Fast approximation of Antarctica’s GIA response to future ice melt with 3-D Earth structure

Abstract. Projections of Antarctic ice mass loss and associated sea level contributions over the coming centuries are intrinsically linked to glacial isostatic adjustment (GIA), a process by which changing ice sheets deform the solid Earth and sea surface. Altering bedrock topography and sea levels at the grounding line, GIA exerts a strong control on marine ice sheet dynamics, especially on the multi-century timescales to be considered in ISMIP7 (Ice Sheet Model Intercomparison Project for the Coupled Model Intercomparison Project - Phase 7). To accurately capture bedrock and sea level changes, ice sheet models must be coupled with GIA models that include realistic spatial variations in solid Earth structure. However, GIA models that incorporate 3-D variations in Earth structure are computationally expensive, limiting their use in ice sheet modelling. Consequently, most ice sheet models still assume rigid bedrock topography or rely on simple GIA models that neglect realistic lateral variations in Earth structure. Here, we assess the performance of FastIsostasy, a computationally-efficient regional 2-D GIA model, relative to Seakon, a state-of-the-art 3-D GIA model, in iteratively coupled ice sheet – GIA simulations of Antarctic Ice Sheet evolution over the next five centuries. Coupled simulations that employ FastIsotasy produce GIA, ice thickness, and grounding line predictions that closely match those from simulations using Seakon, and, more specifically, perform better than ice sheet simulations that rely on overly simplified GIA models. With the protocols for ISMIP7 under active development, FastIsostasy offers a viable approach for ice sheet modellers to accurately and efficiently capture solid Earth – ice sheet feedbacks, permitting improved projections of Antarctic ice mass change and associated sea level contributions over the coming centuries.