Increased importance of methane reduction for a 1.5 degree target

To understand the importance of methane on the levels of carbon emission reductions required to achieve temperature goals, a processed-based approach is necessary rather than reliance on the transient climate response to emissions. We show that plausible levels of methane (CH 4 ) mitigation can make a substantial difference to the feasibility of achieving the Paris climate targets through increasing the allowable carbon emissions. This benefit is enhanced by the indirect effects of CH 4 on ozone (O 3 ). Here the differing effects of CH 4 and CO 2 on land carbon storage, including the effects of surface O 3 , lead to an additional increase in the allowable carbon emissions with CH 4 mitigation. We find a simple robust relationship between the change in the 2100 CH 4 concentration and the extra allowable cumulative carbon emissions between now and 2100 (0.27 ± 0.05 GtC per ppb CH 4 ). This relationship is independent of modelled climate sensitivity and precise temperature target, although later mitigation of CH 4 reduces its value and thus methane reduction effectiveness. Up to 12% of this increase in allowable emissions is due to the effect of surface ozone. We conclude early mitigation of CH 4 emissions would significantly increase the feasibility of stabilising global warming below 1.5 °C, alongside having co-benefits for human and ecosystem health.