The evolution of future Antarctic surface melt using PISM-dEBM-simple

With a volume of 58 m sea-level equivalent, the Antarctic Ice Sheet represents the largest potential source of future sea-level rise under global warming. While the ice sheet gains mass through snowfall at the surface, it loses mass through dynamic discharge and iceberg calving into the ocean, as well as by melting at the surface and underneath its floating ice shelves.

Already today, Antarctica is contributing to sea-level rise. So far, this contribution has been comparatively modest, but is expected to increase in the future. Most of the current mass losses are concentrated in the West Antarctic Ice Sheet, mainly caused by sub-shelf melting and ice discharge. Because air temperatures are low and thus surface melt rates are small, any significant melting at the surface is restricted to the low-elevation coastal zones. At the same time, most of the mass loss is offset by snowfall, which is projected to further increase in a warming atmosphere.

As warming progresses over the coming centuries, the question arises as to how long the mass losses on the one side will be compensated by the gains on the other. In 21st-century projections, increasing surface mass balance is outweighing increased discharge even under strong warming scenarios. However, in long-term (multi-century to millennium scale) warming simulations the positive surface mass balance trend shows a peak and subsequent reversal. Owing to positive feedbacks, like the surface-elevation or the ice-albedo feedback, this effect can be enhanced once a surface lowering is triggered or the surface reflectivity is lowered by initial melt.

Here, we implement a simplified version of the diurnal Energy Balance Model (dEBM-simple) as a surface module in the Parallel Ice Sheet Model (PISM), which extends the conventional positive-degree-day (PDD) approach to include the influence of solar radiation and parameterizes the ice albedo as a function of melting, implicitly accounting for the ice-albedo feedback.

Using a model sensitivity ensemble, we analyze the range of possible surface mass balance evolutions over the 21st century as well as in long-term simulations based on extended end-of-century climatological conditions with the coupled model. The comparison with the PDD approach hints to a strong overestimation of surface melt rates of the latter, even under present day conditions. The dEBM-simple further allows us to disentangle the respective contributions of temperature- and insolation-driven surface melt to future sea level rise.