HYDROELECTRIC DAMS IN THE BRAZILIAN AMAZON AS SOURCES OF GREENHOUSE GASES

Existing hydroelectric dams in Brazilian Amazonia emitted about 0.26 million tons of methane and 38 million tons of carbon dioxide in 1990. The methane emissions represent an essentially permanent addition to gas fluxed from the region, rather than a one-time release. The total area of reservoirs planned in the region is about 20 times the area existing 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 components in the calculation. Methane is also released in significant quantities from open water, macrophyte beds, and above-water decay of forest biomass. Hydroelectric dams release a large pulse of carbon dioxide from above-water decay of trees left standing in the reservoirs, especially during the first decade after closing. This elevates the expected global warming impact of-the dams to levels much higher than would occur by generating the same power from fossil fuels. In 1990, the impoundment behind the Balbina Dam (closed in 1987) had over 20 times more impact on expectable global warming than would generating the same power from fossil fuels, white the Tucurui Dam (closed in 1984) had 0.4 times the impact of fossil fuels. Because of the large area flooded per unit of electricity generated at Balbina, 'greenhouse' gas emissions are expected to exceed avoided fossil-fuel emissions indefinitely.