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Abstract

Ice loss from Antarctica’s vast freshwater reservoir could threaten coastal communities and the global economy if the ice volume decreases by just a few percent. Key processes controlling the fine balance between ice gain and ice loss remain poorly constrained, so that Antarctica’s contribution to future sea-level changes ranks as the most uncertain of all potential contributors. Meanwhile, observed changes in mass balance are limited to ∼40 years and difficult to put into context for an ice sheet with response time scales reaching centuries to millennia. To gain a much longer-term perspective, we here combine transient and equilibrium Parallel Ice Sheet Model simulations of Antarctic Ice Sheet response to repeated glacial-interglacial warming and cooling cycles over the last 800,000 years. We find hysteresis (path-dependent states) that is caused both by the long response time and by crossing of tipping points. Notably, West Antarctic Ice Sheet collapse contributes over 4 m sea-level rise in equilibrium ice sheet states with little (0.25◦C) or even no ocean warming above present. Therefore, today we are likely already at (or almost at) an 'overshoot' scenario, supporting recent studies warning of substantial irreversible ice loss on millennial time-scales with little or no further climate warming.