Editing IPCC:AR6/WGI/Chapter-7 (section)

==== 7.3.2.5 Ozone ====

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Estimates of the pre-industrial to present-day tropospheric ozone radiative forcing are based entirely on models. The lack of pre-industrial ozone measurements prevents an observational determination. There have been limited studies of ozone ERFs ( [[#MacIntosh--2016|MacIntosh et al., 2016]] ; [[#Xie--2016|Xie et al., 2016]] ; [[#Skeie--2020|Skeie et al., 2020]] ). [[#Skeie--2020|Skeie et al. (2020)]] found little net contribution to the ERF from tropospheric adjustment terms for 1850–2000 change in ozone (tropospheric and stratospheric ozone combined), although [[#MacIntosh--2016|MacIntosh et al. (2016)]] suggested that increases in stratospheric or upper tropospheric ozone reduces high-cloud and increases low-cloud, whereas an increase in lower tropospheric ozone reduces low-cloud. Further studies suggest that changes in circulation due to decreases in stratospheric ozone affect Southern Hemisphere clouds and the atmospheric levels of sea salt aerosol that would contribute additional adjustments, possibly of comparable magnitude to the SARF from stratospheric ozone depletion ( [[#Grise--2013|Grise et al., 2013]] , 2014; [[#Xia--2016|Xia et al., 2016]] , 2020). ESM responses to changes in ozone-depleting substances (ODS) in CMIP6 show a much more negative ERF than would be expected from offline calculations of SARF ( [[#Morgenstern--2020|Morgenstern et al., 2020]] ; [[#Thornhill--2021b|Thornhill et al., 2021b]] ) again suggesting a negative contribution from adjustments. However there is insufficient evidence available to quantify this effect.

Without sufficient information to assess whether the ERFs differ from SARF, this assessment relies on offline radiative transfer calculations of SARF for both tropospheric and stratospheric ozone. [[#Checa-Garcia--2018|Checa-Garcia et al. (2018)]] found SARF of 0.30 W m <sup>–2</sup> for changes in ozone (1850–1860 to 2009–2014). These were based on precursor emissions and ODS concentrations from the Coupled Chemistry Model Initiative (CCMI) project ( [[#Morgenstern--2017|Morgenstern et al., 2017]] ). [[#Skeie--2020|Skeie et al. (2020)]] calculated an ozone SARF of 0.41 ± 0.12 W m <sup>–2</sup> (1850–2010; from five climate models and one chemistry transport model) using CMIP6 precursor emissions and ODS concentrations (excluding models without fully interactive ozone chemistry and one model with excessive ozone depletion). The ozone precursor emissions are higher in CMIP6 than in CCMI, which explains much of the increase compared to [[#Checa-Garcia--2018|Checa-Garcia et al. (2018)]] ''.''

Previous assessments have split the ozone forcing into tropospheric and stratospheric components. This does not correspond to the division between ozone production and ozone depletion and is sensitive to the choice of tropopause ( ''high confidence'' ) ( [[#Myhre--2013b|Myhre et al., 2013b]] ). The contributions to total SARF in CMIP6 ( [[#Skeie--2020|Skeie et al., 2020]] ) are 0.39 ± 0.07 and 0.02 ± 0.07 W m <sup>–2</sup> for troposphere and stratosphere respectively (using a 150 ppb ozone tropopause definition). This small positive (but with uncertainty encompassing negative values) stratospheric ozone SARF is due to contributions from ozone precursors to lower stratospheric ozone and some of the CMIP6 models showing ozone depletion in the upper stratosphere, where depletion contributes a positive radiative forcing ( ''medium confidence'' ).

As there is insufficient evidence to quantify adjustments, for total ozone the assessed central estimate for ERF is assumed to be equal to SARF ( ''low confidence'' ) and follows [[#Skeie--2020|Skeie et al. (2020)]] , since that study uses the most recent emissions data. The dataset is extended over the entire historical period following [[#Skeie--2020|Skeie et al. (2020)]] , with a SARF for 1750–1850 of 0.03 W m <sup>–2</sup> and for 2010–2018 of 0.03 W m <sup>–2</sup> , <sup></sup> to give 0.47 [0.24 to 0.70] W m <sup>–2</sup> for 1750–2019. This maintains the 50% uncertainty (5–95% range) from AR5 which is largely due to the uncertainty in pre-industrial emissions ( [[#Rowlinson--2020|Rowlinson et al., 2020]] ). There is also ''high confidence'' that this range includes uncertainty due to the adjustments. The CMIP6 SARF is more positive than the AR5 value of 0.31 W m <sup>–2</sup> for the period 1850–2011 ( [[#Myhre--2013b|Myhre et al., 2013b]] ) which was based on the Atmospheric Chemistry and Climate Intercomparison Project (ACCMIP; [[#Shindell--2013|Shindell et al., 2013]] ) ''.'' The assessment is sensitive to the assumptions on precursor emissions used to drive the models, which are larger in CMIP6 than ACCMIP.

In summary, although there is insufficient evidence to quantify adjustments, there is ''high confidence'' in the assessed range of ERF for ozone changes over the 1750–2019 period, giving an assessed ERF of 0.47 [0.24 to 0.70] W m <sup>–2</sup> .

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