Contrast in Slow Climate Response to Methane and Carbon Dioxide Forcings Identified Using the Radiative Kernel Technique

Abstract Methane is the third most significant anthropogenic driver of climate change after carbon dioxide and aerosols, contributing about one-fourth of carbon dioxide’s effective radiative forcing in the industrial era. Given the role of methane in observed and future climate changes, this study compares the slow climate responses to carbon dioxide (CO 2 ) and methane (CH 4 ) radiative forcings by estimating individual climate feedbacks using radiative kernels. We use the National Center for Atmospheric Research (NCAR) Community Atmosphere Model, version 5 (CAM5), in two configurations (prescribed sea surface temperature and slab ocean) to estimate radiative forcing and climate response, respectively. These experiments show that for comparable radiative forcing, methane forcing’s efficacy in the 10xCH 4 experiment (0.90) is smaller than the efficacy of 1.35xCO 2 (unity), consistent with previous studies. This lower efficacy of the slow response for CH 4 owing to its more negative feedback (−1.08 W m −2 K −1 ) compared to CO 2 (−0.97 W m −2 K −1 ) is attributed to differences in lapse rate, water vapor, shortwave cloud, and albedo feedbacks. Despite the broadly similar meridional distributions of radiative forcing, methane’s larger longwave and shortwave forcing in tropical latitudes gives rise to significant differences in lapse rate and cloud feedbacks, resulting in smaller climate sensitivity than CO 2 forcing. CH 4 also has a smaller albedo feedback, owing to its atmospheric absorption of shortwave radiation. Significance Statement In this study, we compare the climate responses to increases in two different greenhouse gases, carbon dioxide (CO 2 ) and methane (CH 4 ), and show how differences in their meridional patterns of radiative forcing affect the magnitude of global-mean surface warming. Using a climate model and the forcing-feedback framework, we show that CH 4 produces smaller global-mean surface warming than CO 2 for similar magnitudes of global-mean radiative forcing, mainly due to larger warming caused by the net effect of temperature, water vapor, cloud, and surface albedo feedbacks in the case of CO 2 forcing compared to the CH 4 case. Consequently, although the climate response from the same magnitude of radiative forcing is similar to within 10%, there are systematic differences in climate feedbacks and warming patterns between forcing due to CH 4 and CO 2 .