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.
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. It names two rainforest reservoirs as major planet-warmers. One is Balbina in Brazil, which is just four metres deep in parts. Its generating capacity is 112 megawatts and it is estimated that it will produce 3 million tonnes of carbon per year over its first 20 years.