Methane's Solar Radiative Forcing
Rachael Byrom, Keith P. Shine
Abstract
Abstract Methane (CH 4 ) has significant absorption bands at wavelengths of 1.7, 2.3, 3.3 and 7.6 μm which absorb incoming solar ‘shortwave’ (SW) radiation and contribute to radiative forcing (RF). A comprehensive quantification of CH 4 SW RF is presented using a narrow‐band radiative transfer model to calculate spatially and monthly resolved estimates of CH 4 SW RF and its impact on longwave (LW) stratospheric temperature adjusted RF (SARF). These new calculations include satellite measurements of CH 4 distribution and spectrally varying surface albedo, and include absorption of solar mid‐infrared radiation by methane's 7.6 μm band. These factors substantially influence methane's SW effect. For a 750–1,800 ppb perturbation, the all‐sky top‐of‐atmosphere SW instantaneous RF is 0.082 W m −2 ; at the tropopause it is 0.002 W m −2 , considerably smaller than previous estimates. Including the impact of SW absorption on stratospheric temperature increases tropopause SARF by 0.039 W m −2 (or 7%) compared to the LW‐only SARF.