Settled Science? New Climate Study Shifts the Goalposts to 2.6-3.9C

Guest essay by Eric Worrall

A new climate study has dismissed utterly implausible high end climate models. But the new study also seeks to raise the low end of the range of estimated climate sensitivity into the discomfort zone.

The climate won’t warm as much as we feared – but it will warm more than we hoped

July 23, 2020 5.52am AEST

Steven SherwoodARC Laureate Fellow, Climate Change Research Centre, UNSW

Eelco RohlingProfessor of Ocean and Climate Change, Australian National University

Katherine MarvelAssociate Research Scientist, NASA

We know the climate changes as greenhouse gas concentrations rise, but the exact amount of expected warming remains uncertain. 

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A major new assessment has now calculated a range of 2.6–3.9℃. This implies that alarmingly high estimates from some recent climate models are unlikely, but also that comfortingly low estimates from other studies are even less likely.

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In 1979, a farsighted report estimated for the first time that equilibrium climate sensitivity falls somewhere between 1.5℃ and 4.5℃. So if carbon dioxide concentrations doubled, global temperatures would eventually increase by somewhere in that range. 

The width of this range is a problem. If equilibrium climate sensitivity lies at the low end of the range, climate change might be manageable with relatively relaxed national policies.

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Read more: https://theconversation.com/the-climate-wont-warm-as-much-as-we-feared-but-it-will-warm-more-than-we-hoped-143175

The abstract of the study;

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

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

We assess evidence relevant to Earth’s equilibrium climate sensitivity per doubling of atmospheric CO2, 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 (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.

Read more: https://climateextremes.org.au/wp-content/uploads/2020/07/WCRP_ECS_Final_manuscript_2019RG000678R_FINAL_200720.pdf

The study uses an unusual definition of equilibrium climate sensitivity, though they provide a detailed explanation for their choice. From the main body of the study;

In choosing the reference scenario to define sensitivity for this assessment, for practical reasons we depart from the traditional Charney ECS definition (equilibrium response with ice sheets and vegetation assumed fixed) in favor of a comparable and widely used, so-called “effective climate sensitivity” S derived from system behavior during the first 150 years following a (hypothetical) sudden quadrupling of CO2. During this time the system is not in equilibrium, but regression of global-mean top-of-atmosphere energy imbalance onto global-mean near-surface air temperature, extrapolated to zero imbalance, yields an estimate of the long-term warming valid if the average feedbacks active during the first 150 years persisted to equilibrium (Gregory et al., 2004). This quantity therefore approximates the long-term Charney ECS (e.g., Danabasoglu and Gent, 2009), though how well it does so is a matter of active investigation addressed below. Our reference scenario does not formally exclude any feedback process, but the 150-year time frame minimizes slow feedbacks (especially ice sheet changes).

Read more: Same link as above

The treatment of cloud feedback is interesting. The study acknowledges large cloud feedback uncertainties, mentions the Lindzen et al. (2001) “iris effect”, and admits GCMs cannot be trusted to reproduce observed cloud response, yet still appears to attempt to derive a cloud feedback factor based on satellite observations, and mix this observational cloud factor with model predictions.

The treatment of clouds may turn out to be one of the most controversial assumptions in the study – as Pat Frank has pointed out on a number of occasions, the magnitude of model cloud response error is significantly greater than the CO2 driven warming which models attempt to project, which calls into question whether climate models have any predictive skill whatsoever.

To the author’s credit they have described their method in great detail, so I’m looking forward to detailed responses to this study.