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An assessment of Earth's climate sensitivity using multiple lines of evidence

An assessment of Earth's climate sensitivity using multiple lines of evidence
An assessment of Earth's climate sensitivity using multiple lines of evidence

We assess evidence relevant to Earth's equilibrium climate sensitivity per doubling of atmospheric CO 2, characterized by an effective sensitivity S. This evidence includes feedback process understanding, the historical climate record, and the paleoclimate record. An S value lower than 2 K is difficult to reconcile with any of the three lines of evidence. The amount of cooling during the Last Glacial Maximum provides strong evidence against values of S greater than 4.5 K. Other lines of evidence in combination also show that this is relatively unlikely. We use a Bayesian approach to produce a probability density function (PDF) for S given all the evidence, including tests of robustness to difficult-to-quantify uncertainties and different priors. The 66% range is 2.6–3.9 K for our Baseline calculation and remains within 2.3–4.5 K under the robustness tests; corresponding 5–95% ranges are 2.3–4.7 K, bounded by 2.0–5.7 K (although such high-confidence ranges should be regarded more cautiously). This indicates a stronger constraint on S than reported in past assessments, by lifting the low end of the range. This narrowing occurs because the three lines of evidence agree and are judged to be largely independent and because of greater confidence in understanding feedback processes and in combining evidence. We identify promising avenues for further narrowing the range in S, in particular using comprehensive models and process understanding to address limitations in the traditional forcing-feedback paradigm for interpreting past changes.

Bayesian methods, Climate, climate sensitivity, global warming
8755-1209
Sherwood, S.
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Webb, M. J.
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Annan, J. D.
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Armour, K. C.
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Forster, P. M.
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Hargreaves, J. C.
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Hegerl, G.
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Klein, S. A.
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Marvel, K. D.
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Rohling, E. J.
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Watanabe, M.
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Andrews, T.
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Braconnot, P.
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Bretherton, C. S.
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Foster, G. L.
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Hausfather, Z.
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Heydt, A. S. Von Der
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Knutti, R.
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Mauritsen, T.
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Norris, J. R.
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Proistosescu, C.
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Rugenstein, M.
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Schmidt, G. A.
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Tokarska, K. B.
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Zelinka, M. D.
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Sherwood, S.
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Webb, M. J.
a572ad9b-7b1b-418c-87b2-f3e0d4b1e848
Annan, J. D.
dfa1bdc7-bf41-409c-960c-1d96adca782e
Armour, K. C.
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Forster, P. M.
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Hargreaves, J. C.
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Hegerl, G.
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Klein, S. A.
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Marvel, K. D.
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Rohling, E. J.
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Watanabe, M.
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Andrews, T.
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Braconnot, P.
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Bretherton, C. S.
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Foster, G. L.
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Hausfather, Z.
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Heydt, A. S. Von Der
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Knutti, R.
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Mauritsen, T.
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Norris, J. R.
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Proistosescu, C.
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Rugenstein, M.
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Schmidt, G. A.
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Tokarska, K. B.
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Zelinka, M. D.
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Sherwood, S., Webb, M. J., Annan, J. D., Armour, K. C., Forster, P. M., Hargreaves, J. C., Hegerl, G., Klein, S. A., Marvel, K. D., Rohling, E. J., Watanabe, M., Andrews, T., Braconnot, P., Bretherton, C. S., Foster, G. L., Hausfather, Z., Heydt, A. S. Von Der, Knutti, R., Mauritsen, T., Norris, J. R., Proistosescu, C., Rugenstein, M., Schmidt, G. A., Tokarska, K. B. and Zelinka, M. D. (2020) An assessment of Earth's climate sensitivity using multiple lines of evidence. Reviews of Geophysics, 58 (4), [e2019RG000678]. (doi:10.1029/2019RG000678).

Record type: Article

Abstract

We assess evidence relevant to Earth's equilibrium climate sensitivity per doubling of atmospheric CO 2, characterized by an effective sensitivity S. This evidence includes feedback process understanding, the historical climate record, and the paleoclimate record. An S value lower than 2 K is difficult to reconcile with any of the three lines of evidence. The amount of cooling during the Last Glacial Maximum provides strong evidence against values of S greater than 4.5 K. Other lines of evidence in combination also show that this is relatively unlikely. We use a Bayesian approach to produce a probability density function (PDF) for S given all the evidence, including tests of robustness to difficult-to-quantify uncertainties and different priors. The 66% range is 2.6–3.9 K for our Baseline calculation and remains within 2.3–4.5 K under the robustness tests; corresponding 5–95% ranges are 2.3–4.7 K, bounded by 2.0–5.7 K (although such high-confidence ranges should be regarded more cautiously). This indicates a stronger constraint on S than reported in past assessments, by lifting the low end of the range. This narrowing occurs because the three lines of evidence agree and are judged to be largely independent and because of greater confidence in understanding feedback processes and in combining evidence. We identify promising avenues for further narrowing the range in S, in particular using comprehensive models and process understanding to address limitations in the traditional forcing-feedback paradigm for interpreting past changes.

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2020 WCRP_ECS_Final_manuscript_2019RG000678R_FINAL_200720 - Accepted Manuscript
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Accepted/In Press date: 24 June 2020
e-pub ahead of print date: 22 July 2020
Published date: December 2020
Additional Information: ©2020. American Geophysical Union. All Rights Reserved.
Keywords: Bayesian methods, Climate, climate sensitivity, global warming

Identifiers

Local EPrints ID: 443117
URI: http://eprints.soton.ac.uk/id/eprint/443117
ISSN: 8755-1209
PURE UUID: c3f179c6-eb16-44b7-a54c-85c0b218b6b7
ORCID for E. J. Rohling: ORCID iD orcid.org/0000-0001-5349-2158
ORCID for G. L. Foster: ORCID iD orcid.org/0000-0003-3688-9668

Catalogue record

Date deposited: 11 Aug 2020 16:31
Last modified: 26 Nov 2021 07:18

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Contributors

Author: S. Sherwood
Author: M. J. Webb
Author: J. D. Annan
Author: K. C. Armour
Author: P. M. Forster
Author: J. C. Hargreaves
Author: G. Hegerl
Author: S. A. Klein
Author: K. D. Marvel
Author: E. J. Rohling ORCID iD
Author: M. Watanabe
Author: T. Andrews
Author: P. Braconnot
Author: C. S. Bretherton
Author: G. L. Foster ORCID iD
Author: Z. Hausfather
Author: A. S. Von Der Heydt
Author: R. Knutti
Author: T. Mauritsen
Author: J. R. Norris
Author: C. Proistosescu
Author: M. Rugenstein
Author: G. A. Schmidt
Author: K. B. Tokarska
Author: M. D. Zelinka

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