Using a global inversion constrained by isotope measurements to track progress of the Global Methane Pledge

Methane is the 2^nd largest contributor to atmospheric radiative forcing after CO₂.  It is present in the atmosphere at much smaller concentrations than CO₂, but it has a global warming potential of 28-36, and it may have contributed 0.25 to 0.80 ^ºC (IPCC) to historical warming. The large range is due to uncertainties in atmospheric chemical oxidation of methane, also a small source of atmospheric CO₂ contributing ~0.5 PgC/yr to the global atmosphere, comparable to regional estimates of natural fluxes from inversions. 

Observations indicate accelerating growth of atmospheric CH₄, creating a challenge for meeting the Global Methane Pledge that aims for 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.

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.