Global methane emissions derived from two years of TROPOMI observations:
Sensitivity to OH and missing sources
Abstract
The rate of increase in atmospheric methane (CH4) has
accelerated in recent years, reaching 15 ppb/yr in 2020, with causes
that are not well understood. Given methane’s potent global warming
potential (85x that of CO2 on a 20-year timescale), this
indicates a crucial need to better understand its current budget.
Near-global high-precision methane column observations from the TROPOMI
satellite sensor offer a major advance for mapping methane fluxes. Here
we combine two years of TROPOMI data with the GEOS-Chem adjoint model in
a 4D-Var framework to optimize global methane emissions at high spatial
resolution. The inversions converge on distinct sets of solutions
depending on whether methane loss rates are also simultaneously
optimized or not. Findings thus show that even with the dense TROPOMI
coverage, methane budget inferences remain sensitive to the prior
assumptions for OH. The ensemble of solutions adheres to a close linear
relationship between the derived global source and sink terms, with each
distinct result successfully improving the simulation of
globally-available in-situ data. Solutions with methane loss rates
treated as a hard constraint exhibit the best consistency with remote OH
and CO measurements and with the background seasonal cycle in methane.
We further employ multiple inversion formalisms to test the solution
sensitivity to the assumed prior emissions. This presentation will
explore the derived emission adjustments in terms of their implications
for methane flux drivers and potential missing sources.