U.S. Research Team Tracks Main Drivers of Global Methane Rise Using Stable Isotope Analysis

A research team from the U.S. National Oceanic and Atmospheric Administration Global Monitoring Laboratory and the University of Colorado has recently traced the sources of the rapid rise in global atmospheric methane since 2007 through high-precision methane concentration measurements and carbon and hydrogen stable isotope analysis. The findings show that over the past two decades, microbial emissions have been the primary factor driving the increase in methane concentrations, while fossil fuel-related methane emissions may have remained relatively stable overall.

A methane molecule consists of one carbon atom and four hydrogen atoms, and methane from different sources differs in its carbon and hydrogen isotope composition. Researchers used this "chemical fingerprint" to distinguish methane sources: fossil fuel methane such as natural gas typically contains a relatively higher proportion of carbon-13, while methane produced by microbial processes in wetlands, landfills, livestock, and agricultural environments has a lower carbon-13 proportion, exhibiting a "lighter" isotopic signature. The proportion of deuterium in hydrogen isotopes also varies with the way methane is formed, providing a further basis for source identification.

In related research, a team led by Sylvia Michel, a scientist at the University of Colorado's Institute of Arctic and Alpine Research, focused on analyzing atmospheric methane changes from 2020 to 2022. The results showed that as methane concentrations rose, the ratio of heavier carbon-13 to lighter carbon-12 declined. The research team concluded that this finding does not support the explanation that "increased fossil fuel emissions caused the rapid methane rise," nor does it support the hypothesis that changes in atmospheric hydroxyl radicals produced the observed results. The researchers stated that after testing multiple hypotheses, an increase in microbial sources was the most reasonable explanation.

Subsequently, a follow-up study led by Ben Riddell-Young, a scientist at the University of Colorado's Cooperative Institute for Research in Environmental Sciences, incorporated hydrogen isotope analysis. The study noted that carbon isotopes alone cannot always accurately pinpoint methane sources, but incorporating hydrogen isotopes enables a more complete two-dimensional fingerprint identification method. The researchers stated that the two estimates were highly consistent, strengthening the reliability of the earlier conclusions. The study also mentioned that despite rising global oil and natural gas production, related emissions have remained relatively stable, possibly due to methane leak control and emission reduction policy implementation.

In the most recently published study, a team led by scientist Youmi Wu employed a three-dimensional atmospheric inversion modeling approach to infer the locations and intensities of surface emission sources and sinks from observed atmospheric gas concentrations. The research team used data from the NOAA Global Greenhouse Gas Reference Network and other global networks to track global methane emission changes from 2000 to 2022.

The analysis showed that between 2007 and 2013, methane emissions increased across multiple source categories, with temperate Asia contributing significantly. After 2014, tropical microbial emissions in South America and Africa became the primary drivers of global methane emission growth. In recent years, tropical microbial emissions have continued to increase, while fossil fuel-related emissions in the United States and the European Union have declined.

The study also showed that between 2000 and 2022, wetlands contributed approximately 40% of the total global methane emission increase, a finding supported by estimates from terrestrial water storage satellite data. The researchers noted that distinguishing agricultural and waste emissions from wetland emissions is important because the former can be controlled through management measures, while wetland emissions are not directly subject to human control. These findings provide new observational evidence for understanding global methane changes and their environmental responses.