Editing IPCC:AR6/WGIII/Chapter-2 (section)

==== 2.8.4.1 Co-impacts of Air Quality, Sector-specific and Energy Policies on Climate Mitigation ====

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Co-impacts of local or regional air pollution abatement policies for climate mitigation are widely studied in the literature. Cross-border externalities of air pollution have also made these a focus of several international agreements ( [[#Mitchell--2020|Mitchell et al. 2020]] ). Evaluating the effectiveness of such treaties and policies is difficult because deriving causal inferences and accurate attribution requires accounting for several confounding factors, and direct and indirect spillovers ( [[#Isaksen--2020|Isaksen 2020]] ). Nevertheless, several studies assess the effectiveness of such treaties and regulations (De Foy et al. 2016; [[#Li--2017a|Li et al. 2017a]] , 2017b; [[#Morgenstern--2018|Morgenstern 2018]] ; [[#Mardones--2020|Mardones and Cornejo 2020]] ). However, there is little ex-post empirical analysis and a greater focus on ex-ante studies in the literature.

At a local scale, air pollutants are often co-emitted with GHGs in combustion processes. Many air quality policies and regulations focus on local pollution from specific sources that can potentially either substitute or complement global GHG emissions in production and generation processes. Also, policies that reduce certain air pollutants, such as sulphur dioxide (SO 2 ), have a positive radiative forcing effect ( [[#Navarro--2016|Navarro et al. 2016]] ). The evidence on individual air pollution control regulation and policies for GHG emissions is therefore mixed ( ''medium evidence'' , ''medium agreement'' ). Evidence from the USA suggests that increased stringency of local pollution regulation had no statistically detectable co-benefits or costs on GHG emissions ( [[#Brunel--2019|Brunel and Johnson 2019]] ). Evidence from China suggests that the effectiveness of policies addressing local point sources differed from those of non-point sources and the co-benefits for climate are mixed, though policies addressing large industrial point sources have been easier to implement and have had significant impact ( [[#Huang--2016|Huang and Wang 2016]] ; [[#Xu--2016|Xu et al. 2016]] ; [[#van%20der%20A--2017|van der A et al. 2017]] ; [[#Dang--2019|Dang and Liao 2019]] ; [[#Fang--2019|Fang et al. 2019]] ; [[#Yu--2019|Yu et al. 2019]] ). Legislation to reduce emissions of air pollutants in Europe have significantly improved air quality and health but have had an unintended warming effect on the climate ( [[#Turnock--2016|Turnock et al. 2016]] ).

Often, the realisation of potential co-benefits depends on the type of pollutant addressed by the specific policy, and whether complementarities between local pollution and global GHG emissions are considered in policy design ( ''medium evidence'' , ''high agreement'' ) ( [[#Rafaj--2014|Rafaj et al. 2014]] ; [[#Li--2017a|Li et al. 2017a]] ). Effective environmental regulations that also deliver co-benefits for climate mitigation require integrated policies ( [[#Schmale--2014|Schmale et al. 2014]] ; [[#Haines--2017|Haines et al. 2017]] ). Uncoordinated policies can have unintended consequences and even increase emissions ( [[#Holland--2015|Holland et al. 2015]] ). Many studies suggest that policies that target both local and global environmental benefits simultaneously may be more effective ( ''medium evidence'' , ''medium agreement'' ) ( [[#Klemun--2020|Klemun et al. 2020]] ). Furthermore, air pollution policies aimed at inducing structural changes – for example, closure of polluting coal power plants or reducing motorised miles travelled – are more likely to have potential positive spillover effects for climate mitigation, as compared to policies incentivising end-of-pipe controls ( [[#Wang--2021|Wang 2021]] ).

Other policies that typically have potential co-benefits for climate mitigation include those specific to certain sectors and are discussed in Chapters 5–11. Examples of such policies include those that encourage active travel modes, which have been found to have ancillary benefits for local air quality, human health, and GHG emissions ( [[#Fujii--2018|Fujii et al. 2018]] ). Policies to reduce energy use through greater efficiency have also been found to have benefits for air quality and the climate ( ''robust evidence'' , ''medium agreement'' ) ( [[#Tzeiranaki--2019|Tzeiranaki et al. 2019]] ; [[#Bertoldi--2020|Bertoldi and Mosconi 2020]] ). Important air quality and climate co-benefits of renewable or nuclear energy policies have also been found ( ''medium evidence'' , ''medium agreement'' ) ( [[#Lee--2017|Lee et al. 2017]] ; [[#Apergis--2018|Apergis et al. 2018]] ; [[#Sovacool--2021|Sovacool and Monyei 2021]] ).

Policies specific to other sectors, such as encouraging green building design, can also reduce GHG emissions ( [[#Eisenstein--2017|Eisenstein et al. 2017]] ). Evidence from several countries also shows that replacing polluting solid biomass cooking with cleaner gas-burning or electric alternatives have strong co-benefits for health, air quality, and climate change ( ''robust evidence'' , ''high agreement'' ) ( [[#Anenberg--2017|Anenberg et al. 2017]] ; [[#Singh--2017|Singh et al. 2017]] ; [[#Tao--2018|Tao et al. 2018]] ).

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