New advances and new questions for atmospheric methane

Scenarios to keep global warming below 2°C include significant decreases in short lived atmospheric methane to allow time for the much longer-lived atmospheric CO₂ to decrease more slowly. A methane decrease during the 2020s decade has been built into SSP scenarios and the need for this is reinforced by recent studies [Reisinger, 2024; Shindell et al., 2024]. In reality, the atmospheric methane burden has been growing very rapidly since 2006.

Atmospheric methane destruction is predominantly through oxidation by hydroxyl (OH). There is now evidence that since 1997, OH has been increasing in the Southern Hemisphere [Morgenstern et al., 2025]. This is based on 30 years of data for cosmic-ray produced ¹⁴C in atmospheric carbon monoxide (CO). Although most atmospheric chemistry models expect an increase in OH, the observed Southern Hemisphere increase of about +5% per decade is significantly greater than expected. Unfortunately, ¹⁴CO data in the Northern Hemisphere are insufficient to compare with models there.

The increase in methane removal rate inferred from the ¹⁴CO data means that methane sources are larger than prior estimates based on an almost-constant removal rate. If so, this new finding reduces a long standing discrepancy between “top-down” estimates of methane emissions from wetlands and consistently larger “bottom-up” estimates [Saunois et al., 2024].

While the increasing availability of satellite data is leading to better determination of methane’s source distribution, it is also necessary to differentiate between fossil fuel and biogenic sources. The positive trend of atmospheric δ¹³C_CH4 for two centuries prior to 2006 reflected methane emissions from fossil fuel sources, but the strongly negative trend in δ¹³C_CH4 since 2006 is primarily driven by biogenic sources such as wetlands and agriculture [Michel et al., 2024]. The magnitude of the source increase, particularly when the OH increase is taken into account, implies strong growth in wetland emissions, especially from northern tropical Africa.

More recent δ¹³C_CH4 data for 2023 have shown flattening of its post-2006 trend at many Northern Hemisphere sites. While something similar was seen in 2012 this apparent shift in methane sources now appears more pronounced.

Given the urgency of reducing atmospheric methane to keep to the 2°C target, the recent changes in δ¹³C_CH4 show atmospheric methane is in a very dynamic period of change. Future changes in the global methane budget may be less predictable than is currently assumed.

 

References:

Michel, S.E., Lan, X., Miller, J., et al, 2024: Rapid shift in methane carbon isotopes suggests microbial emissions drove record high atmospheric methane growth in 2020–2022. Proceedings of the National Academy of Sciences - PNAS, 121(44), e2411212121.

Morgenstern, O., Moss, R., Manning, M., et al, 2025: Radiocarbon monoxide indicates increasing atmospheric oxidizing capacity. Nature Communications, 16, 249.

Reisinger, A., 2024: Why addressing methane emissions is a non-negotiable part of effective climate policy. Frontiers in Science, 2, 5.

Saunois, M., et al., 2024: Global Methane Budget 2000-2020. Earth System Science Data, https://doi.org/10.5194/essd-2024-115 Discussion started: 6 June 2024, 147.

Shindell, D., Sadavarte, P., Aben, I., et al , 2024: The methane imperative. Frontiers in Science, 2, 1349770.