Hydroelectricity was generally considered to be a clean, green technology - but no longer. The World Commission on Dams, a group of scientists, engineers and environmentalists supported by the World Bank, reported that many dams emit more greenhouse gases than large coal-fired power stations.

Dams creating large stagnant lakes are the worst because vegetation washed into them from upstream decays and produces the gas methane, which is twenty times more 'global warming' than carbon dioxide. One example is the Balbina Dam in Brazil, whose shallow warm waters produce ten times the amount of methane and carbon dioxide as a coal-fired power station producing the same amount of electricity (112 megawatts/year).