Hydroelectric power plants - hailed as providers of clean energy - could be dangerous contributors to climate change. This is the startling claim of scientists who say that the dams the plants use produce large amounts of methane, one of the most potent causes of global warming.
At a meeting of Unesco in Paris this week, scientists from France and Brazil will present evidence to show that organic matter is often trapped in reservoirs when dams are built. This matter then decays with the result that the water emits carbon dioxide and - more importantly - methane, a gas that has 20 times the warming impact of carbon dioxide.
This problem is particularly severe in the tropics, add the scientists, who include Philip Fearnside, of the National Institute for Research in the Amazon, in Manaus. He argues in Nature magazine this week that a typical hydroelectric power station will, during the first 10 years of its life, produce four times the amount of carbon that a comparable fossil-fuel plant would emit.
Other scientists dispute these figures, however. They say that the emission of methane only occurs for the first few years of a dam's operation and that hydroelectric plants settle down to produce relatively modest amounts of methane and carbon dioxide.