An accurate present-day model of the Antarctic ice sheet is necessary but insufficient for an accurate projection of its future evolution

Abstract. Accelerated retreat of the Antarctic ice sheet (AIS) dominates both the magnitude of, and the uncertainty in, high-end sea-level projections. One frequently cited caveat in the computational models used to create these projections is that many of them poorly represent the observed present-day state of the AIS. Following the thought that a model should accurately represent the present before it can be expected to accurately represent the future, there have been discussions in the ice-sheet modelling community to weight or even exclude individual models from ensemble studies based on how well they reproduce present-day observations.

In this study, we create a wide ensemble of realisations of the AIS with a numerical ice-sheet model under different parametric assumptions, utilising inverse methods to ensure every realisation reproduces snapshot observations of the present-day geometry, velocity, and thinning rates of the AIS to a degree that is comparable with the most recent ISMIP6 ensemble. These model realisations project anything between −0.3 m and +1.5 m of sea-level contribution (SLC) by 2300, with the sea-level rate at that time varying from −0.7 to +9.7 mm/yr. There is no clear relation between the degree to which these model realisations reproduce snapshot observations of the present-day AIS, and the amount of SLC they project. The main factors contributing to the wide variance in projected SLC are the sliding law and the distribution of slipperiness of the subglacial bed. Since these quantities are typically derived by inversion from observations of the ice flow, they can only be known as accurately as other processes affecting the flow, such as subglacial hydrology, ice damage, and calving. The resulting compensating errors are known to affect the modelled present-day state of the ice sheet differently than its response to future climate change, explaining why models with a nearly identical initial state can produce such different SLC projections. This implies that weighting or excluding models from ensemble studies based only on their agreement with snapshot observations of the present-day ice sheet might not be a wise choice. Instead, comparing to temporal observations, and to novel observations of the ice sheet’s sub-surface, might be more helpful to reduce the uncertainty in Antarctica’s future contribution to sea-level rise.