Attribution of the accelerating increase in atmospheric methane during 2010–2018 by inverse analysis of GOSAT observations
2021
Abstract. We conduct a global inverse analysis of 2010–2018 GOSAT
observations to better understand the factors controlling
atmospheric methane and its accelerating increase over the 2010–2018
period. The inversion optimizes anthropogenic methane emissions and their
2010–2018 trends on a 4 ∘ × 5 ∘
grid, monthly regional wetland emissions, and annual hemispheric
concentrations of tropospheric OH (the main sink of methane). We use an
analytical solution to the Bayesian optimization problem that provides
closed-form estimates of error covariances and information content for the
solution. We verify our inversion results with independent methane
observations from the TCCON and NOAA networks. Our inversion successfully
reproduces the interannual variability of the methane growth rate inferred
from NOAA background sites. We find that prior estimates of fuel-related
emissions reported by individual countries to the United Nations are too
high for China (coal) and Russia (oil and gas) and too low for Venezuela
(oil and gas) and the US (oil and gas). We show large 2010–2018 increases in
anthropogenic methane emissions over South Asia, tropical Africa, and
Brazil, coincident with rapidly growing livestock populations in these
regions. We do not find a significant trend in anthropogenic emissions over
regions with high rates of production or use of fossil methane, including the US,
Russia, and Europe. Our results indicate that the peak methane growth rates
in 2014–2015 are driven by low OH concentrations (2014) and high fire
emissions (2015), while strong emissions from tropical (Amazon and tropical
Africa) and boreal (Eurasia) wetlands combined with increasing anthropogenic
emissions drive high growth rates in 2016–2018. Our best estimate is that
OH did not contribute significantly to the 2010–2018 methane trend other
than the 2014 spike, though error correlation with global anthropogenic
emissions limits confidence in this result.
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