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

  • McNorton J
  • Wilson C
  • Gloor M
  • et al.
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<p><strong>Abstract.</strong> The atmospheric methane (CH<sub>4</sub>) 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 CH<sub>4</sub> more depleted in <sup>13</sup>C, suggesting changes in sources and/or sinks. Using surface observations of both CH<sub>4</sub> and the isotopologue ratio value (δ<sup>13</sup>CH<sub>4</sub>) 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&amp;ndash;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&amp;ndash;0.96, RMSE = 8.3&amp;ndash;16.5<span class="thinspace"></span>ppb), relative to prior fluxes (R = 0.60&amp;ndash;0.92, RMSE = 48.6&amp;ndash;64.6<span class="thinspace"></span>ppb). 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 = 21.4<span class="thinspace"></span>ppb) compared to the prior (R = 0.71, RMSE = 55.3<span class="thinspace"></span>ppb). 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<span class="thinspace"></span>±<span class="thinspace"></span>0.4<span class="thinspace"></span>% decrease in mean OH, a 12.9<span class="thinspace"></span>±<span class="thinspace"></span>2.7<span class="thinspace"></span>% increase in energy sector emissions, mainly from Africa/Middle East and Southern Asia/Oceania, and a 2.6<span class="thinspace"></span>±<span class="thinspace"></span>1.8<span class="thinspace"></span>% 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<span class="thinspace"></span>±<span class="thinspace"></span>2.7<span class="thinspace"></span>%) and wetland (3.6<span class="thinspace"></span>±<span class="thinspace"></span>1.8<span class="thinspace"></span>%) emissions but also introduces an 11.5<span class="thinspace"></span>±<span class="thinspace"></span>3.8<span class="thinspace"></span>% 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 δ<sup>13</sup>CH<sub>4</sub> and CH<sub>4</sub> 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 δ<sup>13</sup>CH<sub>4</sub> observations limit the accuracy of the posterior estimates.</p>




McNorton, J., Wilson, C., Gloor, M., Parker, R., Boesch, H., Feng, W., … Chipperfield, M. (2018). Attribution of recent increases in atmospheric methane through 3-D inverse modelling. Atmospheric Chemistry and Physics Discussions, 1–34.

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