The international and national dam lobbyists have been fast to adapt their discourse to the changing world situation. Given the widespread concern over climate change related to greenhouse gas emissions, dam promoters are now stressing that hydroelectricity is a clean source of energy, thus being the best candidate to substitute fossil fuel-based energy sources. But: is it really clean?
The existing research shows that hydropower is not only socially and environmentally destructive, but that it can also make a significant contribution to global warming, particularly in the tropics.
Through the processes of growth and decay, soils, forests and wetlands continuously consume and emit large amounts of carbon dioxide and methane, the two most important greenhouse gases. When those ecosystems are flooded by the dams' reservoirs, the pattern of fluxes of CO2 and methane with the atmosphere is totally altered. Plants and soils decompose when flooded and will eventually release almost all their stored carbon. Permanently flooding tropical wetlands will tend to increase their methane emissions as well as making them a net source of CO2.
Researcher Philip Fearnside carried out studies in 1995 on two dams in Brazil: Balbina and Tucuruí. He calculated their impact on global warming by assessing the amount of forest they flooded and the rate at which vegetation would decay at different depths of their reservoirs. His findings were that in 1990 (6 years after Tucuruí started to fill and 3 years after the gates were closed at Balbina), the Tucuruí reservoir had emitted 9,450,000 tonnes of CO2 and 90,000 tons of methane, while Balbina had emitted 23,750,000 tonnes of CO2 and 140,000 tons of methane. His conclusion was that Tucuruí had 60 per cent as much impact on global warming as a coal-fired plant generating the same amount of electricity, while Balbina had 26 times more impact on global warming than the emissions from an equivalent coal-fired power station.
The above should suffice to show that hydropower is not clean regarding climate change. But there's even more. A comprehensive accounting of a dam's contribution to global warming should also include the emissions from the fossil fuels used during dam construction, those from the production of the cement, steel and other materials used in the dam, as well as the changes in greenhouse gas fluxes due to the land use and other changes which the dam encourages, such as deforestation, the conversion of floodplain wetlands to intensive agriculture, the adoption of irrigation on once rainfed lands, and the increased use of fossil-fuel-based artificial fertilizers.
In sum, large hydroelectric dams are not only no solution to climate change but, on the contrary, are part of the problem.
Article based on information from: Patrick McCully, "Silenced Rivers. The Ecology and Politics of large Dams", Zed Books 1996