste, L., Galton-Fenzi, B., Christoffersen, P., and King, M.: Subglacial Water Pressure Reshapes projected Antarctic Sea-Level Rise, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-14499, https://doi.org/10.5194/egusphere-egu24-14499, 2024.

The force balance of an ice steam determines its velocity, and along with cross sectional area, influences total flux of glacial ice to the ocean. Satellite datasets provide us strong time series velocity data, while radar, seismics, and geophysical inversion methods yield width and depth estimates within an ice column. However, should an ice stream widen or narrow, the flux of ice to the ocean could change perceptibly. 
Thwaites Glacier’s eastern shear margin is not topographically bounded, but rather arcs across a shallow subglacial rise of intermittent hills and valleys. To determine the stability of the margin it is imperative to understand the force balance and total energy of the glacier system along the shear zone. Driving forces are balanced by normal and shear stresses along the bed, and longitudinal shearing in the margin. The resistance (or lack thereof) creates deformational (frictional-sliding) heating which add energy to the system altering rate factor and meltwater generation, which in turn alter the viscosity and slip factors, making a complex feedback system.
To analyze this system, we develop a 3D full-Stokes flow model in the Elmer/ICE finite element software. The model resolves at 500 m horizontally over realistic surface and bed topography from BedMachinev3. Model domains cover key data acquisition sites from the International Thwaites Glacier Collaboration, Thwaites Interdisciplinary shear Margin Evolution (ITGC TIME) seismic and radar field studies. To determine energy balance, we employ the enthalpy field equations which efficiently solve the thermal field, solve for water content generation, and couple nicely with variable rate factors. Models are initialized by a suite of 1D thermal profiles, converted to enthalpy values, derived from quartile statistics of RACMO2.3p1 surface specific mass balance data, and then advected to steady state flow. 
These models are then run with a flow-coupled glacier drainage system (GlaDS) and enthalpy relation to determine temperate ice distribution, and basal water production. Non-linear Coulomb and Weertman sliding laws are tested between scenarios of natural melt generation, through full drainage piracy of the upper Pine Island catchment, which will determine the effect of spatiotemporal variation in effective pressure (N) on shear margin stability.
Through the suite of spin up models and scenarios, we aim to determine the stability of Thwaites Glacier’s eastern shear margin from perturbations in enthalpy from both sliding friction, and viscous heating from temperate ice generation over non-idealized bed topography and within the shear margin itself. Results will help inform catchment scale transient flow models which aid in determining sea level contribution and West Antarctic Ice Sheet stability.  

How to cite: Martin, C. and Hehlen, M.: An Enthalpy-Hydrology Coupled 3D full-Stokes Flow Model of Thwaites’ Eastern Shear Margin, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-11319, https://doi.org/10.5194/egusphere-egu24-11319, 2024.

The drainage catchments of the neighbouring Thwaites and Pine Island Glaciers, situated in the Amundson Sea Embayment, West Antarctica, contain sufficient ice to raise global mean sea level by a meter. In recent years, significant scientific attention has been given to the potential for sustained retreat of these glaciers as a possible pathway towards widespread deglaciation of the marine-based West Antarctic Ice Sheet. Some recent studies have demonstrated that the Thwaites Ice Shelf provides only limited buttressing to the upstream grounded ice, suggesting limited sensitivity to the melt-driven loss of the ice shelf.

We use the BISICLES ice sheet model to perform a series of 1000-year model experiments on the Amundson Sea domain. A synthetic sub-shelf melt rate is selectively applied to the Pine Island, Thwaites and Crosson/Dotson basins or combinations of those basins. We find that over millennial timescales, Thwaites is relatively insensitive to melt-driven thinning of its ice shelf, with its grounding line rapidly restabilising ~60km upstream of its current location. The same melt forcing applied to Pine Island Glacier leads to widespread deglaciation of the Pine Island catchment and significant retreat in the neighbouring Thwaites catchment despite the lack of melting there. Applying melting simultaneously in the Thwaites and Crosson/Dotson basins leads to widespread deglaciation that is much greater than the sum of its parts. This retreat is driven by a feedback between the ice fluxes crossing the basin boundary.

We also conduct further experiments to determine the melt rates or additional processes required to trigger retreat of Thwaites Glacier in isolation. The results of our experiments support the suggestion that the Thwaites ice shelf provides limited buttressing, while also demonstrating that Thwaites is still vulnerable to retreat via other pathways.

How to cite: Trevers, M., Payne, A., Cornford, S., and Gasson, E.: Retreat of Thwaites Glacier, West Antarctica, triggered by its neighbours., EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-3996, https://doi.org/10.5194/egusphere-egu24-3996, 2024.

In recent decades glaciers in the Amundsen Sea Embayment in West Antarctica have undergone substantial changes, including widespread retreat and acceleration. The subsequent mass loss caused the largest contribution to sea level rise from the entire Antarctic Ice Sheet. These changes have led to concerns about the stability of the region and the implications of future climate change. Recent modelling results show that one of the largest and fastest flowing of these glaciers, Pine Island Glacier (PIG), has already undergone an unstable and irreversible retreat in its recent history, when it detached from a subglacial ridge between the 1940s and 1970s.

Here we use the ice-flow model Úa to study the sensitivity of this retreat to changes in basal melting. We show that an intermittent increase in basal melting would have been sufficient to force PIG into a retreat from its stable position on the ridge. Once high melting begins upstream of the ridge, only near-zero melt rates can stop the retreat. Our results suggest that unstable and irreversible responses to warm anomalies are possible, and this can lead to substantial changes in ice flux over relatively short periods of only a few decades.

How to cite: Reed, B., Green, M., Jenkins, A., and Gudmundsson, H.: Melt sensitivity of irreversible retreat of Pine Island Glacier, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-4001, https://doi.org/10.5194/egusphere-egu24-4001, 2024.

The Antarctic Ice Sheet (AIS) is the largest ice sheet and hence the potentially largest contributor to future sea-level rise. However, the AIS represents also the largest source of uncertainty regarding future projections. One of the main sources for this uncertainty are the floating ice shelves. While these ice shelves do not directly contribute to sea-level rise, they play a major role in the dynamics of the AIS. By transmitting resistive stress to the grounding line, they are capable of slowing down inland ice. If ice shelves disintegrate, this buttressing effect disappears, promoting the flow of inland ice into the ocean. Thus, the assessment of ice shelf stability in future scenarios becomes crucial for accurate predictions.

One key element which is often overlooked is the formation of ice fractures. Satellite images display increased crevasse formation on Antarctic ice shelves and grounded ice near the grounding line over the past decade. These fractures, referred to as damage, impact the ice flow by reducing its viscosity. Reduced viscosity enhances ice flow, leading to higher strain rates which further promotes more damage formation. However, despite its known effect, its application has been only done on idealized domains so far.

Here we will assess the damage sensitivity of a three-dimensional ice-sheet-shelf model in a simplified, symmetric case (MISMIP+ domain) and a complex real-world scenario such as the Amundsen-Sea Embayment (ASE). For this we will test three different damage formulations from literature which account for explicit crevasse formation. In addition, we will also test a new regularization approximation in our viscosity formulation. This approximation ensures that if no damage is applied, then the effective yield strength of our model cannot exceed the failure strength of ice.

Our findings reveal that incorporating damage or viscosity regularization into future projections of the ASE results in higher sea-level contribution and faster grounding-line migration. This underscores the critical need to enhance our understanding of damage and its implications for future sea-level rise, since current projections do not account for this process.

How to cite: Blasco, J., Li, Y., Coulon, V., and Pattyn, F.: The effect of ice damage on future Antarctic projections, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-15478, https://doi.org/10.5194/egusphere-egu24-15478, 2024.

Calving is a key process in the dynamics of marine-terminating glaciers with large ice shelves, such as those in West Antarctica. However, this process is currently not included in most predictive models, due to its difficulty to implement in a general form which can reliably reproduce rates of calving over a range of scenarios. We set out to investigate how important it is to develop such representation of calving for future modelling.

In this study, we quantify the sensitivity of modelled future mass loss to ice front retreat in the Amundsen Sea Embayment, including Pine Island and Thwaites Glaciers. We find that prescribing constant frontal retreat rates from 0.1 to 1 km a_­­^-1 progressively increases the contribution to sea level rise when compared to experiments with a fixed ice front. The result is up to 80% more loss of ice by 2100, and more than triple the ice loss in projections beyond 2200 with the higher rates of retreat. The spatial pattern of ice loss is non-uniformly distributed, with some regions thinning and others thickening as an initial response to the calving front retreat. We identify specific thresholds in the geometry of the system, which have clear effects on the ice flow and are reached at different times depending on the retreat rate.

We compare variability in the range of our results using different retreat rates to that in the range of ISMIP6 ocean forcing products, as ocean-induced melt is known to be a major factor in determining the future evolution of the Antarctic ice sheet. We find that the variability due to these two factors is initially similar, and that variability due to ice front retreat becomes comparatively greater over time. Our results demonstrate the high importance of accurately representing calving processes in models, showing that they are at least as important as ocean forcing and deserve a similar amount of attention in future model development work.

How to cite: Barnes, J., Gudmundsson, G. H., Goldberg, D., and Sun, S.: The importance of calving in ice sheet models: A sensitivity study of ice front retreat in the Amundsen Sea Embayment, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-12685, https://doi.org/10.5194/egusphere-egu24-12685, 2024.

The West Antarctic Ice Sheet (WAIS) is losing ice and its annual contribution to sea level is increasing. The future behaviour of WAIS will impact societies worldwide, yet deep uncertainty remains in the expected rate of ice loss. High impact low likelihood scenarios of sea level rise are needed by risk-averse stakeholders but are particularly difficult to constrain. Here we combine traditional model simulations of the Amundsen Sea sector of WAIS with Gaussian process emulation to show that ice-sheet models capable of resolving kilometre-scale basal topography will be needed to assess the probability of extreme scenarios of sea level rise. This resolution exceeds many state-of-the-art continent-scale simulations. Our model simulations show that lower resolutions tend to overestimate future sea level contribution and inflate the tails of the distribution. We therefore caution against relying purely upon low resolution simulations when assessing the potential for societally important high impact sea level rise.

How to cite: Williams, C. R., Thodoroff, P., Arthern, R. J., Byrne, J., Hosking, J. S., Kaiser, M., Lawrence, N. D., and Kazlauskaite, I.: Calculating exposure to extreme sea level risk will require high resolution ice sheet models, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-8114, https://doi.org/10.5194/egusphere-egu24-8114, 2024.

Antarctic sea ice extent reached a record minimum in 2023. Whilst the buttressing resistance provided by ice shelves has been quantified through past numerical studies, the degree to which sea ice can buttress and regulate upstream ice flow is not known. If significant, a future decline in sea ice extent would lead to increased ice discharge rates and a higher global mean sea level.

The January 2022 disintegration of landfast sea ice in the Larsen B embayment was closely followed by a significant increase in ice flow velocities and retreat rates of numerous outlet glaciers in the region. Notably, Crane glacier saw an initial ~8km retreat over six weeks, during which time a 5% increase in velocity was observed. Afterwards, the full evacuation of ambient sea ice between October and November was accompanied by the most significant monthly increase in velocities.

We use the numerical ice flow model, Ua, to investigate the buttressing effect of sea ice to Crane glacier. The ice-sheet model was initialised with sea ice included and constrained with observational velocity and geometry data sets. We conducted perturbation experiments on sea ice properties to explore its impact on the glacier. The results suggest that sea ice provided significant buttressing to the glacier before its collapse.

How to cite: Parsons, R., Sun, S., and Gudmundsson, H.: Quantifying the Buttressing Contribution of Sea Ice to Crane Glacier, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-12813, https://doi.org/10.5194/egusphere-egu24-12813, 2024.

Simulating mass changes and ice dynamics for mountain glaciers using Earth System Models (ESMs) is challenging due to their small size, sheer number, and spatial discontinuities in the ground ice coverage. This has resulted in a knowledge and representational gap in ESMs. However, there is a need for scaling ESMs from global to regional domains for comprehensive glacio-hydrological and hydroclimatic assessments.

We introduce new developments for simulating the mass balance and dynamic evolution of mountain glaciers within the Community Earth System Model (CESM). We provide an overview of this work across two spatiotemporal scales related to: (1) the dynamic evolution of the Central European glaciers under the GlacierMIP3 protocol using the Community Ice Sheet Model (CISM) and (2) the energy and water balance of the Upper Indus glaciated basins at daily-monthly timescales using the Hillslope Hydrology configuration of the Community Land Model (CLM).

How to cite: Minallah, S., Lipscomb, W., Swenson, S., and Leguy, G.: Simulating Mass Balance and Dynamics of Mountain Glaciers within an Earth System Modeling Framework, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-14128, https://doi.org/10.5194/egusphere-egu24-14128, 2024.

Basal shear stress on hard-bedded glaciers results from normal stress against bed roughness, which depends on basal water pressure and cavity size. These quantities are related in a steady state but are expected to behave differently under rapid changes in water input, which may lead to a transient frictional response not captured by existing friction laws. Here, we investigate transient friction using GPS vertical displacement and horizontal velocity observations, basal water pressure measurements, and cavitation model predictions during rain-induced speed-up events at Glacier d'Argentière, French Alps. We observe up to a threefold increase in horizontal surface velocity, spatially migrating at rates consistent with subglacial flow drainage, and associated with surface uplift and increased water pressure. We show that frictional changes are mainly driven by changes in water pressure at nearly constant cavity size. We propose a generalized friction law capable of capturing observations in both the transient and steady-state regimes.

How to cite: Togaibekov, A., Gimbert, F., Gilbert, A., and Walpersdorf, A.: Observing and modeling short-term changes in basal friction during rain-induced speed-ups on an Alpine glacier, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-11836, https://doi.org/10.5194/egusphere-egu24-11836, 2024.

We present the concepts and capabilities of IGM (https://github.com/jouvetg/igm), a fast and accessible Python model that simulates the evolution of glaciers at any scale by coupling ice thermomechanics, surface mass balance, and mass conservation. IGM models the ice flow by physics-informed deep learning. Specifically, we use a convolutional neural network, which is trained to minimise the energy associated with high-order ice flow physics. Based on the Tensorflow library, IGM performs a suite of fast, vectorised, and differentiable operations, which can be accelerated with a graphics processing unit (GPU). In turn, this allows fully-parallelised implementations of key model components in glacier modelling applications such as the positive degree day surface mass balance scheme, the enthalpy scheme for modelling the thermal regime of ice, or the integration of a large amount of particle trajectories for modelling debris transportation. As a result, IGM combines coding simplicity and modularity, high computational efficiency, state-of-the-art thermomechanical modelling, and efficient data assimilation thanks to underlying automatic differentiation tools. We demonstrate the capability of IGM for two different applications. First, we present a complete workflow (including OGGM-based data preprocessing, inverse and forward modelling, rendering of results) that allows us to model any mountain glacier in the world within a few minutes requiring only its Randolph Glacier Inventory (RGI) ID. Second, we present an application in paleo glacier modelling by simulating the entire European Alpine ice field (about 800 km long) in high resolution (200 m) during the Last Glacial Maximum.

How to cite: Jouvet, G., Cordonnier, G., Maussion, F., Cook, S., Finley, B., Henz, A., Herrmann, O., Kamleitner, S., Leger, T., Lleshi, K., Mey, J., Scherler, D., and Welty, E.: Capabilities of IGM, a thermo-mechanical glacier evolution model accelerated by deep-learning and GPU, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-8997, https://doi.org/10.5194/egusphere-egu24-8997, 2024.

The representation of ice shelf calving in numerical ice models is a new and emerging field in cryospheric modelling. As yet, there has been no systematic, in-depth study of how this process is implemented and whether the results can be trusted. Calving MIP is an ongoing model intercomparison project that seeks to address this issue by providing a common framework for model simulations of ice shelf calving, so that results between different models and approaches can be compared. We find it helpful to distinguish between a calving algorithm, the numerical implementation in a model of how ice is removed due to calving, and a calving law, a law with some physical basis that determines how much ice needs to be removed due to calving. The first phase of Calving MIP experiments are primarily interested in comparing different calving algorithms, whilst future experiments are planned to investigate different calving laws as well as compare simulated results with real world observations.

We present here results from across the cryospheric modelling community from the first phase of Calving MIP experiments investigating calving algorithms as well as future plans for further experiments.

How to cite: Jordan, J.: Calving MIP: Results and conclusions from the first phase of a model intercomparison project into ice shelf calving, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-5370, https://doi.org/10.5194/egusphere-egu24-5370, 2024.

We present a physical description of the ice-sheet model Nix v1.0, an open-source project intended for collaborative development. Nix is a 2D thermomechanical model written in C/C++ that simultaneously solves for the momentum balance equations, mass conservation and temperature evolution. Nix's velocity solver includes a hierarchy of Stokes approximations: Blatter-Pattyn, depth-integrated higher order, shallow-shelf and shallow-ice. The grounding-line position is explicitly solved by a moving coordinate system that avoids further interpolations. The model can be easily forced with any external boundary conditions, including those of stochastic nature. Nix has been verified for standard test problems. Here we show results for a number of benchmark tests from standard intercomparison projects and assess grounding-line migration with an overdeepened bed geometry. Lastly, we further exploit the thermomechanical coupling by designing a suite of experiments where the forcing is a physical variable, unlike previously idealised forcing scenarios where ice temperatures are implicitly fixed via an ice rate factor. Namely, we use atmospheric temperatures and oceanic temperature anomalies to assess model hysteresis behaviour with active thermodynamics. Our results show that hysteresis in an overdeepened bed geometry is similar for atmospheric and oceanic forcings. We find that not only the particular sub-shelf melting parametrisation determines the temperature anomaly at which the ice sheet retreats, but also the particular value of calibrated heat exchange velocities. Notably, the classical hysteresis loop is narrowed for both forcing scenarios (i.e., atmospheric and oceanic) if the ice sheet is thermomechanically active as a results of the internal feedback among ice temperature, stress balance and viscosity. In summary, Nix combines rapid computational capabilities with a Blatter-Pattyn stress balance fully coupled to a thermomechanical solver, not only validating against established benchmarks but also offering a powerful tool for advancing our insight on ice dynamics and grounding line stability.

How to cite: Moreno-Parada, D., Robinson, A., Montoya, M., and Alvarez-Solas, J.: Description and validation of the ice sheet model Nix v1.0, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-13290, https://doi.org/10.5194/egusphere-egu24-13290, 2024.

The vast majority of ice-sheet modelling studies rely on simplified representations of the Glacial Isostatic Adjustment (GIA), which, among other limitations, do not account for lateral variations of the lithospheric thickness and upper-mantle viscosity. In studies using 3D GIA models, this has however been shown to have major impacts on the dynamics of marine-based sectors of Antarctica, which are likely to be the greatest contributors to sea-level rise in the coming centuries. This gap in comprehensiveness is explained by the fact that 3D GIA models are computationally expensive, seldomly open-source and require the implementation of an iterative coupling scheme to converge with the history of the ice-sheet model. To close this gap between "best" and "tractable" GIA models, we here propose FastIsostasy, a regional GIA model capturing lateral variations of the lithospheric thickness and mantle viscosity. By means of Fast-Fourier transforms and a hybrid collocation scheme to solve its underlying partial differential equation, FastIsostasy can simulate 100,000 years of high-resolution bedrock displacement in only minutes of single-CPU computation, including the changes in sea-surface height due to mass redistribution. Despite its 2D grid, FastIsostasy parametrises the depth-dependent viscosity in a physically meaningful way and therefore represents the depth dimension to a certain extent. FastIsostasy is here benchmarked against analytical, 1D and 3D GIA solutions and shows very good agreement with them. It is fully open-source, documented with many examples and provides a straight-forward interface for coupling to an ice-sheet model. The model is benchmarked here based on its implementation in Julia, while a Fortran version is also provided to allow for compatibility with most existing ice-sheet models. The Julia version provides additional features, including a vast library of time-stepping methods and GPU support.

How to cite: Swierczek-Jereczek, J., Montoya, M., Latychev, K., Robinson, A., Alvarez-Solas, J., and Mitrovica, J.: FastIsostasy - An accelerated regional GIA model for coupled ice-sheet/solid-Earth simulations with laterally-variable solid-Earth structures, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-19141, https://doi.org/10.5194/egusphere-egu24-19141, 2024.

Ice sheets have a memory that is stored within both the geometry and thermal properties of the ice. The current Greenland Ice Sheet is thus not in equilibrium with present-day climate, but is in fact affected by a complex product of past changes that occurred over millennial timescales. Therefore, simulating the late-Pleistocene evolution of the Greenland Ice Sheet accurately is important when running future projections using paleo model initialization procedures. Using a novel model-data comparison procedure, we ran an experiment that aimed to produce numerical model simulations that fit available empirical data on the extent and timing of the grounded margin evolution of the Greenland Ice Sheet from the global LGM (24 kyr BP) to 1850 AD. Given the numerous uncertain parameters and boundary conditions required by numerical ice sheet models, finding simulations which adequately replicate empirical data on past grounded ice extent is a challenging task. In an attempt to address this challenge, we ran a perturbed parameter ensemble of 100 ice-sheet-wide simulations at 5 km spatial resolution using the Parallel Ice Sheet Model. Our simulations are forced by the latest transient paleo-climate and ocean simulations of the isotope-enabled Community Earth System Model (iCESM 1.2 and 1.3). Using quantitative model-data comparison tools and the newly developed, Greenland-wide PaleoGrIS 1.0 isochrone reconstruction of former ice extent, each ensemble simulation’s fit with empirical data was assessed quantitatively across both space and time. Using our best-scoring simulations, we here present new insights into the former Greenland Ice Sheet’s likely response to transitional climatic phases throughout the last deglaciation. Secondly, our results suggest ice temperature, geometry, and glacial isostatic adjustment-induced mechanisms of centennial to millennial-scale inertia in ice-extent response to past climatic forcing, with potential implications for the future evolution of the ice sheet. Thirdly, our results show that different parameter combinations produce a better model-data fit during different time periods and for different regions of the ice sheet – i.e. parameter values that work well at one place or time, produce worse fit at others. We hypothesise that better paleo model initializations may be achieved using time- and space-dependent parameter configurations. Finally, after extending several past ensemble simulations to the end of the 21st century under CMIP6-derived forcing, we find that accounting for the past modifies projections of the future. Using a steady-state contemporary ice sheet as an initial state leads to vastly different projected sea level contributions when compared to simulations that perform well at recreating past glacial history.

How to cite: Leger, T., Ely, J., Clark, C., Bradley, S., Archer, R., and Zhu, J.: Insights into the LGM-to-present evolution of the Greenland Ice Sheet from a data evaluated ensemble of numerical model simulations, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-11040, https://doi.org/10.5194/egusphere-egu24-11040, 2024.

State-of-the-art ice sheet model simulations used in the Ice Sheet Model Intercomparison Project (ISMIP) show a mismatch with recent observations of dynamic ice sheet change. In particular, the difference between the modeled and observed cumulative mass balance trend over the last several decades calls into question the accuracy of current projections of ice sheet contribution to sea level rise. Here, we use one of these models, the Ice-sheet and Sea-level System Model, to investigate how transient calibration may improve model hindcasts of ice dynamics and impact projections. Transient calibration is a relatively new capability in ice flow models; it uses a time series of observations to invert for uncertain model parameters, such as basal friction and ice rheology. With more observational constraints than the common snapshot inversion method, transient calibration has been shown to better capture trends and to have the ability to estimate how parameters evolve through time. We apply this method to Northwest Greenland, a region undergoing rapid changes that also has high-resolution, high-accuracy data for bed topography, ice surface velocity, and ice front positions. We find that transient calibration brings hindcast simulations of cumulative mass balance to within observational error. We also find that, when the basal friction parameter is allowed to vary, transient calibration can help mimic the impacts of subglacial hydrology and reproduce observations of seasonal velocity variability. Future simulations to 2100 using the ISMIP6 protocols show that the use of transient calibration leads to greater mass loss and, in the near term out to 2050, has a greater impact on mass balance than the choice of climate forcing scenario.

How to cite: Badgeley, J., Seroussi, H., and Morlighem, M.: Improving modeled ice dynamics in Northwest Greenland with transient calibration: From multi-decadal trends to seasonal cycles, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-4155, https://doi.org/10.5194/egusphere-egu24-4155, 2024.

Ice fabrics – the alignment of crystal orientations - can cause the ice viscosity to vary by an order of magnitude, consequently having a strong impact on the large-scale flow of ice sheets and glaciers. Because of this, there is a need for fabric models which are computationally efficient enough to be included in large-scale ice sheet models. We examine a range of existing models in this class and show they can be combined into a common equation which is a function of 2-3 parameters. By comparing with observations from the Greenland ice sheet, we get the best model predictions by assuming the ice deforms close to the Sachs hypothesis – that all grains experience the same stress. As these fabric predictions also depend on the flow law used, we provide a test of competing anisotropic flow laws for the first time, making a step towards reliably incorporating the effect of fabric and viscous anisotropy in ice sheet flow models.

How to cite: Richards, D., Mantelli, E., Pegler, S., and Piazolo, S.: Unifying and comparing different models of viscous anisotropy to be included in ice sheet models., EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-13210, https://doi.org/10.5194/egusphere-egu24-13210, 2024.