Comparison of physically- and economically-based CO₂-equivalences for methane

Abstract. There is a controversy on the role methane (and other short-lived species) should play in climate mitigation policies, and there is no consensus on what an optimal methane CO₂-equivalence should be. We revisit this question by discussing some aspects of physically-based (i.e. global- warming potential or GWP and global temperature change potential or GTP) and socio-economically-based climate metrics. To this effect we use a simplified global damage potential (GDP) that was introduced by earlier authors and investigate the uncertainties in the methane CO₂-equivalence that arise from physical and socio-economic factors. The median value of the methane GDP comes out very close to the widely used methane 100-yr GWP because of various compensating effects. However, there is a large spread in possible methane CO₂-equivalences from this metric (1–99% interval: 10.0–42.5; 5–95% interval: 12.5–38.0) that is essentially due to the choice in some socio-economic parameters (i.e. the damage cost function and the discount rate). The main factor differentiating the methane 100-yr GTP from the methane 100-yr GWP and the GDP is the fact that the former metric is an end-point metric, whereas the latter are cumulative metrics. There is some rationale for an increase in the methane CO₂-equivalence in the future as global warming unfolds, as implied by a convex damage function in the case of the GDP metric. We also show that a methane CO₂-equivalence based on a pulse emission is sufficient to inform multi-year climate policies and emissions reductions, as long as there is enough visibility on CO₂ prices and CO₂-equivalences for the stakeholders.