Observationally derived rise in methane surface forcing mediated by water vapour trends

Atmospheric methane (CH 4 ) mixing ratios exhibited a plateau between 1995 and 2006 and have subsequently been increasing. While there are a number of competing explanations for the temporal evolution of this greenhouse gas, these prominent features in the temporal trajectory of atmospheric CH 4 are expected to perturb the surface energy balance through radiative forcing, largely due to the infrared radiative absorption features of CH 4 . However, to date this has been determined strictly through radiative transfer calculations. Here, we present a quantified observation of the time series of clear-sky radiative forcing by CH 4 at the surface from 2002 to 2012 at a single site derived from spectroscopic measurements along with line-by-line calculations using ancillary data. There was no significant trend in CH 4 forcing between 2002 and 2006, but since then, the trend in forcing was 0.026 ± 0.006 (99.7% CI) W m 2 yr −1 . The seasonal-cycle amplitude and secular trends in observed forcing are influenced by a corresponding seasonal cycle and trend in atmospheric CH 4 . However, we find that we must account for the overlapping absorption effects of atmospheric water vapour (H 2 O) and CH 4 to explain the observations fully. Thus, the determination of CH 4 radiative forcing requires accurate observations of both the spatiotemporal distribution of CH 4 and the vertically resolved trends in H 2 O. Observations of the radiative forcing from methane at the Earth’s surface are influenced by absorption effects from water vapour, according to spectroscopic measurements and line-by-line calculations.