Freshwater systems account for half of global emissions of methane, a powerful greenhouse gas contributor to global warming. But there were gaps in knowledge about the role of rivers and streams in emissions.
A study by researchers in Sweden and the USA now provides new insights into the global levels, patterns and drivers of methane emissions from waterways.
Peatlands and wetlands are strong sources of methane
The researchers expected to find the highest emissions in the tropics, where the atmosphere is warm, because biological production of methane is highly sensitive to temperature.
Instead, we found that emissions are also high in the tundra and in the region of coniferous forests, despite the lower temperatures. It’s likely because northern waterways are often associated with peatlands and wetlands, which are strong sources of methane, says researcher Gerard Rocher-Ross at the Swedish University of Agricultural Sciences, SLU.
Streams and rivers are very dynamic ecosystems. To be able to predict global methane emissions required researchers to collect extensive data. Before the study, they spent several years compiling observations of methane in rivers and streams. Then it was entered into a public database.
“By applying machine learning tools to these thousands of worldwide observations, we were finally able to model methane emissions from waterways at large spatial scales,” says Gerard Rocher-Ross.
Contributes to greenhouse gases in the atmosphere
The maps showed that the cumulative emissions of methane from Earth’s streams and rivers are in fact significant fluxes into the atmosphere, and on a par with what has been documented for lakes.
The global pattern was that high discharges from streams are often associated with features in landscapes that enhance methane production through increased organic matter inputs and anoxic conditions in sediments, particularly in soils and wetlands in watersheds.
However, it was found that temperature was not a good predictor of the rise in methane emissions from rivers and streams, neither globally nor when compared within individual streams over time.
different from lakes
Thus, while methane emissions from lakes and wetlands are often regulated by temperature, waterways don’t seem to work the same way.
– This means that other processes can influence methane emissions from rivers, for example changes in runoff or in hydrological connections to land and wetlands, says researcher Ryan Spuller at Umeå University and continues:
– This is important when we calculate future methane emissions. While warming may be a major driver for lakes and wetlands, it may be important for rivers to understand how environmental change increases or reduces methane inputs from the surrounding landscape, but this remains uncertain.
Higher emissions in changing environments
Because methane production relies heavily on organic matter and low oxygen levels, the authors also took advantage of the new database to examine how humans might directly influence methane emissions from rivers. They note that environments that have been significantly altered by humans — such as the streams, canals and rivers below sewage treatment plants — also tend to have higher emissions.
Humans are actively modifying water systems around the world, and these changes generally seem to increase methane emissions. The research results show that a side effect of watercourse restoration could be reduced methane emissions from waterways, says Ryan Bravelier.
They can be used in climate models
Because the researchers were able to produce monthly global maps of methane emissions from rivers, the database is also useful for better understanding the global methane cycle, and is currently being incorporated into the latest edition of the global methane budget.
– This is important because a better understanding of all methane sources and their extent and spatial patterns can really help in optimization Earth system models. Gerard Rocher-Ross says the large-scale models used to predict future climate are what we need to prepare society.
Gerard Rocher-Ros, Postdoctoral Fellow in the Department of Forest Environment and Management, Swedish University of Agriculture in Umeå, [email protected]
Ryan Raeler, Associate Professor, Department of Ecology, Ecology and Earth Sciences, Umea University, [email protected]
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