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Progressing emergent constraints on future climate change

Abstract

In recent years, an evaluation technique for Earth System Models (ESMs) has arisen—emergent constraints (ECs)—which rely on strong statistical relationships between aspects of current climate and future change across an ESM ensemble. Combining the EC relationship with observations could reduce uncertainty surrounding future change. Here, we articulate a framework to assess ECs, and provide indicators whereby a proposed EC may move from a strong statistical relationship to confirmation. The primary indicators are verified mechanisms and out-of-sample testing. Confirmed ECs have the potential to improve ESMs by focusing attention on the variables most relevant to climate projections. Looking forward, there may be undiscovered ECs for extremes and teleconnections, and ECs may help identify climate system tipping points.

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Fig. 1: An illustration of the relationship between the emergent constraint approach and conventional climate model evaluation.
Fig. 2: Emergent relationships for two ECs relating to physical and biogeochemical components of the climate system.
Fig. 3: Illustration of the confirmation process for ECs.
Fig. 4: Four proposed ECs used to constrain Arctic sea ice projections in AR5.

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Acknowledgements

A.H. is supported by the National Science Foundation under Grant No. 1543268, and the U.S. Department of Energy Regional and Global Climate Modelling Program contract B618798:2. P.C. is supported by the European Research Council (ERC) ECCLES project (agreement number 742472) and the EU Horizon2020 CRESCENDO project (agreement number 641816). C.H. is supported by the NERC CEH National Capability Fund. S.K. is supported by the Regional and Global Climate Modelling Program of the Office of Science of the U.S. Department of Energy under the auspices of the U.S. DOE by LLNL under contract DE-AC52-07NA27344. This paper benefited from discussions at the Aspen Global Change Institute (AGCI) in 2017, during a model evaluation workshop that was attended by A.H., P.C. and S.K.

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A.H. drafted large portions of the paper, informed by discussions with C.H., P.C., and S.K., and an earlier manuscript drafted mainly by C.H. C.H., P.C. and S.K. each also drafted pieces of the paper. AH revised the paper in response to reviewer comments, after gathering feedback from C.H., P.C., and S.K. C.H. managed the references throughout the drafting process.

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Correspondence to Alex Hall.

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Journal peer review information: Nature Climate Change thanks Benjamin Sanderson and Tapio Schneider for their contribution to the peer review of this work.

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Hall, A., Cox, P., Huntingford, C. et al. Progressing emergent constraints on future climate change. Nat. Clim. Chang. 9, 269–278 (2019). https://doi.org/10.1038/s41558-019-0436-6

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