Methane emissions in the United States, Canada, and Mexico: evaluation of national methane emission inventories and 2010–2017 sectoral trends by inverse analysis of in situ (GLOBALVIEWplus CH4 ObsPack) and satellite (GOSAT) atmospheric observations
Authors: Lu Xiao , Jacob Daniel J. , Wang Haolin , Maasakkers Joannes D. , Zhang Yuzhong , Scarpelli Tia R. , Shen Lu , Qu Zhen , Sulprizio Melissa P. , Nesser Hannah , Bloom A. Anthony , Ma Shuang , Worden John R. , Fan Shaojia , Parker Robert J. , Boesch Hartmut , Gautam Ritesh , Gordon Deborah , Moran Michael D. , Reuland Frances , Villasana Claudia A. Octaviano , Andrews Arlyn 
Published: January 2022

Abstract. We quantify methane emissions and their 2010–2017 trends by sector in the
contiguous United States (CONUS), Canada, and Mexico by inverse analysis of in
situ (GLOBALVIEWplus CH4 ObsPack) and satellite (GOSAT) atmospheric
methane observations. The inversion uses as a 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 lognormal error
forms. This yields closed-form statistics of error covariances and information
content on the posterior (optimized) estimates, allows better representation
of the high tail of the emission distribution, and enables construction of a
large ensemble of inverse solutions using different observations and
assumptions. We find that GOSAT and in situ observations are largely
consistent and complementary in the optimization of methane emissions for
North America. Mean 2010–2017 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. These are higher
than the most recent reported national inventories of 26.0 Tg a−1
for the US (EPA), 4.0 Tg a−1 for Canada (ECCC), and
5.0 Tg a−1 for Mexico (INECC). The correction in all three
countries is largely driven by a factor of 2 underestimate in emissions from
the oil sector with major contributions from the south-central US, western
Canada, and southeastern Mexico. 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 offsetting effects of increasing
emissions from the oil and landfill sectors, decreasing emissions from the gas
sector, and flat emissions from the livestock and coal sectors. 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 mean 2010–2017 wetland emissions 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 of +2.6 (+1.7 to
+3.8)% a−1 over 2010–2017
correlated with precipitation.