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Much of the knowledge of Antarctic Ice Sheet variations since its inception ~34 Ma derives from marine sediments on the continental shelf, deposited in glacimarine or sub-ice environments by advancing and retreating grounded ice, and observed today by seismic profiling and coring. Here we apply a 3-D coupled ice sheet and sediment model from 40 Ma to the present, with the goal of directly linking ice-sheet variations with the sediment record. The ice-sheet model uses vertically averaged ice dynamics and parameterized grounding-line flux. The sediment model includes quarrying of bedrock, sub-ice transport, and marine deposition. Atmospheric and oceanic forcing are determined by uniform shifts to modern climatology in proportion to records of atmospheric CO2, deep-sea-core δ18O, and orbital insolation variations. The model is run continuously over the last 40 Myr at coarse resolution (80 or 160 km), modeling post-Eocene ice, landscape evolution and off-shore sediment packages in a single self-consistent simulation. Strata and unconformities are tracked by recording times of deposition within the model sediment stacks, which can be compared directly with observed seismic profiles. The initial bedrock topography is initialized to 34 Ma geologic reconstructions, or an iterative procedure is used that yields independent estimates of paleo bedrock topography. Preliminary results are compared with recognized Cenozoic ice-sheet variations, modern sediment distributions and seismic profiles, and modern and paleo bedrock topographies. 

The use of a boundary-layer parameterization of buttressing and ice flux across grounding lines in a two dimensional ice-sheet model is improved by allowing general orientations of the grounding line. This and another modification to the model’s grounding-line parameterization are assessed in three settings: rectangular fjord-like domains – the third Marine Ice Sheet Model Intercomparison Project (MISMIPC) and Marine Ice Sheet Model Intercomparison Project for plan view models (MISMIP3d) – and future simulations of West Antarctic ice retreat under Representative Concentration Pathway (RCP) 8.5-based climates. The new modifications are found to have significant effects on the fjord-like results, which are now within the envelopes of other models in the MISMIP+ and MISMIP3d intercomparisons. In contrast, the modifications have little effect on West Antarctic retreat, presumably because dynamics in the wider major Antarctic basins are adequately represented by the model’s previous simpler one-dimensional formulation. As future grounding lines retreat across very deep bedrock topography in the West Antarctic simulations, buttressing is weak and deviatoric stress measures exceed the ice yield stress, implying that structural failure at these grounding lines would occur. We suggest that these grounding-line quantities should be examined in similar projections by other ice models to better assess the potential for future structural failure. 

Much of the knowledge of Antarctic Ice Sheet variations since its inception ∼34 Ma derives from marine sediments on the continental shelf, deposited in glacimarine or sub-ice environments by advancing and retreating grounded ice, and observed today by seismic profiling and coring. If coupled ice-sheet and sediment models can simulate these deposits explicitly, direct comparisons with the sediment record would help in linking it to Cenozoic ice and climate history. Here we apply an existing 3-D ice sheet and sediment model to the whole period of late Cenozoic Antarctic evolution. The ice-sheet model uses local parameterizations of grounding-line flux, ice-shelf hydrofracture and ice cliff failure. The sediment model includes quarrying of bedrock, sub-ice transport, and marine deposition. Atmospheric and oceanic forcing is determined by uniform shifts to modern climatology in proportion to records of atmospheric CO2, deep-sea-core d18O, and orbital insolation variations. Initial ice-free bedrock topography can either be prescribed from geologic reconstructions for ∼34 Ma (Wilson et al., Palaeo3, 2011) or deduced in an iterative procedure fitting to observed modern topography and total sediment amounts. The model is run continuously from 40 Ma to the present, capturing post-Eocene Antarctic landscape evolution and off-shore sediment packages in a single self-consistent simulation. In order to make these long simulations feasible, the model resolution is very coarse, 80 km. However the ice model’s use of local parameterizations for fine-scale dynamical processes yields results that are not seriously degraded compared to finer resolutions in short tests. The primary goals are (1) to reproduce major recognized ice-sheet trends and fluctuations from the Eocene to today, and (2) to produce a 3-D model map of modern sediment deposits. "Strata" are tracked by recording times of deposition within the model sediment stacks, which can be compared with observed seismic profiles. Initial results are presented, and preliminary overall comparisons are made with observed sediment packages and the modern ice and bedrock state. 

Much of the knowledge of Antarctic Ice Sheet variations since its inception ~34 Ma derives from marine sediments on the continental shelf, deposited in glacimarine or sub-ice environments by advancing and retreating grounded ice, and observed today by seismic profiling and coring. If coupled ice-sheet and sediment models can simulate these deposits explicitly, direct comparisons with the sediment record would be valuable in linking it to Cenozoic ice and climate history. Here we apply an existing 3-D ice sheet and sediment model to the whole period of late Cenozoic Antarctic evolution. The ice-sheet model uses local parameterizations of grounding-line flux, ice-shelf hydrofracture and ice cliff failure. The sediment model includes quarrying of bedrock, sub-ice transport, and marine deposition. Atmospheric and oceanic forcing is determined by uniform shifts to modern climatology in proportion to records of atmospheric CO2, deep-sea-core d18O, and orbital insolation variations. Initial ice-free and sediment-free bedrock topography is prescribed from the 34 Ma reconstruction of Wilson et al., Palaeo3, 2011, and their estimated rate of tectonic subsidence is applied in West Antarctica. The model is run continuously from 34 Ma to the present, to capture the entire post-Eocene Antarctic landscape evolution and off-shore sediment packages in a single self-consistent simulation. In order to make these long simulations feasible, the model resolution is very coarse, 80 km. However the ice model's use of local parameterizations for fine-scale dynamical processes yields results that are not seriously degraded compared to finer resolutions in short tests. The primary goals are (1) to reproduce major recognized ice-sheet trends and fluctuations from the Eocene to today, and (2) to produce a 3-D model map of modern sediment deposits. "Strata" are tracked by recording times of deposition within the model sediment stacks. Unconformities in these strata occur in the model that can be compared with observed profiles. Initial results are presented, and preliminary overall comparisons are made with observed sediment packages, focusing on sensitivities to climate forcing, quarrying rates, and sediment parameters that stand in for alternate sediment rheologies. 

Theory, modeling and observations point to the prospect of runaway grounding-line retreat and marine ice loss from West Antarctica and major East Antarctic basins, in response to climate warming. These rapid retreats are associated with geologic evidence of past high sea-level stands, and pose a threat of drastic sea-level rise in the future. Rapid calving of ice from deep grounding lines generates substantial downstream melange (floating ice debris). It is unknown whether this melange has a significant effect on ice dynamics during major Antarctic retreats, through clogging of seaways and back pressure at the grounding line. Observations in Greenland fjords suggest that melange can have a significant buttressing effect, but the lateral scales of Antarctic basins are an order of magnitude larger (100's km compared to 10's km), with presumably much less influence of confining margins. Here we attempt to include melange as a prognostic variable in a 3-D Antarctic ice sheet-shelf model. Continuum mechanics is used as a heuristic representation of discrete particle physics. Melange is created by ice calving and cliff failure. Its dynamics are treated similarly to ice flow, but with little or no resistance to divergence. Melange provides back pressure where adjacent to grounded tidewater ice faces or ice-shelf edges. We examine the influence of the new melange component during rapid Antarctic retreat in warm-Pliocene and future warming scenarios. 

Seismic tomography models indicate highly variable Earth structure beneath Antarctica with anomalously low shallow mantle viscosities below West Antarctica. An improved projection of the contribution of the Antarctic Ice Sheet to sea‐level change requires consideration of this complexity to precisely account for water expelled into the ocean from uplifting marine sectors. Here we build a high‐resolution 3‐D viscoelastic structure model based on recent inferences of seismic velocity heterogeneity below the continent. The model serves as input to a global‐scale sea‐level model that we use to investigate the influence of solid Earth deformation in Antarctica on future global mean sea‐level (GMSL) rise. Our calculations are based on a suite of ice mass projections generated with a range of climate forcings and suggest that water expulsion from the rebounding marine basins contributes 4%–16% and 7%–14% to the projected GMSL change at 2100 and 2500, respectively.

 

Observational evidence indicates that the West Antarctic Ice Sheet (WAIS) is losing mass at an accelerating rate. Impacts to global climate resulting from changing ocean circulation patterns due to increased freshwater runoff from Antarctica in the future could have significant implications for global heat transport, but to-date this topic has not been investigated using complex numerical models with realistic freshwater forcing. Here, we present results from a high resolution fully coupled ocean-atmosphere model (CESM 1.2) forced with runoff from Antarctica prescribed from a high resolution regional ice sheet-ice shelf model. Results from the regional simulations indicate a potential freshwater contribution from Antarctica of up to 1 m equivalent sea level rise by the end of the century under RCP 8.5 indicating that a substantial input of freshwater into the Southern Ocean is possible. Our high resolution global simulations were performed under IPCC future climate scenarios RCP 4.5 and 8.5. We will present results showing the impact of WAIS collapse on global ocean circulation, sea ice, air temperature, and salinity in order to assess the potential for abrupt climate change triggered by WAIS collapse. 

The agreement reached at the 21st Conference of the Parties (COP21) of the United Nations Framework Conven- tion on Climate Change (UNFCC) is aimed at limiting the post-preindustrial rise in global mean temperature to less than 2 oC at the end of this century, and to promote further efforts to limit the warming to 1.5 oC. Here, we use a numerical ice sheet-shelf model, with physics tested and calibrated against modern and past ice-sheet behavior and coupled to highly resolved atmospheric and ocean components, to test the Antarctic Ice Sheet’s response to a range of future climate scenarios representing COP21 aspirations versus a fossil-fuel intensive RCP8.5 emissions scenario. Assuming COP21 temperature targets are achievable and those temperatures will not be exceeded beyond 2100, we find that a global mean temperature rise less than 2 oC substantially reduces both the short term (decadal- century) and long-term risk of catastrophic sea level rise from Antarctica. In contrast, we find that the current, Intended Nationally Determined Contributions (INDCs), allowing global mean temperature to approach ∼3 oC by the end of this century, results in a substantial increase in Antarctica’s contribution to sea-level rise, relative to 1.5 or 2 oC. The results suggest that the current INCDs might not be sufficient to save the West Antarctic Ice Sheet and some East Antarctic outlets from substantial retreat. 

The treatment of surface melt, runoff, and the snow-firn-ice transition in ice-sheet models (ISMs) is becoming increasingly important, as mobile liquid on Greenland and Antarctic flanks increases due to climate warming in the next century and beyond. Simple Positive Degree Day (PDD)-based box models used in some ISMs crudely capture liquid storage and refreezing, but need to be extended to include vertical structure through the whole firn-ice column, as in some regional climate models (RCMs). This is a necessary prelude to modeling the flow of mobile meltwater in channel-river-moulin systems, and routing to the base and/or margins of the ice sheet. More detailed column models of snow and firn exist, that include compaction, grain size, and other processes. Some focus on dry-snow zones, and have fine vertical resolution spanning the entire firn column with Lagrangian tracking of annual snow layers (e.g., FirnMICE: Lundin et al., J. Glac., 2017). However, they are mostly too computationally expensive for ISM applications, and are not designed for ablation zones with meltwater and bare ice in summer. More general models are used in some RCMs that include similar physics but with fewer layers, and are applicable both to accumulation and ablation zones. Here we formulate a new snow-firn model, similar to those in RCMs, for use within an ice-sheet model. A limited number of vertical layers is used (∼10), with Lagrangian tracking of layers, grain size evolution, compaction, ice lenses, liquid melting, storage, percolation and runoff. Surface melting is computed from linearized net atmospheric energy fluxes, not from PDDs. The model is tested using the FirnMICE experiments, and using gridded RACMO2 modern climate input over Greenland, seeking to balance model performance with computational efficiency.