Global methane emissions from rivers and streams.

Methane (CH) is a potent greenhouse gas and its concentrations have tripled in the atmosphere since the industrial revolution. There is evidence that global warming has increased CH emissions from freshwater ecosystems, providing positive feedback to the global climate. Yet for rivers and streams, the controls and the magnitude of CH emissions remain highly uncertain. Here we report a spatially explicit global estimate of CH emissions from running waters, accounting for 27.9 (16.7-39.7) Tg CH per year and roughly equal in magnitude to those of other freshwater systems. Riverine CH emissions are not strongly temperature dependent, with low average activation energy (E = 0.14 eV) compared with that of lakes and wetlands (E = 0.96 eV). By contrast, global patterns of emissions are characterized by large fluxes in high- and low-latitude settings as well as in human-dominated environments. These patterns are explained by edaphic and climate features that are linked to anoxia in and near fluvial habitats, including a high supply of organic matter and water saturation in hydrologically connected soils. Our results highlight the importance of land-water connections in regulating CH supply to running waters, which is vulnerable not only to direct human modifications but also to several climate change responses on land.