So says the World Commission on Dams, a group of scientists, engineers and environmentalists supported by the World Bank, the world's biggest funder of large dams. The commission will report its findings later this month at a meeting in Bonn to discuss the Clean Development Mechanism, a key part of the Protocol aimed at reducing carbon emissions worldwide. One surprise finding is that organic matter washed into a reservoir from upstream generates much of the greenhouse gas. This means that the emissions don't disappear when the flooded forest has rotted away, but may continue for the lifetime of the reservoir.
Hydroelectric reservoirs cover an area of the world the size of France.
They release carbon dioxide and methane. Stagnant water produces the worst emissions because the decaying vegetation generates methane. This is 20 times as potent a greenhouse gas as CO2, which is produced when there is oxygen in the water. So a reservoir will produce more methane than the river did before the dam was built. But what appeared at first to be a problem for a handful of reservoirs now looks much more general.