It depends on how it’s produced according to a study soon to be published. Dams are particularly harmful, it seems:
« This is because large amounts of carbon tied up in trees and other plants are released when the reservoir is initially flooded and the plants rot. Then after this first pulse of decay, plant matter settling on the reservoir’s bottom decomposes without oxygen, resulting in a build-up of dissolved methane. This is released into the atmosphere when water passes through the dam’s turbines. »
Large dams contribute up to 4 per cent of global warming by some estimates.
Still, there are alternatives, such as « run of river » which, theoretically at least, results in a smaller ecological footprint.