Yale School of the Environment scientists, working with an international team of researchers, provide the most comprehensive estimate to date of monthly methane emissions from rivers and streams worldwide, an understanding that is key to climate change modeling and mitigation.
Methane levels in the atmosphere are now more than two and a half times their pre-industrial level, accounting for 25% of global warming to date. Understanding the many sources of methane emissions and how they may change in the future is vital to climate modeling and mitigation. Freshwater ecosystems account for about half of global methane emissions in the atmosphere but quantifying the specific amount has been difficult because estimates from rivers and streams vary widely and such estimates were not well documented.
A breakthrough study, co-authored by Yale School of the Environment Professor of Ecosystem Ecology Peter Raymond, research scientist Guiseppe Amatulli, postdoctoral associate Shaoda Liu, and a team of international scientists, provides the most comprehensive estimate to date of monthly methane emissions from rivers and streams worldwide.
It is a big step forward. We now have a much better predictive capacity for methane.”
The research, published in Nature, confirmed that rivers are an important source of methane in the atmosphere, similar to emissions levels from lakes, which have a larger surface area. The study estimates that 27 teragrams of methane is emitted globally from rivers and streams, which is about a quarter of the methane produced by fossil fuels.
“This is the first global estimate that is spatially and temporally resolved, which we needed in order to get a firmer understanding of the global methane budget. It is a big step forward. We now have a much better predictive capacity for methane, which is a really difficult greenhouse gas to model,” Raymond says.
To obtain the estimate, the researchers first created a Global River Methane Database comprised of all empirical observations of methane rivers, 24,000 records of methane concentrations, and more than 8,000 measurements of methane emissions. They then combined those observations with high-resolution hydrological datasets that capture the movement of water in rivers and applied machine learning tools to predict global methane concentrations and emissions and identify the main drivers of river methane concentrations.
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The research will help scientists working on the Global Carbon Project, which estimates global budgets for three main greenhouse gases — methane, carbon dioxide, and nitrous oxide — and investigates the complex system of the carbon cycle, including anthropogenic emissions, redistribution in the atmosphere, ocean and terrestrial biosphere, and natural sinks.
“Methane has been gaining recognition in the global community as an important greenhouse gas and its concentration is increasing very fast. We want to understand the global drivers of methane emissions, natural sources of the emissions, and be able to predict future emissions. That’s a key step in being able to mitigate emissions at a global scale,” says Gerard Rocher-Ros, lead author of the study and a postdoctoral researcher with the Swedish University of Agricultural Sciences and the Centre for Advanced Studies of Blanes..
A difficult element to estimating methane emissions is the amount of gas that can be emitted by ebullition, in which bubbles with high methane concentration are released from sediments, the researchers say. The study accounted for that flux, but the authors suggest that “more detailed and sophisticated models are needed to predict methane emissions and integrate them into comprehensive Earth system models.”
The researchers note that one of their most critical, and surprising, findings was that temperature is not a key driver of methane emissions in rivers and streams, although it is an important factor in lakes and wetlands. River and stream emissions are instead more sensitive to land-water connections and human activity that is impacting river networks. Many direct and indirect human modifications of stream and rivers, such as wastewater treatment plants, ditches, or concrete stormwater canals can lead to high methane concentrations.
Study co-author Emily Stanley, a professor at the University of Wisconsin-Madison’s Center for Limnology, who first began compiling data on rivers and stream emissions in 2015, says slowing the flow of pollutants like fertilizer and human and animal waste and restoring rivers and streams could curb methane emissions that are contributing to global warming.
