1124. The Recent Surge in Atmospheric Methane Driven by Increased Emissions from Microbes

The increase in atmospheric methane has accelerated over the past decade, with the highest increase recorded from 2020 to 2022. As recently introduced, a paper published in the journal PNAS showed that inundation in the humid tropics was likely one of the major drivers of the methane surge in 2020-2022.

A new paper, published in PNAS, estimates that the recent surge in atmospheric methane was primarily caused by increased emissions from microbes associated with wetlands, waste, and agriculture.

The authors used measurements and models of the ¹³C:¹²C ratio (expressed as δ¹³C_CH4) of CH₄ from the National Oceanic and Atmospheric Administration's (NOAA) Global Greenhouse Gas Reference Network to analyze potential drivers of rapid CH₄ growth. The carbon isotopic composition of atmospheric CH₄ (δ¹³C_CH4) is a powerful tool for tracking the sources and sinks of atmospheric CH₄.

According to the paper, different CH₄ sources have unique δ¹³C_CH4 values. CH₄ emissions from microbial origin (e.g., wetlands, livestock, landfills) have lower δ¹³C_CH4 values than pyrogenic (burning of biomass and biofuels) and fossil fuels. Various sinks of CH₄ in the atmosphere also have unique isotopic effects. Therefore, by combining the CH₄ molar fraction in the atmosphere with the observation of δ¹³C_CH4, it is possible to capture changes in global CH₄ sources and sinks.

In 2020, 2021, and 2022, the annual global average methane growth rate reached record levels of 15.2 ± 0.45, 17.9 ± 0.45, and 13.1 ± 0.8 ppb, significantly higher than the average annual growth rate of 9.2 ppb from 2014 to 2020 and the average growth rate of 5.3 ppb from 2008 to 2014. Meanwhile, the global annual growth rate of δ¹³C_CH4 from 2020 to 2022 was -0.09±0.01‰, which was a much faster decrease than -0.04±0.02‰ from 2014 to 2020 and -0.03±0.02‰ from 2008 to 2014.

To test the isotope response to the potential drivers of CH₄ increase, the authors used a model to simulate a decrease in OH in the troposphere, an increase in fossil fuel emissions, and an increase in microbial emissions, and showed that only cases of increased microbial emissions showed a decrease in δ¹³C_CH4.

On the other hand, according to the paper, more research was needed to investigate the potential climate feedback hypothesis, as atmospheric δ¹³C_CH4 cannot distinguish between anthropogenic microbial sources (livestock, landfills) and natural sources (wetlands). However, the model used in the paper suggests that microbial emissions played an even more important role in 2020-2022 than in the years following 2008. This is broadly in line with research highlighting the important role of increased wetland-derived emissions in the global CH₄ balance in recent years.

Reference
Michel, Sylvia Englund, 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 (2024). https://www.pnas.org/doi/10.1073/pnas.2411212121

Contributor: IIYAMA Miyuki, Information Program