A Barrage Of Criticism
FOR the past century or so, governments and development
agencies alike have been keen on dams. Built to store water and generate
electricity, they are often emblems of national pride. And surely driving
turbines from flowing rivers is a greener way to generate energy than burning
coal, oil or gas? But now serious doubts are emerging. For the first time a
comprehensive effort has been made to analyse the environmental, economic and
social impacts of the world’s 45,000 large dams. The World Commission on
Dams has spent two years gathering data on 1,000 of them, soliciting the
views of governments and non-governmental groups. This week South Africa’s
Nelson Mandela unveiled the result. The conclusion is bleak.
Dams make valuable economic contributions, of course: a third of countries
depend on hydropower for over half their electricity, and over a third of
irrigated land depends on dams. Farmers benefit from cheap irrigation water
which, in turn, subsidises much of the world’s food. But the overall costs of
dams, to both man and nature, have never before been considered. The report
concludes that dams’ impacts on ecosystems are "mostly negative".
The building of dams is often destructive. It usually means clearing forests or
other habitats in areas to be flooded. Water in reservoirs, especially in
water-storage dams, becomes silted with vegetation and matter washed off land
upstream. As that rots, it emits carbon dioxide and methane, contributing to the
greenhouse effect. How much is emitted is not known, but some estimates say
reservoirs could account for more than a quarter of the "global-warming
potential" of gases in the atmosphere. It was assumed that hydropower, by
substituting for burning fossil fuels, would reduce emissions of greenhouse
gases. In hot, tropical countries, where reservoirs appear to produce most
gases, that claim now needs reassessing.
Two studies of a single hydropower project in Brazil show why that will not be
easy. The first, in 1998, found that the massive (2,600 km2) Tucurui reservoir
emitted 76 tonnes of methane per km2 and 3,808 tonnes of carbon dioxide each
year; but a second study last year put the figures at a mere five tonnes of
methane and only 2,378 tonnes of CO2. On the first analysis it would have been
cleaner to burn gas, oil or even coal for electricity; on the second, the dam is
probably cleaner. Both analyses could even be correct, if gas emissions from
reservoirs vary greatly.
There are other problems, too. Some large dams alter flood cycles and downstream
flows; some pollute rivers, remove nutrients and alter the water temperature.
All these can affect the survival of plants, fish and animals. Reservoirs may
serve as hosts for mosquitoes and the diseases, including malaria, that they
spread. Blocked rivers disrupt the migration and breeding of fish, causing some
species to become extinct.
Assessing the net benefits of dams requires far more attention to be paid to
these difficulties. Nearly half the world’s rivers have at least one large
dam; more are now being built in China, India, Turkey and elsewhere. Over $2
trillion has been invested, and as many as 80m people have been displaced by
dams. Yet the projects have often proved unprofitable, slow to deliver energy or
water, and prone to corruption. In poor countries, even when dams deliver as
promised, the better-off often benefit most.
Will this barrage of criticisms improve the way dams are built? The involvement
of the World Bank and industry in the report should ensure closer scrutiny of
big dam projects, and help make the report’s recommendations more palatable to
the private sector. But a decision to build a dam is often made for political
reasons, to create jobs, or to benefit a particular group. So better
information, or pressure from outsiders, may not help. At least the report
offers a yardstick for measuring future failures.
Reprinted from The
Economist - 16 November 2000
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