The Belo Monte and Altamira hydroelectric dams (Babaquara) as sources of greenhouse gases

Estimating the greenhouse-gas emissions from hydroelectric dams is important as an input to the decision-making process on public investments in the various options for electricity generation and conservation. Brazil's proposed Belo Monte Dam and its upstream counterpart, the Altamira Dam are at the center of controversies regarding how greenhouse-gas emissions from dams should be counted. The Belo Monte Dam by itself would have a small reservoir area (440 km(2)) and large installed capacity (11,181.3 MW), but the Babaquara Dam that would regulate the flow of the Xingu River (thereby increasing power generation at Belo Monte) would flood a vast area (6140 km(2)). The water level in Babaquara would rise and fall by 23 m each year, annually exposing a drawdown area of 3580 km(2) on which soft easily decomposed vegetation would quickly grow. This vegetation would decompose each year at the bottom of the reservoir when the water level rises, producing methane. The methane from drawdown-zone vegetation represents a permanent source of this greenhouse gas, unlike the large peak of emission from decomposition of initial stocks of carbon in the soil and in the leaves and litter of the original forest. The present analysis indicates that the Belo Monte/Babaquara complex would not break even in terms of greenhouse gas emissions until 41 years after the first dam is filled in a calculation with no discounting, and that any annual discount rate above 1.5% results in the complex failing to perform as well as natural gas by the end of the 50-year time horizon used in Brazil's assessments of proposed energy projects.