The main source of the greenhouse gas (ghg), according to a report published by the World Commission on Dams, is organic matter washed into reservoirs from upstream. The decay of forests submerged when the reservoirs fill up creates ‘only a fraction’ of the gas. This means that the emissions don’t disappear when the flooded forest has rotted away, but may continue for the lifetime of the reservoir.

“Greenhouse gases are emitted for decades from all dam reservoirs in the boreal and tropical regions for which measurements have been made. This is in contrast to the widespread assumption that such emissions are zero,” says the Commission. “There is no justification for claiming that hydroelectricity does not contribute significantly to global warming.”

The report, first reported in New Scientist magazine, could cause some waves at the World Bank: the Bank funds the Commission and is also the world’s biggest investor in large dams. What’s more, both dam engineers and environmentalists agree on the report’s conclusions.

The report comes just as engineers are arguing that dams should qualify for support as a ‘clean’ technology under the Kyoto Protocol. The Commission will report its findings later this month at a meeting in Bonn to discuss the Clean Development Mechanism (CDM), which allows industrialised countries to offset their carbon dioxide emissions by investing in energy efficiency projects, renewable energy supplies and tree planting in the developing world.

Hydroelectric reservoirs release carbon dioxide and methane. Stagnant water produces the worst emissions because the decaying vegetation generates methane. This is 20 times as potent a greenhouse gas as CO2, which is produced when there is oxygen in the water. So a reservoir will produce more methane than the river did before the dam was built.

Warnings about the gas emissions from reservoirs surfaced in the mid-1990s. But what appeared at first to be a problem for a handful of reservoirs now looks much more general.

Tropical reservoirs that are shallow and uncleared of biomass before flooding appear to pose the most risk. The Commission names two rainforest reservoirs as major planet-warmers. One is Balbina in Brazil, which is just four metres deep in parts. Its generating capacity is 112MW and it is estimated that it will produce 3 million tonnes of carbon per year over its first 20 years. A coal-fired power station of the same capacity would produce 0.35 million tonnes per year. Petit-Saut in French Guyana, which has a similar capacity and powers the launch site for Europe’s Ariane rocket, will produce 0.9 million tonnes per year in its first 20 years.

The report’s authors have only studied a handful of reservoirs so far, in just four countries, so they believe there may be many more offenders. They warn, however, that emissions from reservoirs seem erratic and unpredictable: one study of nine reservoirs in Brazil found that their emissions per unit of electricity vary by a factor of 500.

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