A case for ensuring reductions in CO₂ emissions are given priority over reductions in CH₄ emissions in the near term

Abstract

For several base scenarios, the impacts, on the global mean temperature, of the time of instigation of reductions in emissions of carbon dioxide (CO₂) and methane (CH₄) were estimated using standard methods. It was confirmed that the necessary bending over of the global temperature curve is crucially dependent on the rapid phaseout of CO₂ emissions. Reductions in CH₄ emissions could be delayed till the period 2050–2080 without appreciably affecting the global temperature in 2100. For pathways consistent with the Paris Agreement, the analysis highlighted the dependence of the abatement costs associated with a temperature overshoot on whether the overshoot is primarily due to a delay in reducing CO₂ emissions or primarily due to a delay in reducing CH₄ emissions. A delay in reducing CO₂ emissions increases both the overshoot and the abatement costs accrued over the century, whereas although it also increases the overshoot, a delay in reducing CH₄ emissions until the period from 2050 to 2080 actually lowers overall abatement costs. To minimize the risk of the enabling resources being inadequate for the essential near-term reductions in CO₂ emissions, a new instrument is needed to ensure that if the rate of reduction of CO₂ emissions is deemed to be insufficient for achieving a 1.5 °C consistent pathway with limited overshoot, near-term reductions in CO₂ emissions are given priority over near-term reductions in CH₄ emissions. The replacement of the default GWP₁₀₀ metric for CH₄ by the time-dependent GTP metric, with the year 2100 as the endpoint, is proposed as the basis of the new instrument.