Surface warming and wetting due to methane’s long-wave radiative effects muted by short-wave absorption 

Prior studies show that methane (CH4) shortwave (SW) absorption can increase the stratospheric adjusted radiative forcing by 20-25%. However, a comprehensive analysis of how CH4 SW absorption impacts the climate system remains to be conducted. In the case of black carbon (BC), a very strong absorber of SW radiation, the rapid adjustments—largely due to changes in clouds and UTLS temperature—act to offset much of the positive instantaneous radiative forcing, dampening the total surface temperature signal. Thus, the climate impacts of methane SW absorption may not be straightforward. This work is to better understand the methane (CH4) shortwave (SW) absorption effect and its impact on the climate system. NCAR CAM6, which includes CH4 SW absorption are used.

We find that methane short-wave absorption counteracts ~30% of the surface warming associated with its long-wave radiative effects. An even larger impact occurs for precipitation as methane short-wave absorption offsets ~60% of the precipitation increase relative to its long-wave radiative effects. The methane short-wave-induced cooling is due largely to cloud rapid adjustments, including increased low-level clouds, which enhance the reflection of incoming short-wave radiation, and decreased high-level clouds, which enhance outgoing long-wave radiation. The cloud responses, in turn, are related to the profile of atmospheric solar heating and corresponding changes in temperature and relative humidity. (Allen, R.J., Zhao, X., Randles, C. A., Kramer, R. J., Samset, B. H., & Smith, C. J., 2023, Nature Geoscience).