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


Atmospheric methane (CH4) 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 CH4 are expected to perturb the surface energy balance through radiative forcing, largely due to the infrared radiative absorption features of CH4. 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 CH4 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 CH4 forcing between 2002 and 2006, but since then, the trend in forcing was 0.026 ± 0.006 (99.7% CI) W m2 yr−1. The seasonal-cycle amplitude and secular trends in observed forcing are influenced by a corresponding seasonal cycle and trend in atmospheric CH4. However, we find that we must account for the overlapping absorption effects of atmospheric water vapour (H2O) and CH4 to explain the observations fully. Thus, the determination of CH4 radiative forcing requires accurate observations of both the spatiotemporal distribution of CH4 and the vertically resolved trends in H2O.

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Fig. 1: Recent evolution of atmospheric methane at the surface study site.
Fig. 2: Factors affecting CH4 long-wave surface instantaneous radiative forcing.
Fig. 3: CH4 long-wave surface radiative forcing time series at SGP.
Fig. 4: Thermodynamic dependence of CH4 surface forcing.
Fig. 5: Forcing time-series decomposition and reconstruction from predictors.


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This material is based on work supported by the Director, Office of Science, Office of Biological and Environmental Research of the US Department of Energy (DOE) under contract no. DE-AC02-05CH11231 as part of their Atmospheric System Research (ASR) Program, the Atmospheric Radiation Measurement (ARM) Program, the Terrestrial Ecosystem Sciences (TES) Programs, and the ARM Aerial Facility (AAF). Resources of the National Energy Research Scientific Computing Center (NERSC) were used under the same contract. Work at LLNL was performed under the auspices of the US DOE by Lawrence Livermore National Laboratory under contract no. DE-AC52-07NA27344. M. Alvarado, K. Cady-Pereira, L. Riihimaki, I. Simpson and P. Novelli also contributed. NOAA GMD provided flask CH4, C2H6 and N2O data.

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D.R.F. led the research, performed all calculations and wrote the manuscript; W.D.C. proposed the study concept, provided research guidance and conceived methods for isolating the CH4 signal; S.C.B. provided CH4 and N2O data and associated support; M.D.R. provided the statistical analysis; D.D.T. provided guidance on AERI instrument performance and research focus; P.J.G. helped interpret AERI data; J.T. analysed thermodynamic contributions to the observed forcing. D.H. provided C2H6 data and associated support; S.X. provided ARMBE data and associated support; E.J.M. and T.R.S. provided clear-sky error analysis; M.S.T. provided research feedback and guidance and served as the principal investigator of the grant supporting this research. All authors discussed the results and commented on the manuscript.

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Correspondence to D. R. Feldman.

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Feldman, D.R., Collins, W.D., Biraud, S.C. et al. Observationally derived rise in methane surface forcing mediated by water vapour trends. Nature Geosci 11, 238–243 (2018). https://doi.org/10.1038/s41561-018-0085-9

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