Large regional shortwave forcing by anthropogenic methane informed by Jovian observations

Creators: Collins, William D.; Feldman, Daniel R.; Kuo, Chaincy; Nguyen, Newton H.

Abstract

Recently, it was recognized that widely used calculations of methane radiative forcing systematically underestimated its global value by 15% by omitting its shortwave effects. We show that shortwave forcing by methane can be accurately calculated despite considerable uncertainty and large gaps in its shortwave spectroscopy. We demonstrate that the forcing is insensitive, even when confronted with much more complete methane absorption spectra extending to violet light wavelengths derived from observations of methane-rich Jovian planets. We undertake the first spatially resolved global calculations of this forcing and find that it is dependent on bright surface features and clouds. Localized annual mean forcing from preindustrial to present-day methane increases approaches +0.25 W/m^2, 10 times the global annualized shortwave forcing and 43% of the total direct CH_4 forcing. Shortwave forcing by anthropogenic methane is sufficiently large and accurate to warrant its inclusion in historical analyses, projections, and mitigation strategies for climate change.

Additional Information

© 2018 The Authors, some rights reserved; exclusive licensee American Association for the Advancement of Science. No claim to original U.S. Government Works. Distributed under a Creative Commons Attribution NonCommercial License 4.0 (CC BY-NC). Submitted 9 January 2018; Accepted 22 August 2018; Published 26 September 2018. We acknowledge M. Alvarado and K. Cady-Pereira of AER (Atmospheric and Environmental Research) for their line list and radiative transfer code support and thank E. Karkoschka and M. Tomasko for providing machine-readable tabulations of the methane optical properties in the Supplementary Materials from their synthesis of optical properties from Jovian planetary atmospheres (9). This research was supported by the Director, Office of Science, Office of Biological and Environmental Research of the U.S. Department of Energy Atmospheric Systems Research Program and used resources of the National Energy Research Scientific Computing Center. This study was also supported by the Office of Science of the U.S. Department of Energy under contract no. DE-AC02-05CH11231 as part of their Earth System Modeling Program. This research and the supporting climate OSSE have also been funded by NASA grants NNX10AK27G, NNX11AE65G, NNH11AQ75I, and NNL16AA60I and by NASA High-End Computing grant SMD-16-7156. Author contributions: W.D.C. conceived the study, the utilization of Jovian measurements, and the supporting numerical experiments. D.R.F. and C.K. conducted the experiments. N.H.N. prepared model data sets describing time-dependent three-dimensional concentrations of methane. All authors reviewed the manuscript. The authors declare that they have no competing interests. Data and materials availability: The data used to generate all the figures, tables, and supplementary figures are freely available from https://github.com/cloudwilliam/Collins_et_al_Sci_Adv_2018 or https://bit.ly/2wQ46kE. The specialized computer code used to conduct the analysis and to produce the figures will be made readily available upon request. All data needed to evaluate the conclusions in the paper are present in the paper and/or the Supplementary Materials. Additional data related to this paper may be requested from the authors.

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