FAR from being "green", many hydroelectric power schemes release more greenhouse gases into the atmosphere than large coal-fired power stations, because of the rotting vegetation they contain. So says the World Commission on Dams, a group of scientists, engineers and environmentalists supported by the World Bank, the world's biggest funder of large dams.
The report comes just as engineers are arguing that dams should qualify for support as a "clean" technology under the Kyoto Protocol agreed in 1997. The commission will report its findings later this month at a meeting in Bonn to discuss the Clean Development Mechanism, a key part of the Protocol aimed at reducing carbon emissions worldwide.
One surprise finding is that organic matter washed into a reservoir from upstream generates much of the greenhouse gas. 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.
Hydroelectric reservoirs cover an area of the world the size of France. They 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 (New Scientist, 4 May 1996, p 29). 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 most at risk," says the commission. It 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 112 megawatts 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 0á35 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 0á9 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.
"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."
Jamie Skinner, environmental adviser to the commission, which is based in Cape Town, South Africa, says the report is significant because both dam engineers and environmentalists agree on its conclusions.
Author: Fred Pearce
New Scientist issue: 3 June 2000
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