M.: The reversibility of sea ice loss in a state-of-the-art climate model, Geophys

Rapid Arctic sea ice retreat has fueled speculation about the possibility of threshold (or ‘tipping point’) behavior and irreversible loss of the sea ice cover. We test sea ice re-versibility within a state-of-the-art atmosphere–ocean global climate model by increasing atmospheric carbon dioxide un-til the Arctic Ocean becomes ice-free throughout the year and subsequently decreasing it until the initial ice cover re-turns. Evidence for irreversibility in the form of hysteresis outside the envelope of natural variability is explored for the loss of summer and winter ice in both hemispheres. We find no evidence of irreversibility or multiple ice-cover states over the full range of simulated sea ice conditions between the modern climate and that with an annually ice-free Arc-tic Ocean. Summer sea ice area recovers as hemispheric temperature cools along a trajectory that is indistinguish-able from the trajectory of summer sea ice loss, while the recovery of winter ice area appears to be slowed due to the long response times of the ocean near the modern winter ice edge. The results are discussed in the context of previous studies that assess the plausibility of sea ice tipping points by other methods. The findings serve as evidence against the existence of threshold behavior in the summer or winter ice cover in either hemisphere. 1