Global Methane Emission Estimates from a Dual-Isotope Inversion: New Constraints from δD-CH₄

Abstract. Methane (CH₄) is a potent greenhouse gas; however, the causes of its growth since 2006 are a subject of debate. While measurements of CH₄ mole fraction and carbon isotopic composition (δ¹³C-CH₄) have been extensively used to investigate the global CH₄ budget, the hydrogen isotopic composition (δD-CH₄) remains underutilised despite its unique sensitivity to source types and oxidation processes. Here, we assimilate a newly harmonised 35-year dataset of dual isotope measurements from high-latitude monitoring stations in both hemispheres within a two-box Bayesian inversion to quantify global CH₄ sources and sinks. The model integrates prior emissions from five source categories based on global bottom-up inventories. Methane removal processes are represented by sink-specific kinetic isotope effects as tropospheric and stratospheric loss, and soil uptake.

We find that the inclusion of δD-CH₄ improves the model's ability to constrain emission apportionment between biogenic and thermogenic sources, particularly for fossil fuel emissions during the late 1990s and early 2000s, which affects CH₄ lifetime estimate. CH₄ increase post-2006 is driven mainly by rising wetland emissions, while fossil-fuel growth is modest, biomass burning declines, and agriculture and waste make smaller, regionalised contributions. The optimised inversion results favour a strong ¹³C kinetic isotope effect in the total CH₄ removal in the troposphere and a net shortening of NH lifetime of CH₄ by 0.2 years. This study demonstrates the added value of incorporating δD-CH₄ into inverse modelling frameworks and underscores the importance of long-term δD-CH₄ measurements for advancing our understanding of CH₄ biogeochemistry and its role in the global carbon cycle.