What do we learn about regional and global methane budgets using stable methane isotope measurements in a global inverse model?

Observations indicate accelerating growth of atmospheric CH₄, creating a challenge for meeting the Global Methane Pledge that aims to achieve 30% cuts in global emissions by 2030. A recent UNEP report proposes that feasible CH₄ emission cuts could result in a 45% reduction in anthropogenic emissions, avoiding 0.3 ºC of warming by mid-century while having a positive impact on human health through air quality improvements. However, given that the most feasible methane emissions reductions are in the oil and gas sector, it will be difficult to achieve the goals of the Global Methane Pledge with current signatories without also considering emissions from agriculture and waste. It is therefore important to be able to quantify and monitor anthropogenic and natural microbial emissions.

Measurements of the ¹³C stable isotope of CH₄ could be useful for partitioning emissions between fossil fuel and microbial sources, and global analyses imply that recent increases in atmospheric growth are dominated by microbial sources. Atmospheric observations of methane and ¹³CH₄ were used to constrain the NOAA CarbonTracker-CH₄ inversion modeling system. Results show that the largest share of recent growth in CH₄ is due to increasing microbial and fossil fuel emissions in the developing economies of Asia. A smaller contribution to the recent growth in atmospheric CH₄ is also from increasing microbial emissions in tropical South America and Africa, possibly a combination of emissions from natural wetlands and agriculture. At global scale there is little change in fossil fuel emissions, however this result is highly dependent in how stable isotope measurements are used in the inversion. In this presentation we highlight uncertainties associated with using isotope measurements and how they affect our understanding of the atmospheric methane budget.