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

=== 7.6.2 Applications of Emissions Metrics ===

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One prominent use of emissions metrics is for comparison of efforts measured against climate change goals or targets. One of the most commonly discussed goals is in Article 2 of the Paris Agreement which aims to limit the risks and impacts of climate change by setting temperature goals. In addition, the Paris Agreement has important provisions which relate to how the goals are to be achieved, including making emissions reductions in a manner that does not threaten food production (Article 2), an early emissions peaking target, and the aim to ‘achieve a balance between anthropogenic emissions by sources and removals by sinks of greenhouse gases in the second half of this century’ (Article 4). Article 4 also contains important context regarding international equity, sustainable development, and poverty reduction. Furthermore, the United Nations Framework Convention on Climate Change (UNFCCC) sets out as its ultimate objective, the ‘stabilization of greenhouse gas concentrations in the atmosphere at a level that would prevent dangerous anthropogenic interference with the climate system.’

How the interpretation of the Paris Agreement and the meaning of ‘net zero’ emissions, reflects on the appropriate choice of metric is an active area of research ( [[#Schleussner--2016|Schleussner et al., 2016]] , 2019; [[#Fuglestvedt--2018|Fuglestvedt et al., 2018]] ; [[#Collins--2020|Collins et al., 2020]] ). Several possible scientific interpretations of the Article 2 and 4 goals can be devised, and these, along with emissions metric choice, have implications both for when a balance in GHG emissions, net zero CO <sub>2</sub> emissions or net zero GHG emissions are achieved, and for their meaning in terms of temperature outcome ( [[#Fuglestvedt--2018|Fuglestvedt et al., 2018]] ; [[#Rogelj--2018|Rogelj et al., 2018]] ; [[#Wigley--2018|Wigley, 2018]] ). In AR6 net zero GHG emissions is defined as the condition in which metric-weighted anthropogenic GHG emissions are balanced by metric-weighted anthropogenic GHG removals over a specified period (see Box 1.4 and Appendix VII: Glossary). The quantification of net zero GHG emissions depends on the GHG emissions metric chosen to compare emissions and removals of different gases, as well as the time horizon chosen for that metric. As the choice of emissions metric affects the quantification of net zero GHG emissions, it therefore affects the resulting temperature outcome after net zero emissions are achieved ( [[#Lauder--2013|Lauder et al., 2013]] ; [[#Rogelj--2015|Rogelj et al., 2015]] ; [[#Fuglestvedt--2018|Fuglestvedt et al., 2018]] ; [[#Schleussner--2019|Schleussner et al., 2019]] ). [[#Schleussner--2019|Schleussner et al. (2019)]] note that declining temperatures may be a desirable outcome of net zero. [[#Rogelj--2019|Rogelj and Schleussner (2019)]] also point out that the use of physical metrics raises questions of equity and fairness between developed and developing countries.

Based on SR1.5 ( [[#Allen--2018a|Allen et al., 2018a]] ), there is ''high confidence'' that achieving net zero CO <sub>2</sub> emissions and declining non-CO <sub>2</sub> radiative forcing would halt human-induced warming. Based on ( [[#Bowerman--2013|Bowerman et al., 2013]] ; [[#Pierrehumbert--2014|Pierrehumbert, 2014]] ; [[#Fuglestvedt--2018|Fuglestvedt et al., 2018]] ; [[#Tanaka--2018|Tanaka and O’Neill, 2018]] ; [[#Schleussner--2019|Schleussner et al., 2019]] ) there is also ''high confidence'' that reaching net zero GHG emissions as quantified by GWP-100 typically leads to reductions from peak global surface temperature after net zero GHGs emissions are achieved, depending on the relative sequencing of mitigation of short-lived and long-lived species. If both short- and long-lived species are mitigated together, then temperatures peak and decline. If mitigation of short-lived species occurs much earlier than that of long-lived species, then temperatures stabilize very near peak values, rather than decline. Temperature targets can be met even with positive net GHG emissions based on GWP-100 ( [[#Tanaka--2018|Tanaka and O’Neill, 2018]] ). As demonstrated by [[#Allen--2018b|Allen et al. (2018b)]] , [[#Cain--2019|Cain et al. (2019)]] , [[#Schleussner--2019|Schleussner et al. (2019)]] and [[#Collins--2020|Collins et al. (2020)]] reaching net zero GHG emissions when quantified using the new emissions metric approaches such as CGTP or GWP* would lead to an approximately similar temperature evolution as achieving net zero CO <sub>2</sub> . Hence, net zero CO <sub>2</sub> and net zero GHG, quantified using these new approaches, would both lead to approximately stable contributions to temperature change after net zero emissions are achieved ( ''high confidence'' ).

Comparisons with emissions or global surface temperature stabilization goals are not the only role for emissions metrics. Other important roles include those in pricing approaches where policymakers choose to compare short-lived and long-lived climate forcers (e.g., [[#Manne--2001|Manne and Richels, 2001]] ), and in life cycle analyses (e.g., [[#Hellweg--2014|Hellweg and Milà i Canals, 2014]] ). Several papers have reviewed the issue of metric choice for life cycle analyses, noting that analysts should be aware of the challenges and value judgements inherent in attempting to aggregate the effects of forcing agents with different time scales onto a common scale (e.g., [[#Mallapragada--2017|Mallapragada and Mignone, 2017]] ) and recommend aligning metric choice with policy goals as well as testing sensitivities of results to metric choice ( [[#Cherubini--2016|Cherubini et al., 2016]] ). Furthermore, life cycle analyses approaches which are sensitive to choice of emissions metric benefit from careful communication of the reasons for the sensitivity ( [[#Levasseur--2016|Levasseur et al., 2016]] ).

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