Inverse modelling of global CH4 emissions using surface based measurements and GOSAT satellites retrievals.

Atmospheric methane (CH₄) is a significant greenhouse gas with a warming potential 84 times greater than that of CO₂ over a 20-year time horizon. Given its relatively short atmospheric lifetime of approximately 10 years and its high warming potential, reducing anthropogenic methane emissions is crucial for limiting near-term increases in global temperatures. Methane is emitted from both natural and anthropogenic sources and is primarily consumed through reactions with hydroxyl (OH) radicals in the atmosphere. To a lesser extent, it is also removed through soil interactions. The limited understanding of the interplay between sources and sinks leads to an unclear explanation of the interannual variability in atmospheric methane concentrations over the past decades. Moreover, there are growing concerns about the possibility that climate change could amplify natural CH₄ fluxes. Here we present an inverse model-based reanalysis of global CH₄ emissions (2018-2021). To achieve this, we employ the TM5-4DVAR inverse model system, which is driven by ECMWF-ERA5 meteorological data at a resolution of 1° x 1° for both latitude and longitude, and encompasses 137 vertical levels. This four-dimensional inverse system generates monthly global fields of CH₄ fluxes across four source categories: wetlands, rice fields, biomass burning, and anthropogenic activities. The methane fluxes are optimized using high-resolution surface-based measurements from the NOAA Earth System Research Laboratory (ESRL) global cooperative air sampling network, as well as column-averaged dry mixing ratio XCH₄ data from the GOSAT satellite. The primary aim of this work is to identify the major geographical areas and source categories driving the interannual variability and trends of global CH₄ fluxes during the study period. Moreover, the temporal variability of natural methane fluxes is analysed in relation to physical parameters to investigate how natural CH₄ emissions respond to climate factors (e.g. temperature).