Hydroelectric dams in tropical forest areas emit carbon dioxide and methane. How these emissions and their impacts should be calculated, and how comparisons should be made with global warming contributions of alternative energy sources such as fossil fuels, can lead to sharp differences in conclusions on the relative advantages of these options. The example of Brazil's Tucuruí Dam is examined to clarify these differences. The present paper extends an earlier analysis to 100 years and explores the differences between these and comparable fossil fuel emissions.

Factors considered here in calculating emissions for Tucuruí Dam include the initial stock and distribution of carbon, decay rates and pathways (leading to carbon dioxide and methane), and losses of power in transmission lines. Factors not considered include forest degradation on islands and reservoir shores, nitrous oxide sources in drawdown zones and transmission lines, additional methane emission pathways for release from standing trees, water passing through the turbines, etc. Construction-phase emissions are also not included; neither are emissions from deforestation by people displaced by and attracted to the project. A complete accounting of the alternative landscape is also lacking. 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 contribution to emissions, the annual balance of emissions in a given year, and emissions over a long time horizon (such as 100 years). The timing of emissions differs between hydroelectric and thermal generation, hydro producing a large pulse of carbon dioxide emissions in the first years after filling the reservoir while thermal produces a constant flux of gases in proportion to the power generated. The impacts of emissions are related to the atmospheric load (stocks) of the gases rather than to the emissions (flows), and therefore last over a long time. According to the calculations in the present paper, the average carbon dioxide molecule in the atmospheric load contributed by Tucuruí was present in the atmosphere 15 years earlier than the average molecule in the comparable load from fossil fuel generation. This means that, considering a 100-year time horizon, a tonne of CO2 emitted by Tucuruí has 15% more global warming impact than a tonne emitted by fossil fuel, assuming no discounting. If discounting is applied, then the relative impact of the hydroelectric option is increased.

Time preference, either by discounting or by an alternative procedure, is a key factor affecting the attractiveness of hydroelectric power. At low annual discount rates (say 1–2%), the attractiveness of Tucuruí, although less than without discounting, is still 3–4 times better than fossil-fuel generation. If the discount rate reaches 15%, the situation is reversed, and fossil-fuel generation becomes more attractive from a global-warming perspective. Tucuruí, 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 for Brazilian Amazonia's 5500 km2 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.