Seasonality and scenario dependence of rapid Arctic sea ice loss events in CMIP6 simulations

Abstract. The end-of-summer Arctic Ocean is projected to face at least one occurrence of practically ice-free conditions (sea ice extent < 1 million km²) by the middle of the century under all Climate Model Intercomparison Project phase 6 (CMIP6) scenarios. Climate models indicate that this transition toward a nearly ice-free Arctic Ocean in late summer will be punctuated by rapid ice loss events (RILEs), i.e., reductions in sea ice extent that occur at a much faster rate than expected from the forced contribution. The extreme sea ice loss associated with RILEs in climate models is larger than what has been observed since the start of the satellite era (−0.28 million km² per year over 2001–2008). As such, it could lead to a much faster transition toward practically ice-free conditions than expected based on a linear trend. RILEs are not well understood and it is currently impossible to predict their occurrence a season to several years ahead. It is therefore essential to improve our understanding of these events. This study presents the first comprehensive analysis of RILEs in a diverse set of 26 CMIP6 models, including 5 large ensembles,  following both low and high warming scenarios over the period from 1970 to 2100. Our analysis shows that RILEs are expected to occur year-round. However, the timing and duration of the events are found to be season-dependent, with less frequent but longer-lived RILEs in winter and spring, and more frequent but shorter-lived RILEs in summer and autumn under a high emission scenario. In addition, we find that the warming scenario has a greater influence on RILE characteristics in the winter/spring season than in the summer/autumn season. Our results also emphasize that model uncertainty is larger regarding the probability and characteristics of RILEs for winter/spring events compared to summer/autumn ones. Finally, while the initial sea ice extent at which RILEs are triggered depends on whether they occur in September or March, the initial sea ice volume is similar for both months, which emphasizes the critical role of sea ice thickness as a preconditioning factor for RILEs. Based on CMIP6 models, there is 63 % chance that at least one summer RILE starts before 2030 in September. The study of RILEs is particularly opportune as, after more than ten years of relatively stable conditions between 2012–2023, the current summer Arctic sea ice state has an increased probability to be on the verge of a rapid reduction for the coming decade.