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Hydroelectric Dams Contribute to Global Warming

Dams are not necessarily more environmentally friendly than coal or other other methods of creating electricity.

Many environmentalists have long put forth the view that hydro-electric dams are preferable to coal, atomic energy, or other options for generating electricity. That may not necessarily be the case.

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.

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. 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 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 release only about 12 per cent as much carbon into the atmosphere.

The report's authors have only studied a handful of reservoirs so far, 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."

Extracted from an article in New Scientist