Hydropower: Large Contributor of Methane?

Reservoirs That Store Water and Produce Electricity Might Be Among Some of the World's Largest Contributors of Greenhouse Gas Emissions

Reservoirs and hydropower are often thought of as climate friendly because they don't burn fossil fuels to produce electricity. But what if reservoirs that store water and produce electricity were among some of the world's largest contributors of greenhouse gas emissions?

Scientists are searching for answers to that question, as they study how much methane is emitted into the atmosphere from man-made reservoirs built for hydropower and other purposes. Until recently, it was believed that about 20 percent of allman-made methane emissionscome from the surface of reservoirs.

New research suggests that figure may be much higher than 20 percent, but it's unclear how much higher because too little data is available to estimate. Methane is about35 times as potenta greenhouse gas as carbon dioxide over the span of a century.

Think about man-made lakes in terms of cows passing gas: Harsha Lake, a large reservoir near Cincinnati, Ohio, emitted as much methane in 2012 as roughly 5,800 dairy cows would have emitted over an entire year, University of Cincinnati biogeochemist Amy Townsend-Small told Climate Central.

Methane emissions from livestock are the second-largest source of methane emissions in the U.S., behind crude oil and natural gas,according tothe U.S. Environmental Protection Agency. But the EPA's greenhouse gas emissions estimates do not yet account formethane emissionscoming from man-made reservoirs.

Part of the reason is that, generally, very little is known about reservoirs and their emissions, especially in temperate regions, such as in the U.S., where few studies have been conducted.

In2012 study, researchers in Singapore found that greenhouse gas emissions from hydropower reservoirs globally are likely greater than previously estimated, warning that "rapid hydropower development and increasing carbon emissions from hydroelectric reservoirs to the atmosphere should not be downplayed."

Those researchers suggest all large reservoirs globally could emit up to 104 teragrams of methane annually. By comparison,NASA estimatesthat global methane emissions associated with burning fossil fuels totals between 80 and 120 teragrams annually.

But how much reservoirs contribute to global greenhouse gas emissions is "still a big question mark," because the issue remains relatively unstudied and emission rates are highly uncertain, said John Harrison, an associate professor in the School of the Environment at the Washington State University-Vancouver whose research focuses on how reservoirs can be managed to reduce greenhouse gas emissions.

"So I don't think we really know what the relative greenhouse gas effect of reservoirs is compared to other sources of energy in the U.S.," he said.

Research at Harsha Lake may help scientists better understand how reservoirs contribute to climate change.

In astudy published in August, Townsend-Small and researchers from the EPA found that Harsha Lake emitted more methane into the atmosphere in 2012 than had ever been recorded at any other reservoir in the U.S.

"When you compare the annual scale of the methane emission rate of this reservoir (Harsha Lake) to other studies, it's really much higher than people would predict," EPA research associate and Harsha Lake study lead author Jake Beaulieu told Climate Central.

Source: Scientific American

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