The energy sources used to generate grid electricity can be a major source of greenhouse-gas emissions, and thus they affect the environmental impact of electric cars.

The cleaner the electricity used to charge an electric car, the cleaner that car becomes in operation.

And among potential energy sources, hydroelectric power from dams would seem to be a fairly clean option.

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But dams and reservoirs present their own climate issues, according to a recent study cited by Science magazine.

Reservoirs already contributed about 1.3 percent of human-produced greenhouse gases, and their share will likely rise because of an ongoing boom in dam construction, according to the study, which is to be published in the journal BioScience.

The source of these emissions is methane—a greenhouse gas 34 times more powerful than carbon dioxide—that is produced by microbes eating organic matter that builds up in reservoir lakes.

Researchers compiled and analyzed the findings of more than 100 studies of more than 250 reservoirs worldwide, and found methane emissions to be more significant than previously thought.

That's because, until recently, methane emissions were harder to track.

While greenhouse gases such as carbon dioxide and nitrogen oxides dissolve in water and enter the atmosphere in a fairly uniform way, methane surfaces sporadically in bubbles.

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Use of new tools—including sonar to track bubbles—led researchers to conclude that reservoirs were emitting up to 25 percent more methane per square meter than previously thought.

The findings come as nations around the world are working on building an estimated 847 new hydroelectric dams.

It may be possible to minimize the negative impact of dams by taking methane emissions into consideration when choosing locations, though.

Hoover Dam [photo: U.S. Bureau of Reclamation via Flickr]

Hoover Dam [photo: U.S. Bureau of Reclamation via Flickr]

Dams on rivers with fewer of the nutrients that feed algae growth may produce less methane.

Engineers could also avoid locating dams in dry areas, which can absorb methane, but may then begin producing the gas once a reservoir is filled.

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Greenhouse-gas emissions from bodies of water generally aren't included in overall estimates of emissions, but that may soon change.

The Task Force on National Greenhouse Gas Inventories—part of the United Nations' Intergovernmental Panel on Climate Change—is reportedly weighing the question of whether to include reservoirs in a new set of guidelines it plans to issue in 2019.

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