Global methane budget and trend, 2010–2017: complementarity of inverse analyses using in situ (GLOBALVIEWplus CH<sub>4</sub> ObsPack) and satellite (GOSAT) observations

Abstract

Abstract. We use satellite (GOSAT) and in situ (GLOBALVIEWplus 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 assess the consistency and complementarity (or redundancy) of the satellite and in situ datasets. 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 mid-latitudes and for relaxing global error correlations between methane emissions and the main methane sink (oxidation by OH radicals). The GOSAT observations achieve 212 independent pieces of information (DOFS) for quantifying mean 2010–2017 anthropogenic emissions on 1009 global model grid elements, and a DOFS of 122 for 2010–2017 emission trends. Adding the in situ data increases the DOFS by about 20–30 %, to 262 and 161 respectively for mean emissions and trends. Our joint inversion finds that oil/gas emissions in the US and Canada are underestimated relative to the values reported by these countries to the United Nations Framework Convention on Climate Change (UNFCCC) and used here as prior estimates, while coal emissions in China are overestimated. Wetland emissions in North America are much lower than in the mean WetCHARTs inventory used as prior estimate. Oil/gas emissions in the US increase over the 2010–2017 period but decrease in Canada and Europe. Our joint GOSAT+in situ inversion yields a global methane emission 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).

Estimating 2010–2015 Anthropogenic and Natural Methane Emissions in Canada using ECCC Surface and GOSAT Satellite Observations

“…These statistics are consistent with previous inversions using GEOS-Chem that showed relatively unbiased conditions against NOAA surface stations globally (Turner et al, 2015;Maasakkers et al, 2019). A high resolution inversion over North America over the same 2010-2015 time-period using the same prior have shown adjustments to US emissions near the Canadian border are relatively minimal (Maasakkers et al, 2020), so we treat US emissions as constant in the inversion. The acceptable reproducibility of background methane at this site allows us to attribute much larger differences observed at other sites, up to a maximum of ~1000 ppb in the summer ( Figure 6), to…”

“…The posterior with GOSAT data does not corroborate this result, the overall emissions trend using GOSAT data is not robust and shows a decreasing trend of ~0.2 Tg a -1 per year. The lack of corroboration of trends between ECCC and GOSAT data may be reflective of the lower overall sensitivity of total column methane to these surface fluxes (Sheng et al, 2017;Lu et al, 2020) or the inability of this inverse system to constrain trends sufficiently. Poulter et al (2017) with ECCC data shows a peak 1-month later in August in most years instead of July, with lower than prior emissions in the spring months before the peak (March to May) and similar emissions to the prior in the autumn months after the peak (September to November).…”

Comment on acp-2020-1195

“…These statistics are consistent with previous inversions using GEOS-Chem that showed relatively unbiased conditions against NOAA surface stations globally (Turner et al, 2015;Maasakkers et al, 2019). A high resolution inversion over North America over the same 2010-2015 time-period using the same prior have shown adjustments to US emissions near the Canadian border are relatively minimal (Maasakkers et al, 2020), so we treat US emissions as constant in the inversion. The acceptable reproducibility of background methane at this site allows us to attribute much larger differences observed at other sites, up to a maximum of ~1000 ppb in the summer (Figure 6), to 2009-2015 at the same site using a reduced major axis (RMA) regression along with the 1:1 line (black).…”

“…We show in Section 3.1 that this configuration of the model reliably reproduces the global growth rate in atmospheric methane with adjustments only needed for 2014 and 2015 primarily due to differences in tropical wetland emissions (Maasakkers et al, 2019). A high resolution inversion over North America over the 2010-2015 time-period using the same prior has shown adjustments to US emissions near the Canadian border are also relatively minimal, (Maasakkers et al, 2020), so we treat US emissions as constant. This gives a well-represented background for methane which is checked using global GOSAT data and in situ data at Canadian background site ESP.…”

“…observational error is ppb where the instrument error is ppb, showing most of the observational error is from the instrument rather than the forward model representation of the total column. Column-averaged methane concentrations are less sensitive to surface emissions resulting in the lower model error (Lu et al, 2020).…”