Constraining Antarctica's contribution to sea-level change: development of a new glacial isostatic adjustment model for Antarctica

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The Antarctic Ice Sheet is currently melting and undergoing rapid dynamic change. However, the rate at which it is melting, and hence its contribution to sea-level rise, is currently poorly known. The aim of my Fellowship is to derive a better estimate of current ice-mass change across Antarctica. Such an estimate can be derived from satellite gravity data, which are used to infer ice-mass changes from measurements of the Earth's gravity field. However, these data must be corrected for the ongoing effects of postglacial rebound (also known as glacial isostatic adjustment; GIA).Satellite gravity measurements cannot distinguish between changes in ice mass and mass movement of the solid Earth. During the last glaciation the Antarctic Ice Sheet was much larger and the extra mass caused the land beneath the ice to subside. As the ice sheet shrank towards its present size, the land began to rebound. This process - GIA - continues today due to the viscous nature of Earth's mantle. In fact, the GIA signal, as recorded by satellite gravity data, can be of the same magnitude as the signal due to current ice-mass change. Therefore, it is vital to constrain the pattern of GIA as accurately as possible in order to determine the pattern of ice-mass change.I will address two fundamental problems with the methods currently used to model GIA in Antarctica, thus significantly reducing the uncertainty on Antarctica's sea-level contribution.In the first part of my Fellowship I will model the 3D Earth structure beneath Antarctica. Current Antarctic GIA models do not account for variations in crustal thickness or mantle viscosity. However, such variations are known to be large because East and West Antarctica have very different geological histories: the crust beneath East Antarctica is old and 'stiff' and thus will not deform easily under loading. In contrast, the crust beneath West Antarctica is young and 'weak' and so deforms more easily under loading. In order to realistically determine the rebound signal, spatial variations in Earth structure must be accounted for so that the correct response to loading is calculated; not doing so can alter the rebound signal by up to 30%. The recent acquisition of new data relating to Antarctic Earth structure, and GIA model developments, make this aspect of my proposal very timely.The second problem I will address is related to sea-level change. West Antarctica is a marine-based ice sheet meaning that its bed is grounded below present sea level. In order to model changes in ice extent over time, and hence determine the GIA signal, it is necessary to consider feedbacks between sea-level change and ice dynamics. Sea level directly controls the position where an ice sheet begins to float and thus where ice is lost to the ocean. However, due to GIA, sea level does not vary uniformly over time: In areas where the ice sheet grows the land is pushed down by the extra load and the sea surface is drawn upwards due to the increased gravitational attraction of the ice sheet, resulting in an increase in sea level. The opposite happens when ice retreats. In both cases, feedbacks influence the dynamical behaviour of the ice sheet. Such feedbacks are not accounted for in current ice-sheet models, despite their potential to increase the stability of an ice sheet: Their inclusion could fundamentally alter predictions of ice-sheet behaviour under future sea-level rise scenarios. In the second component of my project I will therefore develop a coupled GIA-ice-sheet model. Both fields of research will benefit from improved model capabilities.These model improvements will be used to derive a more realistic estimate of GIA across Antarctica. The model will be calibrated to fit field constraints relating to former ice extent, past sea-levels, and present-day rebound. It will then be used to correct satellite data for the effects of GIA, and hence determine a more accurate map of current Antarctic ice-mass change.