permaculture

[Jeffrey Newman <newman@bcinternet.net>]

RAISING A STINK
By Fred Pearce
New Scientist magazine
03 June 2000

Rotting vegetation in hydroelectric dams stokes global warming

http://www.newscientist.com/nl/0603/stink.html

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 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.

"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.



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