Global methane budget and trend, 2010-2017: Complementarity of inverse analyses using in situ (globalviewplus ch4 obspack) and satellite (gosat) observations

Abstract

We use satellite (GOSAT) and in situ (GLOB ALVIEWplus CH4 ObsPack) observations of atmospheric methane in a joint global inversion of methane sources, sinks, and trends for the 2010-2017 period. The inversion is done by analytical solution to the Bayesian optimization problem, yielding closed-form estimates of information content to as sess the consistency and complementarity (or redundancy) of the satellite and in situ data sets. We find that GOSAT and in situ observations are to a large extent complementary, with GOSAT providing a stronger overall constraint on the global methane distributions, but in situ observations being more important for northern midlatitudes and for relaxing global error correlations between methane emissions and the main methane sink (oxidation by OH radicals). The in-situ only and the GOSAT-only inversions alone achieve 113 and 212 respective independent pieces of information (DOFS) for quantifying mean 2010-2017 anthropogenic emissions on 1009 global model grid elements, and respective DOFS of 67 and 122 for 2010-2017 emission trends. The joint GOSAT+ in situ inversion achieves DOFS of 262 and 161 for mean emissions and trends, respectively. Thus, the in situ data in crease the global information content from the GOSAT-only inversion by 20 %-30 %. The in-situ-only and GOSAT-only inversions show consistent corrections to regional methane emissions but are less consistent in optimizing the global methane budget. The joint inversion finds that oil and gas emissions in the US and Canada are underestimated relative to the values reported by these countries to the United Na tions Framework Convention on Climate Change (UNFCCC) and used here as prior estimates, whereas coal emissions in China are overestimated. Wetland emissions in North Amer ica are much lower than in the mean WetCHARTs inventory used as a prior estimate. Oil and gas emissions in the US in crease over the 2010-2017 period but decrease in Canada and Europe. The joint inversion yields a global methane emis sion of 551 Tg a-1 averaged over 2010-2017 and a methane lifetime of 11.2 years against oxidation by tropospheric OH (86 % of the methane sink).