Background

To quantify and compare the climate impacts of various emissions, it is necessary to choose a climate parameter by which to measure the effects; that is, radiative forcing, temperature response, and so forth. Thus, various choices are needed for the steps down the cause–effect chain from emissions to climate change and impacts. Each step in the cause effect chain requires a modelling framework. For assessments and evaluation one may—as an alternative to models that explicitly include physical processes resulting in forcing and responses—apply simpler measures or metrics that are based on results from complex models. Metrics are used to quantify the contributions to climate change of emissions of different substances and can thus act as ‘exchange rates’ in multi-component policies or comparisons of emissions from regions/countries or sources/sectors.

Metrics can be given in absolute terms (e.g. K/kg) or in relative terms by normalizing to a reference gas — usually CO₂. To transform the effects of different emissions to a common scale — often called ‘CO₂ equivalent emissions’ (CO₂ eq) —the emission (E_i) of component _i can be multiplied with the adopted normalized metric (M_i):

M_i× E_i= CO₂ eq_i

Metrics do not define goals and policy—they are tools that enable evaluation and implementation of multi-component policies (i.e. which emissions to abate). The most appropriate metric will depend on which aspects of climate change are most important to a particular application, and different climate policy goals may lead to different conclusions about what is the most suitable metric with which to implement that policy. One of the more well-known emission metric is the Global Warming Potential (GWP), which has become the default metric for transferring emissions of different gases to a common scale. The GWP for a time horizon of 100 years from the IPCC was adopted as a metric to implement the multi-gas approach embedded in the UNFCCC and made operational in the Kyoto Protocol.

IPCC Fifth Assessment Report – Climate Change 2013: The Physical Science Basis Chapter 8 - Anthropogenic and Natural Radiative Forcing

The Kyoto Protocol in its Article 5, paragraph 3 defined that the GWPs used to calculate the CO₂ equivalence of anthropogenic emissions and removals by sinks of GHGs listed in Annex A to the Protocol shall be those accepted by the IPCC and agreed upon by the COP. Also it defined that based on the work of the IPCC and advice provided by the SBSTA, the CMP shall regularly review and revise the GWP of each such GHG, taking fully into account any relevant decisions by the COP. Decision 2/CP.3 on methodological issues related to the Kyoto Protocol, reaffirmed that GWPs used by Parties should be those provided by the IPCC in its Second Assessment Report (“1995 IPCC GWP values”) based on the effects of the GHGs over a 100-year time horizon.

In 2002 the COP by its decision 17/CP.8 decided that non-Annex I Parties wishing to report on aggregated GHG emissions and removals expressed in CO₂ eq should use the “1995 IPCC GWP Values” based on the effects of GHGs over a 100-year time horizon. At the same time, by its decision 18/CP.8, it decided that Annex I Parties should report aggregate emissions and removals of GHGs, expressed in CO₂ eq using the same “1995 IPCC GWP Values” based on the effects of GHGs over a 100-year time horizon.

In 2011, by decision 4/CMP.7, Parties agreed that for the second commitment period of the Kyoto Protocol, the GWPs used to calculate the CO₂ equivalence of emissions and removals of the GHGs listed in the Annex A to the Kyoto Protocol shall be those listed in the column entitled “Global Warming Potential for Given Time Horizon” in table 2.14 of the errata to the contribution of Working Group I to the Fourth Assessment Report of the IPCC, based on the effects of GHGs over a 100-year time horizon. Also, Parties requested the SBSTA to conduct an assessment, based on the work of the IPCC, of the implications of the choice of metrics used to calculate the CO₂ equivalence of the GHGs listed in Annex A for the third or subsequent commitment periods and to initiate such an assessment no later than 2015 and to present to the CMP its recommendations on the most appropriate metric and related values to be used by Parties.

In 2013, under the framework of the UNFCCC reporting guidelines on annual inventories for Annex I Parties, the COP, by decision 24/CP.19, decided that, from 2015, the GWPs used to calculate the CO₂ equivalence of emissions and removals of GHGs shall be, also those listed in the column entitled “Global warming potential for given time horizon” in table 2.14 of the errata to the contribution of Working Group I to the Fourth Assessment Report of the IPCC, based on the effects of GHGs over a 100-year time horizon.

Developments under the SBSTA

At SBSTA 30 (2009), Parties initiated the consideration of common metrics to calculate the CO₂ equivalence of the GHGs emissions by sources and removals by sinks under the methodological issues under the Kyoto Protocol.

SBSTA 34 (2011) expressed its appreciation to the IPCC for holding an expert meeting, held in Oslo, Norway, from 18 to 20 March 2009, on the science of alternative metrics, following an invitation to the IPCC by the Ad Hoc Working Group on Further Commitments for Annex I Parties under the Kyoto
Protocol (FCCC/KP/AWG/2008/5, paragraph 45) . At that session, the SBSTA welcomed the report on that meeting and its conclusions and recommendations relating to the Convention and to the scientific community regarding research needs and the scoping phase of the Fifth Assessment Report (AR5) of the IPCC. At the same session, the SBSTA requested the secretariat to organize a workshop on common metrics in 2012, with a focus on uncertainties, new and refined areas or metrics, policy goals and the relationship between policy frameworks and metrics. SBSTA 34 also decided to continue its consideration of common metrics under the Convention at SBSTA 36.

Also, the SBSTA acknowledged:

(a) That alternative common metrics and shortcomings in the use of GWPs are still being assessed by the IPCC in the context of its work on its AR5;

(b) That GWPs are a well-defined metric based on radiative forcing that continues to be useful in a multigas approach. However, GWPs were not designed with a particular policy goal in mind and, depending on the specific policy goals, alternative metrics may be preferable;

(c) The limitations in the use of GWPs based on the 100-year time horizon in evaluating the contribution to climate change of emissions of greenhouse gases with short lifetimes.

The workshop on common metrics mandated by SBSTA 34 was held on 3–4 April 2012 in Bonn, Germany.

SBSTA 36 (2012) welcomed the report on the workshop on common metrics held in April 2012, and acknowledged the relevance of the workshop and its report. The SBSTA expressed its appreciation to the representatives of IPCC working groups I and III for providing information on the status of the work of the IPCC on common metrics in the context of the AR5 and to the members of the scientific community that provided updated and relevant information on the scientific knowledge on common metrics. The SBSTA invited the IPCC to present its findings on common metrics from its AR5 at SBSTA 40 and requested the secretariat to organize a special event for this purpose.

Importantly, the SBSTA acknowledged the relevance of common metrics to climate change policy.

The SBSTA - IPCC Special Event mandated by SBSTA 36 was held on 7 June 2014 in Bonn, Germany.

At SBSTA 42 (2015) Parties considered aspects of common metrics under both the Convention and the Kyoto Protocol, recalling the mandate in decision 4/CMP.7 to conduct an assessment, based on the work of the IPCC, of the implications of the choice of metrics used to calculate the CO₂ equivalence of the GHGs listed in Annex A for the third or subsequent commitment periods of the Kyoto Protocol.

The SBSTA at this occasion welcomed the special event on common metrics held on June 2014, during which the IPCC presented its findings on common metrics from the AR5. The SBSTA expressed its appreciation to the representatives of IPCC working groups I and III for providing information on their work and findings on common metrics in the context of the AR5, which included updated and relevant information on the scientific knowledge of common metrics.

During SBSTA 44 (2016) Parties considered common metrics separately in the framework of the methodological aspects under the Convention and the Kyoto Protocol.

Under the Kyoto Protocol, SBSTA 44 continued its assessment of the implications of the choice of metrics, and acknowledged that this assessment is an important process that is linked to and dependent on consideration by Parties of a third commitment period of the Kyoto Protocol. Therefore, it agreed to defer this assessment and would only return to it if the CMP initiates a consideration of a third commitment period of the Kyoto Protocol.

Under the Convention, SBSTA 44 noted that the COP requested the Ad Hoc Working Group on the Paris Agreement (APA) to elaborate guidance for accounting for Parties’ nationally determined contributions which ensures that accounting is in accordance with common metrics assessed by the IPCC (1/CP.21, paragraph 31). Therefore, the SBSTA agreed to defer the consideration of common metrics to SBSTA 46 (May 2017) so as to be able to take into account deliberations under the APA.

During SBSTA 47 (November 2017) Parties recognized that further consideration of common metrics by the APA was necessary and agreed to resume consideration of common metrics at SBSTA 50 (June 2019), so as to be able to take into account the deliberations under the APA on this matter.

The APA concluded its deliberations at CMA 1 (December 2018), and Parties agreed that Parties account for anthropogenic emissions and removals in accordance with common metrics assessed by the IPCC and in accordance with decision 18/CMA.1 (decision 4/CMA.1, annex II, paragraph 1(a)). Pursuant the modalities, procedures and guidelines (MPGs) for the transparency framework for action and support adopted by decision 18/CMP.1, Parties agreed to use the 100-year time-horizon GWP values from the Fifth Assessment Report of the IPCC (see table 8.A.1), or 100-year time-horizon GWP values from a subsequent IPCC assessment report as agreed upon by the CMA, to report aggregate emissions and removals of GHGs, expressed in CO₂ eq 
( decision 18/CMA.1, annex, paragraph 37).