Abstract
Methane (CH4) 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 cause a radiative forcing (RF) of
climate. This letter presents a comprehensive quantification of
CH4 SW RF using a narrow-band radiative transfer model
to calculate spatially and monthly resolved estimates of the direct
CH4 SW effect and its impact on longwave (LW) RF via
stratospheric temperature change. This significantly extends previous
assessments. Calculations include satellite measurements of
CH4 distribution, spectrally-varying surface albedo and
absorption of solar mid-infrared radiation by methane’s 7.6 µm band.
These factors substantially influence the SW effect; all-sky
CH4 SW tropopause instantaneous RF is significantly
smaller than previous estimates at 0.002 W m-2
(±25%). In total, the CH4 SW effect enhances
CH4 LW-only RF by 7% from 0.574 W m-2
to 0.613 W m-2 for a 750-1800 ppb perturbation.