The premise of the report is that gases such as methane and carbon dioxide that are commonly dissolved in river water, get released from the spillways of dams as the water is churned up going through the turbines. In addition, large quantities of green house gases are emitted from the surface of the reservoirs. When the reservoirs are filled, large quantities of vegetation are typically flooded at the bottom. As this material decomposes it releases large amounts of carbon into the water and eventually the atmosphere.
Frequently cited global warming expert, Philip Fearnside estimates that in 1990, hydropower dams in the Amazon caused between 3 and 54 times more global warming than modern natural gas plants generating the same amount of energy. Before we start relying on more hydro-electric power, maybe we should be looking at these claims more closely and looking at other technologies like reforming all the methane coming from our land-fills and livestock farms into hydrogen and sequestering the carbon back into the earth. The full report is available as a pdf for download.
[Source: International Rivers Network via TreeHugger.com]