An Estimation of the Efficacy of Methane Radiative Forcing Using Radiative Kernels

Methane (CH₄) and carbon dioxide (CO₂) are two major greenhouse gases with distinct radiative properties and climate responses. Using the National Centre for Atmospheric Research (NCAR) Community Atmosphere Model (CAM5) in two configurations (prescribed sea surface temperature and slab ocean) to estimate radiative forcing and climate response and radiative kernel analyses, we compare their slow feedback mechanisms and implications for climate sensitivity. We perform simulations with a 10X increase in CH₄ and 1.35X CO₂ concentration to simulate global mean warming of about 1.5 K in both cases. We find that CH₄ requires a larger effective radiative forcing, indicating a lower efficacy relative to CO₂.

We attribute CH₄'s lower efficacy to differences in slow feedback processes. CH₄ exhibits more negative lapse rate feedback (difference of -0.10 Wm^-2K^-1) and more positive water vapor feedback (difference of 0.06 Wm^-2K^-1) due to equatorially concentrated radiative forcing and stronger upper-tropospheric warming. Feedback differences also include weaker positive shortwave cloud (difference of -0.05 Wm^-2K^-1) and smaller albedo (difference of -0.04 Wm^-2K^-1) feedback responses for CH₄, resulting in a net feedback difference of -0.12 Wm^-2K^-1. These findings underscore the role of spatial forcing patterns, including CH₄’s near-infrared shortwave absorption bands and low-latitude warming, in shaping feedback processes.

We find that CO₂ forcing results in relatively stronger polar warming, enhancing albedo feedback, and induces larger mid-latitude cloud reductions, amplifying shortwave cloud feedback. Both gases have comparable positive longwave cloud feedback, broadly consistent with fixed anvil temperatures. All individual feedback differences are statistically significant. Our results highlight that distinct feedback responses arise from basic physical mechanisms, such as differing meridional warming patterns and small but distinct relative humidity changes.

Our study advances the understanding of radiative forcing structure and feedbacks in determining greenhouse gas impacts on climate sensitivity. It also highlights the need for multi-model assessments and Earth system modeling to evaluate feedback uncertainties and refine projections of long-term climate responses as relative contributions to radiative forcing evolve.