Reservoirs: Another Troubling Source of Greenhouse Gas Emissions
Water reservoirs are a more significant source of greenhouse gas emissions than previously estimated, but there’s a relatively straightforward solution.
Researchers from Washington State University and the University of Quebec at Montreal conducted a study published in Global Biogeochemical Cycles, which found that greenhouse gas emissions from reservoirs are 29% higher than previously thought. In total, the world’s reservoirs produce about 1.07 gigatons of carbon dioxide equivalent annually, in the form of methane and carbon dioxide.
While not the world’s top polluter, reservoirs produce more emissions than Germany, ranking sixth on the list of highest emitting countries.
The discrepancy in the amount of carbon reservoirs release into the atmosphere stems from the prior lack of accounting for methane degassing, a process in which methane in reservoir waters runs through a dam and then bubbles to the surface downstream. The research led by WSU Vancouver School of the Environment’s John Harrison is reportedly the first to study methane degassing from these artificial bodies of water.
"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," Harrison said. "It gives us the ability to start working toward understanding what we could do about methane emissions from these types of systems.”
Methane, a potent greenhouse gas estimated to have 34 times the warming potential of carbon dioxide over a 100-year timespan, gets produced as a byproduct of plant decomposition at the bottom of the reservoirs. If it stayed at the low depths, it wouldn’t pose a problem; however, once the gas makes its way near the surface, it’s quickly released into the atmosphere, where it wreaks havoc on the Earth’s climate.
While the study found that the amount of carbon dioxide released from reservoirs is in line with previous estimates, the amount of methane accounted for about 40% of total emissions. The researchers also found that the highest emissions levels come from reservoirs in tropical climate zones, places where the bulk of new reservoirs are in the construction pipeline.
"We aren't saying that reservoirs are necessarily bad. Many provide important services like electrical power, flood control, navigation and water," Harrison said. "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.”
There may be a relatively simple way to reduce the impact reservoirs are having on driving climate change. By drawing water from the surface of a reservoir where there’s less methane than at greater depths where the gas is produced, operators can help keep a significant amount of methane restrained in the water. A simulation showed that withdrawing water by as little as three meters closer to the surface results in a 92% reduction in emissions from a Malaysian reservoir.
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