Does shortwave absorption by methane influence its effectiveness?

In this study, using idealized step-forcing simulations, we examine the effective radiative forcing of CH₄ relative to that of CO₂ and compare the effects of CH₄ and CO₂ forcing on the climate system. A tenfold increase in CH₄ concentration in the NCAR CAM5 climate model produces similar long term global mean surface warming (~ 1.7 K) as a one-third increase in CO₂ concentration. However, the radiative forcing estimated for CO₂ using the prescribed-SST method is ~ 81% that of CH₄, indicating that the efficacy of CH₄ forcing is ~ 0.81. This estimate is nearly unchanged when the CO₂ physiological effect is included in our simulations. Further, for the same long-term global mean surface warming, we simulate a smaller precipitation increase in the CH₄ case compared to the CO₂ case. This is because of the fast adjustment processes—precipitation reduction in the CH₄ case is larger than that of the CO₂ case. This is associated with a relatively more stable atmosphere and larger atmospheric radiative forcing in the CH₄ case which occurs because of near-infrared absorption by CH₄ in the upper troposphere and lower stratosphere. Within a month after an increase in CH₄, this shortwave heating results in a temperature increase of ~ 0.8 K in the lower stratosphere and upper troposphere. In contrast, within a month after a CO₂ increase, longwave cooling results in a temperature decrease of ~ 3 K in the stratosphere and a small change in the upper troposphere. These fast adjustments in the lower stratospheric and upper tropospheric temperature, along with the adjustments in clouds in the troposphere, influence the effective radiative forcing and the fast precipitation response. These differences in fast climate adjustments also produce differences in the climate states from which the slow response begins to evolve and hence they are likely associated with differing feedbacks. We also find that the tropics and subtropics are relatively warmer in the CH₄ case for the same global mean surface warming because of a larger longwave clear-sky and shortwave cloud forcing over these regions in the CH₄ case. Further investigation using a multi-model intercomparison framework would permit an assessment of the robustness of our results.