Climate feedback on methane from wetlands

Atmospheric methane rose rapidly in the late twentieth century, but it stabilized in the first years of this century as sinks balanced emissions in a convex equilibration curve^1,2,3. In 2007, growth returned, but now with a concave growth curve, sharply accelerating, especially from 2020 (refs. ^1,2). Synchronously, the carbon isotopic content of atmospheric methane expressed as δ¹³C_CH4 now trends negative (becoming richer in ¹²C). This reverses a previous multi-decadal trend^1,3 towards more positive values that are characteristic of methane emissions from fossil fuels such as coal and natural gas, which are typically relatively enriched in ¹³C. Some of methane’s recent negative isotopic trend may be attributed to changes in the main sink, atmospheric hydroxyl OH, but the isotopic trend is likely to be dominantly caused by emissions from increasing biogenic sources and mainly wetland methane feedbacks^3,5. Isotopic data in an atmospheric inversion framework showed that around 85% of the 2007–2016 increase in methane emissions came from wetlands, more than half from the tropics⁵. Wetland and ruminant methane emissions from land rich in C3 plants (dominant in temperate and boreal vegetation) have relatively negative δ¹³C_CH4. By contrast, methane from tropical wetlands rich in C4 plants such as papyrus (Fig. 1) is less negative in δ¹³C_CH4. Process-based modelling⁶ showed microbial emissions are the main driver of methane’s post-2006 growth. The spatial distribution of wetland methane’s δ¹³C_CH4 ratio^6,7 is consistent with the observed latitudinal gradient of atmospheric δ¹³C_CH4, supporting the hypothesis that wetland microbial emissions are an important driver of methane’s recent growth.