Freshwater ecosystems contribute to half of global emissions of methane, a powerful greenhouse gas significantly contributing to global warming. However, although rivers and streams are known to emit a substantial amount of this gas, the rates and patterns of these emissions at a global level remain largely undocumented.
In a new study published in the journal Nature, an international team of scientists has provided a comprehensive description of the global rates, patterns, and drivers of methane emissions from running waters.
The findings will improve methane estimates and existing climate change models, and potentially lead to land-management changes and restoration opportunities which could reduce the amount of methane leaking into the atmosphere.
The study confirms that rivers and streams produce a massive amount of methane and thus play a leading role in climate change dynamics. The research also reveals surprising results about how and where that methane is actually produced.
“We expected to find the highest methane emissions at the tropics, because the biological production of methane is highly sensitive to temperature,” said study co-author Emily Stanley, an expert in Limnology at the University of Wisconsin-Madison.
To their surprise, Stanley and her colleagues discovered that methane emissions in the tropics were in fact comparable to those emerging from the colder streams and rivers of boreal forests stretching around the Northern Hemisphere, as well as Arctic tundra habitats.
Thus, temperature does not appear to be the main variable driving aquatic methane emissions. Rather, as Stanley explains, “the amount of methane coming out of streams and rivers regardless of their latitude or temperature was primarily controlled by the surrounding habitat connected to them.”
According to the scientists, rivers and streams from high latitude boreal forests and polar regions are frequently linked to peatlands and wetlands, while the dense forests of the Amazon and Congo river basins also supply the waters running through them with soils replete with organic matter.
Since both systems often result in low-oxygen conditions preferred by microbes that produce methane by breaking down such organic matter, substantial amounts of methane emerge from both ecosystems.
Yet, in many parts of the world, freshwater methane emissions are mainly controlled by urban and rural human communities.
“Humans are actively modifying river networks worldwide and, in general, these changes seem to favor methane emissions,” said lead author Gerard Rocher, a postdoctoral researcher in Ecology and Complex Systems at the Swedish University of Agricultural Sciences and the Blanes Center of Advanced Studies in Spain.
Regions that have been substantially modified by humans, including ditched streams draining agricultural fields, rivers located below wastewater treatment plants, or concrete stormwater canals, also frequently result in organic matter-rich, oxygen-poor conditions promoting high methane production.
Yet, according to Rocher, the fact that humans play a significant role in methane emissions could be considered good news. “One implication of this finding is that freshwater conservation and restoration efforts could lead to a reduction in methane emissions,” he explained.
For instance, curbing the flow of pollutants such as fertilizers, human and animal waste, or excessive topsoil into freshwater systems could help control the amount of ingredients leading to high methane production.
“From a climate change perspective, we need to worry more about systems where humans are creating circumstances that produce methane than the natural cycles of methane production,” Stanley said.
The experts hope that their results will lead to a better understanding of the magnitude and spatial patterns of all methane sources around the globe, and an improvement of large-scale models used to comprehend the dynamics of climate change and predict its future.
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