GREENHOUSE GAS emissions from hydropower projects can be up to 38 times greater per unit of electricity produced than from modern coal power stations, according to a report from pressure group International Rivers Network (IRN).
The report, Flooding the land, warming the earth , said huge volumes of methane, a gas closely linked with global warming, are produced as submerged vegetation decomposes when dams fill up.
Based on a study of 30 reservoirs in Canada and Brazil, IRN estimates emissions in temperate regions are roughly half those of a modern gas-fired station.
But in tropical regions emissions from reservoirs were between five and 20 times higher than in temperate regions.
'The worst tropical reservoirs can contribute many times more to global warming than coal plants generating the same amount of power, ' IRN said.
Emissions from the Tucurui reservoir in Brazil were calculated to be up to 38 times greater than those from a coal-fired power station.
Including dams built for irrigation, reservoirs worldwide release about 70Mt of methane and 1bn tonnes of carbon dioxide every year. This accounts for 20% of methane emissions from all human activities, including farming, and 4% of carbon dioxide emission, according to the report.
'Combined, the two gases contribute an estimated 7% of global warming impact of human activities over a 100-year period. The contribution to global warming would be considerably higher if measured over a shorter time span, ' IRN said.