HYDROELECTRIC DAMS IN THE BRAZILIAN AMAZON AS SOURCES OF GREENHOUSE GASES

Existing hydroelectric dams in Brazilian Amazonia emitted about 0.26 m illion tons of methane and 38 million tons of carbon dioxide in 1990. The methane emissions represent an essentially permanent addition to g as fluxed from the region, rather than a one-time release. The total a rea of reservoirs planned in the region is about 20 times the area exi sting in 1990, implying a potential annual methane release of about 5. 2 million tons. About 40% of this estimated release is form underwater decay of forest biomass, which is the most uncertain of the component s in the calculation. Methane is also released in significant quantiti es from open water, macrophyte beds, and above-water decay of forest b iomass. Hydroelectric dams release a large pulse of carbon dioxide fro m above-water decay of trees left standing in the reservoirs, especial ly during the first decade after closing. This elevates the expected g lobal warming impact of-the dams to levels much higher than would occu r by generating the same power from fossil fuels. In 1990, the impound ment behind the Balbina Dam (closed in 1987) had over 20 times more im pact on expectable global warming than would generating the same power from fossil fuels, white the Tucurui Dam (closed in 1984) had 0.4 tim es the impact of fossil fuels. Because of the large area flooded per u nit of electricity generated at Balbina, 'greenhouse' gas emissions ar e expected to exceed avoided fossil-fuel emissions indefinitely.