A fixed methane filter maximizes freshwater emissions under warming

Approximately half of all methane (CH4) emissions come from freshwaters, where they are regulated by the microbial ‘CH4 filter’ whose efficiency describes the fraction of CH4 produced that is subsequently oxidized back to CO2 (methanotrophy) before emission. How the CH4 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. CH4 production becomes more efficient with warming, linked to increased abundance of methanogens and underpinned by community shifts. In contrast, while CH4 oxidation activity increases, its process-level efficiency does not, and methanotrophs shift towards less efficient taxa. Consequently, the system-level CH4 filter efficiency remains fixed, and CH4 emissions increase. If this fixed CH4 filter efficiency under warming is common to freshwaters worldwide (wetlands, lakes and rivers), then an upward trajectory for CH4 emissions through future climate change appears inevitable.