Research ArticleCLIMATOLOGY

Cost-effective implementation of the Paris Agreement using flexible greenhouse gas metrics

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Science Advances  28 May 2021:
Vol. 7, no. 22, eabf9020
DOI: 10.1126/sciadv.abf9020
  • Fig. 1 The use of greenhouse gas emission metrics in climate policies and assessments and its relations to modeling approaches.

    The figure shows our interpretation that climate policies and assessments (orange-to-gray area) regard CO2-equivalent emissions as a surrogate indicator of climate impacts, while modeling approaches (blue-to-gray area) look more directly into the temperature change and other physical and social impacts as an indicator of climate impacts. Gray boxes show factors, such as emissions and temperature change, along the cause-effect chain of climate change from left to right (green-to-gray bar), following Fig. 8.27 of the IPCC AR5. Solid arrows represent cause-effect relationships between such factors. The arrow where emission metrics are applied is highlighted in red. If GWP is used, the conversion from non-CO2 emissions to CO2-equivalent emissions implicitly uses radiative forcing calculations using models (dashed arrow). Likewise, if GTP and GCP are used, the CO2-equivalent conversion relies on temperature and mitigation cost calculations using models, respectively (dashed arrows). For the purpose of clarity, only first-order relationships are shown. Temporal and spatial aspects are suppressed in the figure.

  • Fig. 2 Temperature stabilization and overshoot pathways and cost-effective metrics for CH4.

    In (A), dashed green lines show the case in which the 2°C target is achieved without overshoot. Blue and pink lines indicate the cases in which the temperature stabilizes at the 2° and 1.5°C warming levels after overshoot, respectively. Solid and dotted lines correspond to the cases where overshoot is assumed unavoidable before 2100 and 2150, respectively (termed “medium” and “high” overshoot, respectively, in reference to “low” overshoot considered in the IPCC SR15). Gray lines in the background indicate the range of temperature pathways considered in SR15 (Methods). In (B), the CH4 GCPs under the five pathways [line designations as in (A)] are shown. GWPs and GTPs with the time horizons of 20, 50, and 100 years are shown in solid and dashed gray lines, respectively, as a reference for comparison. The metric values indicated in parentheses are taken from Tables 8.A.1 and 8.SM.17 of the IPCC AR5 (i.e., those without inclusion of climate-carbon feedbacks for non-CO2), unless noted otherwise. For GWP100, the panel shows four different values including those from earlier IPCC Assessment Reports (ARs). The GWP100 value in AR5 with inclusion of climate-carbon feedbacks for non-CO2 is indicated by * and taken from Table 8.7 of AR5.

  • Fig. 3 Additional mitigation costs of using GWP and GTP with a range of time horizons under the stabilization and overshoot pathways.

    The additional mitigation costs (in percent) with the use of GWP and GTP relative to the lowest costs without the use of metrics (or equivalently, with the use of GCP) are shown in (A) and (B), respectively. The results with the time horizons between 1 and 150 years are presented. The minimum under each pathway, which is marked by a filled circle, indicates the optimal time horizon and the residual additional mitigation costs. The short horizontal bars vertically aligned with each minimum point indicate the additional mitigation costs of using best available GWPs (from the default set of three metrics GWP100, GWP50, and GWP20) (A) or best available GTPs (from the default set of GTP100, GTP50, and GTP20) (B) under each pathway. All horizontal bars follow the legend in (A).

  • Fig. 4 Choices of representative GWPs and GTPs most proximate to cost-effective metrics under the stabilization and overshoot pathways.

    GWPs and GTPs with three representative time horizons (i.e., 20, 50, and 100 years) from the IPCC AR5 are considered. One of the three GWPs and GTPs [in (A) and (B), respectively], whose value is closest to the corresponding GCP in absolute terms, is shown under each pathway. The color is designated according to the time horizon as indicated in the legend at the bottom of each panel. On the basis of (80), we refer to the IPCC metric values without inclusion of climate-carbon feedbacks for non-CO2 gases (Tables 8.A.1 and 8.SM.17 of the IPCC AR5) while noting that it is unclear whether the COP24 decision [paragraph 37 of the Annex to Decision 18/CMA.1; (20)] refers to metric values with or without inclusion of climate-carbon feedbacks for non-CO2 gases. It should also be noted that the IPCC AR5 does not endorse any metrics assessed.

  • Fig. 5 Additional mitigation costs of shifting from the permanent use of GWP100 to the more flexible use of GWPs.

    This scatterplot shows the additional mitigation costs (in percent) with the permanent use of GWP100 (x axis) versus those with the use of best available GWPs (y axis), both relative to the lowest costs without the use of metrics (or, equivalently, with the use of GCP). The 1:1 line is indicated in black. The outcomes under default assumptions are indicated in large color circles as representative outcomes. The sensitivity ranges are shown in both horizontal and vertical directions and characterized by categories. In the default set of assumptions, the number of available metrics (or time horizons) is three (i.e., GWP100, GWP50, and GWP20), the equilibrium climate sensitivity is 3°C, and the discount rate is 4%. The assumptions in the sensitivity cases are indicated in parentheses in the legend. Note that horizontal error bars indicate their respective sensitivity ranges only for the fixed approach. The same goes for vertical error bars indicating sensitivity ranges only for the flexible approach. By definition, sensitivity ranges, with respect to the number of available metrics, appear only to the vertical direction. Note also that the 2°C stabilization pathway and the 1.5°C medium overshoot pathway are not considered in the case of 4.5°C climate sensitivity because these pathways are infeasible with the high climate sensitivity.

Supplementary Materials

  • Supplementary Materials

    Cost-effective implementation of the Paris Agreement using flexible greenhouse gas metrics

    Katsumasa Tanaka, Olivier Boucher, Philippe Ciais, Daniel J. A. Johansson, Johannes Morfeldt

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