A Decrease in Atmospheric δ¹³C_CH4 Suggests a Continued Increase in Microbial Emissions in Recent Years.

S. Michel¹, X. Lan^2,3, J.B. Miller³, J. Li^2,3, R. Clark¹, H. Moossen⁴, P. Sperlich⁵, G. Brailsford⁵, H. Schaefer⁵ and S. Morimoto⁶

¹Institute of Arctic and Alpine Research (INSTAAR), University of Colorado, Boulder, CO 80309; 303-492-5495, E-mail: sylvia.michel@colorado.edu
²Cooperative Institute for Research in Environmental Sciences (CIRES), University of Colorado, Boulder, CO 80309
³NOAA Global Monitoring Laboratory (GML), Boulder, CO 80305
⁴Max Planck Institute (MPI) for Biogeochemistry, Jena, Germany
⁵National Institute of Water and Atmospheric Research (NIWA), Wellington, New Zealand
⁶Tohoku University, Sendai, Japan

The atmospheric abundance of methane increased at a record high rate from 2020 to 2022. During this time there was also a sharp decline in δ¹³C _CH4 (a measure of the ¹³C:¹²C ratio relative to a standard). Other independent groups that measure δ ¹³C _{CH4 -}NIWA (National Institute of Water and Atmosphere, Lower Hutt, New Zealand), MPI-BGC (Max Planck Institute for Biogeochemistry, Jena Germany), and TU (Tohoku University, Sendai, Japan) all see this change, despite having independent sampling schemes, analytical systems, and ties to primary reference materials.

This continued pattern of increasing atmospheric CH₄ and decreasing δ ¹³C _CH4 that began in 2007/2008 raises concerns of the possibility of emerging methane-climate feedbacks. δ ¹³C _CH4 is an excellent tracer for methane emissions from different sources; microbial emissions (natural sources such as wetlands and other fresh water systems, as well as anthropogenic sources such as livestock, waste/landfills, and rice paddies) lower atmospheric δ ¹³C _CH4 while fossil and biomass burning emissions increase atmospheric δ ¹³C _CH4. Quantitative analysis suggests that the increase in CH₄ abundance since 2007 equates to an emissions increase of 78 Tg/yr, ~ 14% above previous levels. Using a global box model constrained by global CH₄ and δ ¹³C _CH4, we were able to match the observations only by increasing predominantly microbial sources; of the 78 Tg, 67 Tg/yr were from microbial sources, and only 11 Tg/yr were from fossil sources. To explain the changes in 2020-2022, microbial sources increase by 27 Tg/yr from the previous period, to a total of 648 Tg/yr.  

Figure 1. a) Discrete d¹³C_CH4data from three sites: Alert station in Nunavut, Canada (82.45° N, in red), Zeppelin Station, Svalbard, Norway (78.90° N, green) and South Pole station, Antarctica (89.98° S, blue). Data from INSTAAR are plotted in open circles and lighter shades. Data from Alert Station, measured at the Max Planck Institute, are plotted in dark red solid circles. Data from Arrival Heights, Antarctica, and measured at NIWA, are in blue solid circles. Data from Nye Alesund, Svalbard, and measured by Tohoku University, Japan, are plotted in dark green. Each dataset is fitted with a trend in the same color. Comparison data are corrected for known offsets due to different traceability to primary reference materials as per Umezawa et al. (2018). b) same as (a) but zoomed in to highlight recent years.