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

===== 7.4.2.5.1 Non-CO <sub>2</sub> biogeochemical feedbacks =====

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The chemical composition of the atmosphere (beyond CO <sub>2</sub> and water vapour changes) is expected to change in response to a warming climate. These changes in greenhouse gases (methane, nitrous oxide and ozone) and aerosol amount (including dust) have the potential to alter the TOA energy budget and are collectively referred to as ‘non-CO <sub>2</sub> biogeochemical feedbacks’. Methane (CH <sub>4</sub> ) and nitrous oxide (N <sub>2</sub> O) feedbacks arise partly from changes in their emissions from natural sources in response to temperature change; these are assessed in ( [[IPCC:Wg1:Chapter:Chapter-5|Chapter 5]] [[IPCC:Wg1:Chapter:Chapter-5#5.4.7|Section 5.4.7]] ; see also Figure 5.29c). Here we exclude the permafrost CH <sub>4</sub> feedback ( [[IPCC:Wg1:Chapter:Chapter-5#5.4.9.1.2|Section 5.4.9.1.2]] ) because, although associated emissions are projected to increase under warming on multi-decadal to centennial time scales, on longer time scales these emissions would eventually substantially decline as the permafrost carbon pools were depleted ( [[#Schneider%20von%20Deimling--2012|Schneider von Deimling et al., 2012]] , 2015). This leaves the wetland CH <sub>4</sub> , land N <sub>2</sub> O, and ocean N <sub>2</sub> O feedbacks, the assessed mean values of which sum to a positive feedback parameter of +0.04 [0.02 to 0.06] W m <sup>–2</sup> °C <sup>–1</sup> [[IPCC:Wg1:Chapter:Chapter-5#5.4.7|Section 5.4.7]] . Other non-CO <sub>2</sub> biogeochemical feedbacks that are relevant to the net feedback parameter are assessed in [[IPCC:Wg1:Chapter:Chapter-6|Chapter 6]] (Section 6.4.5 and Table 6.8). These feedbacks are associated with sea salt, dimethyl sulphide, dust, ozone, biogenic volatile organic compounds, lightning, and CH <sub>4</sub> lifetime, and sum to a negative feedback parameter of –0.20 [–0.41 to +0.01] W m <sup>–2</sup> °C <sup>–1</sup> . The overall feedback parameter for non-CO <sub>2</sub> biogeochemical feedbacks is obtained by summing the [[IPCC:Wg1:Chapter:Chapter-5|Chapter 5]] and [[IPCC:Wg1:Chapter:Chapter-6|Chapter 6]] assessments, which gives –0.16 [–0.37 to +0.05] W m <sup>–2</sup> °C <sup>–1</sup> . However, there is ''low confidence'' in the estimates of both the individual non-CO <sub>2</sub> biogeochemical feedbacks as well as their total effect, as evident from the large range in the magnitudes of α from different studies, which can be attributed to diversity in how models account for these feedbacks and limited process-level understanding.

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