Why has the building of dams created controversy and attracted criticism?
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DamsA barrage of criticism Print edition | Science and technologyNov 16th 2000
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.
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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.
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