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

==== 6.7.1.2 Future Evolution of Surface Ozone and PM Concentrations ====

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The projection of air-quality relevant abundances (surface ozone and PM <sub>2.5</sub> ) under the SSP scenarios are assessed here. Future changes in global and regional annual mean surface ozone and PM <sub>2.5</sub> driven by the evolution of emissions as well as by climate change have been quantified by CMIP6 models analysed in AerChemMIP ( [[#Allen--2020|Allen et al., 2020]] , 2021; [[#Turnock--2020|Turnock et al., 2020]] ).

Surface ozone increases continuously until 2050 across most regions in SSP3-7.0 and SSP5-8.5, ( [[#Turnock--2020|Turnock et al., 2020]] ), particularly over Eastern Asia, Southern Asia, the Middle East, Africa, South East Asia and Developing Pacific, where this increase can reach and even exceed 5 ppb for annual mean averaged over land areas (Figure 6.20). After 2050, surface ozone concentrations decrease in SSP5-8.5, reaching levels below their 2005–2014 mean levels in most regions, but level off or continue to increase under SSP3-7.0. The increase in surface ozone in the SSP5-8.5 scenario occurs despite an emissions decrease of several ozone precursors because the methane emissions increase until about 2080 in the absence of climate change mitigation. Ozone decreases over all regions in response to strong emissions mitigation in SSP1-1.9 and SSP1-2.6 ( [[#Turnock--2020|Turnock et al., 2020]] ), with decreases of 5 to 10 ppb as soon as 2030 in North America, Europe, Eurasia, Eastern Asia, the Middle East and Southern Asia in their annual means over land areas. In most regions surface ozone is reduced slightly or remains near present day values in the middle of the road scenario, SSP2-4.5. In 2100, the largest differences in surface ozone changes across the scenarios occur for the Middle East, Southern Asia and Eastern Asia with differences ranging up to 40 ppb between SSP3-7.0 and SSP1-1.9 at the end of the century. In each scenario, despite discrepancies in the magnitude of changes, especially over North America, Europe, Eurasia, Eastern Asia and Southern Asia, the models are in high agreement regarding the signs of the changes and are thus assessed as of ''high confidence'' .

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'''Figure 6.20 |''' '''Projected changes in regional annual mean surface ozone (O''' <sub>3</sub> '''; ppb) from 2015 to 2100 in different shared socio-economic pathways (SSPs)''' . Each panel represents values averaged over the corresponding land area (except for ‘Global’) shown on the map in Figure 6.7. Solid coloured lines and shading indicate the multi-model mean and ±1 ''standard deviation'' across the available CMIP6 models (Turnock et al. 2020; Allen et al. 2021) for each scenario. Changes are relative to annual mean values calculated over the period 2005–2014 from the historical experiment as indicated in the top left of each regional panel along with ±1 standard deviation. For each model all available ensemble members are averaged before being used to calculate the multi-model mean. Ozone changes are also displayed in the Interactive Atlas. Further details on data sources and processing are available in the chapter data table (Table 6.SM.3).

The strong abatement of ozone precursor emissions (except those of methane; SSP3-7.0-lowSLCF-highCH <sub>4</sub> ) lead to a decrease of global average surface ozone by 15% (6 ppb) between 2015 and 2055 ( [[#Allen--2020|Allen et al., 2020]] ), and ozone decreases in all regions except Southern Asia. However, this decrease is twice as large when methane emissions are abated simultaneously (SSP3-7.0-lowSLCF-lowCH <sub>4</sub> ), underlying the importance of methane emissions reduction as an important lever to reduce ozone pollution ( ''high confidence'' ) (Section 6.6.4).

A decrease in surface PM <sub>2.5</sub> concentrations is estimated for SSP1-1.9, SSP1-2.6 and SSP2-4.5 ( [[#Turnock--2020|Turnock et al., 2020]] ) (Figure 6.21). A decrease in PM <sub>2.5</sub> , is also projected in SSP5-8.5, which does not consider any climate change mitigation but has a strong air pollution control. The decrease is largest in the regions with the highest 2005–2014 mean concentrations (the Middle East, Southern Asia and Eastern Asia). Under the SSP3-7.0 scenario, PM <sub>2.5</sub> is predicted to increase or remain at near present-day values across Asia; regions where present-day concentrations are currently the highest. There is large model spread over regions with large natural aerosol sources, for example, in North Africa, where dust sources are important. The mitigation of non-methane SLCFs in the SSP3-7.0-lowSLCF-highCH <sub>4</sub> scenario is predicted to reduce PM <sub>2.54</sub> by 25% (in 2055, relative to the SSP3-7.0 scenario) over global land surface areas ( [[#Allen--2020|Allen et al., 2020]] ).

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'''Figure 6.21 |''' '''Future changes in regional five-year mean surface PM''' <sub>2.5</sub> '''from 2015 to 2100 in different shared socio-economic pathways (SSPs).''' PM <sub>2.5</sub> stands for micrograms per cubic meter of aerosols with diameter less than 2.5 μm and is calculated by summing up individual aerosol mass components from each model as: black carbon + particulate organic matter + sulphate + 0.25 × sea salt + 0.1 × dust. Since not all CMIP6 models reported nitrate aerosol, it is not included here. See Figure 6.20 for further details. PM <sub>2.5</sub> changes are also displayed in the Interactive Atlas. Further details on data sources and processing are available in the chapter data table (Table 6.SM.3).

The magnitude of the annual mean change in surface ozone and PM <sub>2.5</sub> for all the SSPs (accounting for both emissions and climate change) is greater than that expected from climate change in isolation ( [[#Turnock--2020|Turnock et al., 2020]] ). The uncertainty in the projections comes from how natural emissions will respond to climate change. However, multiple lines of evidence (along with Sections 6.2.2, 6.5, and 6.7.1) provide ''high confidence'' (compared to ''medium'' in AR5) that changes in emissions, and in particular in human-induced emissions, will drive future air pollution levels rather than physical climate change.

In summary, future air pollution levels are strongly driven by precursor emissions trajectories in the SSPs with substantial reductions in global surface ozone and PM (when air pollution and climate change are both strongly mitigated, e.g., SSP1-2.6) to no improvement and even degradation ( ''high confidence'' ) (when no climate change mitigation and only weak air pollution control are considered, SSP3-7.0). In the latter case, PM levels are estimated to increase until 2050 over large parts of Asia and surface ozone pollution worsens over all continental areas throughout the whole century ( ''high confidence'' ). In scenarios without climate change mitigation but with strong air pollution control (SSP5-8.5), high methane levels hamper the decline in global surface ozone in the near term and only PM levels decrease ( ''high confidence'' ).

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