Editing IPCC:AR6/WGI/Chapter-7 (section)

==== 7.5.4.3 Assessed ECS and TCR Based on Emergent Constraints ====

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The available emergent constraint studies have been divided into two classes: (i) those that are based on global or near-global indices, such as global surface temperature and the TOA energy budget; and (ii) those that are more focussed on physical processes, such as the fidelity of phenomena related to low-level cloud feedbacks or present-day climate biases. The former class is arguably superior in representing ECS, since it is a global surface temperature or energy budget change, whereas the latter class is perhaps best thought of as providing constraints on individual climate feedbacks, for example, the determination that low-level cloud feedbacks are positive. The latter result is consistent with and confirms process-based estimates of low-cloud feedbacks ( [[#7.4.2.4|Section 7.4.2.4]] ), but are potentially biased as a group by missing or biased feedbacks in ESMs and is accordingly not taken into account here. A limiting case here is [[#Dessler--2018|Dessler and Forster (2018)]] which is focused on monthly co-variability in the global TOA energy budget with mid-tropospheric temperature, at which time scale the surface-albedo feedback is unlikely to operate, thus implicitly assuming it is unbiased in the model ensemble.

In the first group of emergent constraints there is broad agreement on the best estimate of ECS ranging from 2.4°C–3.3°C. At the lower end, nearly all studies find lower bounds (5th percentiles) around 1.5°C, whereas several studies indicate 95th percentiles as low as 4°C. Considering both classes of studies, none of them yield upper ''very'' ''likely'' bounds above 5°C. Since several of the emergent constraints can be considered nearly independent one could assume that emergent constraints provide very strong evidence on ECS by combining them. Nevertheless, this is not done here because there are sufficient cross-dependencies, as for instance models are re-used in many of the derived emergent constraints, and furthermore the methodology has not yet reached a sufficient level of maturity since systematic biases may not have been accounted for. Uncertainty is therefore conservatively added to reflect these potential issues. This leads to the assessment that ECS inferred from emergent constraints is ''very likely'' 1.5 to 5 °C with ''medium confidence'' .

Emergent constraints on TCR with a focus on the instrumental temperature record, though less abundant, have also been proposed. These can be influenced by internal variability and pattern effects, as discussed in ( [[#7.5.4.1|Section 7.5.4.1]] , although the influence is smaller because uncertainty in forced pattern effects correlates between transient historical warming and TCR. In the simplest form [[#Gillett--2012|Gillett et al. (2012)]] regressed the response of one model to individual historical forcing components to obtain a tight range of 1.3°C–1.8°C, but later when an ensemble of models was used the range was widened to 0.9°C–2.3°C ( [[#Gillett--2013|Gillett et al., 2013]] ), and updated by [[#Schurer--2018|Schurer et al. (2018)]] . A related data-assimilation-based approach that accounted also for uncertainty in response patterns gave 1.33°C–2.36°C ( [[#Ribes--2021|Ribes et al., 2021]] ), but is dependent on the choice of prior ensemble distribution (CMIP5 or CMIP6). Another study used the response to the Pinatubo volcanic eruption to obtain a range of 0.8°C–2.3°C ( [[#Bender--2010|Bender et al., 2010]] ). A tighter range, notably at the lower end, was found in an emergent constraint focusing on the post-1970s warming exploiting the lower spread in aerosol forcing change over this period ( [[#Jiménez-de-la-Cuesta--2019|Jiménez-de-la-Cuesta and Mauritsen, 2019]] ). Their estimate was 1.67 [1.17 to 2.16] °C. Two studies tested this idea: [[#Tokarska--2020|Tokarska et al. (2020)]] estimates TCR was 1.60 [0.90 to 2.27] °C based on CMIP6 models, whereas [[#Nijsse--2020|Nijsse et al. (2020)]] found 1.68 [1.0 to 2.3] °C. In both cases there was a small sensitivity to choice of ensemble, with CMIP6 models yielding slightly lower values and ranges. Combining these studies gives a best estimate of 1.7°C and a ''very likely'' range of TCR of 1.1 to 2.3 °C with ''high confidence'' .

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