Hysteresis of the Antarctic Ice Sheet

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The Antarctic Ice Sheet is the largest reservoir of freshwater on the planet with an ice volume that is equivalent to approximately 58 m global sea level rise. Even small changes in the mass balance of the ice sheet thus have enormous global impacts, which makes the question of its future evolution highly relevant for society. The current net ice loss from Antarctica is accelerating, bearing the risk to become a major, if not the dominant contributor to sea level rise in the 21st century. Several positive feedbacks, including the marine ice-sheet instability, marine ice-cliff instability as well as the ice-albedo and melt-elevation feedbacks, could lead to abrupt and potentially irreversible decay of certain regions around Antarctica. In this project we propose to explore the stability landscape of the Antarctic Ice Sheet with the Parallel Ice Sheet Model (PISM). Central focus will be given to include the mechanisms relevant for rapid ice dynamics and surface mass balance feedbacks that can drive Antarctic instability, in particular introducing a surface module which will enable PISM to capture the melt-elevation and ice-albedo feedbacks. Furthermore, we will implement hydrofracturing and ice-cliff instability parametrizations. Based on the updated physics in PISM, we plan to create an ensemble of plausible present-day states for the entire Antarctic Ice Sheet. For the best-fit initial state, we will then create a hysteresis diagram through the slow change of the climatic boundary conditions. The time scale of change herein needs to be slower than the dynamic time scale of the ice sheet, so that the ice sheet remains in quasi-equilibrium and transient effects are minimized. The hysteresis simulations will thus be run on a million-year timescale. We expect to find threshold behavior for a set of different ice sheet basins at different levels of warming and aim at quantifying the uncertainty range of thresholds. The proposed project will thus provide a first comprehensive analysis of the stability of the present-day Antarctic Ice Sheet and yield new insights concerning the disentanglement of driving forces behind (abrupt) ice loss. This knowledge of critical temperature thresholds and related uncertainties are key in the current discussions and negotiations on the compliance with ambitious climate targets and will help to clarify our responsibility for the consequences of long-term climate change.