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Climate constraint reflects forced signal

Nature volume 563, pages E6–E9 (2018)Cite this article

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arising from P. M. Cox, C. Huntingford & M. S. Williamson Nature 553, 319–322 (2018); https://doi.org/10.1038/nature25450

A recent paper by Cox et al.1 introduces Ψ, “a theoretically informed metric of global temperature variability”, which scales with equilibrium climate sensitivity (ECS) across 16 general circulation models (GCMs). Cox et al.1 report that Ψ provides a strong constraint on ECS, ruling out both high and low values. Our analysis shows that this constraint is sensitive to the GCMs considered, primarily reflects the forced climate response rather than climate variability and does not narrow the uncertainty in ECS. It is therefore premature to rule out the possibility of large ECS values. There is a Reply to this Comment by Cox, P. M. et al. Nature 563, https://doi.org/10.1038/s41586-018-0641-x (2018).

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Fig. 1: The ECS constraint depends on the underlying forcing.
Fig. 2: Forced temperature changes contaminate Ψ.

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Author information

Authors and Affiliations

  1. Lawrence Livermore National Laboratory, Livermore, CA, USA

    Stephen Po-Chedley & Benjamin D. Santer

  2. Joint Institute for the Study of the Atmosphere and the Ocean, University of Washington, Seattle, WA, USA

    Cristian Proistosescu

  3. Department of Atmospheric Sciences and School of Oceanography, University of Washington, Seattle, WA, USA

    Kyle C. Armour

Authors
  1. Stephen Po-Chedley
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  2. Cristian Proistosescu
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  3. Kyle C. Armour
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  4. Benjamin D. Santer
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Contributions

All authors collaborated on the design of the study, the interpretation of the results and writing the manuscript. S.P. performed the analysis using CMIP5 data.

Corresponding author

Correspondence to Stephen Po-Chedley.

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Declared none.

Extended data figures and tables

Extended Data Fig. 1 The strength of the \(\bar{{\boldsymbol{\Psi }}}\)–ECS relationship depends on the models considered.

a, \(\bar{\Psi }\) versus ECS for the pre-industrial control experiment (as in Fig. 1a), but including six additional models listed in extended data table 1 of Cox et al.1 (grey) and five additional models not included in their original analysis (red; see Supplementary Information). The black line represents the regression obtained with the original 16-model subset of Cox et al.1, the grey line represents the regression with the 22-model subset (grey and black dots) and the red line represents the regression using all 27 models. The dotted grey lines connect models from a common modelling centre. The correlation coefficient is listed in parentheses for each set of models considered. b, As in a, but for the historical experiment. Using all models and the early historical period (1880–1962) to compute \(\bar{\Psi }\) (as in Fig. 1c), we arrive at a median ECS of 3.4 °C (95% confidence interval of 1.9–4.9 °C).

Supplementary information

Supplementary Methods

This file contains Supplementary Methods, References and Acknowledgements.

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Po-Chedley, S., Proistosescu, C., Armour, K.C. et al. Climate constraint reflects forced signal. Nature 563, E6–E9 (2018). https://doi.org/10.1038/s41586-018-0640-y

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