Editing IPCC:AR6/SR15/Chapter-3 (section)

==== 3.6.2.3 Atmospheric compounds (aerosols and methane) ====

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There are multiple pathways that could be used to limit anthropogenic climate change, and the details of the pathways will influence the impacts of climate change on humans and ecosystems. Anthropogenic-driven changes in aerosols cause important modifications to the global climate (Bindoff et al., 2013a; Boucher et al., 2013b; P. Wu et al., 2013; Sarojini et al., 2016; H. Wang et al., 2016) <sup>[[#fn:r1315|1315]]</sup> . Enforcement of strict air quality policies may lead to a large decrease in cooling aerosol emissions in the next few decades. These aerosol emission reductions may cause a warming comparable to that resulting from the increase in greenhouse gases by mid-21st century under low CO <sub>2</sub> pathways (Kloster et al., 2009; Acosta Navarro et al., 2017) <sup>[[#fn:r1316|1316]]</sup> . Further background is provided in Sections 2.2.2 and 2.3.1; Cross Chapter Box 1 in Chapter 1). Because aerosol effects on the energy budget are regional, strong regional changes in precipitation from aerosols may occur if aerosol emissions are reduced for air quality reasons or as a co-benefit from switches to sustainable energy sources (H. Wang et al., 2016) <sup>[[#fn:r1317|1317]]</sup> . Thus, regional impacts, especially on precipitation, are very sensitive to 1.5°C-consistent pathways (Z. Wang et al., 2017) <sup>[[#fn:r1318|1318]]</sup> .

Pathways which rely heavily on reductions in methane (CH <sub>4</sub> ) instead of CO <sub>2</sub> will reduce warming in the short term because CH <sub>4</sub> is such a stronger and shorter-lived greenhouse gas than CO <sub>2</sub> , but will lead to stronger warming in the long term because of the much longer residence time of CO <sub>2</sub> (Myhre et al., 2013; Pierrehumbert, 2014) <sup>[[#fn:r1319|1319]]</sup> . In addition, the dominant loss mechanism for CH <sub>4</sub> is atmospheric photo-oxidation. This conversion modifies ozone formation and destruction in the troposphere and stratosphere, therefore modifying the contribution of ozone to radiative forcing, as well as feedbacks on the oxidation rate of methane itself (Myhre et al., 2013) <sup>[[#fn:r1320|1320]]</sup> . Focusing on pathways and policies which both improve air quality and reduce impacts of climate change can provide multiple co-benefits (Shindell et al., 2017) <sup>[[#fn:r1321|1321]]</sup> . These pathways are discussed in detail in Sections 4.3.7 and 5.4.1 and in Cross-Chapter Box 12 in Chapter 5.

Atmospheric aerosols and gases can also modify the land and ocean uptake of anthropogenic CO <sub>2</sub> ; some compounds enhance uptake while others reduce it (Section 2.6.2; Ciais et al., 2013) <sup>[[#fn:r1322|1322]]</sup> . While CO <sub>2</sub> emissions tend to encourage greater uptake of carbon by the land and the ocean (Ciais et al., 2013) <sup>[[#fn:r1323|1323]]</sup> , CH <sub>4</sub> emissions can enhance ozone pollution, depending on nitrogen oxides, volatile organic compounds and other organic species concentrations, and ozone pollution tends to reduce land productivity (Myhre et al., 2013; B. Wang et al., 2017) <sup>[[#fn:r1324|1324]]</sup> . Aside from inhibiting land vegetation productivity, ozone may also alter the CO <sub>2</sub> , CH <sub>4</sub> and nitrogen (N <sub>2</sub> O) exchange at the land–atmosphere interface and transform the global soil system from a sink to a source of carbon (B. Wang et al., 2017) <sup>[[#fn:r1325|1325]]</sup> . Aerosols and associated nitrogen-based compounds tend to enhance the uptake of CO <sub>2</sub> in land and ocean systems through deposition of nutrients and modification of climate (Ciais et al., 2013; Mahowald et al., 2017b) <sup>[[#fn:r1326|1326]]</sup> .

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