Hydropower is often seen as a clean, green form of energy, because a dam generates electricity without burning fossil fuels. A new study, though, finds that the man-made reservoirs created by hydro projects are producing more greenhouse gases than anyone realized. As the Washington Post reports, the world’s reservoirs—constructed not just for electricity generation but also to meet irrigation and other needs—collectively contribute about 1.3 percent of the world’s annual greenhouse gas emissions, about the same amount as Canada. And they do so mostly in the form of methane, which has a particularly acute warming effect.
Scientists already knew that man-made reservoirs emitted greenhouse gases. (When microbes break down the organic matter in newly-flooded areas, carbon dioxide and methane emerge as byproducts.) But the authors of the new study found that methane emissions per area of reservoir were about 25 percent higher than previously thought.
That’s potentially worrisome, considering that as of 2015, some 3,700 major power dams were either planned or under construction around the world. The study may strengthen anti-dam campaigns, which often focus on damage to ecosystems caused by flooding. It may also bring more clarity to debates over which energy sources contribute least to global warming.