Hydroelectric reservoirs emit about one-sixth of the greenhouse gases previously attributed to them, says an international team of scientists.
They emit 48 million metric tonnes of carbon annually, a downgrade from earlier estimates of 321 million metric tonnes, according to a study of 85 reservoirs published in this week's online version of Nature Geoscience.
"Our analysis indicates that hydroelectric reservoirs are not major contributors to the greenhouse gas problem," Jonathan Cole, a limnologist at New York State's Cary Institute of Ecosystem Studies, said in a release.
"But there are some caveats," he warned. "To date, only 17 per cent of potential hydroelectric reservoir sites have been exploited, and impacts vary based on reservoir age, size, and location."
In particular, emissions are correlated with latitude and the amount of vegetation being flooded.
"Reservoirs in tropical locations, such as the Amazon, emit more methane and carbon throughout their life cycles," said lead author Nathan Barros of the Federal University of Juiz de Fora in Brazil.
Hydro reservoirs are created by damming rivers and flooding large swaths of land so that when water is released it can turn turbines and generate electricity. The water not only displaces wildlife and people, but drowned vegetation and soil also give off the greenhouse gases carbon dioxide and methane.
As reservoirs age, emissions decline, with cold-water systems giving off fewer emissions than warm water ones.
Hydroelectricity supplies an estimated 20 per cent of the world's electricity and accounts for more than 85 per cent of electricity from renewable sources.
Hydro reservoirs produce less CO2 than believed - Technology & Science - CBC News
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