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

==== 7.6.1.3 Carbon Cycle Responses and Other Indirect Contributions ====

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The effect of a compound on climate is not limited to its direct radiative forcing. Compounds can perturb the carbon cycle affecting atmospheric CO <sub>2</sub> concentrations. Chemical reactions from emitted compounds can produce or destroy other GHGs or aerosols.

Any agent that warms the surface perturbs the terrestrial and oceanic carbon fluxes (Sections 5.4.3 and 5.4.4), typically causing a net flux of CO <sub>2</sub> into the atmosphere and hence further warming. This aspect is already included in the carbon cycle models that are used to generate the radiative effects of a pulse of CO <sub>2</sub> ( [[#Joos--2013|Joos et al., 2013]] ), but was neglected for non-CO <sub>2</sub> compounds in the conventional metrics so this introduces an inconsistency and bias in the metric values ( [[#Gillett--2010|Gillett and Matthews, 2010]] ; [[#MacDougall--2015|MacDougall et al., 2015]] ; [[#Tokarska--2018|Tokarska et al., 2018]] ). A simplistic account of the carbon cycle response was tentatively included in AR5 based on a single study (W.J. [[#Collins--2013|]] [[#Collins--2013|Collins et al., 2013]] ). Since AR5 this understanding has been revised ( [[#Gasser--2017b|Gasser et al., 2017b]] ; [[#Sterner--2017|Sterner and Johansson, 2017]] ) using simple parametrized carbon cycle models to derive the change in CO <sub>2</sub> surface flux for a unit temperature pulse as an impulse response function to temperature. In W.J. [[#Collins--2013|]] [[#Collins--2013|Collins et al. (2013)]] this response function was assumed to be simply a delta function, whereas the newer studies include a more complete functional form accounting for subsequent re-uptake of CO <sub>2</sub> after the removal of the temperature increase. Accounting for re-uptake has the effect of reducing the carbon-cycle responses associated with the metrics compared to AR5, particularly at large time horizons. The increase in any metric due to the carbon cycle response can be derived from the convolution of the global surface temperature response with the CO <sub>2</sub> flux response to temperature and the equivalent metric for CO <sub>2</sub> (Equation 7.SM.5.5 in the Supplementary Material). Including this response also increases the duration of the effect of short-lived GHGs on climate ( [[#Fu--2020|Fu et al., 2020]] ). An alternative way of accounting for the carbon cycle temperature response would be to incorporate it into the temperature response function (the response functions used here and given in Supplementary Material 7.SM.5.2 do not explicitly do this). If this were done, the correction could be excluded from both the CO <sub>2</sub> and non-CO <sub>2</sub> forcing responses as, in [[#Hodnebrog--2020a|Hodnebrog et al. (2020a)]] .

Including the carbon cycle response for non-CO <sub>2</sub> treats CO <sub>2</sub> and non-CO <sub>2</sub> compounds consistently and therefore we assess that its inclusion more accurately represents the climate effects of non-CO <sub>2</sub> species. There is ''high confidence'' in the methodology of using carbon cycle models for calculating the carbon cycle response. The magnitude of the carbon cycle response contributions to the emissions metrics varies by a factor of two between [[#Sterner--2017|Sterner and Johansson (2017)]] and [[#Gasser--2017b|Gasser et al. (2017b)]] . The central values are taken from [[#Gasser--2017b|Gasser et al. (2017b)]] as the OSCAR 2.2 model used is based on parameters derived from CMIP5 models, and the climate–carbon feedback magnitude is therefore similar to the CMIP5 multi-model mean ( [[#Arora--2013|Arora et al., 2013]] ; [[#Lade--2018|Lade et al., 2018]] ). As values have only been calculated in two simple parametrized carbon cycle models the uncertainty is assessed to be ±100%. Due to there being few studies and a factor of two difference between them, there is ''low confidence'' that the magnitude of the carbon cycle response is within the higher end of this uncertainty range, but ''high confidence'' that the sign is positive. Carbon cycle responses are included in all the metrics presented in Table 7.15 and Supplementary Table 7.SM.7. The carbon cycle contribution is lower than in AR5, but there is ''high confidence'' in the need for its inclusion and the method by which it is quantified.

Emissions of non-CO <sub>2</sub> species can affect the carbon cycle in other ways: emissions of ozone precursors can reduce the carbon uptake by plants (W.J. [[#Collins--2013|]] [[#Collins--2013|Collins et al., 2013]] ); emissions of reactive nitrogen species can fertilize plants and hence increase the carbon uptake ( [[#Zaehle--2015|Zaehle et al., 2015]] ); and emissions of aerosols or their precursors can affect the utilisation of light by plants ( [[#Cohan--2002|Cohan et al., 2002]] ; [[#Mercado--2009|Mercado et al., 2009]] ; [[#Mahowald--2017|Mahowald et al., 2017]] ; see Section 6.4.4 for further discussion). There is ''robust evidence'' that these processes occur and are important, but ''insufficient evidence'' to determine the magnitude of their contributions to emissions metrics. Ideally, emissions metrics should include all indirect effects to be consistent, but limits to our knowledge restrict how much can be included in practice.

Indirect contributions from chemical production or destruction of other GHGs are quantified in ( [[IPCC:Wg1:Chapter:Chapter-6|Chapter 6]] (Section 6.4). For methane (CH <sub>4</sub> ), AR5 ( [[#Myhre--2013b|Myhre et al., 2013b]] ) assessed that the contributions from effects on ozone and stratospheric water vapour add 50% ± 30% and 15% ± 11% to the emissions-based ERF, which were equivalent to 1.8 ± 0.7 ×10 <sup>–4</sup> and 0.5 ± 0.4 ×10 <sup>–4</sup> W m <sup>–2</sup> ppb (CH <sub>4</sub> ) <sup>–1</sup> . In AR6 the radiative efficiency formulation is preferred as it is independent of the assumed radiative efficiency for methane. The assessed contributions to the radiative efficiency for methane due to ozone are 1.4 ± 0.7 ×10 <sup>–4</sup> W m <sup>–2</sup> ppb (CH <sub>4</sub> ) <sup>–1</sup> , based on 0.14 W m <sup>–2</sup> forcing from a 1023 ppb (1850–2014) methane change ( [[#Thornhill--2021b|Thornhill et al., 2021b]] ). The contribution from stratospheric water vapour is 0.4 ± 0.4 ×10 <sup>–4</sup> W m <sup>–2</sup> ppb (CH <sub>4</sub> ) <sup>–1</sup> , based on 0.05 W m <sup>–2</sup> forcing from a 1137 ppb (1750–2019) methane change ( [[#7.3.2.6|Section 7.3.2.6]] ). Nitrous oxide (N <sub>2</sub> O) depletes upper stratospheric ozone (a positive forcing) and reduces the methane lifetime. In AR5 the methane lifetime effect was assessed to reduce methane concentrations by 0.36 ppb per ppb increase in N <sub>2</sub> O, with no assessment of the effective radiative forcing from ozone. This is now increased to –1.7 ppb methane per ppb N <sub>2</sub> O (based on a methane lifetime decrease of 4% ± 4% for a 55 ppb increase in N <sub>2</sub> O ( [[#Thornhill--2021b|Thornhill et al., 2021b]] ) and a radiative efficiency of 5.5 ± 0.4 ×10 <sup>–4</sup> W m <sup>–2</sup> ppb (N <sub>2</sub> O) <sup>–1</sup> through ozone ( [[#Thornhill--2021b|Thornhill et al., 2021b]] )). In summary, GWPs and GTPs for methane and nitrous oxide are slightly lower than in AR5 ( ''medium confidence'' ) due to revisions in their lifetimes and updates to their indirect chemical effects.

Methane can also affect the oxidation pathways of aerosol formation ( [[#Shindell--2009|Shindell et al., 2009]] ) but the available literature is insufficient to make a robust assessment of this. Hydrocarbon and molecular hydrogen oxidation also leads to tropospheric ozone production and change in methane lifetime ( [[#Collins--2002|Collins et al., 2002]] ; [[#Hodnebrog--2018|Hodnebrog et al., 2018]] ). For reactive species the emissions metrics can depend on where the emissions occur, and the season of emission ( [[#Aamaas--2016|Aamaas et al., 2016]] ; [[#Lund--2017|Lund et al., 2017]] ; [[#Persad--2018|Persad and Caldeira, 2018]] ). The AR5 included a contribution to the emissions metrics for ozone-depleting substances (ODSs) from the loss of stratospheric ozone. The assessment of ERFs from ODSs in ( [[IPCC:Wg1:Chapter:Chapter-6|Chapter 6]] (Section 6.4.2) suggests the quantification of these terms may be more uncertain than the formulation in AR5 so these are not included here.

Oxidation of methane leads ultimately to the net production of atmospheric CO <sub>2</sub> ( [[#Boucher--2009|Boucher et al., 2009]] ). This yield is less than 100% (on a molar basis) due to uptake by soils and some of the reaction products (mainly formaldehyde) being directly removed from the atmosphere before being completely oxidized. Estimates of the yield are 61% ( [[#Boucher--2009|Boucher et al., 2009]] ) and 88% ( [[#Shindell--2017|Shindell et al., 2017]] ), so the assessed range is 50–100% with a central value of 75% ( ''low confidence'' ) ''.'' For methane and hydrocarbons from fossil sources, this will lead to additional fossil CO <sub>2</sub> in the atmosphere whereas for biogenic sources of methane or hydrocarbons, this replaces CO <sub>2</sub> that has been recently removed from the atmosphere. Since the ratio of molar masses is 2.75, 1 kg of methane generates 2.1 ± 0.7 kgCO <sub>2</sub> for a 75% yield. For biogenic methane the soil uptake and removal of partially oxidized products is equivalent to a sink of atmospheric CO <sub>2</sub> of 0.7 ± 0.7 kg per kg methane. The contributions of this oxidation effect to the methane metric values allow for the time delay in the oxidation of methane. Methane from fossil fuel sources has therefore slightly higher emissions metric values than those from biogenic sources ( ''high confidence'' ). The CO <sub>2</sub> can already be included in carbon emissions totals ( [[#Muñoz--2016|Muñoz and Schmidt, 2016]] ) so care needs to be taken when applying the fossil correction to avoid double counting.

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