Abstract
Abstract. Marine ice-sheet stability is mostly controlled by the dynamics of the grounding line, i.e. the junction between the grounded ice sheet and the floating ice shelf. Grounding line migration has been investigated within the framework of MISMIP (Marine Ice Sheet Model Intercomparison Project), which mainly aimed at investigating steady state solutions. Here we focus on transient behaviour, executing short-term simulations (200 yr) of a steady ice sheet perturbed by the release of the buttressing restraint exerted by the ice shelf on the grounded ice upstream. The transient grounding line behaviour of four different flowline ice-sheet models has been compared. The models differ in the physics implemented (full Stokes and shallow shelf approximation), the numerical approach, as well as the grounding line treatment. Their overall response to the loss of buttressing is found to be broadly consistent in terms of grounding line position, rate of surface elevation change and surface velocity. However, still small differences appear for these latter variables, and they can lead to large discrepancies (> 100%) observed in terms of ice sheet contribution to sea level when cumulated over time. Despite the recent important improvements of marine ice-sheet models in their ability to compute steady state configurations, our results question the capacity of these models to compute short-term reliable sea-level rise projections.
Highlights
A range of observational methodologies have shown that significant loss of Antarctic ice mass has occurred over the past decade (Wingham et al, 2006; Rignot et al, 2008, 2011; Velicogna, 2009; Pritchard et al, 2012)
All models have successfully participated in the MISMIP benchmark (Pattyn et al, 2012a), exhibiting unique stable positions on downward sloping beds, unstable grounding line (GL) positions on retrograde slopes and related hysteresis behaviour over an undulated bedrock
We first evaluate the response of the various models regarding the variables that are currently observed over actual ice sheets, namely GL position (Fig. 2), surface elevation change (Fig. 3) and surface velocity (Fig. 4)
Summary
A range of observational methodologies have shown that significant loss of Antarctic ice mass has occurred over the past decade (Wingham et al, 2006; Rignot et al, 2008, 2011; Velicogna, 2009; Pritchard et al, 2012). Increased basal melt of ice shelves appears to be the primary control on Antarctic ice sheet loss. The dynamical response of the grounding line (GL), where ice loses contact with bed and, downstream, begins to float over the ocean, is an essential control on the mass balance of a marine ice sheet. While observations are crucial in diagnosing the state of balance of an ice sheet, extrapolation of current trends is a limited technique in predicting ice-sheet future behaviour. Increasing complexity has been regularly added, enabling progressive improvements from 1-D flowline models based on shallow-ice approximations to full numerical solutions of the Stokes equations for an actual 3-D geometry
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