Present-day mass loss rates are a precursor for West Antarctic Ice Sheet collapse

Abstract. Observations of recent mass loss rates of the West Antarctic Ice Sheet (WAIS) raise concerns about its stability since a collapse would increase global sea levels by several meters. Future projections of these mass loss trends are often estimated using numerical ice sheet models. However, most current models display low skill in reproducing observed mass change rates accurately. Here, we develop a new initialization method that optimizes agreement not only with observations of ice thickness and surface velocity, but also with satellite-based estimates of mass change rates. Starting from this improved present-day state, we generate an ensemble of future projections of Antarctic mass change, covering uncertainties in model choices, parameter values and (observational) input data. Our ensemble displays a slow retreat over several centuries followed by a speed-up that lasts around 200 years. We find that for all ensemble members, the Thwaites and Pine Island glaciers collapse, even though the climate is held constant at present-day values. Our results imply that today’s mass loss rates are a precursor of the deglaciation of large parts of the WAIS, which would raise sea levels by at least a meter in the coming centuries, without additional climate forcing.