Greenhouse gas emissions from hydroelectric dams are often portrayed as nonexistent by the hydropower industry, and have been largely ignored in global calculations of emissions from land-use change. Brazil's Tucuruí Dam provides an example with important lessons for policy debates on Amazonian development and on how to assess the global warming impact of different energy options. Tucuruí is better from the point of view of power density, and hence greenhouse gas emissions per unit of electricity, than both the average for existing dams in Amazonia and the planned dams that, if all built, would flood 3% of Brazil's Amazon forest. Tucuruí's emission of greenhouse gases in 1990 is equivalent to 7.0-10.1 × 106 tons of CO2-equivalent carbon, an amount substantially greater than the fossil fuel emission of Brazil's biggest city, São Paulo. Emissions need to be properly weighed in decisions on dam construction. Although many proposed dams in Amazonia are expected to have positive balances as compared to fossil fuels, substantial emissions indicated by the present study reduce the benefits often attributed to the planned dams.
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