A study published in Nature Geoscience has analyzed the greenhouse gas (GHG) emissions, i.e. carbon dioxide and methane, from 85 hydropower reservoirs located around the world and found that the carbon output was highest in new reservoirs in the tropics.

The study, led by Nathan Barros from the Federal University of Juiz de Fora in Brazil, estimates that 48 megatons of carbon per year are emitted as carbon dioxide from reservoirs, with a further 3 megatons of carbon emitted as methane.

The highest emissions came from the tropics, the region around the equator, and especially the Amazon region, where the bottom waters and sediments tend to be oxygen-free, leading to anaerobic decomposition that contributes significantly to methane production.

Dr David Harries, a professor at the University of Western Australia, said: “The study shows that while greenhouse gas emissions associated with hydropower reservoirs are relatively small, they need to be taken into account when planning new hydro projects, especially schemes in tropical areas or the Amazon.”

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