Abrupt reductions in the future Arctic ice cover

We examine the trajectory of Arctic summer sea ice in future climate projections and find that abrupt transitions are a common feature of 21st century model simulations. These events have decreasing trends in September ice extent that are typically 4 times larger than comparable observed trends. One event exhibits a decrease from 6 million km2 to 2 million km2 in 10 years, reaching essentially ice-free September conditions by 2040. In the simulations, ice retreat accelerates as thinning increases the efficiency of open water formation for a given melt rate and the ice-albedo feedback increases shortwave absorption. In one climate model, the retreat is abrupt when ocean heat transport to the Arctic is rapidly increasing. Analysis from multiple climate models and three different future forcing scenarios suggests that reductions in future greenhouse gas emissions moderate the likelihood and severity of these events.