Methane emissions in the United States, Canada, and Mexico: Evaluation of national methane emission inventories and sectoral trends by inverse analysis of in situ (GLOBALVIEWplus CH4 ObsPack) and satellite (GOSAT) atmospheric observations

We quantify methane emissions by sector and their 2010-2017 trends in the contiguous United States (CONUS), Canada, and Mexico by inverse analysis of in situ (GLOBALVIEWplus CH4 ObsPack) and the Greenhouse Gases Observing Satellite (GOSAT) atmospheric methane observations. The inversion uses as prior estimate the national anthropogenic emission inventories for the three countries reported by the US Environmental Protection Agency (EPA), Environment and Climate Change Canada (ECCC), and the Instituto Nacional de Ecología y Cambio Climático (INECC) in Mexico to the United Nations Framework Convention on Climate Change (UNFCCC), and thus serves as an evaluation of these inventories in terms of their magnitudes and trends. Emissions are optimized with a Gaussian mixture model (GMM) at 0.5°×0.625° resolution and for individual years. Optimization is done analytically using log-normal error forms. We find that GOSAT and in situ observations are largely consistent and complementary in the optimization of methane emissions for North America. Optimized anthropogenic emissions from our base GOSAT + in situ inversion, with ranges from the inversion ensemble, are 36.9 (32.5-37.8) Tg a-1 for CONUS, 5.3 (3.6-5.7) Tg a-1 for Canada, and 6.0 (4.7-6.1) Tg a-1 for Mexico for the 2010-2017 mean period. These are higher than the reported national inventories of 25.5 Tg a-1 for the US (EPA), 3.7 Tg a-1 for Canada (ECCC), and 5.0 Tg a-1 for Mexico (INECC). The correction is largely driven in all three countries by a factor of 2 or more underestimate in emissions from oil/gas sectors, including in the south-central US, western Canada, and Mexico Sureste. Total CONUS anthropogenic emissions in our inversion peak in 2014, in contrast to the EPA report of a steady decreasing trend over 2010-2017. This reflects combined effects of increases in emissions from oil and landfill sectors, decrease from gas emissions, and flat livestock and coal emissions. We find decreasing trends in Canadian and Mexican anthropogenic methane emissions over the 2010-2017 period, mainly driven by oil and gas emissions. Our best estimates of wetland emissions for the 2010-2017 mean are 8.4 (6.4-10.6) Tg a-1 for CONUS, 9.9 (7.8-12.0) Tg a-1 for Canada, and 0.6 (0.4-0.6) Tg a-1 for Mexico. Wetland emissions in CONUS show an increasing trend over 2010-2017 correlated with precipitation.