premise: The problem is not so great with hydroelectric schemes in temperate regions. But, before more hydropower schemes are built in tropical zones, the United Nations wants experts to examine the emissions produced by existing schemes and to recommend ways in which they can be made more environmentally friendly. A lot of the tropical hydropower plants were created by flooding forests, and as drowned plants and trees rot millions of tons of greenhouse gases are released. Despite burning no fossil fuels, the greenhouse emissions from these hydropower plants are higher than comparable fossil fuel-burning power plants. The most polluting hydropower installations are those that were created by the flooding of vast areas of carbon-rich land. These are usually shallow reservoirs, and over the first 10 years of their life it is estimated that they generate four times more greenhouse gases than equivalent modern coal-burning power stations to produce the same power output.
hypothesis: The greenhouse gas that is largely responsible for the problem is methane, produced as the lush vegetation flooded by tropical reservoirs rots.