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

===== 7.4.2.5.3 Synthesis of biogeophysical and non-CO 2 biogeochemical feedbacks =====

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The non-CO <sub>2</sub> biogeochemical feedbacks are assessed in ( [[#7.4.2.5.1|Section 7.4.2.5.1]] to be –0.16 [–0.37 to +0.05] W m <sup>–</sup> <sup>2</sup> °C <sup>–1</sup> and the biogeophysical feedbacks are assessed in ( [[#7.4.2.5.2|Section 7.4.2.5.2]] to be +0.15 [0.0 to +0.3] W m <sup>–2</sup> °C <sup>–1</sup> . The sum of the biogeophysical and non-CO <sub>2</sub> biogeochemical feedbacks is assessed to have a central value of –0.01 W m <sup>–2</sup> °C <sup>–1</sup> and a ''very likely'' range from –0.27 to +0.25 W m <sup>–2</sup> °C <sup>–1</sup> (Table 7.10). Given the relatively long time scales associated with the biological processes that mediate the biogeophysical and many of the non-CO <sub>2</sub> biogeochemical feedbacks, in comparison with the relatively short time scale of many of the underlying model simulations, combined with the small number of studies for some of the feedbacks, and the relatively small signals, this overall assessment has ''low confidence'' .

Some supporting evidence for this overall assessment can be obtained from the CMIP6 ensemble, which provides some pairs of instantaneous 4×CO <sub>2</sub> simulations carried out using related models, with and without biogeophysical and non-CO <sub>2</sub> biogeochemical feedbacks. This is not a direct comparison because these pairs of simulations may differ by more than just their inclusion of these additional feedbacks; furthermore, not all biogeophysical and non-CO <sub>2</sub> biogeochemical feedbacks are fully represented. However, a comparison of the pairs of simulations does provide a first-order estimate of the magnitude of these additional feedbacks. [[#Séférian--2019|Séférian et al. (2019)]] find a slightly more negative feedback parameter in CNRM-ESM2-1 (with additional feedbacks) then in CNRM-CM6-1 (a decrease of 0.02 W m <sup>–2</sup> °C <sup>–1</sup> , using the linear regression method from years 10–150). [[#Andrews--2019|Andrews et al. (2019)]] also find a slightly more negative feedback parameter when these additional feedbacks are included (a decrease of 0.04 W m <sup>–2</sup> °C <sup>–1</sup> in UKESM1 compared with HadGEM3-GC3.1). Both of these studies suggest a small but slightly negative feedback parameter for the combination of biogeophysical and non-CO <sub>2</sub> biogeochemical feedbacks, but with relatively large uncertainty given (i) interannual variability and (ii) that feedbacks associated with natural terrestrial emissions of CH <sub>4</sub> and N <sub>2</sub> O were not represented in either pair.

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