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

=== 6.4.2 Emissions-based Radiative Forcing and Effect on Global Surface Air Temperature (GSAT) ===

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The ERFs attributable to emissions versus concentrations for several SLCFs including ozone and methane are different. A concentration change, used to assess the abundance - based ERF, results from The changes in emissions of multiple species and subsequent chemical reactions. The corollary is that the perturbation of a single emitted compound can induce subsequent chemical reactions and affect the concentrations of several climate forcers (chemical adjustments); this is what is accounted for in emissions-based ERF. Due to non-linear chemistry (Section 6.3) and non-linear aerosol–cloud interactions ( [[IPCC:Wg1:Chapter:Chapter-7#7.3.3.2|Section 7.3.3.2]] ), the ERF attributed to the individual species cannot be precisely defined and can only be estimated through model simulations. For example, the ERF attributed to methane emissions, which includes indirect effects through ozone formation and oxidation capacity with feedbacks on the methane lifetime, depend non-linearly on the concentrations of NO <sub>x</sub> , CO and NMVOCs. This means that the results from the model simulations depend to some extent on the chosen methodology. In AR5 (based on [[#Shindell--2009|Shindell et al., 2009]] ; [[#Stevenson--2013|Stevenson et al., 2013]] ) the attribution was done by removing the anthropogenic emissions of individual species one by one from a control simulation for present-day conditions. Further, only the radiative forcings, and not the ERF (mainly including the effect of aerosol–cloud interactions) were attributed to the emitted compounds.

Since AR5, the emissions estimates have been revised and extended for CMIP6 ( [[#Hoesly--2018|Hoesly et al., 2018]] ), the models have been further developed, the period has been extended (1750–2019, versus 1750–2011 in AR5) and the experimental setup for the model simulations has changed ( [[#Collins--2017|Collins et al., 2017]] ), making a direct comparison of results difficult. Figure 6.12a shows the global and annual mean ERF attributed to emitted compounds over the period 1750–2019 based on AerChemMIP simulations (Thornhill et al., 2021b) where anthropogenic emissions or concentrations of individual species were perturbed from 1850 to 2014 levels (methodology described in Supplementary Material 6.SM.1).

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Figure 6.12 | '''Contribution to effective radiative forcing (ERF) (a) and global mean surface air temperature (GSAT) change (b) from component emissions between 1750 to 2019 based on CMIP6 models (Thornhill et al.''' ''', 2021b).''' ERFs for the direct effect of well-mixed greenhouse gases (WMGHGs) are from the analytical formulae in section 7.3.2, H <sub>2</sub> O (strat) is from Table 7.8. ERFs for other components are multi-model means from [[#Thornhill--2021b|Thornhill et al. (2021b)]] and are based on ESM simulations in which emissions of one species at a time are increased from 1850 to 2014 levels. The derived emissions-based ERFs are rescaled to match the concentration-based ERFs in Figure 7.6. Error bars are 5–95% and for the ERF account for uncertainty in radiative efficiencies and multi-model error in the means. ERFs due to aerosol–radiation (ERFari) and cloud effects are calculated from separate radiation calls for clear-sky and aerosol-free conditions ( [[#Ghan--2013|Ghan, 2013]] ; [[#Thornhill--2021b|Thornhill et al., 2021b]] ). ‘Cloud’ includes cloud adjustments (semi-direct effect) and ERF from indirect aerosol-cloud to –0.22 W m <sup>–2</sup> for ERFari and –0.84 W m <sup>–2</sup> interactions (ERFaci). The aerosol components (SO <sub>2</sub> , organic carbon and black carbon) are scaled to sum to –0.22 W m <sup>–2</sup> for ERFari and –0.84 W m <sup>–2</sup> for ‘cloud’ ( [[IPCC:Wg1:Chapter:Chapter-7#7.3.3|Section 7.3.3]] ). For GSAT estimates, time series (1750–2019) for the ERFs have been estimated by scaling with concentrations for WMGHGs and with historical emissions for SLCFs. The time variation of ERFaci for aerosols is from Chapter 7. The global mean temperature response is calculated from the ERF time series using an impulse response function (Cross-Chapter Box 7.1) with a climate feedback parameter of –1.31 W m <sup>–2</sup> °C <sup>–1</sup> . Contributions to ERF and GSAT change from contrails and light-absorbing particles on snow and ice are not represented, but their estimates can be seen on Figure 7.6 and 7.7, respectively. Further details on data sources and processing are available in the chapter data table (Table 6.SM.3).

The ERF based on primary CO <sub>2</sub> emissions is slightly lower than the abundance-based estimate ( [[IPCC:Wg1:Chapter:Chapter-7#7.3.2.1|Section 7.3.2.1]] ) because the abundance-based ERF combines the effect of primary CO <sub>2</sub> emissions and a small additional secondary contribution from atmospheric oxidation of methane, CO, and NMVOCs (4%) of fossil origin, consistent with AR5 findings.

Ozone-depleting substances, such as N <sub>2</sub> O and halocarbons, cause a reduction in stratospheric ozone, which affects ozone and OH production in the troposphere through ultraviolet radiation changes (and thus affect methane). They also have indirect effects on aerosols and clouds ( [[#Karset--2018|Karset et al., 2018]] ), since changes in oxidants induce changes in the oxidation of aerosol precursors.

The net ERF from N <sub>2</sub> O emissions is estimated to be 0.24 [0.13 to 0.34] W m <sup>–2</sup> , which is very close to the abundance-based estimate of 0.21 W m <sup>–2</sup> [[IPCC:Wg1:Chapter:Chapter-7#7.3.2.3|Section 7.3.2.3]] ). The indirect contributions from N <sub>2</sub> O are relatively minor with negative (methane-lifetime) and positive (ozone-and-cloud) effects nearly compensating each other. Emissions of halogenated compounds, including CFCs and HCFCs, were assessed as very likely causing a net-positive ERF in the AR5. However, recent studies ( [[#Morgenstern--2020|Morgenstern et al., 2020]] ; [[#O’Connor--2021|O’Connor et al., 2021]] ; [[#Thornhill--2021b|Thornhill et al., 2021b]] ) find strong adjustments in Southern Hemisphere aerosols and clouds, such that the ''very likely'' range in the emission-based ERF for CFC + HCFCs + HFCs now also include negative values.

For methane emissions, in addition to their direct effect, there are indirect positive ERFs from methane enhancing its own lifetime, causing ozone production, enhancing stratospheric water vapour, and influencing aerosols and the lifetimes of HCFCs and HFCs ( [[#Myhre--2013|Myhre et al., 2013]] ; [[#O’Connor--2021|O’Connor et al., 2021]] ). The ERF from methane emissions is considerably higher than the ERF estimate resulting from its abundance change. The central estimate with the ''very likely'' range is 1.19 [0.81 to 1.58] W m <sup>–2</sup> for the emissions-based estimate <sup></sup> compared with 0.54 W m <sup>–2</sup> for the abundance-based estimate ( [[IPCC:Wg1:Chapter:Chapter-7#7.3.5|Section 7.3.5]] ). The abundance-based ERF estimate for methane results from contributions of its own emissions and the effects of several other compounds, some decreasing methane lifetime, notably NO <sub>x</sub> , which importantly reduce the methane abundance-based ERF. Emissions of CO and NMVOCs both indirectly contribute to a positive ERF through enhancing ozone production in the troposphere and increasing the methane lifetime. For CO and NMVOCs of fossil origin there is also a 0.07 W m <sup>–2</sup> contribution to CO <sub>2</sub> from their oxidation. The ''very likely'' total ERF of CO and NMVOCs emissions is estimated to be 0.44 [0.22 to 0.67] W m <sup>–2</sup> .

NO <sub>x</sub> causes a positive ERF through enhanced tropospheric ozone production and a negative ERF through enhanced OH concentrations that reduce the methane lifetime. There is also a small negative ERF contribution through the formation of nitrate aerosols, although only three of the AerChemMIP models include nitrate aerosols. The best estimate of the net ERF from changes in anthropogenic NO <sub>x</sub> emissions is –0.27 [–0.55 to 0.01] W m <sup>–2</sup> . The magnitude is somewhat greater than the AR5 estimate (–0.15 [–0.34 to +0.02] W m <sup>–2</sup> ) but with a similar level of uncertainty. The difference between AR6 and AR5 estimates is possibly due to the different modeling protocols (see Supplementary Material: 6.SM.1).

Anthropogenic emissions of SO <sub>2</sub> lead to the formation of sulphate aerosols and a negative ERF through aerosol–radiation and aerosol–cloud interactions. The emissions-based ERFaci, which was not previously considered in AR5, is now included. The estimated ERF is thus considerably more negative than the AR5 estimate with a radiative forcing of –0.4 W m <sup>–2</sup> , despite the decline of ERF due to aerosols since 2011 ( [[IPCC:Wg1:Chapter:Chapter-7#7.3.3.1.3|Section 7.3.3.1.3]] , Figure 6.12a). SO <sub>2</sub> emissions are estimated to contribute to a negative ERF of –0.94 [–1.63 to –0.25] W m <sup>–2</sup> , with –0.23 W m <sup>–2</sup> from aerosol–radiation interactions and –0.70 W m <sup>–2</sup> from aerosol–cloud interactions. Emissions of NH <sub>3</sub> lead to formation of ammonium-nitrate aerosols with an estimated ERF of –0.03 W m <sup>–2</sup> .

The best estimate for the ERF due to emissions of BC is reduced from the AR5, and is now estimated to be 0.11 [–0.20 to 0.42] W m <sup>–2</sup> with an uncertainty also including negative values. As discussed in [[IPCC:Wg1:Chapter:Chapter-7#7.3.3.1.2|Section 7.3.3.1.2]] , a significant portion of the positive BC forcing from aerosol–radiation interactions is offset by negative atmospheric adjustments due to cloud changes, as well as lapse rate and atmospheric water vapour changes, resulting in a smaller positive net ERF for BC compared with AR5. The large range in the forcing estimate stems from variation in the magnitude and sign of atmospheric adjustments across models and is related to the differences in the model treatment of different processes affecting BC (e.g., ageing, mixing) and its interactions with clouds and cryosphere ( [[IPCC:Wg1:Chapter:Chapter-7#7.3.3|Section 7.3.3]] ; [[#Thornhill--2021b|Thornhill et al., 2021b]] ). The emissions-based ERF for organic carbon aerosols is –0.21 [–0.44 to +0.02] W m <sup>–2</sup> , a weaker estimate compared with AR5 attributed to stronger absorption by OC ( [[IPCC:Wg1:Chapter:Chapter-7#7.3.3.1.2|Section 7.3.3.1.2]] ).

The emissions-based contributions to GSAT change (Figure 6.12b) were not assessed in AR5, but with the ERF from aerosol–cloud interactions attributed to the emitted compounds there is now a better foundation for this assessment. The contribution to emissions-based ERF at 2019 (Figure 6.12a) is scaled by the historical emissions (over the period 1750–2019) of each compound to reconstruct the historical time series of ERF. An impulse response function (Cross-Chapter Box 7.1, Supplementary Material 7.SM5.2) is then applied to obtain the contribution of SLCF emissions to the GSAT response. Due to the non-linear chemical and physical processes described above relating emissions to ERF, and the additional non-linear relations between ERF and GSAT, these emissions-based estimates of GSAT responses strongly depend on the methodology applied to estimate ERF and GSAT (Supplementary Material 6.SM.2). Therefore, the relative contribution of each compound through its primary emissions versus secondary formation or destruction (e.g., for methane emissions its ozone versus methane contributions), by construction (omitting the non-linear processes), will be equal for ERF and GSAT. Uncertainties in the GSAT response are estimated using the assessed range of the equilibrium climate sensitivity (ECS) from [[IPCC:Wg1:Chapter:Chapter-7|Chapter 7]] of this report. For most of the emitted compounds the uncertainty in the GSAT response is dominated by the uncertainty in the relationship between emissions and the ERF.

The contributions from the emitted compounds to GSAT broadly follow their contributions to the ERF, mainly because their evolution over the past decades have been relatively similar and slow enough compared to their lifetimes to be reflected similarly in their ERF and GSAT despite the delay of the GSAT response to ERF changes (Section 6.6.1). However, for some SLCFs (e.g., SO <sub>2</sub> ) that have been reduced globally, their contribution to GSAT change is slightly higher compared with that of CO <sub>2</sub> than their relative contribution to ERF because the peak in their ERF change has already occurred (Section 6.4.1) whereas the peak of their GSAT effect started to decline recently (Figure 7.9). This is due to the inertia of the climate system delaying the full response of GSAT to a change in forcing (Figure 6.15).

In summary, emissions of SLCFs, especially methane, NO <sub>x</sub> and SO <sub>2</sub> , have substantial effects on effective radiative forcing (ERF) ( ''high confidence'' ). The net global emissions-based ERF of NO <sub>x</sub> is negative and that of NMVOCs is positive, in agreement with the AR5 assessment ( ''high confidence'' ). For methane, the emissions-based ERF is twice as high as the abundance-based ERF ( ''high confidence'' ). SO <sub>2</sub> emissions make the dominant contribution to the ERF associated with the aerosol–cloud interaction ( ''high'' ''confidence'' ). The contributions from the emitted compounds to GSAT broadly follow their contributions to the ERF ( ''high confidence'' ). However, due to the inertia of the climate system delaying the full GSAT response to a change in forcing, the contribution to GSAT change due to SO <sub>2</sub> emissions is slightly higher compared with that due to CO <sub>2</sub> emissions (than their relative contributions to ERF) because the peak in emission-induced SO <sub>2</sub> eRF has already occurred.

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