Editing IPCC:AR6/WGIII/Chapter-2 (section)

=== Cross-Chapter Box 2 | GHG Emissions Metrics ===

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'''Authors:''' Andy Reisinger (New Zealand), Alaa Al Khourdajie (United Kingdom/Syria), Kornelis Blok (the Netherlands), Harry Clark (New Zealand), Annette Cowie (Australia), Jan S. Fuglestvedt (Norway), Oliver Geden (Germany), Veronika Ginzburg (the Russian Federation), Céline Guivarch (France), Joanna I. House (United Kingdom), Jan Christoph Minx (Germany), Rachid Mrabet (Morocco), Gert-Jan Nabuurs (the Netherlands), Glen P. Peters (Norway/Australia), Keywan Riahi (Austria), Roberto Schaeffer (Brazil), Raphael Slade (United Kingdom), Anders Hammer Strømman (Norway), Detlef P. van Vuuren (the Netherlands)

Comprehensive mitigation policy relies on consideration of all anthropogenic forcing agents, which differ widely in their atmospheric lifetimes and impacts on the climate system. GHG emission metrics [[#footnote-008|6]] provide simplified information about the effects that emissions of different GHGs have on global temperature or other aspects of climate, usually expressed relative to the effect of emitting CO 2 (see emission metrics in Annex I: Glossary). This information can inform prioritisation and management of trade-offs in mitigation policies and emission targets for non-CO 2 gases relative to CO 2 , as well as for baskets of gases expressed in CO 2 -eq. This assessment builds on the evaluation of GHG emission metrics from a physical science perspective by WGI (Forster et al. 2021b). For additional details and supporting references, see [https://www.ipcc.ch/report/ar6/wg3/chapter/chapter-2 Chapter 2] Supplementary Material (2.SM.3) and Annex II.8.

The global warming potential (GWP) and the global temperature change potential (GTP) werethe main metrics assessed in AR5 ( [[#Myhre--2013|Myhre et al. 2013]] ; Kolstad et al. 2014). The GWP with a lifetime of 100 years (GWP100) continues to be the dominant metric used in the scientific literature on mitigation assessed by WGIII. The assessment by WGI (Forster et al. 2021) includes updated values for these metrics based on updated scientific understanding of the response of the climate system to emissions of different gases, including changing background concentrations. It also assesses new metrics published since AR5. Metric values in AR6 include climate-carbon cycle feedbacks by default; this provides an important update and clarification from AR5 which reported metric values both with and without such feedbacks.

The choice of metric, including time horizon, should reflect the policy objectives for which the metric is applied ( [[#Plattner--2009|Plattner et al. 2009]] ). Recent studies confirm earlier findings that the GWP is consistent with a cost-benefit framework (Kolstad et al. 2014), which implies weighting each emission based on the economic damages that this emission will cause over time, or conversely, the avoided damages from avoiding that emission. The GWP time horizon can be linked to the discount rate used to evaluate economic damages from each emission. For methane, GWP100 implies a social discount rate of about 3–5% depending on the assumed damage function, whereas GWP20 implies a much higher discount rate, greater than 10% ( ''medium confidence'' ) (Mallapragada and Mignone 2019; [[#Sarofim--2018|Sarofim and Giordano 2018]] ). The dynamic GTP is aligned with a cost-effectiveness framework, as it weights each emission based on its contribution to global warming in a specified future year (e.g., the expected year of peak warming for a given temperature goal). This implies a shrinking time horizon and increasing relative importance of SLCF emissions as the target year is approached (Johansson 2011; [[#Aaheim--2017|Aaheim and Mideksa 2017]] ). The GTP with a static time horizon (e.g., GTP100) is not well-matched to either a cost-benefit or a cost-effectiveness framework, as the year for which the temperature outcome is evaluated would not match the year of peak warming, nor the overall damages caused by each emission ( [[#Edwards--2014|Edwards and Trancik 2014]] ; [[#Strefler--2014|Strefler et al. 2014]] ; [[#Mallapragada--2017|Mallapragada and Mignone 2017]] ).

A number of studies since AR5 have evaluated the impact of various GHG emission metrics and time horizons on the global economic costs of limiting global average temperature change to a pre-determined level (e.g. [[#Strefler--2014|Strefler et al. 2014]] ; [[#Harmsen--2016|Harmsen et al. 2016]] ; Tanaka et al. 2021) (see 2.SM.3 for additional detail). These studies indicate that, for mitigation pathways that limit warming to 2°C (&lt;67%) above pre-industrial levels or lower, using GWP100 to inform cost-effective abatement choices between gases would achieve such long-term temperature goals at close to least global cost within a few percent ( ''high confidence'' ). Using the dynamic GTP instead of GWP100 could reduce global mitigation costs by a few percent in theory ( ''high confidence'' ), but the ability to realise those cost savings depends on the temperature limit, policy foresight and flexibility in abatement choices as the weighting of SLCF emissions increases over time ( ''medium confidence'' ) ( [[#van%20den%20Berg--2015|van den Berg et al. 2015]] ; [[#Huntingford--2015|Huntingford et al. 2015]] ). Similar benefits as for the dynamic GTP might be obtained by regularly reviewing and potentially updating the time horizon used for GWP in light of actual emissions trends compared to climate goals ( [[#Tanaka--2020|Tanaka et al. 2020]] ).

The choice of metric and time horizon can affect the distribution of costs and the timing of abatement between countries and sectors in cost-effective mitigation strategies. Sector-specific lifecycle assessments find that different emission metrics and different time horizons can lead to divergent conclusions about the effectiveness of mitigation strategies that involve reductions of one gas but an increase of another gas with a different lifetime (e.g., [[#Tanaka--2019|Tanaka et al. 2019]] ). Assessing the sensitivity of conclusions to different emission metrics and time horizons can support more robust decision-making ( [[#Levasseur--2016|Levasseur et al. 2016]] ; [[#Balcombe--2018|Balcombe et al. 2018]] ) (see 2.SM.3 for details). Sectoral and national perspectives on GHG emission metrics may differ from a global least-cost perspective, depending on other policy objectives and equity considerations, but the literature does not provide a consistent framework for assessing GHG emission metrics based on equity principles.

Literature since AR5 has emphasised that the GWP100 is not well-suited to estimating the warming effect at specific points in time from sustained SLCF emissions (e.g., [[#Allen--2016|Allen et al. 2016]] ; [[#Cain--2019|Cain et al. 2019]] ; [[#Collins--2019|Collins et al. 2019]] ). This is because the warming caused by an individual SLCF emission pulse diminishes over time and hence, unlike CO 2 , the warming from SLCF emissions that are sustained over multiple decades to centuries depends mostly on their ongoing rate of emissions rather than their cumulative emissions. Treating all gases interchangeably based on GWP100 within a stated emissions target therefore creates ambiguity about actual global temperature outcomes ( [[#Fuglestvedt--2018|Fuglestvedt et al. 2018]] ; [[#Denison--2019|Denison et al. 2019]] ). Supplementing economy-wide emission targets with information about the expected contribution from individual gases to such targets would reduce the ambiguity in global temperature outcomes.

Recently developed step/pulse metrics such as the combined global temperature change potential (CGTP) ( [[#Collins--2019|Collins et al. 2019]] ) and GWP* ( [[#Allen--2018|Allen et al. 2018]] ; [[#Cain--2019|Cain et al. 2019]] ) recognise that a sustained increase/decrease in the rate of SLCF emissions has a similar effect on global surface temperature over multiple decades as a one-off pulse emission/removal of CO 2 . These metrics use this relationship to calculate the CO 2 emissions or removals that would result in roughly the same temperature change as a sustained change in the rate of SLCF emissions (CGTP) over a given time period, or as a varying time series of CH 4 emissions (GWP*). From a mitigation perspective, these metrics indicate greater climate benefits from rapid and sustained methane reductions over the next few decades than if such reductions are weighted by GWP100, while conversely, sustained methane increases have greater adverse climate impacts ( [[#Collins--2019|Collins et al. 2019]] ; [[#Lynch--2020|Lynch et al. 2020]] ). The ability of these metrics to relate changes in emission rates of short-lived gases to cumulative CO 2 emissions makes them well-suited, in principle, to estimating the effect on the remaining carbon budget from more, or less, ambitious SLCF mitigation over multiple decades compared to a given reference scenario ( ''high confidence'' ) ( [[#Collins--2019|Collins et al. 2019]] ; Forster et al. 2021).

The potential application of GWP* in wider climate policy (e.g., to inform equitable and ambitious emission targets or to support sector-specific mitigation policies) is contested, although relevant literature is still limited ( [[#Rogelj--2019|Rogelj and Schleussner 2019]] , 2021; [[#Schleussner--2019|Schleussner et al. 2019]] ; [[#Allen--2021|Allen et al. 2021]] ; [[#Cain--2021|Cain et al. 2021]] ). Whereas GWP and GTP describe the marginal effect of each emission relative to the absence of that emission, GWP* describes the equivalent CO 2 emissions that would give the same temperature change as an emissions trajectory of the gas considered, starting at a (user-determined) reference point. The warming based on those cumulative CO 2 -equivalent emission at any point in time is relative to the warming caused by emissions of that gas before the reference point. Because of their different focus, GWP* and GWP100 can equate radically different CO 2 emissions to the same CH 4 emissions: rapidly declining CH 4 emissions have a negative CO 2 -warming-equivalent value based on GWP* (rapidly declining SLCF emissions result in declining temperature, relative to the warming caused by past SLCF emissions at a previous point in time) but a positive CO 2 -equivalent value based on GWP or GTP (each SLCF emission from any source results in increased future radiative forcing and global average temperature than without this emission, regardless of whether the rate of SLCF emissions is rising or declining). The different focus in these metrics can have important distributional consequences, depending on how they are used to inform emission targets ( [[#Lynch--2021|Lynch et al. 2021]] ; [[#Reisinger--2021|Reisinger et al. 2021]] ), but this has only begun to be explored in the scientific literature.

A key insight from WGI is that, for a given emissions scenario, different metric choices can alter the time at which net zero GHG emissions are calculated to be reached, or whether net zero GHG emissions are reached at all (2.SM.3). From a mitigation perspective, this implies that changing GHG emission metrics but retaining the same numerical CO 2 -equivalent emissions targets would result in different climate outcomes. For example, achieving a balance of global anthropogenic GHG emissions and removals, as stated in Article 4.1 of the Paris Agreement could, depending on the GHG emission metric used, result in different peak temperatures and in either stable, or slowly or rapidly declining temperature after the peak ( [[#Allen--2018|Allen et al. 2018]] ; [[#Fuglestvedt--2018|Fuglestvedt et al. 2018]] ; [[#Tanaka--2018|Tanaka and O’Neill 2018]] ; [[#Schleussner--2019|Schleussner et al. 2019]] ). A fundamental change in GHG emission metrics used to monitor achievement of existing emission targets could therefore inadvertently change their intended climate outcomes or ambition, unless existing emission targets are re-evaluated at the same time ( ''very'' ''high confidence'' ).

The WGIII contribution to AR6 reports aggregate emissions and removals using updated GWP100 values from AR6 WGI unless stated otherwise. This choice was made on both scientific grounds (the alignment of GWP100 with a cost-benefit perspective under social discount rates and its performance from a global cost-effectiveness perspective) and for procedural reasons, including continuity with past IPCC reports and alignment with decisions under the Paris Agreement Rulebook (Annex II.8). A key constraint in the choice of metric is also that the literature assessed by WGIII predominantly uses GWP100 and often does not provide sufficient detail on emissions and abatement of individual gases to allow translation into different metrics. Presenting such information routinely in mitigation studies would enable the application of more diverse GHG emission metrics in future assessments to evaluate their contribution to different policy objectives.

All metrics have limitations and uncertainties, given that they simplify the complexity of the physical climate system and its response to past and future GHG emissions. No single metric is well-suited to all applications in climate policy. For this reason, the WGIII contribution to AR6 reports emissions and mitigation options for individual gases where possible; CO 2 -equivalent emissions are reported in addition to individual gas emissions where this is judged to be policy-relevant. This approach aims to reduce the ambiguity regarding mitigation potentials for specific gases and actual climate outcomes over time arising from the use of any specific GHG emission metric.

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