Freshwater ecosystems account for half of global emissions of methane, a potent greenhouse gas that contributes to global warming.
Rivers and streams, especially, are thought to emit a substantial amount of that methane, but the rates and patterns of these emissions at global scales remain largely undocumented.
The new study confirms that rivers and streams do, indeed, produce a lot of methane and play a major role in climate change dynamics. But the study also reveals some surprising results about how – and where – that methane is produced.“We expected to find the highest methane emissions at the tropics, because the biological production of methane is highly sensitive to temperature,” says Emily Stanley, a professor at UW–Madison’s Center for Limnology and co-author of the Nature report.
Rivers and streams in boreal forests and polar regions at high latitudes are often tied to peatlands and wetlands, while the dense forests of the Amazon and Congo river basins also supply the waters running through them with soils rich in organic matter. Both systems produce substantial amounts of methane because they often result in low-oxygen conditions preferred by microbes that produce methane while breaking down all that organic matter.
“Humans are actively modifying river networks worldwide and, in general, these changes seem to favor methane emissions,” says Gerard Rocher, lead author of the report and a postdoctoral researcher with both the Swedish University of Agricultural Sciences and the Blanes Centre of Advanced Studies in Spain.