Constraining the First Year of Ice‐Free Arctic: Importance of Regional Perspective.

The ice‐free Arctic in summer not only symbolizes human‐induced climate change but also highlights the need to critically consider climate change adaptation policies. To constrain projections when the Arctic Ocean will first become ice‐free, studies have typically combined the historical observations of the Arctic sea ice area (SIA) with future emission scenario simulations. However, these studies primarily relied on the historical climatology and trend of the Arctic SIA, without considering regional variations. In this study, we analyze September SIA projections using the Coupled Model Intercomparison Project Phase 6 (CMIP6) model simulations, while considering Arctic sub‐regions. Additionally, we assess the impact of incorporating sub‐region September SIA when constraining the first year of ice‐free Arctic in September. CMIP6 models generally overestimate the historical September SIA decreasing trend in the central Arctic, whereas they underestimate this trend in the surrounding shelf seas. The central Arctic, where the region expected to retain sea ice for the longest period in the future, holds particular significance for projecting when the Arctic will first become ice‐free. Consequently, when we employ the historical trend of September SIA in the central Arctic as a constraint, observationally‐constrained projections suggest a delay of 12 years (2056) for the first ice‐free September compared to raw/unconstrained projections (2044 in model average) under a high‐emission scenario. These findings underscore the importance of considering model biases in central Arctic SIA when constraining projections of the first year of ice‐free Arctic, which may occur later than previously projected in many studies. Plain Language Summary: The Arctic sea ice area (SIA) has steadily decreased over the historical period, and future projections indicate a continuous decrease throughout the twenty‐first century, which can potentially lead to an ice‐free Arctic in September. However, there are large uncertainties in climate model projections regarding when the first year of ice‐free Arctic in September will emerge, even under specific emissions scenarios. This study employs a statistical analysis that incorporates historically observed information to generate observationally‐constrained future projections. Particularly, we focus on the decline in September central Arctic SIA, a critical region expected to be the last to become ice‐free Arctic in September in the future. The latest climate model simulations mostly overestimate the observed September SIA decrease in the central Arctic during the historical period. Moreover, the climate model simulations exhibit robust statistical relationship between the historical trends in September central Arctic SIA and the projected year of the first ice‐free Arctic in September. Utilizing this relationship combined with the observed information for the historical trend of September central Arctic SIA, our constrained projections suggest as 12 years delay in the first year of ice‐free Arctic in September compared to the original model projections under a high‐emission scenario. Key Points: Coupled Model Intercomparison Project Phase 6 (CMIP6) models generally overestimate the observed September central Arctic sea ice area (SIA) decreaseHistorical central Arctic SIA trend positively correlates with the first year of ice‐free Arctic in September, serving as a constraintConstrained projections suggest 12 years delayed September ice‐free Arctic emergence than raw projections under a high‐emission scenario [ABSTRACT FROM AUTHOR]