When you think of renewable energy, you probably think of solar and wind. But hydroelectric power, in which electricity is generated by moving water, accounts for about 16 percent of the world's electricity supply, and it's touted as one of the most promising sources of clean energy. In the U.S., hydro is the biggest single source of renewable energy, providing about a quarter of the supply.
But new research suggests that dams and the reservoirs they create, some of which are used to generate hydroelectric power, may be more significant sources of methane, a greenhouse gas that is many times more potent than carbon dioxide, emissions than previously believed.
In an article published in the journal Bioscience, researchers, who analyzed data from 100 other studies published over the last 16 years, concluded that methane emissions from dams and reservoirs were roughly the equivalent of the 100 million tons generated by the world's rice paddies, and that over a 100-year-period, the man-made bodies of water would contribute about 1.3 percent of the total human-generated greenhouse gas emissions.
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The methane from reservoirs comes from microbial decomposition of organic material in the water, which also occurs in natural lakes. But the man-made bodies of water emit more methane than natural ones because the water level changes more often, and the change in pressure during draw-downs of the water stimulates bubbling.
"These emissions appear to be linked to the biological productivity of the system," Bridget Deemer, a doctoral candidate at the Washington State University and the study's lead author, said in an email.
These findings suggest that reservoirs sited in locations downstream of "nutrient" (i.e. nitrogen and phosphorus) inputs will produce more methane than those receiving fewer nutrient inputs. It's also possible that reducing nutrient inputs to existing reservoirs could reduce methane emissions, but this remains to be tested in the field.
The researchers found that reservoirs' methane emissions are about 25 percent higher than previous estimates.
The findings could be significant, since the world increasingly is turning to hydro as a source of energy. A 2015 article in Aquatic Sciences reported that least 3,700 major hydroelectric dams were planned or under construction across the planet.
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LeRoy Coleman, a spokesperson for the Washington, D.C.-based National Hydropower Association, questioned the study's relevance. "Even if hydropower reservoirs - like all freshwater bodies - produce GHG emissions, hydropower generation does not. And the fact that many reservoirs in the U.S. will continue to exist for other significant public purposes and benefits such as municipal water supply and irrigation, the responsible and logical approach would be to capture the energy potential of these reservoirs, and combat climate change, by installing hydropower equipment."
Coleman said that hydroelectric power helps reduce carbon dioxide emissions by about 200 million metric tons each year.
Deemer said she was hopeful that improvements in reservoir design and operation could reduce the amount of methane emitted.
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