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

=== 6.7.3 Effect of SLCF Mitigation in SSP Scenarios ===

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Air-quality policies lead to a decrease in emissions of both warming and cooling SLCFs. Here we assess the contribution of SLCFs to the total warming (also including the LLGHGs) in the case of stringent SLCF mitigation to improve air quality in scenarios with continued high use of fossil fuels (e.g., SSP3-7.0-lowSLCF and SSP5-8.5). Conversely, we also assess the effect on air quality of strategies aiming to mitigate air pollution or climate change under the SSP3-7.0 framework (using the SSP3-7.0-lowSLCF-lowCH <sub>4</sub> , SSP3-7.0-lowSLCF-highCH <sub>4</sub> and SSP3-3.4 scenarios).

As illustrated in Figure 2.2 of SR1.5 ( [[#Rogelj--2018a|Rogelj et al., 2018a]] ), the total aerosol ERF change in stringent mitigation pathways is expected to be positive and to contribute to a warming since it is dominated by the effects from the phase-out of SO 2 (Figure 6.24, Section 6.7.2.2). Recent emissions inventories and observations of trends in AOD (Sections 2.2.6 and 6.2.1) show that it is ''very likely'' that there have been reductions in global SO <sub>2</sub> emissions and in aerosol burdens over the last decade. Here, we use 2019 as the reference year rather than the ‘Recent Past’ defined as the average over 1995–2014 ( [[IPCC:Wg1:Chapter:Chapter-4#4.1|Section 4.1]] ) in order to exclude the recent emissions reductions when discussing the different possible futures.

The role of the different SLCFs, and also the net of all the SLCFs relative to the total warming in the scenarios, is quite different across the SSP scenarios varying with the summed levels of climate change mitigation and air pollution control (Figure 6.24). In the scenario without climate change mitigation but with strong air pollution control (SSP5-8.5) all the SLCFs (methane, aerosols and tropospheric ozone) and the HFCs (with lifetimes less than 50 years) add to the warming, while in the strong climate change and air pollution mitigation scenarios (SSP1-1.9 and SSP1-2.6), the emissions controls act to reduce methane, ozone and BC, and these reductions thus contribute to cooling. In all scenarios, except SSP3-7.0, emissions controls lead to a reduction of the aerosols relative to 2019, causing a warming. However, the warming from aerosol reductions is stronger in the SSP1 scenarios (with best estimates of 0.21°C in 2040 and 0.4°C in 2100 in SSP1-2.6) because of higher emissions reductions from stronger decrease of fossil fuel use in these scenarios than in SSP5-8.5 (0.13°C in 2040 and 0.22°C in 2100). The changes in methane abundance contribute a warming of 0.14°C in SSP5-8.5, but a cooling of 0.14°C in SSP1-2.6 by the end of the 21st century relative to 2019. Furthermore, under SSP5-8.5, HFCs induce a warming of 0.06°C with a ''very'' ''likely'' range of [0.04 to 0.08] °C in 2050 and 0.2 [0.1 to 0.3] °C by the end of the 21st century, relative to 2019, while under SSP1-2.6, warming due to HFCs is negligible (below 0.02°C) ( ''high confidence'' ). This assessment relies on these estimates, which are based on updated ERFs and HFC lifetimes. It is in accordance with previous estimates (Section 6.6.3.2) of the efficiency of the implementation of the Kigali Amendment and national regulations. It is ''very likely'' that under a stringent climate and air pollution mitigation scenario (SSP1-2.6), the warming induced by changes in methane, ozone, aerosols and HFCs towards the end of the 21st century, will be very low compared with the warming they would cause under the SSP5-8.5 scenario (0.14°C in SSP1-2.6 versus 0.62°C in SSP5-8.5).

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[[File:ab55f93a96af4801e067c83b49900846 IPCC_AR6_WGI_Figure_6_24.png]]

'''Figure 6.24 |''' '''Effects of changes in short-lived climate forcers (SLCFs) and hydrofluorocarbons (HFCs) on global surface air temperature (GSAT) across the WGI core set of Shared Socio-economic Pathways (SSPs)''' . Effects of net aerosols, methane, tropospheric ozone and hydrofluorocarbons (HFCs; with lifetimes &lt;50years), are compared with those of total anthropogenic forcing for 2040 and 2100 relative to the year 2019. The GSAT changes are based on the assessed historic and future evolution of effective radiative forcing (ERF; [[IPCC:Wg1:Chapter:Chapter-7#7.3.5|Section 7.3.5]] ). The temperature responses to the ERFs are calculated with an impulse response function with an equilibrium climate sensitivity of 3.0°C for a doubling of atmospheric CO <sub>2</sub> (feedback parameter of –1.31 W m <sup>–2</sup> °C <sup>–1</sup> ; Cross-Chapter Box 7.1). Uncertainties are 5–95% ranges. The scenario total (grey bar) includes all anthropogenic forcings (long- and short-lived climate forcers, and land-use changes) whereas the white diamonds and bars show the net effects of SLCFs and HFCs and their uncertainties. Further details on data sources and processing are available in the chapter data table (Table 6.SM.3).

For the SSP3-7.0-lowSLCF-highCH <sub>4</sub> and SSP3-7.0-lowSLCF-lowCH <sub>4</sub> scenarios, a five-ESM ensemble has been analysed relative to the standard SSP3-7.0 scenario ( [[#Allen--2020|Allen et al., 2020]] , 2021). For SSP3-7.0-lowSLCF-highCH <sub>4</sub> , in which the methane emissions are as in the standard SSP3-7.0 scenario, [[#Allen--2021|Allen et al. (2021)]] found an enhanced global mean surface warming of 0.23°C ± 0.05°C by mid-century and 0.21°C ± 0.03°C by 2100 relative to the warming in the standard SSP3-7.0 scenario. Also including strong mitigation of methane emissions, the same models ( [[#Allen--2021|Allen et al., 2021]] ) find that the warming is offset resulting in a net cooling of 0.15°C ± 0.05°C at mid-century (2050–2059) and 0.50°C ± 0.02°C at the end of the century (2090–2099) relative to SSP3-7.0.

There is ''robust evidence'' and ''high agreement'' that non-methane SLCF mitigation measures, through reductions in aerosols and non-methane ozone precursors to improve air quality (SSP3-7.0-lowSLCF-highCH <sub>4</sub> vs SSP3-7.0), would lead to additional near-term warming with a range of 0.1°C–0.3°C. Methane mitigation that also reduces tropospheric ozone, stands out as an option that combines near- and long-term gains on surface temperature ( ''high confidence)'' . With stringent but realistic methane mitigation (SSP3-7.0-lowSLCF-lowCH <sub>4</sub> ), it is ''very likely'' that warming (relative to SSP3-7.0) from non-methane SLCFs can be offset (Figure 6.24; [[#Allen--2021|Allen et al., 2021]] ). Due to the slower response to the methane mitigation, this offset becomes more robust over time and it is ''very likely'' to be an offset after 2050. However, when comparing to the present day, it is ''unlikely'' that methane mitigation can fully cancel out the warming over the 21st century from reduction of non-methane cooling SLCFs.

The SSP3 storyline assumes ‘regional rivalry’ ( [[IPCC:Wg1:Chapter:Chapter-1#1.6.1.1|Section 1.6.1.1]] ) with weak air pollution legislation and no climate change mitigation, and is compared here against SSP3-7.0-lowSLCF-lowCH <sub>4</sub> (strong air pollution control) and SSP3-3.4 (the most ambitious climate policy feasible under the SSP3 narrative). In the SSP3-3.4 scenario, all emissions follow the SSP3-7.0 scenario until about 2030 and then deep and rapid cuts in fossil fuel use are imposed ( [[#Fujimori--2017|Fujimori et al., 2017]] ). In the case of climate change mitigation, such as in the SSP3-3.4 scenario, the decrease of SLCF emissions is a co-benefit from the targeted decrease of CO <sub>2</sub> (when SLCFs are co-emitted), but also directly targeted as in the case of methane. For SLCFs, this means that emissions of aerosols and methane increase until 2030 and are reduced quickly thereafter ( [[#Fujimori--2017|Fujimori et al., 2017]] ). The effect on GSAT (relative to 2019) is shown in Figures 6.22 and 6.24. The net GSAT response to the SLCFs is dominated by the aerosols, with an initial cooling until 2030, then a fast rebound for 15 years followed by a very moderate warming reaching 0.21°C in 2100. The ozone reduction causes a slight cooling (up to 0.06°C), in contrast to the warming in the SSP3-7.0-lowSLCF-highCH <sub>4</sub> scenario in which the methane emissions increase until 2100.

To assess the effect of dedicated air-quality versus climate policy on air quality, PM <sub>2.5</sub> and ozone indicators were estimated for three SSP3 scenarios by applying a widely used approach for the analysis of air-quality implications for given emissions scenarios ( [[#Rao--2017|Rao et al., 2017]] ; [[#Van%20Dingenen--2018|Van Dingenen et al., 2018]] ; [[#Vandyck--2018|Vandyck et al., 2018]] ) and whose sensitivity of surface concentrations to emissions changes is comparable to that in the ESM ensemble (Supplementary Material 6.SM.5). The assessment shows that both strong air pollution control and strong climate change mitigation, implemented independently, lead to a large reduction of exposure to PM <sub>2.5</sub> and ozone by the end of the century ( ''high confidence'' ) (Figures 6.25 and 6.26). However, implementation of air pollution control, relying on the deployment of existing technologies, leads to benefits more rapidly than climate change mitigation ( ''high confidence'' ), which requires systemic changes and is thus implemented later in this scenario. Notably, under the underlying SSP3 context, significant parts of the population remain exposed to air quality exceeding the WHO guidelines for PM <sub>2.5</sub> over the whole century ( ''high confidence'' ), in particular in Africa, Eastern and Southern Asia and the Middle East, and for ozone only a small improvement in population exposure is expected in Africa and Asia. Confidence levels here result from expert judgement on the whole chain of evidence.

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[[File:f2a3bb2c54c35a1bd8579a4b0c6cbba1 IPCC_AR6_WGI_Figure_6_25.png]]

'''Figure 6.25 |''' '''Effect of dedicated air pollution or climate policy on population-weighted PM''' <sub>2.5</sub> '''concentrations (µg m''' <sup>–3</sup> ''') and share of population (%) exposed to different PM''' <sub>2.5</sub> '''levels across 10 world regions.''' Thresholds of 10 µg m <sup>–3</sup> and 35 µg m <sup>–3</sup> represent the WHO air quality guideline and the WHO interim target 1, respectively; [[#WHO--2017|WHO (2017)]] . Results are compared for SSP3-7.0 (no major improvement of current legislation is assumed), SSP3-lowSLCF (strong air pollution controls are assumed), and a climate change mitigation scenario SSP3-3.4; details of scenario assumptions are discussed in [[#Riahi--2017|Riahi et al. (2017)]] and [[#Rao--2017|Rao et al. (2017)]] . Analysis performed with the TM5-FASST model ( [[#Van%20Dingenen--2018|Van Dingenen et al., 2018]] ) using emissions projections from the Shared economic Pathway (SSP) database ( [[#Riahi--2017|Riahi et al., 2017]] ; [[#Rogelj--2018a|Rogelj et al., 2018a]] ; [[#Gidden--2019|Gidden et al., 2019]] ). Further details on data sources and processing are available in the chapter data table (Table 6.SM.3).

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[[File:383534a75c6196a557e8a37ed592543d IPCC_AR6_WGI_Figure_6_26.png]]

'''Figure 6.26 |''' '''Effect of dedicated air pollution or climate policy on population-weighted ozone concentrations (ppb) and share of population (%) exposed to different ozone levels across 10 world regions.''' Results are compared for SSP3-7.0 (no major improvement of current legislation is assumed), SSP3-low SLCF (strong air pollution controls are assumed), and a climate change mitigation scenario (SSP3-3.4); details of scenario assumptions are discussed in [[#Riahi--2017|Riahi et al. (2017)]] and [[#Rao--2017|Rao et al. (2017)]] . Analysis performed with the TM5-FASST model ( [[#Van%20Dingenen--2018|Van Dingenen et al., 2018]] ) using emissions projections from the Socio-economic Pathway (SSP) database ( [[#Riahi--2017|Riahi et al., 2017]] ; [[#Rogelj--2018a|Rogelj et al., 2018a]] ; [[#Gidden--2019|Gidden et al., 2019]] ). Further details on data sources and processing are available in the chapter data table (Table 6.SM.3).

In summary, the warming induced by SLCF changes is stable after 2040 in the WGI core set of SSP scenarios associated with lower global air pollution as long as methane emissions are also mitigated, but the overall warming induced by SLCF changes is higher in scenarios in which air quality continues to deteriorate (caused by growing fossil fuel use and limited air pollution control) ( ''high confidence'' ). In the SSP3-7.0 context, applying an additional strong air pollution control resulting in reductions in anthropogenic aerosols and non-methane ozone precursors would lead to an additional near-term global warming of 0.08 °C with a ''very likely'' range of [–0.05 to 0.25] °C (compared with SSP3-7.0 for the same period). A simultaneous methane mitigation consistent with SSP1’s stringent climate change mitigation policy implemented in the SSP3 world, could entirely alleviate this warming and even lead to a cooling of 0.07°C with a ''very likely'' range of [–0.08 to +0.18] °C (compared with SSP3-7.0 for the same period) . Across the SSPs, the reduction of methane , ozone precursors and HFCs can make a 0.2 [0.1 to 0.4] °C difference on GSAT in 2040 and a 0.8 [0.5 to 1.3] °C difference at the end of the 21st century (Figure 6.24), which is substantial in the context of the Paris Agreement. Sustained methane mitigation, wherever it occurs, stands out as an option that combines near- and long-term gains on surface temperature ( ''high confidence)'' and leads to air pollution benefits by reducing surface ozone level globally ( ''high confidence'' ).

Strong air pollution control as well as strong climate change mitigation, implemented independently, lead to large reduction of the exposure to air pollution by the end of the century ( ''high confidence'' ). Implementation of air pollution control, relying on the deployment of existing technologies, leads more rapidly to air-quality benefits than climate change mitigation which requires systemic changes but, in both cases, significant parts of the population remain exposed to air pollution exceeding the WHO guidelines ( ''high confidence'' ).

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