| Summary: | State-of-the-art climate models simulate a large spread in the projected decline of Arctic sea-ice area (SIA) over the 21st century. Here we diagnose causes of this intermodel spread using a simple model that approximates future SIA based on present SIA and the sensitivity of SIA to Arctic temperatures. This model accounts for 70%–95% of the intermodel variance, with the majority of the spread arising from present-day biases. The remaining spread arises from intermodel differences in Arctic warming, with some contribution from differences in the local sea-ice sensitivity. Using observations to constrain the projections moves the probability of an ice-free Arctic forward by 10–35 years when compared to unconstrained projections. Under a high-emissions scenario, an ice-free Arctic will likely (urn:x-wiley:00948276:media:grl62946:grl62946-math-000166% probability) occur between 2036 and 2056 in September and between 2050 and 2068 from July to October. Under a medium-emissions scenario, the "likely" date occurs between 2040 and 2062 in September and much later in the 21st century from July to October. © 2021. American Geophysical Union. Issue Online: 16 September 2021; Version of Record online: 16 September 2021; Accepted manuscript online: 29 August 2021; Manuscript accepted: 24 August 2021; Manuscript revised: 13 August 2021; Manuscript received: 12 May 2021. The authors thank Elizabeth Hunke, Caroline Holmes, and François Massonnet for helpful comments on an earlier draft of this work. The authors also thank two anonymous reviewers and the Editor, Gudrun Magnusdottir, for helpful comments. D. B. Bonan was supported by an American Meteorological Society (AMS) Graduate Fellowship and the National Science Foundation Graduate Research Fellowship Program (NSF Grant DGE-1745301). Part of this research was supported by Eric and Wendy Schmidt by recommendation of the Schmidt Futures program. I. Eisenman acknowledges support from the National Science Foundation (Grant OPP-1643445. R. C. J. Wills also acknowledges ...
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