Water reservoirs generate much higher emissions than expected

In a new study from Washington State University, experts report that greenhouse gas emissions from the world’s water reservoirs are around 29 percent higher than previously estimated. 

Much of this increase in emissions is tied to a process where methane passes through a dam and bubbles up downstream, known as methane degassing. 

In collaboration with the University of Quebec at Montreal, the researchers found that water reservoirs are producing the equivalent of 1.07 gigatons of carbon dioxide in greenhouse gases each year. This figure is larger than the annual emissions from the entire country of Germany.

In addition to methane degassing, the researchers considered numerous other variables such as water temperature, water depth, and the amount of sediment entering into thousands of different reservoirs located around the world. 

“While a number of papers have pointed out the importance of aquatic systems as sources of methane to the atmosphere, this is the first paper that I know of to look explicitly at which kinds of reservoirs are big sources and why,” said study lead author Professor John Harrison. 

“It gives us the ability to start working toward understanding what we could do about methane emissions from these types of systems.”

The production of methane is fueled by decomposing plant matter near the bottom of reservoirs. This greenhouse gas is 34 times more potent than carbon dioxide over the course of a century.

Professor Harrison and colleagues determined that methane degassing accounts for roughly 40 percent of overall emissions from water reservoirs. The team’s estimates of carbon dioxide emissions were found to be similar to those in previous reports. 

The study showed that the highest rates of greenhouse gas emissions from reservoirs occur in the tropics and subtropics, which is also where the majority of ongoing and planned new reservoir construction projects are taking place.

According to the experts, it may be possible to reduce methane emissions from reservoirs by selectively withdrawing water from near the surface rather than from greater depths, where methane often accumulates.

In a related study, a simulated decrease in water withdrawal depth by as little as 3 meters yielded a 92 percent reduction in methane degassing emissions from a Malaysian reservoir.

“We aren’t saying that reservoirs are necessarily bad. Many provide important services like electrical power, flood control, navigation and water,” said Professor Harrison. “Rather, we want to bring attention to a source of greenhouse gas emissions that we think can be reduced in the years ahead as we work towards carbon neutral emissions.”

The research has helped lead the Intergovernmental Panel on Climate Change to recognize water reservoirs as an integral part of each country’s overall emissions.

“We’re interested in using this work to improve these models and global estimates,” said Professor Harrison. “One end goal of this work is to improve our ability to estimate the amount of greenhouse gases coming from reservoirs on a per country basis so that countries can address this source and include it in the way that they are managing their greenhouse gas liabilities.”

The study is published in the journal Global Biogeochemical Cycles.

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By Chrissy Sexton, Earth.com Staff Writer