Newly established hydroelectric reservoirs in low latitudes are the largest hydropower contributors to emissions of the greenhouse gases carbon dioxide and methane, reports a paper published online this week in Nature Geoscience. However, overall emissions from hydroelectric reservoirs are lower than previously thought.

From a meta-analysis of studies on 85 globally distributed reservoirs, Fabio Roland and colleagues note that carbon emissions from hydroelectric reservoirs decrease with reservoir age, and with latitude. They find that the highest emission rates occur in the Amazon region and conclude that locations of new reservoirs should be chosen carefully to minimize carbon emissions.

In an accompanying News & Views, Bernhard Wehrli writes: “The global greenhouse gas emission rates derived by [the researchers] […] represent only about 4% of the estimated global carbon emissions from inland waters. […] It would be premature to conclude that greenhouse gas emissions of hydropower operations could be dismissed as insignificant.”