B. B. Cael, Philip Goodwin

Methane (CH$_4$) is a potent greenhouse gas whose contribution to anthropogenic radiative forcing of the climate system is second only to carbon dioxide (CO$_2$). CH$_4$ emission reduction has become central to global climate mitigation policy, resulting most notably in the Global Methane Pledge (GMP), pledging a 30\% reduction of CH$_4$ emissions by 2030. Methane is, however, much shorter-lived in the atmosphere than CO$_2$, so emissions reductions may have different impacts on global warming over time. We quantify the difference over time in global annual mean surface temperature of the GMP versus the equivalent amount of CO$_2$ emission reduction. The avoidance of CH$_4$ emissions in the 2020s due to the GMP initially results in greater relative cooling than the avoidance of the equivalent amount of CO$_2$ emissions over the same period, but less relative cooling after $\sim$2060, when almost all CH$_4$ emitted during the 2020s has been removed from the atmosphere but much of the CO$_2$ emitted during the 2020s remains. However, the GMP placing the world on a lower CH$_4$ emissions trajectory after 2030 results in a persistently and substantially greater reduction to global warming than the equivalent change in the CO$_2$ emissions trajectory, with a maximum difference of 0.22$\pm$0.06$^\circ$C in 2055 and relative cooling for well over a century. This equates to a large difference in avoided climate change damages. While the greatest reduction in warming is obtained by reducing both CH$_4$ and CO$_2$ emissions, our results underscore the striking global societal benefits of sustained reduction in CH$_4$ emissions.