Changes in methane source signatures inferred from long-term CH₄ and δ¹³C-CH₄ observations

Methane is the second most important anthropogenic greenhouse gas. Since 2007, atmospheric methane concentrations have resumed growth following a period of relative stabilization, with the growth rate accelerating in recent years. At the same time, a reversal in the isotopic trend of atmospheric methane suggests possible changes in sources driving the observed increase in atmospheric CH₄.

In this study, we use a novel approach based on the Miller-Tans method, which uses short-term devations from a smooth background signal in observed time series of CH₄ mole fraction and isotopic composition to infer δ¹³C-CH₄ source signatures of this short-term component. This allows us to focus on regional scale changes in methane sources .

We use 20 years of long-term atmospheric methane and carbon isotope (δ¹³C-CH₄) observations from a global sampling network to conduct a comprehensive analysis of the spatial distribution, seasonal variability, and long-term trends of δ¹³C-CH₄ source signatures. In particular, we observe a pronounced decrease in δ¹³C-CH₄ source signatures at high northern latitudes after 2007, indicating a relative increase in isotopically depleted, biogenic methane sources in this region. By comparing our observation-based results with inverse modeling studies, we further discuss how this regional shift is reflected at the global scale and explore possible explanations underlying the substantial shifts in the source mix at high northern latitudes.