A new study by the Environmental Defense Fund (EDF) analyzed the climate impacts of 1,500 facilities across the globe that account for about half of hydropower generation worldwide. Many release almost no greenhouse gases, but some can be worse than burning fossil fuels. The team looked at whether the facilities behave as a greenhouse gas sink or as a source by investigating the components that determine emissions such as the reservoir itself, when vegetation and soils are submerged underwater in the dam used to generate power.

Ilissa Ocko, a senior climate scientist at EDF and co-author of the study in Environmental Science & Technology, notes that as the submerged vegetation decays, it releases methane or carbon dioxide. “The larger the surface area of the reservoir, the more greenhouse gases are going to be emitted from that reservoir. Also, the temperature plays a role, as well; how warm the reservoir is will affect how much greenhouse gases are produced and emitted from the reservoir.” Some facilities perform just as well as do wind and solar, but shockingly, more than 100 facilities turned out to be worse for the climate than fossil fuels.