EOMF-03. CarbonTracker-CH4 (v2023): An atmospheric assimilation system for estimating methane emissions consistent with atmospheric measurements of methane and its stable carbon isotopes

Abstract
Atmospheric methane (CH4) set another record for growth in 2021, the second year in a row, according to observations from NOAA’s Global Greenhouse Gas Reference Network. Since the beginning of these systematic measurements, global mean atmospheric CH4 increased and then flattened between 1999 and 2006, followed by a rapid increase reaching 1,896 parts per billion (ppb) in 2021. Current global mean CH4 is more than 160% greater than the pre-industrial level and the continuing CH4 increase is a challenge for reaching the climate mitigation goals of the Paris Agreement and COP26, which requires steep cuts in atmospheric CH4 emissions. However, the causes of this rapid growth are widely debated, with suggested mechanisms including increases in emissions from fossil and microbial sources and changes in atmospheric chemical sinks. The ratio of stable carbon isotopes of CH4 (13C/12C), expressed as delta notation relative to a standard (δ13C-CH4), is a useful tool for disentangling the contribution of the sources and sinks of CH4. Our previous CarbonTracker-CH4 was published in 2014 to estimate emissions of CH4 from 2000 to 2010 using atmospheric measurements of CH4. We improved our inversion system by jointly assimilating measurements of CH4 and δ13C-CH4, optimizing fluxes at a grid scale, incorporating spatially- and temporally- resolved source signature of δ13C-CH4, and extending our flux estimation to 2021. Our inversion shows that the total global CH4 emission increased more than 100 Tg yr-1 since 2000 and reached ~650 Tg yr-1 in 2021. We find that the dual-tracer inversion of CH4 and δ13C-CH4 attributes growth in atmospheric CH4 in 2020-2021 to a large increase in microbial sources and a slight decrease in fossil sources. In contrast, the inversion that uses CH4 data only does not match atmospheric δ13C-CH4 and simulates increase in fossil sources in 2020-2021. We further apply changes in atmospheric concentrations of hydroxyl radical, primary atmospheric oxidants of CH4, and find that [OH] decrease can contribute to atmospheric CH4 increase.