Abstract Approximately half of all methane (CH 4 ) emissions come from freshwaters, where they are regulated by the microbial ‘CH 4 filter’ whose efficiency describes the fraction of CH 4 produced that is subsequently oxidized back to CO 2 (methanotrophy) before emission. How the CH 4 filter efficiency responds to natural warming over centuries or millennia remains unknown. Here we address this question using a natural experiment comprising high-latitude, geothermally warmed streams in five regions spanning the Northern Hemisphere. CH 4 production becomes more efficient with warming, linked to increased abundance of methanogens and underpinned by community shifts. In contrast, while CH 4 oxidation activity increases, its process-level efficiency does not, and methanotrophs shift towards less efficient taxa. Consequently, the system-level CH 4 filter efficiency remains fixed, and CH 4 emissions increase. If this fixed CH 4 filter efficiency under warming is common to freshwaters worldwide (wetlands, lakes and rivers), then an upward trajectory for CH 4 emissions through future climate change appears inevitable.
Harpenslager et al. (Fri,) studied this question.