Editing IPCC:AR6/WGI/Chapter-6 (section)

==== 6.6.3.3 Assessment of SLCF Mitigation Strategies and Opportunities ====

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There is a consensus in the literature that mitigation of SLCF emissions plays a central role in simultaneous mitigation of climate change, air quality and other development goals including SDG targets (UNEP and WMO, 2011; Shindell et al. , 2012, 2017b; Rogelj et al. , 2014b, 2018b; [[#AMAP--2015a|AMAP, 2015a]] ; Haines et al. , 2017; Klimont et al. , 2017b; McCollum et al. , 2018; Rafaj et al. , 2018; UNEP and CCAC, 2018; [[#UNEP--2019|UNEP, 2019]]) . There is less agreement in the literature with respect to the actual mitigation potential (or its potential rate of implementation), necessary policies to trigger successful implementation, and resulting climate impacts. Most studies agree that climate policies, especially those aiming to keep warming below 1.5°C or 2°C, trigger large SLCF mitigation co-benefits, (e.g., [[#Rogelj--2014b|Rogelj et al., 2014b]] , 2018b), however, discussion of practical implementation of respective policies and SDGs has only started ([[#Haines--2017|Haines et al., 2017]]). Note that mitigation scenarios outside of the SSP framework are assessed here while those within the SSPs are assessed in Section 6.7.3.

Focusing on air quality, specifically addressing aerosols, by introducing the best available technology reducing PM <sub>2.5</sub> , SO <sub>2</sub> and NO <sub>x</sub> in most Asian countries within the 2030–2050 time frame (a strategy that has indeed shown reduction in PM <sub>2.5</sub> exposure in China) comes, in many regions, short of national regulatory PM <sub>2.5</sub> concentration standards (often set at 35 µg m <sup>–3</sup> for annual mean; [[#UNEP--2019|UNEP, 2019]]). Similarly, global studies ([[#Rafaj--2018|Rafaj et al., 2018]] ; [[#Amann--2020|Amann et al., 2020]]) show that strengthening current air-quality policies, that address primarily aerosols and their precursors, will not enable the achievement of WHO air quality guidelines (annual average concentration of PM <sub>2.54</sub> below 10 µg m <sup>–3</sup>) in many regions.

A multi-model study (four ESMs and six CTMs) found a consistent response to the removal of SO <sub>2</sub> emissions that resulted in a global mean surface temperature increase of 0.69°C (0.4°C–0.84°C). However, results are mixed for a global BC-focused deep SLCF reduction without SO <sub>2</sub> and methane mitigation which remain as in the baseline (see ECLIPSE in Figure 6.18). BC contributed about –0.022°C temperature reduction for the decade 2041–2050 based on the assumption that mitigation of the non-methane species contributed only about 10% of the global temperature reduction for the strategy where methane mitigation was also included (–0.22°C ± 0.07°C; [[#Stohl--2015|Stohl et al., 2015]]). These results are consistent with studies analysing similar strategies using emulators (e.g., [[#Smith--2013|Smith and Mizrahi, 2013]] ; [[#Rogelj--2014b|Rogelj et al., 2014b]]). [[#Stohl--2015|Stohl et al. (2015)]] also analysed the impact of BC-focused mitigation on air quality, estimating large-scale regional reduction in PM <sub>2.5</sub> mean concentration from about 2% in Europe to 20% over India for the decade 2041–2050.

Local response to global reduction can be higher than the global temperature response, particularly for regions subjected to rapid changes. Hence, mitigation of rapid warming in the Arctic has been subject to an increasing number of studies (Sand et al. , 2013b, 2016; Jiao et al. , 2014; [[#AMAP--2015a|AMAP, 2015a]] , b; Mahmood et al. , 2016; Christensen et al. , 2019) . Considering maximum technically feasible reductions (MTFR) for methane globally and an idealized strategy reducing key global anthropogenic sources of BC (about 80% reduction by 2030 and sustained thereafter) and precursors of ozone was estimated to jointly bring a reduction of Arctic warming, averaged over the 2041–2050 period, between 0.2°C and 0.6°C ([[#AMAP--2015a|AMAP, 2015a]] ; [[#Sand--2016|Sand et al., 2016]]). [[#Stohl--2015|Stohl et al. (2015)]] have estimated that a global SLCF mitigation strategy (excluding further reduction of SO <sub>2</sub>) would lead to about twice as high a temperature reduction (–0.44 (–0.39 to –0.49) °C) in the Arctic than the global response to such mitigation.

While there is robust evidence that air-quality policies resulting in reductions of aerosols and ozone can be beneficial for human health but can lead to ‘disbenefits’ for near-term climate change, the existence of such trade-offs in response to climate change mitigation policies is less certain ([[#Shindell--2019|Shindell and Smith, 2019]]). Recent studies show that very ambitious but plausible gradual phasing out of fossil fuels in 1.5°C-compatible pathways with little or no overshoot, lead to a near-term future warming of less than 0.1°C, when considering associated emissions reduction of both warming and cooling species. This suggests that there may not be a strong conflict, at least at the global scale, between climate and air-quality benefits in the case of a worldwide transition to clean energy ([[#Shindell--2019|Shindell and Smith, 2019]] ; [[#Smith--2019|Smith et al., 2019]]). However, at the regional scale, the changes in spatially variable emissions and abundance changes might result in different responses, including implications for precipitation and monsoons (Chapter 8), especially over Southern Asia (e.g., [[#Wilcox--2020|Wilcox et al., 2020]]).

Decarbonization of energy supply and end-use sectors is among key pillars of any ambitious climate change mitigation strategy and it would result in improved air quality owing to associated reduction of co-emitted SLCF emissions (e.g., [[#McCollum--2013|McCollum et al., 2013]] ; [[#Rogelj--2014b|Rogelj et al., 2014b]] ; [[#Braspenning%20Radu--2016|Braspenning Radu et al., 2016]] ; [[#Rao--2016|Rao et al., 2016]] ; [[#Stechow--2016|Stechow et al., 2016]] ; [[#Lelieveld--2019|Lelieveld et al., 2019]] ; [[#Shindell--2019|Shindell and Smith, 2019]]). Regional studies ([[#Lee--2016|Lee et al., 2016]] ; [[#Shindell--2016|Shindell et al., 2016]] ; [[#Chen--2018|Chen et al., 2018]] ; [[#Li--2018|Li et al., 2018]]), where significant CO <sub>2</sub> reductions were assumed for 2030 and 2050, show consistently reduced of PM <sub>2.5</sub> and ozone concentrations resulting in important health benefits. However, these improvements are not sufficient to bring PM <sub>2.5</sub> levels in agreement with the WHO air-quality guidelines in several regions. [[#Amann--2020|Amann et al. (2020)]] and [[#UNEP--2019|UNEP (2019)]] highlight that only the combination of strong air-quality, development and climate policies, including societal transformations, could pave the way towards the achievement of such a target at a regional and global level.

At a global level, [[#Rao--2016|Rao et al. (2016)]] showed that climate policies, compatible with Copenhagen pledges and a long-term CO <sub>2</sub> target of 450 ppm, result in important air-quality benefits, reducing the share of the global population exposed to PM <sub>2.5</sub> levels above the WHO Tier 1 standard (35 µg m <sup>–3</sup>) in 2030 from 21% to 5%. The impacts are similar to a strong air-quality policy but still leave large parts of population, especially in Asia and Africa, exposed to levels well above the WHO air quality guideline level of 10 µg m <sup>–3</sup> . The latter can be partly alleviated by combining such climate policy with strong air-quality policy. [[#Shindell--2018|Shindell et al. (2018)]] analysed more ambitious climate change mitigation scenarios than [[#Rao--2016|Rao et al. (2016)]] and highlighted the opportunities to improve air quality and avert societal effects associated with warmer climate by accelerated decarbonization strategies. Most climate change mitigation strategies compatible with limiting global warming to well below 2°C rely on future negative CO <sub>2</sub> emissions postponing immediate reduction. Alternatively, a faster decarbonization could allow the achievement of a 2°C goal without negative CO <sub>2</sub> emissions and, with currently known and effectively applied emissions-control technologies, this would also have immediate and significant air-quality benefits, reducing premature deaths worldwide ([[#Shindell--2018|Shindell et al., 2018]]). For a 2°C-compatible pathway, [[#Vandyck--2018|Vandyck et al. (2018)]] estimated 5% and 15% reduction in premature mortality due to PM <sub>2.5</sub> in 2030 and 2050, respectively, compared to reference scenarios.

There is robust evidence that reducing atmospheric methane will benefit climate and improve air quality through near-surface ozone reduction ([[#Fiore--2015|Fiore et al., 2015]] ; [[#Shindell--2017a|Shindell et al., 2017a]]) and wide agreement that strategies reducing methane offer larger (and less uncertain) climate benefits than policies addressing BC (e.g., Smith andMizrahi, 2013; [[#Rogelj--2014b|Rogelj et al., 2014b]] , 2018b ; [[#Stohl--2015|Stohl et al., 2015]] ; [[#Christensen--2019|Christensen et al., 2019]] ; [[#Shindell--2019|Shindell and Smith, 2019]]). SR1.5 ([[#Rogelj--2018b|Rogelj et al., 2018b]]) highlighted the importance of methane mitigation in limiting warming to 1.5ºC in addition to net zero CO <sub>2</sub> emissions by 2050. Implementation of the identified maximum technically feasible reductions (MTFR) potential for methane globally, estimated at nearly 50% reduction (or 205 Tg CH <sub>4</sub> in 2050) of anthropogenic emissions from the baseline, would lead to a reduction in warming, calculated as the differences between the baseline and MTFR scenario, for the 2036–2050 period of about 0.20°C ± 0.02°C globally ([[#AMAP--2015b|AMAP, 2015b]]). Plausible levels of methane mitigation, achieved with proven technologies, can increase the feasibility of achieving the Paris Agreement goal through slightly slowing down the pace of CO <sub>2</sub> reductions (but not changing the final CO <sub>2</sub> reduction goal) while this benefit is enhanced by the indirect effects of methane mitigation on ozone levels ([[#Collins--2018|Collins et al., 2018]]). Adressing methane mitigation appears even more important in view of recently observed growth in atmospheric concentrations that is linked to increasing anthropogenic emissions ([[IPCC:Wg1:Chapter:Chapter-5#5.2.2|Section 5.2.2]]).

Neither ambitious climate change policy nor air-quality abatement policy can automatically yield co-benefits without integrated policies aimed at co-beneficial solutions ([[#Zusman--2013|Zusman et al., 2013]] ; [[#Schmale--2014a|Schmale et al., 2014a]] ; [[#Melamed--2016|Melamed et al., 2016]]), particularly in the energy generation and transport sectors (Rao et al. , 2013; Thompson et al. , 2016; Shindell et al. , 2018; Vandyck et al. , 2018) . Integrated policies are necessary to yield multiple benefits of mitigating climate change, improving air quality, protecting human health and achieving several SDGs.

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