GREENHOUSE-GAS EMISSIONS FROM AMAZONIAN HYDROELECTRIC RESERVOIRS - THE EXAMPLE OF BRAZIL TUCURUI DAM AS COMPARED TO FOSSIL-FUEL ALTERNATIVES

Hydroelectric dams in tropical forest areas emit carbon dioxide and me thane. How these emissions and their impacts should be calculated, and how comparisons should be made with global warming contributions of a lternative energy sources such as fossil fuels, can lead to sharp diff erences in conclusions on the relative advantages of these options. Th e example of Brazil's Tucurui Dam is examined to clarify these differe nces. The present paper ex-tends an earlier analysis to 100 years and explores the differences between these and comparable fossil fuel emis sions. Factors considered here in calculating emissions for Tucurui Da m include the initial stock and distribution of carbon, decay rates an d pathways (leading to carbon dioxide and methane), and losses of powe r in transmission lines. Factors not considered include forest degrada tion on islands and reservoir shores, nitrous oxide sources in drawdow n zones and transmission lines, additional methane emission pathways f or release from standing trees, water passing through the turbines, et c. Construction-phase emissions are also not included; neither are emi ssions from deforestation by people displaced by and attracted to the project. A complete accounting of the alternative landscape is also la cking. Standardization of the level of reliability of the electricity supply is needed to compare hydroelectric and thermoelectric options. Types of emission calculations commonly used include the ultimate cont ribution to emissions, the annual balance of emissions in a given year , and emissions over a long time horizon (such as 100 years). The timi ng of emissions differs between hydroelectric and thermal generation, hydro producing a large pulse of carbon dioxide emissions in the first Sears after filling the reservoir while thermal produces a constant f lux of gases in proportion to the power generated. The impacts of emis sions are related to the atmospheric load (stocks) of the gases rather than to the emissions (flows), and therefore last over a long time. A ccording to the calculations in the present paper, the average carbon dioxide molecule in the atmospheric load contributed by Tucurui was pr esent in the atmosphere 15 years earlier than the average molecule in the comparable load from fossil fuel generation This means that, consi dering a 100-year time horizon, a tonne of CO2 emitted by Tucurui has 15% more global warming impact than a tonne emitted by fossil fuel, as suming no discounting. If discounting is applied, then the relative im pact of the hydroelectric option is increased. Time preference, either by discounting or by an alternative procedure, is a key factor affect ing the attractiveness of hydroelectric power. At low annual discount rates (say 1-2%), the attractiveness of Tucurui, although less than wi thout discounting, is still 3-4 times better than fossil-fuel generati on. If the discount rate reaches 15%, the situation is reversed, and f ossil-fuel generation becomes more attractive from a global-warming pe rspective. Tucurui, with a power density (installed capacity/reservoir area) of 1.63 W m(-2) is better than both the 0.81 W m(-2) average fo r Brazilian Amazonia's 5500 km(2) of existing reservoirs and the 1 W m (-2) estimated by Brazil's electrical authorities as the mean for all planned hydroelectric development in the region.