the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 24 Mar 2025
Comment on "Can uncertainty in climate sensitivity be narrowed further?" by Sherwood and Forest (2024)
Abstract. This comment addresses assertions made by Sherwood and Forest (2024) [SF24] regarding the narrowing of the range of equilibrium climate sensitivity (ECS), particularly at the low end. SF24 challenged a previous study by Lewis (2022) [L22] that found a narrower and substantially lower ECS level. This comment clarifies that, contrary to SF24's claims, L22 did not rule out a high ECS level based on historical evidence, and did identify and correct errors in Sherwood et al. (2020). Those errors included use of an invalid likelihood estimation method that, ironically, substantially underestimated likelihood at high ECS levels for their historical evidence. This comment also discusses the role of priors in Bayesian ECS estimation and explains why the subjective Bayesian approach favoured by SF24 risks producing unreliable inference for uncertain parameters such as ECS. Finally, the importance of considering structural uncertainties in climate models, particularly concerning tropical warming patterns, is extended beyond the points raised by SF24. Such uncertainties could affect ECS estimation not only from historical period evidence but also from climate process understanding, paleoclimate data and emergent constraints, but seem more likely to suggest existing ECS estimates are too high than too low.
Status: open (until 15 May 2025)
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RC1: 'Comment on egusphere-2025-1179', Anonymous Referee #1, 15 Apr 2025
reply
The estimation of ECS is an extremely important element of climate science, with direct policy relevance, and the failure to date to reduce uncertainty much below that which the Charney Report expressed in 1979 has been the subject of extensive comment. The commentary by Sherwood and Forest (SF24) offers many useful insights. Since the lead author of SF24 was also a lead author of the IPCC AR6 chapter that relied heavily, in turn, on a paper of which he was likewise lead author (Sherwood et al. 2020, referred to herein as S20), his perspective is influential. Lewis’ paper (herein L25) pushes back on certain points and the resulting exchange is valuable for readers. Also LF24 made a number of critical remarks about L22 so it is fitting to have Lewis’ reply. In particular LF24 deny that L22 identified any errors in their method, dismissing them instead as mere differences in opinion. But L22 did indeed assert that there were errors in the S20 methodology and went into some detail about them. The correction of errors in empirical methodology is necessary for any branch of science so assertions of error need to be resolved in appropriate detail. This exchange does not go into sufficient detail to do so but helpfully outlines for readers what some of the issues are, leaving the details to the underlying papers.
My sense of the IPCC AR6 discussion of ECS was that it placed too much reliance on a hypothesized pattern effect bias, referring to a projected weakening of the west-to-east temperature gradient in the tropical Pacific which, were it to happen, would raise climate sensitivity to subsequent GHG emissions. The evidence of Seager et al. (2019), available at the time of the AR6 and cited by L25 (but not SF24) was that empirical evidence does not support such a pattern effect and that tropical Pacific dynamics do not imply such a pattern effect should be expected in the future, climate models notwithstanding. Consequently I found LF24’s continued assumption that the pattern effect bias should be taken for granted merits challenge, and L25 engages the point. LF24 cited some evidence in favour of the pattern effect. L25 acknowledges it but draws on other studies not cited by LF24 which provide counterevidence, and I think readers will find the exchange illuminating. However the discussion in L25 is broken up, occurring in lines 90-100 and then in lines 139-157, so editing is needed to combine these into a single discussion.
Much of L25 is focused on the issue of using Subjective versus Objective priors in Bayesian analysis. LF24 defended the Subjective approach and L25 advocates for the Objective approach. Here again the contrast in views is illuminating for readers, especially since L24 acknowledge that “most physical scientists are uncomfortable with explicitly subjective judgments”. L25 rightly argues that use of a Subjective prior that relies on the same evidence contained in the data sample is duplicative. There is much more needing to be said on this debate, but both author groups acknowledge that few climate scientists have had formal training in statistical theory so the discussion to date has been limited. I would recommend removing the second sentence of Section 3 so as not to sound antagonistic. However I think the points made in the concluding section are valid: little progress can be made if the methodology allows for arbitrary choices of subjective priors that strongly influence the results.
Paper appraisal:
Scientific significance: Excellent
Scientific quality: Excellent
Presentation quality: Good
- Does the paper address relevant scientific questions within the scope of ACP? Yes
- Does the paper present novel concepts, ideas, tools, or data? Yes
- Are substantial conclusions reached? Yes
- Are the scientific methods and assumptions valid and clearly outlined? Yes
- Are the results sufficient to support the interpretations and conclusions? yes
- Is the description of experiments and calculations sufficiently complete and precise to allow their reproduction by fellow scientists (traceability of results)? N/A
- Do the authors give proper credit to related work and clearly indicate their own new/original contribution? Yes
- Does the title clearly reflect the contents of the paper? Yes
- Does the abstract provide a concise and complete summary? Yes
- Is the overall presentation well structured and clear? Yes
- Is the language fluent and precise? Yes
- Are mathematical formulae, symbols, abbreviations, and units correctly defined and used? N/A
- Should any parts of the paper (text, formulae, figures, tables) be clarified, reduced, combined, or eliminated? 2 suggested edits noted above
- Are the number and quality of references appropriate? Yes
- Is the amount and quality of supplementary material appropriate? N/A
References:
Lewis, N.: Objectively combining climate sensitivity evidence, Clim. Dynam., 60, 3139-3165, https://doi.org/10.1007/s00382-022-06468-x, 2022.
Lewis, N.: Comment on "Can uncertainty in climate sensitivity be narrowed further?" by Sherwood and Forest (2024). Atmos. Chem. Phys., preprint https://doi.org/10.5194/egusphere-2025-1179
Seager, R., Cane, M., Henderson, N., Lee, D. E., Abernathey, R., and Zhang, H.: Strengthening tropical Pacific zonal sea surface temperature gradient consistent with rising greenhouse gases, Nat. Clim. Change, 9, 517-522, https://doi.org/10.1038/s41558-019-0505-x, 2019
Sherwood, S. C., and Forest, C. E.: Opinion: Can uncertainty in climate sensitivity be narrowed further?, Atmos. Chem. Phys., 24, 2679-2686, https://doi.org/10.5194/acp-24-2679-2024, 2024.
Sherwood, S. C., 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., Knutti, R., Mauritsen, T., Norris, J. R., Proistosescu, C., Rugenstein, M., Schmidt, G. A., Tokarska, K. B., and Zelinka, M. D.: An Assessment of Earth’s Climate Sensitivity Using Multiple Lines of Evidence, Rev. Geophys., 58, https://doi.org/10.1029/2019RG000678, 2020.
Citation: https://doi.org/10.5194/egusphere-2025-1179-RC1
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