Reduced probability of ice-free summers for 1.5 °C compared to 2 °C warming

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Arctic sea ice has declined rapidly with increasing global temperatures. However, it is largely unknown how Arctic summer sea-ice impacts would vary under the 1.5 °C Paris target compared to scenarios with greater warming. Using the Community Earth System Model, I show that constraining warming to 1.5 °C rather than 2.0 °C reduces the probability of any summer ice-free conditions by 2100 from 100% to 30%. It also reduces the late-century probability of an ice cover below the 2012 record minimum from 98% to 55%. For warming above 2 °C, frequent ice-free conditions can be expected, potentially for several months per year. Although sea-ice loss is generally reversible for decreasing temperatures, sea ice will only recover to current conditions if atmospheric CO2 is reduced below present-day concentrations. Due to model biases, these results provide a lower bound on summer sea-ice impacts, but clearly demonstrate the benefits of constraining warming to 1.5 °C.

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This work was funded by CU and NSF OPP grant no. 1504348. I thank J. Kay, D. M. Hall and F. Massonnet for valuable feedback on an earlier version of the manuscript, E. Rosenblum for sharing her CMIP5 SIEs and temperatures, D. Notz, S. Sigmond, B. Sanderson and the CESM Polar Climate Working Group members for useful discussions, and the PIs and members of the three CESM1.1 ensemble projects for planning and running the ensembles and freely sharing the output. The CESM project is supported by the NSF and the Office of Science (BER) of the US Department of Energy. Computing resources for the CESM ensembles were provided by the Climate Simulation Laboratory at NCAR’s Computational and Information Systems Laboratory, sponsored by the NSF and other agencies. Five of the CESM LE simulations were produced at the University of Toronto under the supervision of P. Kushner. NCL was used for data analysis.

Author information


  1. Department of Atmospheric and Oceanic Sciences and Institute of Arctic and Alpine Research, University of Colorado at Boulder, Boulder, CO, USA

    • Alexandra Jahn


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A.J. designed the study, performed the analysis, and wrote the manuscript.

Competing interests

The author declares no competing interests.

Corresponding author

Correspondence to Alexandra Jahn.

Supplementary information

  1. Supplementary Information

    Additional ice-free definitions, Supplementary Figures 1–5 and Supplementary References