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

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.