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Antarctica's ice shelves modulate the grounded ice flow, and weakening of ice shelves due to climate forcing will decrease their ‘buttressing’ effect, causing a response in the grounded ice. While the processes governing ice-shelf weakening are complex, uncertainties in the response of the grounded ice sheet are also difficult to assess. The Antarctic BUttressing Model Intercomparison Project (ABUMIP) compares ice-sheet model responses to decrease in buttressing by investigating the ‘end-member’ scenario of total and sustained loss of ice shelves. Although unrealistic, this scenario enables gauging the sensitivity of an ensemble of 15 ice-sheet models to a total loss of buttressing, hence exhibiting the full potential of marine ice-sheet instability. All models predict that this scenario leads to multi-metre (1–12 m) sea-level rise over 500 years from present day. West Antarctic ice sheet collapse alone leads to a 1.91–5.08 m sea-level rise due to the marine ice-sheet instability. Mass loss rates are a strong function of the sliding/friction law, with plastic laws cause a further destabilization of the Aurora and Wilkes Subglacial Basins, East Antarctica. Improvements to marine ice-sheet models have greatly reduced variability between modelled ice-sheet responses to extreme ice-shelf loss, e.g. compared to the SeaRISE assessments.
Basal motion of ice sheets depends in part on the roughness and material properties of the subglacial bed and the occurrence of water. To date, basal motion represents one of the largest uncertainties in ice-flow models. It is that component of the total flow velocity that can change most rapidly and can, therefore, facilitate rapid variations in dynamic behaviour. In this study, we investigate the subglacial properties of the East Antarctic Ice Sheet by statistically analysing the roughness of the bed topography, inferred from radio-echo sounding measurements. We analyse two sets of roughness parameters, one derived in the spatial and the other in the spectral domain, with two roughness parameters each. This enables us to compare the suitability of the four roughness parameters to classify the subglacial landscapes below the ice sheet. We further investigate the relationship of the roughness parameters with observed surface flow velocity and modelled basal temperatures of the ice sheet. We find that one of the roughness parameters, the Hurst exponent derived in the spatial domain, coincides with the thermal condition at the base of the ice sheet for slow flow velocities and varies with flow velocity.
The cryosphere is an essential component of the global climate system, equally affecting climate processes significantly and being subject, and particularly sensitive, to changes in climate conditions. Numerical models are an important tool for assessing climate-change impacts on the Antarctic ice–sheet–ice–shelf–ocean system. They not only complement field and satellite remotesensing investigations but are often the only feasible alternative for addressing some of the important parameters and processes. Over the last few years, our group has made significant progress in developing and applying innovative numerical methods. In this paper, we provide a brief overview of some of the methods employed and the major results obtained for a number of case studies in the Atlantic sector of Antarctica.
Two diagnostic, dynamic/thermodynamic ice-shelf models are applied to the Brunt Ice Shelf/Stancomb-Wills Ice Tongue system, located off Caird Coast, Coats Land, Antarctica. The Brunt Ice Shelf/Stancomb-Wills Ice Tongue system is characterized as a thin, unbounded ice shelf with an atypical and highly heterogeneous structure. In contrast to other ice shelves, a composite mass of icebergs that calved at the grounding line and were then locked within fast (sea) ice exists between the fast-moving Stancomb-Wills Ice Stream and the slow-moving Brunt Ice Shelf. We simulate the present flow regime of the ice shelf that results from the ice-thickness distribution and the inflow at the grounding line with two different models, and compare the model results with feature tracking and InSAR flow velocities. We then incorporate two observed features, a rift and a shear margin, into the models with two different approaches, and demonstrate the effects of variations in numerical values for the shear strength and viscosity in these zones on the simulated velocity field. A major result is that both kinds of implementation of the rifts lead to similar effects on the entire velocity field, while there are discrepancies in the vicinity of the rifts.
We study the presence and effect of subglacial water on the motion of inland ice in western Dronning Maud Land, Antarctica. A full-Stokes model including three routing schemes for a thin film of subglacial water and a modification of a Weertman-type sliding relation, to account for higher sliding velocities under wet basal conditions, were used to perform 200 ka spin-up simulations on a 2.5 km grid. Subsequent 30 ka simulations with wet and dry basal conditions were analysed for the effects of sliding on the thermal regime and velocities. The occurrence of the major ice streams in this area is mainly controlled by the ice and bedrock geometry. Smaller glaciers only appear as pronounced individual glaciers when subglacial water is taken into account. The thermal regime is affected by creep instabilities produced by an ice rheology including a microscopic water content, leading to cyclic behaviour on millennial timescales of the ice flow and occurrence of temperate ice at the base.
Predictions of marine ice-sheet behaviour require models able to simulate grounding-line migration. We present results of an intercomparison experiment for plan-view marine ice-sheet models. Verification is effected by comparison with approximate analytical solutions for flux across the grounding line using simplified geometrical configurations (no lateral variations, no buttressing effects from lateral drag). Perturbation experiments specifying spatial variation in basal sliding parameters permitted the evolution of curved grounding lines, generating buttressing effects. The experiments showed regions of compression and extensional flow across the grounding line, thereby invalidating the boundary layer theory. Steady-state grounding-line positions were found to be dependent on the level of physical model approximation. Resolving grounding lines requires inclusion of membrane stresses, a sufficiently small grid size (<500 m), or subgrid interpolation of the grounding line. The latter still requires nominal grid sizes of <5 km. For larger grid spacings, appropriate parameterizations for ice flux may be imposed at the grounding line, but the short-time transient behaviour is then incorrect and different from models that do not incorporate grounding-line parameterizations. The numerical error associated with predicting grounding-line motion can be reduced significantly below the errors associated with parameter ignorance and uncertainties in future scenarios.
Ice-stream dynamics are strongly controlled by processes taking place at the ice/bed interface where subglacial water both lubricates the base and saturates any existing, underlying sediment. Large parts of the former Eurasian ice sheet were underlain by thick sequences of soft, marine sediments and many areas are imprinted with geomorphological features indicative of fast flow and wet basal conditions. Here, we study the effect of subglacial water on past Eurasian ice-sheet dynamics by incorporating a thin-film model of basal water flow into the ice-sheet model SICOPOLIS and use it to better represent flow in temperate areas. The adjunction of subglacial hydrology results in a smaller ice-sheet building up over time and generally faster ice velocities, which consequently reduces the total area fraction of temperate basal ice and ice streaming areas. Minima in the hydraulic pressure potential, governing water flow, are used as indicators for potential locations of past subglacial lakes and a probability distribution of lake existence is presented based on estimated lake depth and longevity.
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