[DEBATE] : (Fwd) Hydropower no answer for climate change
pbond at mail.ngo.za
Sat Dec 2 06:49:43 GMT 2006
Hydropower: a greenhouse gas culprit?
*The green image of hydropower may have been seriously overstated, warn
1 December 2006
Hydropower plants have long been a byword for clean energy. But
researchers warn that tropical reservoirs might release more greenhouse
gases than fossil-fuel power stations.
Philip Fearnside, a conservation biologist at the National Institute for
Research in the Amazon in Manaus, Brazil, has shown that in the first
ten years of operation, a typical reservoir will emit four times as much
carbon as a fossil-fuel station.
The culprit is organic matter trapped when land is flooded to create a
dam. As this decays, it releases significant amounts of carbon dioxide
It is a topic of vigorous debate, fuelled by a lack of data for tropical
dams and the implications for energy strategies in developing countries,
reports Jim Giles.
It calls into question the wisdom of promoting dam construction in
developing countries, including a US$5 billion project proposed for the
Congo river. Another concern is the funding of hydropower projects
through the Kyoto Protocol's Clean Development Mechanism.
These concerns are likely to influence discussions of greenhouse gas
emissions at a UNESCO meeting in Paris, France, next week (5-6 December)
as well as future analyses by the Intergovernmental Panel on Climate
Nature 444, 524-525 (30 November 2006) | doi:10.1038/444524a; Published
online 29 November 2006
Methane quashes green credentials of hydropower
Emissions from tropical dams can exceed fossil-fuel plants.
At the time, it must have sounded like a sensible case of sustainable
development. During the 1980s, about 2,500 square kilometres of
Amazonian rainforest was flooded to create the Balbina dam to feed the
energy demands of the Brazilian city of Manaus. A sizeable chunk of
rainforest was lost, but Brazil gained access to a non-polluting energy
source. It's a compromise Brazil has made many times; more than 80% of
the country's domestic electricity is generated by hydropower plants.
Yet the clean, green image of dams may have been seriously overstated.
Researchers are gathering in Paris next week to discuss greenhouse-gas
emissions from tropical reservoirs. Some of the latest findings point to
a disturbing conclusion: that the global-warming impact of hydropower
plants can often outweigh that of comparable fossil-fuel power stations.
If that's correct, current energy strategies, particularly in developing
nations, will need to be rethought.
The problem lies with the organic matter in the reservoir. Large amounts
are trapped when land is flooded to create the dam, and more is flushed
in after that. In the warm water of tropical dams, this matter decays to
form methane and carbon dioxide. Although both are greenhouse gases, the
main worry is methane, which has more than 20 times the warming impact
of carbon dioxide over a 100-year period. In the specific case of
Balbina, there is now a rough consensus: in terms of avoiding
greenhouse-gas emissions, a fossil-fuel plant would have been better.
But that is where the agreement ends. On one side of the debate is
Philip Fearnside, a conservation biologist at the National Institute for
Research in the Amazon in Manaus. His work, based mainly on theoretical
calculations, looks at water leaving dams. Many dams release water from
several metres below the surface, so the flow goes through an abrupt
pressure change. Fearnside calculates that this causes methane release,
much as carbon dioxide fizzes out when carbonated drinks are opened. His
latest results suggest that a typical tropical hydropower plant will,
during the first ten years of its life, emit four times as much carbon
as a comparable fossil-fuel station.
Lining up against him in a decade-long dispute are Luiz Pinguelli Rosa
and his colleagues at the Federal University of Rio de Janeiro, who
accuse Fearnside of exaggerating reservoir emissions. They complain in
particular that Fearnside has extrapolated from measurements taken on
the Petit Saut dam in French Guiana; the data were taken in the years
immediately after the reservoir was created, when the store of organic
matter would have been greatest.
With few data sets available on tropical dams, the debate has increased
in acrimony without approaching a conclusion. Environmental groups
question the impartiality of Rosa's work, which is funded in part by the
hydropower industry. Rosa strongly denies any bias, and in turn accuses
Fearnside of seeking to show that "something is wrong with dams".
If these estimates are correct, figures for annual global methane
emissions need to be increased by a fifth.
The Paris meeting, which runs on 5–6 December and is organized by the
United Nations Educational, Scientific and Cultural Organization
(UNESCO), is unlikely to settle their dispute, but researchers will
discuss new methane data. On 14 November, for example, Frédéric Guérin
of the Laboratory of Meteorology in Toulouse, France, and his colleagues
published results on methane release from sites downstream of three
tropical dams1. They found that so much methane builds up in the dam
that downstream emissions, which are rarely factored into estimates of a
reservoir's impact, should account for between a tenth and a third of
total emissions. Another new paper estimates that, for Balbina,
downstream emissions alone have the same greenhouse warming potential as
6% of all the fossil fuels consumed by São Paulo, a city of more than 11
Even without these downstream emissions, the global impact of dams may
be significant. Danny Cullenward, an energy-policy expert at Stanford
University, has made preliminary calculations of the impact of
Fearnside's findings. Cullenward stresses that more data are needed, but
his estimates suggest that dams release between 95 million and 122
million tonnes of methane per year. If correct, estimates of annual
global methane emissions (which do not generally include dam emissions)
need to be increased by a fifth. Even extrapolating Rosa's figures gives
Cullenward a total of 23 million tonnes.
Many think enough is known to start acting now. Some worry about the
huge dam projects tentatively planned for tropical areas, such as a
$5-billion project on the Congo river. Another concern is the Clean
Development Mechanism (CDM), a system that allows developed nations to
fund clean-energy projects in developing nations in return for credits
that can be used to meet Kyoto Protocol targets. Current rules allow
certain hydropower projects to be funded under the CDM, a situation some
scientists and environmental groups would like to see revised.
But matters are unlikely to change without more data, so researchers at
the UNESCO meeting will discuss which questions to prioritize and how
best to work together. More substantial progress could begin in 2008,
when the Intergovernmental Panel on Climate Change (IPCC) will decide
whether or not to start work on a special report on renewable energy.
Previous IPCC special reports have had significant political impact, and
the dams question is likely to fit very well into the scope of the
proposed energy study, says Bert Metz, a climate-policy expert at the
Netherlands Environmental Assessment Agency and co-chair of one of the
IPCC's three working groups.
1. Guérin, F. et al. Geophys. Res. Lett. 33, L21407 (2006).
2. Kemenes, A., Forsberg, B. R. & Melack, J. M. in Proc. 8th Int. Conf.
Southern Hemisphere Meteorology and Oceanography, Foz do Iguaçu, Brazil,
24–28 April 2006, 663–668 (INPE, São José dos Campos, Brazil, 2006).
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