Abstract/Summary

Methane (CH4) is a potent greenhouse gas with a global-mean surface mixing ratio that has more than doubled in the past century. Its ability to alter thermal infrared irradiances has been well studied. CH4 also absorbs incoming shortwave (SW) radiation, predominantly at near-infrared wavelengths (0.7 – 4 µm). However, this effect is not included in many climate models, or in the Intergovernmental Panel on Climate Change 5 th assessment estimate of CH4 radiative forcing (RF). Recent studies indicate that this SW effect enhances total CH4 RF and related emission metrics, but estimates are sensitive to the specification of clouds and spectral overlap with water vapour. These studies did not examine the impact of CH4 SW forcing on the spatial or seasonal variation of CH4 stratospheric temperature-adjusted RF, nor did they quantify in detail the sensitivity of the SW effect to surface albedo or vertical profile specification. This thesis provides the most comprehensive quantification of the CH4 SW effect to date using a narrow-band radiative transfer model. It investigates key sensitivities, including vertical profile representation, spectrally-resolved surface albedo specification and the effect of SW absorption at mid-infrared wavelengths between 5 – 10 µm. Recent satellite measurements are exploited to derive a best estimate of CH4 global-mean SW forcing and its impact on total CH4 forcing from seasonally and spatially-resolved calculations, including the impact of SW absorption on CH4 longwave forcing via stratospheric temperature change. All-sky CH4 SW tropopause instantaneous RF is found to be significantly smaller than previous estimates at 0.002 W m-2 with an uncertainty of ±25%. This is caused by a combination of factors which alter the magnitude of the downward and upward SW tropopause forcing components. In total, the CH4 SW radiative effect is found to enhance the CH4 longwave-only RF by 7% from 0.574 W m-2 to 0.613 W m-2 .