Under the Paris Agreement, emissions scenarios are pursued that would stabilize the global mean temperature at 1.5–2.0 °C above pre-industrial levels, but current emission reduction policies are expected to limit warming by 2100 to approximately 3.0 °C. Whether such emissions scenarios would prevent a summer sea-ice-free Arctic is unknown. Here we employ stabilized warming simulations with an Earth System Model to obtain sea-ice projections under stabilized global warming, and correct biases in mean sea-ice coverage by constraining with observations. Although there is some sensitivity to details in the constraining method, the observationally constrained projections suggest that the benefits of going from 2.0 °C to 1.5 °C stabilized warming are substantial; an eightfold decrease in the frequency of ice-free conditions is expected, from once in every five to once in every forty years. Under 3.0 °C global mean warming, however, permanent summer ice-free conditions are likely, which emphasizes the need for nations to increase their commitments to the Paris Agreement.
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We acknowledge the Canadian Sea Ice and Snow Evolution Network for proposing the CanESM2 Large Ensemble simulations and the modelling groups, the Program for Climate Model Diagnosis and Intercomparison, and the WCRP Working Group on Coupled Modelling for their roles in making available the WCRP CMIP5 multimodel data set. We thank W. Lee and Y. Jiao for technical assistance and B. Merryfield and S. Howell for their helpful comments on an earlier draft.