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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.

We present a novel and computationally efficient subglacial hydrological model that represents in a unified way both hard and soft bed rheologies as well as a dynamic switch between efficient and inefficient subglacial discharge. The subglacial model is dynamically linked to a regularized Coulomb friction law, allowing for a coupled evolution of the ice sheet on decadal to centennial time scales. The hydrological model is tested on an idealized marine ice sheet and subsequently applied to the drainage basin of Thwaites Glacier, West Antarctica, that is composed of a heterogeneous (hard/soft) bed. We find that subglacial hydrology embedded in the sliding law accelerates the grounding line retreat of Thwaites Glacier under present-day climatic conditions. Highest retreat rates are obtained for hard bed configurations and/or inefficient drainage systems.We show that the sensitivity is particularly driven by large gradients in effective pressure, more so than the value of effective pressure itself in the vicinity of the grounding line. Clearly, a better understanding of the subglacial system is needed with respect to both the spatial and temporal variability in effective pressure and bed rheological conditions. [ABSTRACT FROM AUTHOR]

Current observed mass loss of the Antarctic ice sheet is tightly linked to the loss of buttressing of ice shelves, leading to increased inland mass flux and concomitant grounding-line retreat. However, the rate of mass loss is also a function of the basal friction regime of the grounded ice sheet, exemplified by the type of sliding law governing basal sliding and/or sediment deformation. Recent studies (Ritz et al., 2015; Pattyn, 2017; Bulthuis et al., 2018) clearly advocate for higher mass losses with plastic sliding laws compared to viscous sliding. However, more elaborated processes leading to increased lubrication at the bed, such as subglacial hydrology, are often neglected.In this study we employ a basic subglacial hydrology model (Lebrocq et al., 2009) coupled to a thermomechanical ice-sheet model (f.ETISh; Pattyn, 2017) to investigate both the effect of sliding law power laws and subglacial water distribution on the sensitivity of Antarctic mass loss to ocean perturbations (represented by sub-shelf melting). Results show that while subglacial hydrology changes the sensitivity of the ice-sheet system, the choice of sliding law impacts sensitivity to a larger extent. [ABSTRACT FROM AUTHOR]