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

==== 7.4.2.7 Synthesis ====

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Table 7.10 summarizes the estimates and the assessment of the individual and the net feedbacks presented in the above sections. The uncertainty range of the net climate feedback was obtained by adding standard deviations of individual feedbacks in quadrature, assuming that they are independent and follow the Gaussian distribution. It is ''virtually certain'' that the net climate feedback is negative, primarily due to the Planck temperature response, indicating that climate acts to stabilize in response to radiative forcing imposed to the system. Supported by the level of confidence associated with the individual feedbacks, it is also ''virtually certain'' that the sum of the non-Planck feedbacks is positive. Based on Table 7.10 these climate feedbacks amplify the Planck temperature response by about 2.8 [1.9 to 5.9] times ''.'' Cloud feedback remains the largest contributor to uncertainty of the net feedback, but the uncertainty is reduced compared to AR5. A secondary contribution to the net feedback uncertainty is the biogeophysical and non-CO <sub>2</sub> biogeochemical feedbacks, which together are assessed to have a central value near zero and thus do not affect the central estimate of ECS. The net climate feedback is assessed to be –1.16 W m <sup>–2</sup> °C <sup>–1</sup> , ''likely'' from –1.54 to –0.78 W m <sup>–2</sup> °C <sup>–1</sup> , and ''very likely'' from –1.81 to –0.51 W m <sup>–2</sup> °C <sup>–1</sup> ''.''

Feedback parameters in climate models are calculated assuming that they are independent of each other, except for a well-known co-dependency between the water vapour (WV) and lapse rate (LR) feedbacks. When the inter-model spread of the net climate feedback is computed by adding in quadrature the inter-model spread of individual feedbacks, it is 17% wider than the spread of the net climate feedback directly derived from the ensemble. This indicates that the feedbacks in climate models are partly co-dependent. Two possible co-dependencies have been suggested ( [[#Huybers--2010|Huybers, 2010]] ; [[#Caldwell--2016|Caldwell et al., 2016]] ). One is a negative covariance between the LR and longwave cloud feedbacks, which may be accompanied by a deepening of the troposphere ( [[#O’Gorman--2013|O’Gorman and Singh, 2013]] ; [[#Yoshimori--2020|Yoshimori et al., 2020]] ) leading both to greater rising of high-clouds and a larger upper-tropospheric warming. The other is a negative covariance between albedo and shortwave cloud feedbacks, which may originate from the Arctic regions: a reduction in sea ice enhances the shortwave cloud radiative effect because the ocean surface is darker than sea ice ( [[#Gilgen--2018|Gilgen et al., 2018]] ). This covariance is reinforced as the decrease of sea ice leads to an increase in low-level clouds ( [[#Mauritsen--2013|Mauritsen et al., 2013]] ). However, the mechanism causing these co-dependences between feedbacks is not well understood yet and a quantitative assessment based on multiple lines of evidence is difficult. Therefore, this synthesis assessment does not consider any co-dependency across individual feedbacks.

The assessment of the net climate feedback presented above is based on a single approach (i.e., process understanding) and directly results in a value for ECS given in ( [[#7.5.1|Section 7.5.1]] ; this is in contrast to the synthesis assessment of ECS in ( [[#7.5.5|Section 7.5.5]] which combines multiple approaches. The total (net) feedback parameter consistent with the final synthesis assessment of the ECS and Equation 7.1 (Box 7.1) is provided there.

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