Yale Daily News

Like cows and humans, even our rivers can be gassy.

A “Nature” paper co-authored by researchers at the Yale School of the Environment last month quantified the methane contributions of all the world’s rivers and streams. Drawing on more than 20,000 measurements of methane emissions across approximately 6,000 sites, they produced a machine-learning model to visualize how local geography and latitude might influence the methane emissions of rivers and streams.

The results point to a complex group of factors including human activity, climate and groundwater levels that determine the amount of methane released from our rivers.

“One of the bigger takeaways for me is that the methane in an individual place, in a stream, has a lot to do with … landscape-level predictors,” Luke Loken, U.S. Geological Survey hydrologist and coauthor, told the News.

Methane is currently the second-most abundant greenhouse gas in the atmosphere and over 25 times more potent than its carbon-dioxide counterpart. However, methane has a shorter life and degrades more quickly in the atmosphere than carbon dioxide. In water systems, methane can either diffuse into the atmosphere or enter the air through bubbles. 

What most surprised the researchers in the study — beyond the model’s estimated 27.9 million metric tons of methane emitted by rivers and streams globally — was how emissions varied depending on the location.

Emily Stanley ’84, limnology professor at the University of Madison-Wisconsin and coauthor of the study, wrote that temperature is not a “primary determinant” of methane emissions for streams and rivers, unlike lakes and wetlands, where warmer weather usually causes greater methane release. 

The paper’s spatial map of methane emissions across sites instead suggests that the temperature of a site is independent from its methane emissions, as the highest levels of riverine methane emissions came from both tropical and Arctic biomes. Since methane is produced by the breakdown of organic, carbon-rich materials, the outsized biomass presence in northern peatlands and tropical forests would have likely contributed to rivers instead, per the paper. 

However, Loken said that the model had to account for many other variables. The chemical processes of methanogenesis, the synthesis of methane, requires that methane only be produced in anoxic, or oxygen-free sites. The physics of running water also require that methane models for rivers and streams account for more factors than models for other environments, Loken added.  

Anoxic conditions, where methane would be produced within a river or stream, are usually found below the groundwater table. However, in shallow groundwater, for instance, these anoxic conditions — and by extension, methane production — happen closer to rivers or streams and can cause increased emissions. 

Loken added that altitude is another common culprit for varying methane emissions. Cascading waterfalls and turbulent rivers can cause more “gas exchange” between the water and air, in turn releasing more methane from the river. 

In addition to these environmental factors, human activity was the seventh-most important variable in predicting methane emissions in the model from the study. Wastewater-processing centers and farmland have some of the highest levels of human-caused methane emissions.

“Now with this global estimate, we can see what areas in the world have high methane fluxes,” said Swedish researcher Gerard Rocher-Ros, lead author of the study.

According to the paper, however, global warming could alter some of these trends. The study notes that thawing soils in the north could cause high methane concentrations in water streams, thereby increasing the region’s contributions.

While the amount of methane emitted by rivers and streams is much lower than that of wetlands, Stanley added that the findings emphasize the environmental significance of the surrounding land. 

“Keeping soil and manure from running off of fields” and ensuring proper sewage treatment would help prevent excess organic-rich materials from entering the water, she explained.

Stanley wrote that the sensitivity of methane emissions to nearby land means that land management and changing land use are likely to affect emissions more than warming of rivers or streams. 

The project has been the culmination of years of research, according to Stanley. Stanley also added that the project had started during the pandemic in hopes of compiling new methane data released after 2015. Since then, the team scoured the internet and gathered methane emissions data from over 100 articles, bonding over group Zoom calls.

Rocher-Ros said that the current model is not yet “process-based” and cannot make predictions, but, moving forward, the researchers hope that modifications might allow them to use the existing data to project into the future. 

The current model also explains only half the variability, according to Loken. Refining the model to account for areas with insufficient data and forecast future emission levels will be the group’s ongoing focus.

Loken also told the News that he hopes to integrate the research with current studies of standing lakes and reservoirs, creating a “watershed model” that can track the dynamics of carbon cycling. According to Stanley, future projects might also consider the relationship between methane and carbon dioxide in addition to assessing the impact of human development on emissions.

Agriculture is currently the largest contributor to atmospheric methane.