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

=== 6.5.1 Effect of Climate Change on Surface Ozone ===

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The AR5 assessed with ''high confidence'' that in unpolluted regions, higher water vapour abundances and temperatures in a warmer climate would enhance ozone chemical destruction, leading to lower baseline <sup>[[#footnote-000|5]]</sup> surface ozone levels ( [[#Kirtman--2013|Kirtman et al., 2013]] ). In polluted regions, AR5 assessed with ''medium confidence'' that higher surface temperatures will trigger regional feedbacks in chemistry and local emissions that will increase surface ozone and the intensity of surface ozone peaks.

The response of surface ozone to climate-induced Earth system changes is complex due to counteracting effects. Studies considering the individual effects of climate-driven changes in specific precursor emissions or processes show increases in surface ozone under warmer atmosphere for some processes. This is indeed the case for enhanced STE and stratospheric ozone recovery ( [[#Sekiya--2014|Sekiya and Sudo, 2014]] ; [[#Hess--2015|Hess et al., 2015]] ; [[#Banerjee--2016|Banerjee et al., 2016]] ; [[#Meul--2018|Meul et al., 2018]] ; [[#Morgenstern--2018|Morgenstern et al., 2018]] ; [[#Akritidis--2019|Akritidis et al., 2019]] ) or the increase of soil NO <sub>x</sub> emissions ( [[#Wu--2008|Wu et al., 2008]] ; [[#Romer--2018|Romer et al., 2018]] ), which can each lead to 1 to 2 ppb increase in surface ozone. Other processes, in particular deposition or those related to emissions from natural systems (Section 6.2.2) are expected to play a key role in future surface ozone and even the occurrence of pollution events (e.g., in the case of wildfires) but their effects are difficult to quantify in isolation.

Since the AR5, several studies have investigated the net effect of climate change on surface ozone, based on either global or regional model projections. A systematic and quantitative comparison of the ozone change, however, is difficult due to the variety of models with different complexities in the representation of natural emissions, chemical mechanisms and physical processes, as well as the surface ozone metrics applied for analysis. Ozone response to climate change has been shown to be particularly sensitive to model representation of processes like BVOC emissions, deposition, and isoprene chemistry (Squire et al. , 2015; Val Martin et al. , 2015; Schnell et al. , 2016; Pommier et al. , 2018) . More robust protocols are now used more commonly comprising, notably, longer simulations necessary to separate change from interannual variability (Barnes et al. , 2016; Lacressonnière et al. , 2016; Garcia-Menendez et al. , 2017) . However, the amplitude of climate change penalty on ozone over polluted regions may be different in high-resolution (regional- and urban-scale) models in comparison to coarse-resolution global models, because a number of controlling processes are resolution-dependent including for example, local emissions and sensitivity to the chemical regime (NMVOC limited versus NO <sub>x</sub> limited; [[#Lauwaet--2014|Lauwaet et al., 2014]] ; [[#Markakis--2014|Markakis et al., 2014]] , 2016).

Consistent with AR5 findings, global mean surface ozone concentration decreases range from 0.69 ± 0.16 ppb to 2.28 ± 0.24 ppb due to the dominating role of ozone destruction by water vapour in a four-member ensemble of CMIP6 ESM for surface warmings of 1.5°C–2.5°C (Figure 6.14). This decrease is driven by the ozone decrease over oceans, especially in the tropics (decrease of 1–5 ppb) and large parts of the continental unpolluted regions. The sensitivity of annual mean surface ozone to the level of surface warming over these remote areas varies spatially from –2 to –0.2 ppb <sup>o</sup> C <sup>–1</sup> (Supplementary Material Figure 6.SM.1).

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[[File:622b088318696710265aebab0c22e65c IPCC_AR6_WGI_Figure_6_14.png]]

'''Figure 6.14 |''' '''Multi-model annual mean change in surface O''' <sub>3</sub> '''(ppb) concentrations at different warming levels.''' Changes are shown for '''''(a)''''' 1.0°C, '''''(b)''''' 1.5°C, '''''(c)''''' 2.0°C and '''''(d)''''' 2.5°C increases in global mean surface air temperature. CMIP6 models include GFDL-ESM4, GISS-E2-1-G, MRI-ESM2-0 and UKESM1-0-LL. For each model, the change in surface O <sub>3</sub> is calculated as the difference between two AerChemMIP experiments – one with evolving future emissions and sea surface temperatures (SSTs) under the SSP3-7.0 scenario and the other with the same setup but with fixed present-day SSTs. The difference is calculated as a 20-year mean in surface O <sub>3</sub> around the year when the temperature threshold in each model is exceeded. The multi-model change in global annual mean surface O <sub>3</sub> concentrations with ± 1 ''standard deviation'' are shown within parentheses. Uncertainty is represented using the simple approach: no overlay indicates regions with high model agreement, three out of four models agree on sign of change; diagonal lines indicate regions with low model agreement, where three out of four models agree on sign of change. For more information on the simple approach, please refer to the Cross-Chapter Box Atlas.1. Further details on data sources and processing are available in the chapter data table (Table 6.SM.3).

Over ozone-producing regions of the world, such as in North America, Europe and Eastern Asia, AR5 and post-AR5 model studies project a general increase of surface ozone levels (climate change penalty on ozone) in a future warmer climate particularly during summer ( [[#Fu--2019|Fu and Tian, 2019]] ) . However, in current regional models, using more robust protocols, this increase of surface ozone, attributable to climate change, is of lower magnitude than in previous estimates ( [[#Lacressonnière--2016|Lacressonnière et al., 2016]] ; [[#Garcia-Menendez--2017|Garcia-Menendez et al., 2017]] ). Climate change enhances the efficiency of precursor emissions to generate surface ozone in polluted regions (Schnell et al. , 2016), and thus the magnitude of this effect will depend on the emissions considered in the study (present or future, and mitigated or not; Colette et al. , 2015; Fiore et al. , 2015) .

Considering anthropogenic emissions of precursors globally higher than the current emissions ( SSP3-7.0 in 2050; Figure 6.20), the CMIP6 ensemble confirms the surface ozone penalty due to climate change o ver regions close to anthropogenic pollution sources or close to natural emissions sources of ozone precursors (e.g., biomass-burning areas), with a penalty of a few ppb for the annual mean, proportional to warming levels (Figure 6.14). This rate ranges regionally from 0.2–2 ppb °C <sup>–1</sup> (Supplementary Material Figure 6.SM.1). The CMIP6 ESMs show this consistently for South East Asia (in line with [[#Hong--2019|Hong et al. (2019)]] and [[#Schnell--2016|Schnell et al. (2016)]] ) and for India (in line with [[#Pommier--2018|Pommier et al., 2018]] ) as well as in parts of Africa and South America, close to enhanced BVOC emissions (at least three out of four ESMs agree on the sign of change). The results are mixed in polluted regions of Europe and US because of lower anthropogenic precursor emissions which leads to a very low sensitivity of surface ozone to climate change (–0.5 ppb °C <sup>–1</sup> to 0.5 ppb °C <sup>–1</sup> ; Supplementary Material Figure 6.SM.1) and thus the ESMs can disagree on sign of changes for a given warming level. This heterogeneity in the results is also found in regional studies over North America (Gonzalez-Abraham et al. , 2015; Val Martin et al. , 2015; Schnell et al. , 2016; He et al. , 2018; Nolte et al. , 2018; Rieder et al. , 2018) or over Europe (Colette et al. , 2015; Lacressonnière et al. , 2016; Schnell et al. , 2016; Fortems-Cheiney et al. , 2017) .

Overall, warmer climate is expected to reduce surface ozone in unpolluted regions as a result of greater water vapour abundance accelerating ozone chemical loss ( ''high confidence'' ). Over regions with high anthropogenic and/or natural ozone precursor emissions, there is prevailing evidence that climate change will introduce a surface O <sub>3</sub> penalty increasing with increasing warming levels (with a magnitude ranging regionally from 0.2–2 ppb °C <sup>–1</sup> ) ( ''medium'' to ''high confidence'' ). Yet, there are uncertainties in processes affected in a warmer climate which can impact and modify future baseline and regional/local surface ozone levels. The response of surface ozone to future climate change through stratosphere–troposphere exchange, soil NO <sub>x</sub> emissions and wildfires is positive ( ''medium confidence'' ). In addition, there is ''low confidence'' in the magnitude of the effect of climate change on surface ozone through biosphere interactions (natural methane, non-methane BVOC emissions and ozone deposition) and lightning NO <sub>x</sub> emissions.

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