Sarah F. Harpenslager, Kate Randall, Yizhu Zhu, Michelle C. Jackson, Ian Sanders, Bruno Gallo, Danielle Harris, Hannah Prentice, Yulia V. Bespalaya, Olga V. Aksenova, Alexander Milner, Tom C. Cameron, Boyd A. McKew, Eoin J. O’Gorman, Gabriel Yvon-Durocher, Nikolai Friberg, Kevin Purdy, Guy Woodward, Alex J. Dumbrell, Mark Trimmer
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.