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© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 25 Jun 2018

Research article | 25 Jun 2018

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This discussion paper is a preprint. A revision of the manuscript is under review for the journal Atmospheric Chemistry and Physics (ACP).

Attribution of recent increases in atmospheric methane through 3-D inverse modelling

Joe McNorton1,2, Chris Wilson1,3, Manuel Gloor4, Rob Parker3,5, Hartmut Boesch3,5, Wuhu Feng1,6, Ryan Hossaini7, and Martyn Chipperfield1,3 Joe McNorton et al.
  • 1School of Earth and Environment, University of Leeds, Leeds, UK
  • 2Research Department, European Centre for Medium-Range Weather Forecasts, Reading, UK
  • 3National Centre for Earth Observation, University of Leeds, Leeds, UK
  • 4School of Geography, University of Leeds, Leeds, UK
  • 5Earth Observation Science Group, Department of Physics and Astronomy, University of Leicester, Leicester, UK
  • 6National Centre for Atmospheric Science, University of Leeds, Leeds, UK
  • 7Lancaster Environment Centre, Lancaster University, Lancaster UK

Abstract. The atmospheric methane (CH4) growth rate has varied considerably in recent decades. Unexplained renewed growth after 2006 followed seven years of stagnation and coincided with an isotopic trend toward CH4 more depleted in 13C, suggesting changes in sources and/or sinks. Using surface observations of both CH4 and the isotopologue ratio value (δ13CH4) to constrain a global 3D chemical transport model (CTM), we have performed a synthesis inversion for source and sink attribution. Our method extends on previous studies by providing monthly and regional attribution of emissions from 6 different sectors and changes in atmospheric sinks for the extended 2003–2015 period. Regional evaluation of the model CH4 tracer with independent column observations from the Greenhouse gases Observing SATellite (GOSAT) shows improved performance when using posterior fluxes (R = 0.94–0.96, RMSE = 8.3–16.5ppb), relative to prior fluxes (R = 0.60–0.92, RMSE = 48.6–64.6ppb). Further independent validation with data from the Total Carbon Column Observing Network (TCCON) shows a similar improvement in the posterior fluxes (R = 0.90, RMSE = 21.4ppb) compared to the prior (R = 0.71, RMSE = 55.3ppb). Based on these improved posterior fluxes, the inversion results suggest the most likely cause of the renewed methane growth is a post-2006 1.8±0.4% decrease in mean OH, a 12.9±2.7% increase in energy sector emissions, mainly from Africa/Middle East and Southern Asia/Oceania, and a 2.6±1.8% increase in wetland emissions, mainly from Northern Eurasia. The posterior wetland increases are in general agreement with bottom-up estimates, but the energy sector growth is greater than estimated by bottom-up methods. The model results are consistent across a range of sensitivity analyses performed. When forced to assume a constant (annually repeating) OH distribution, the inversion requires a greater increase in energy sector (13.6±2.7%) and wetland (3.6±1.8%) emissions but also introduces an 11.5±3.8% decrease in biomass burning emissions. Assuming no prior trend in sources and sinks slightly reduces the posterior growth rate in energy sector and wetland emissions and further increases the amplitude of the negative OH trend. We find that possible tropospheric Cl variations do not to influence δ13CH4 and CH4 trends, although we suggest further work on Cl variability is required to fully diagnose this contribution. While the study provides quantitative insight into possible emissions variations which may explain the observed trends, uncertainty in prior source and sink estimates and a paucity of δ13CH4 observations limit the accuracy of the posterior estimates.

Joe McNorton et al.
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Status: final response (author comments only)
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Joe McNorton et al.
Joe McNorton et al.
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Publications Copernicus
Short summary
Since 2007 atmospheric methane (CH4) has been unexpectedly increasing following a 6 year hiatus. We have used an atmospheric model to attribute regional sources and global sinks of CH4 using observations for the 2003–2015 period. Model results show the renewed growth is best explained by decreased atmospheric removal, decreased biomass burning emissions and increased energy sector (mainly from Africa/Middle East and Southen Asia/Oceania) and wetland emissions (mainly from Northern Eurasia).
Since 2007 atmospheric methane (CH4) has been unexpectedly increasing following a 6 year hiatus....