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

• Published in: Nature geoscience Vol. 11; no. 4; pp. 238 - 243
• Main Authors: Feldman, D. R., Collins, W. D., Biraud, S. C., Risser, M. D., Turner, D. D., Gero, P. J., Tadić, J., Helmig, D., Xie, S., Mlawer, E. J., Shippert, T. R, Torn, M. S.
• Format: Journal Article
• Language: English
• Published: London Nature Publishing Group UK 01-04-2018; Nature Publishing Group
• Subjects: 704/106/35/824; 704/106/694; Absorption; Atmospheric methane; Atmospheric water; Atmospheric water vapor; Earth and Environmental Science; Earth Sciences; Earth System Sciences; Energy balance; ENVIRONMENTAL SCIENCES; Evolution; Geochemistry; Geology; Geophysics/Geodesy; Greenhouse effect; Greenhouse gases; Mathematical analysis; Methane; Mixing ratio; Radiative forcing; Radiative transfer; Radiative transfer calculations; Seasonal variation; Sky; Spatial distribution; Surface chemistry; Surface energy; Surface energy balance; Surface properties; Temporal distribution; Trends; Water vapor; Water vapour
• ISSN: 1752-0894; 1752-0908
• DOI: 10.1038/s41561-018-0085-9

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.