FAR from being 鈥済reen鈥�, 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鈥檚 biggest funder of large dams.
The report comes just as engineers are arguing that dams should qualify for
support as a 鈥渃lean鈥� 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 鈥渙nly a fraction鈥� of the gas. This means
that the emissions don鈥檛 disappear when the flooded forest has rotted away, but
may continue for the lifetime of the reservoir.
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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
(快猫短视频, 4 May 1996, p 29).
But what appeared at first to be a problem for a handful of reservoirs now looks much more general.
鈥淭ropical reservoirs that are shallow and uncleared of biomass [before
flooding] appear most at risk,鈥� 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
0.35 million tonnes per year. Petit-Saut in French Guyana, which has a similar
capacity and powers the launch site for Europe鈥檚 Ariane rocket, will produce 0.9
million tonnes per year in its first 20 years.
The report鈥檚 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.
鈥淕reenhouse 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,鈥� says the
commission. 鈥淭here is no justification for claiming that hydroelectricity does
not contribute significantly to global warming.鈥�
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.
