Isotopic constraints in methane inversions reveal larger trends in wetland emissions with improved linkage to terrestrial water storage

Abstract

Abstract Accurately separating the contributions of different sources to recent atmospheric methane (CH 4 ) growth is crucial for better quantifying the present and future responses of CH 4 emissions to changing climate and anthropogenic activity. Here, we run atmospheric inversions to assess global and regional CH 4 emissions from microbial, fossil, and pyrogenic sources from 2000 to 2022, using measurements of atmospheric CH 4 and its stable carbon isotope ratio ( 13 C: 12 C, expressed relative to a standard as δ 13 C-CH 4 ). We confirm that global total CH 4 emissions has increased by 15% from 2000 to 2022, with dominated contribution from microbial emissions. Both microbial and fossil emissions increased during 2007–2013 relative to 2000–2006, with the largest contributions from temperate Asia. From 2014–2017, microbial emissions from tropical regions, particularly South America and Africa, were the dominant driver of the overall increase, while fossil emissions remained stable in Europe and North America. Between 2020 and 2022, microbial emissions surged in the African and Asian tropics, whereas fossil emissions declined across most industrial regions. Notably, inversions constrained only by CH 4 observations did not capture the decline in fossil emissions during 2020–2022, a decline potentially related to the COVID-19 pandemic, policy-driven changes, and decreases in CH 4 emissions intensity, highlighting the critical role of isotopic measurements in independently verifying changes in fossil emissions. Further, our inversion with isotopic constraints estimates a more prominent increase in wetland emissions that is 20–30% more strongly correlated with variations in terrestrial water storage during 2003–2022, demonstrating the importance of climate-driven natural sources in explaining long-term CH 4 growth.