An Assessment of Earth's Climate Sensitivity Using Multiple Lines of Evidence

Abstract

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.

Keywords

Affiliated Institutions

• University of Southampton GB
• Met Office GB
• Centre National de la Recherche Scientifique FR
• University of Illinois Urbana-Champaign US
• Utrecht University NL
• National Oceanography Centre GB
• Scripps Institution of Oceanography US
• Columbia University US
• Max Planck Institute for Meteorology DE
• Australian National University AU
• University of Leeds GB
• Goddard Institute for Space Studies US
• University of Washington US
• Université Paris-Saclay FR
• The University of Tokyo JP
• Université de Versailles Saint-Quentin-en-Yvelines FR
• Breakthrough Institute US
• Laboratoire des Sciences du Climat et de l'Environnement FR
• Stockholm University SE
• UNSW Sydney AU
• CEA Paris-Saclay FR
• University of Edinburgh GB
• ARC Centre of Excellence for Climate System Science AU
• Commissariat à l'Énergie Atomique et aux Énergies Alternatives FR

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