More Like this

1. Improvements in one-dimensional grounding-line parameterizations in an ice-sheet model with lateral variations (PSUICE3D v2.1)

   https://doi.org/10.5194/gmd-13-6481-2020  

   Pollard, David; DeConto, Robert M. (January 2020, Geoscientific Model Development)

   null (Ed.)

   Abstract. The use of a boundary-layer parameterization ofbuttressing and ice flux across grounding lines in a two-dimensionalice-sheet model is improved by allowing general orientations of thegrounding line. This and another modification to the model's grounding-lineparameterization are assessed in three settings: rectangular fjord-likedomains – the third Marine Ice Sheet Model Intercomparison Project (MISMIP+) and Marine Ice Sheet Model Intercomparison Project for plan view models (MISMIP3d) – and future simulations of West Antarcticice retreat under Representative Concentration Pathway (RCP)8.5-based climates. The new modifications are found tohave significant effects on the fjord-like results, which are now within theenvelopes of other models in the MISMIP+ and MISMIP3d intercomparisons. Incontrast, the modifications have little effect on West Antarctic retreat,presumably because dynamics in the wider major Antarctic basins areadequately represented by the model's previous simpler one-dimensionalformulation. As future grounding lines retreat across very deep bedrocktopography in the West Antarctic simulations, buttressing is weak anddeviatoric stress measures exceed the ice yield stress, implying thatstructural failure at these grounding lines would occur. We suggest thatthese grounding-line quantities should be examined in similar projections byother ice models to better assess the potential for future structuralfailure. 

   more » « less
2. Simple model of melange and its influence on rapid ice retreat in a large-scale Antarctic ice sheet model

   Pollard, D; DeConto, R. (December 2018, AGU Fall Meeting 2017)

   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. 

   more » « less
3. Model insights into bed control on retreat of Thwaites Glacier, West Antarctica

   https://doi.org/10.1017/jog.2023.13  

   Schwans, Emily; Parizek, Byron R.; Alley, Richard B.; Anandakrishnan, Sridhar; Morlighem, Mathieu M. (October 2023, Journal of Glaciology)

   Abstract Thwaites Glacier (TG) plays an important role in future sea-level rise (SLR) contribution from the West Antarctic Ice Sheet. Recent observations show that TG is losing mass, and its grounding zone is retreating. Previous modeling has produced a wide range of results concerning whether, when, and how rapidly further retreat will occur under continued warming. These differences arise at least in part from ill-constrained processes, including friction from the bed, and future atmosphere and ocean forcing affecting ice-shelf and grounding-zone buttressing. Here, we apply the Ice Sheet and Sea-level System Model (ISSM) with a range of specifications of basal sliding behavior in response to varying ocean forcing. We find that basin-wide bed character strongly affects TG's response to sub-shelf melt by modulating how changes in driving stress are balanced by the bed as the glacier responds to external forcing. Resulting differences in dynamic thinning patterns alter modeled grounding-line retreat across Thwaites' catchment, affecting both modeled rates and magnitudes of SLR contribution from this critical sector of the ice sheet. Bed character introduces large uncertainties in projections of TG under equal external forcing, pointing to this as a crucial constraint needed in predictive models of West Antarctica. 

   more » « less
4. Climatic Thresholds for Widespread Ice Shelf Hydrofracturing and Ice Cliff Calving In Antarctica: Implications for Future Sea Level Rise

   DeConto, R. (December 2018, AGU Fall Meeting)

   The loss or thinning of buttressing ice shelves and accompanying changes in grounding-zone stress balance are commonly implicated as the primary trigger for grounding-line retreat, such as that observed in Amundsen Sea outlet glaciers today. Ice-shelf thinning is mostly attributed to the presence of warm ocean waters beneath the shelves. However, climate model projections show that summer air temperatures could soon exceed the threshold for widespread meltwater production on ice-shelf surfaces. This has serious implications for their future stability, because they are vulnerable to water-induced flexural stresses and water-aided crevasse penetration, termed ‘hydrofracturing’. Once initiated, the rate of shelf loss through hydrofracturing can far exceed that caused by sub-surface melting, and could result in the complete loss of some buttressing ice shelves, with marine grounding lines suddenly becoming calving ice fronts. In places where those exposed ice fronts are thick (>900m) and crevassed, deviatoric stresses can exceed the strength of the ice and the cliff face will fail mechanically, leading to rapid calving like that seen in analogous settings such as Jakobshavn on Greenland. Here we explore the implications of hydrofacturing and subsequent ice-cliff collapse in a warming climate, by parameterizing these processes in a hybrid ice sheet-shelf model. Model sensitivities to meltwater production and to ice-cliff calving rate (a function of cliff height above the stress balance threshold triggering brittle failure) are calibrated to match modern observations of calving and thinning. We find the potential for major ice-sheet retreat if global mean temperature rises more than ~2ºC above preindustrial. In the model, Antarctic calving rates at thick ice fronts are not allowed to exceed those observed in Greenland today. This may be a conservative assumption, considering the very different spatial scales of Antarctic outlets, such as Thwaites. Nonetheless, simulations following a ‘worst case’ RCP8.5 scenario produce rates of sea-level rise measured in cm per year by the end of this century. Clearly, the potential for brittle processes to deliver ice to the ocean, in addition to viscous and basal processes, needs to be better constrained through more complete, physically based representations of calving. 

   more » « less
5. A protocol for calculating basal melt rates in the ISMIP6 Antarctic ice sheet projections

   https://doi.org/10.5194/tc-14-3111-2020  

   Jourdain, Nicolas C.; Asay-Davis, Xylar; Hattermann, Tore; Straneo, Fiammetta; Seroussi, Hélène; Little, Christopher M.; Nowicki, Sophie (January 2020, The Cryosphere)

   null (Ed.)

   Abstract. Climate model projections have previously been used to compute ice shelf basal melt rates in ice sheet models, but the strategies employed – e.g., ocean input, parameterization, calibration technique, and corrections – have varied widely and are often ad hoc. Here, a methodology is proposed for the calculation of circum-Antarctic basal melt rates for floating ice, based on climate models, that is suitable for ISMIP6, the Ice Sheet Model Intercomparison Project for CMIP6 (6th Coupled Model Intercomparison Project). The past and future evolution of ocean temperature and salinity is derived from a climate model by estimating anomalies with respect to the modern day, which are added to a present-day climatology constructed from existing observational datasets. Temperature and salinity are extrapolated to any position potentially occupied by a simulated ice shelf. A simple formulation is proposed for a basal melt parameterization in ISMIP6, constrained by the observed temperature climatology, with a quadratic dependency on either the nonlocal or local thermal forcing. Two calibration methods are proposed: (1) based on the mean Antarctic melt rate (MeanAnt) and (2) based on melt rates near Pine Island's deep grounding line (PIGL). Future Antarctic mean melt rates are an order of magnitude greater in PIGL than in MeanAnt. The PIGL calibration and the local parameterization result in more realistic melt rates near grounding lines. PIGL is also more consistent with observations of interannual melt rate variability underneath Pine Island and Dotson ice shelves. This work stresses the need for more physics and less calibration in the parameterizations and for more observations of hydrographic properties and melt rates at interannual and decadal timescales. 

   more » « less