I'd like to return to Fred Pearce’s book When the Rivers Run Dry, reviewed here last month, for another excellent and counterintuitive insight.

Hydroelectric energy is generally thought to be low on greenhouse gas emissions, but Pearce argues that hydro-supporters are missing a critical piece of the puzzle: rotting vegetation. Drifting vegetation gets caught in dam reservoirs and produces massive amounts of methane when it rots. The World Commission on Dams warned:

Green house gases bubble up from every one of the reservoirs in the world where measurements have been made… There is no justification for claiming that hydroelectricity does not contribute significantly to global warming.

How significant? Check this out:

French Guiana has a small population and its industrial emissions are miniscule. But a new dam built in the jungle … produces three times as much greenhouse gas as an equivalent coal-burning power station. As a result, French Guiana’s real per capita emissions of greenhouse gases are three times those of France and even greater than those of the United States.”