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

==== 7.6.1.5 Emissions Metrics by Compounds ====

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Emissions metrics for selected compounds are presented in Table 7.15, with further compounds presented in the Supplementary Material, Table 7.SM.7. The evolution of the CO <sub>2</sub> concentrations in response to a pulse emission is as in AR5 ( [[#Joos--2013|Joos et al., 2013]] ; [[#Myhre--2013b|Myhre et al., 2013b]] ), the perturbation lifetimes for CH <sub>4</sub> and N <sub>2</sub> O are from ( [[#7.6.1.1|Section 7.6.1.1]] . The lifetimes and radiative efficiencies for halogenated compounds are taken from [[#Hodnebrog--2020a|Hodnebrog et al. (2020a)]] . Combined metrics (CGTPs) are presented for compounds with lifetimes less than 20 years. Note that CGTP has units of years and is applied to a change in emissions rate rather than a change in emissions amount. Changes since AR5 are due to changes in radiative properties and lifetimes ( [[#7.6.1.1|Section 7.6.1.1]] ), and indirect contributions ( [[#7.6.1.3|Section 7.6.1.3]] ). Table 7.15 also gives overall emissions uncertainties in the emissions metrics due to uncertainties in radiative efficiencies, lifetimes and the climate response function (Supplementary Material, Tables 7.SM.8 to 7.SM.13).

Following their introduction in AR5 the assessed metrics now routinely include the carbon cycle response for non-CO <sub>2</sub> gases ( [[#7.6.1.3|Section 7.6.1.3]] ). As assessed in this earlier section, the carbon cycle contribution is lower than in AR5. Contributions to CO <sub>2</sub> formation are included for methane depending on whether or not the source originates from fossil carbon, thus methane from fossil fuel sources has slightly higher emissions metric values than that from non-fossil sources.

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'''Box 7.3 | Physical Considerations in Emissions Metric Choice'''

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Following AR5, this Report does not recommend an emissions metric because the appropriateness of the choice depends on the purposes for which gases or forcing agents are being compared. Emissions metrics can facilitate the comparison of effects of emissions in support of policy goals. They do not define policy goals or targets but can support the evaluation and implementation of choices within multi-component policies (e.g., they can help prioritize which emissions to abate). The choice of metric will depend on which aspects of climate change are most important to a particular application or stakeholder and over which time horizons. Different international and national climate policy goals may lead to different conclusions about what is the most suitable emissions metric ( [[#Myhre--2013b|Myhre et al., 2013b]] ).

Global warming potentials (GWP) and global temperature-change potentials (GTP) give the relative effect of pulse emissions, that is, how much more energy is trapped (GWP) or how much warmer (GTP) the climate would be when unit emissions of different compounds are compared ( [[#7.6.1.2|Section 7.6.1.2]] ). Consequently, these metrics provide information on how much energy accumulation (GWP) or how much global warming (GTP) could be avoided (over a given time period, or at a given future point in time) by avoiding the emission of a unit of a short-lived greenhouse gas compared to avoiding a unit of CO <sub>2</sub> . By contrast, the new metric approaches of combined GTP (CGTP) and GWP* closely approximate the additional effect on climate from a time series of short-lived GHG emissions, and can be used to compare this to the effect on temperature from the emission or removal of a unit of CO <sub>2</sub> [[#7.6.1.4|Section 7.6.1.4]] ; [[#Allen--2018b|Allen et al., 2018b]] ; [[#Collins--2020|Collins et al., 2020]] ).

Box 7.3

If global surface temperature stabilization goals are considered, cumulative CO <sub>2</sub> equivalent emissions computed with the GWP-100 emissions metric would continue to rise when short-lived GHG emissions are reduced but remain above zero (Figure 7.22b). Such a rise would not match the expected global surface temperature stabilization or potential decline in warming that comes from a reduction in emissions of short-lived greenhouse gases ( [[#Pierrehumbert--2014|Pierrehumbert, 2014]] ; [[#Allen--2018b|Allen et al., 2018b]] ; [[#Cain--2019|Cain et al., 2019]] ; [[#Collins--2020|Collins et al., 2020]] ; [[#Lynch--2020|Lynch et al., 2020]] , 2021). This is relevant to net zero GHG emissions goals ( [[#7.6.2|Section 7.6.2]] and Box 1.4).

When individual gases are treated separately in climate model emulators (Cross-Chapter Box 7.1), or weighted and aggregated using an emissions metric approach (such as CGTP or GWP*) which translate the distinct behaviour from cumulative emissions of short-lived gases, ambiguity in the future warming trajectory of a given emissions scenario can be substantially reduced ( [[#Cain--2019|Cain et al., 2019]] ; [[#Denison--2019|Denison et al., 2019]] ; [[#Collins--2020|Collins et al., 2020]] ; [[#Lynch--2021|Lynch et al., 2021]] ). The degree of ambiguity varies with the emissions scenario. For mitigation pathways that limit warming to 2°C with an even chance, the ambiguity arising from using GWP-100 as sole constraint on emissions of a mix of greenhouse gases (without considering their economic implications or feasibility) could be as much as 0.17°C, which represents about one-fifth of the remaining global warming in those pathways ( [[#Denison--2019|Denison et al., 2019]] ). If the evolution of the individual GHGs is not known, this can make it difficult to evaluate how a given global multi-gas emissions pathway specified only in CO <sub>2</sub> equivalent emissions would achieve (or not) global surface temperature goals. This is potentially an issue as Nationally Determined Contributions frequently make commitments in terms of GWP-100-based CO <sub>2</sub> equivalent emissions at 2030 without specifying individual gases ( [[#Denison--2019|Denison et al., 2019]] ). Clear and transparent representation of the global warming implications of future emissions pathways including Nationally Determined Contributions could be achieved either by their detailing pathways for multiple gases or by detailing a pathway of cumulative carbon dioxide equivalent emissions approach aggregated across GHGs evaluated by either GWP* or CGTP metric approaches ( [[#Cain--2019|Cain et al., 2019]] ; [[#Collins--2020|Collins et al., 2020]] ; [[#Lynch--2021|Lynch et al., 2021]] ). It should be noted that although the Paris Agreement Rulebook asks countries to report emissions of individual GHGs separately for the global stocktake (Decision 18/CMA.1, annex, paragraph 38), which can allow the current effects of their emissions on global surface temperature to be accurately estimated, estimates of future warming are potentially ambiguous where emissions are aggregated using GWP-100 or other pulse metrics.

Although there is significant history of using single-basket approaches, supported by emissions metrics such as GWP-100, in climate policies such as the Kyoto Protocol, multi-basket approaches also have many precedents in environmental management, including the Montreal Protocol ( [[#Daniel--2012|Daniel et al., 2012]] ). Further assessment of the performance of physical and economics-based metrics in the context of climate change mitigation is provided in the contribution of Working Group III to AR6.

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