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Ambiguity in the causes for decadal trends in atmospheric methane and hydroxyl

Turner, Alexander J. and Frankenberg, Christian and Wennberg, Paul O. and Jacob, Daniel J. (2017) Ambiguity in the causes for decadal trends in atmospheric methane and hydroxyl. Proceedings of the National Academy of Sciences of the United States of America, 114 (21). pp. 5367-5372. ISSN 0027-8424. PMCID PMC5448216. doi:10.1073/pnas.1616020114.

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Methane is the second strongest anthropogenic greenhouse gas and its atmospheric burden has more than doubled since 1850. Methane concentrations stabilized in the early 2000s and began increasing again in 2007. Neither the stabilization nor the recent growth are well understood, as evidenced by multiple competing hypotheses in recent literature. Here we use a multispecies two-box model inversion to jointly constrain 36 y of methane sources and sinks, using ground-based measurements of methane, methyl chloroform, and the C^(13)/C^(12) ratio in atmospheric methane (δ^(13)CH_4) from 1983 through 2015. We find that the problem, as currently formulated, is underdetermined and solutions obtained in previous work are strongly dependent on prior assumptions. Based on our analysis, the mathematically most likely explanation for the renewed growth in atmospheric methane, counterintuitively, involves a 25-Tg/y decrease in methane emissions from 2003 to 2016 that is offset by a 7% decrease in global mean hydroxyl (OH) concentrations, the primary sink for atmospheric methane, over the same period. However, we are still able to fit the observations if we assume that OH concentrations are time invariant (as much of the previous work has assumed) and we then find solutions that are largely consistent with other proposed hypotheses for the renewed growth of atmospheric methane since 2007. We conclude that the current surface observing system does not allow unambiguous attribution of the decadal trends in methane without robust constraints on OH variability, which currently rely purely on methyl chloroform data and its uncertain emissions estimates.

Item Type:Article
Related URLs:
URLURL TypeDescription Information CentralArticle
Turner, Alexander J.0000-0003-1406-7372
Frankenberg, Christian0000-0002-0546-5857
Wennberg, Paul O.0000-0002-6126-3854
Jacob, Daniel J.0000-0002-6373-3100
Additional Information:© 2017 National Academy of Sciences. Freely available online through the PNAS open access option. Edited by Mark H. Thiemens, University of California, San Diego, La Jolla, CA, and approved December 28, 2016 (received for review September 26, 2016). Published online before print April 17, 2017. We thank E. Dlugokencky for providing methane data; S. Montzka, R. Prinn, S. O’Doherty, and R. Weiss for providing MCF data; and I. Levin, C. Veidt, B. Vaughn, J. White, and S. Englund for providing δ^(13)CH_4 data. This work was supported by a Department of Energy Computational Science Graduate Fellowship (to A.J.T.) and by a NASA Carbon Monitoring System grant (to D.J.J.). Author contributions: A.J.T. and C.F. designed research; A.J.T. and C.F. performed research; A.J.T. contributed new reagents/analytic tools; A.J.T., C.F., P.O.W., and D.J.J. analyzed data; and A.J.T., C.F., P.O.W., and D.J.J. wrote the paper. The authors declare no conflict of interest. This article is a PNAS Direct Submission. Data deposition: The model and data reported in this paper have been deposited in GitHub, This article contains supporting information online at
Funding AgencyGrant Number
Department of Energy (DOE)UNSPECIFIED
Subject Keywords:methane; renewed growth; hydroxyl; oxidative capacity; troposphere
Issue or Number:21
PubMed Central ID:PMC5448216
Record Number:CaltechAUTHORS:20170418-113144693
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Official Citation:Alexander J. Turner, Christian Frankenberg, Paul O. Wennberg, and Daniel J. Jacob Ambiguity in the causes for decadal trends in atmospheric methane and hydroxyl PNAS 2017 114 (21) 5367-5372; published ahead of print April 17, 2017, doi:10.1073/pnas.1616020114
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:76636
Deposited By: Tony Diaz
Deposited On:18 Apr 2017 18:57
Last Modified:28 Mar 2022 22:12

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