Future abrupt reductions in the summer Arctic sea ice

TLDR

Abrupt summer ice loss occurs when thinning boosts open‑water formation efficiency and ice‑albedo feedback amplifies shortwave absorption, and when ocean heat transport to the Arctic rises rapidly. The simulations show that abrupt summer sea‑ice reductions are common, with September ice extent declining four times faster than observed, and one scenario dropping from 6 to 2 million km² by 2040; over half of the models predict such events, and lower greenhouse‑gas emissions could reduce their likelihood.

Abstract

We examine the trajectory of Arctic summer sea ice in seven projections from the Community Climate System Model and find that abrupt reductions are a common feature of these 21st century simulations. These events have decreasing September ice extent trends that are typically 4 times larger than comparable observed trends. One event exhibits a decrease from 6 million km 2 to 2 million km 2 in a decade, reaching near ice‐free September conditions by 2040. In the simulations, ice retreat accelerates as thinning increases the open water formation efficiency for a given melt rate and the ice‐albedo feedback increases shortwave absorption. The retreat is abrupt when ocean heat transport to the Arctic is rapidly increasing. Analysis from multiple climate models and three forcing scenarios indicates that abrupt reductions occur in simulations from over 50% of the models and suggests that reductions in future greenhouse gas emissions moderate the likelihood of these events.