David Adam

Generating electricity with water appears to be a clean option, so why are soaring emissions being recorded at hydropower plants?

THEY ARE considered a clean source of renewable energy and, to some, a model of sustainable development. Hydropower plants supply up to 80 per cent of the electricity in power-hungry tropical countries such as Brazil, but their murky waters could harbour a dirty secret. Contrary to popular belief, and despite burning no fossil fuels, hydroelectric plants release millions of tonnes of polluting greenhouse gases into the atmosphere each year, as bacteria degrade the vegetation submerged in their giant reservoirs.

The United Nations has now pledged to tackle the issue, and with it the thorny question of whether some countries that have invested heavily in hydropower might have been better off building coal and gas power stations instead. Before more hydropower systems are built in tropical zones, it wants experts to examine the emissions from existing schemes and to work out how to make them more environmentally friendly.

Mike Acreman, a professor at the U.K. Centre for Ecology and Hydrology in Wallingford, Oxfordshire, says: "Perhaps hydropower is not as green as we thought. A lot of these tropical hydropower schemes would have been made by simply flooding a forest. There would have been a lot of trees and plants, and you need to think about what happens to all that carbon."

According to Vincent St Louis, a scientist at the University of Alberta, Canada, manmade reservoirs across the world about a quarter of which feed hydroelectric dams together release around one billion tonnes of CO2 each year. They also release some 70 million tonnes of methane, a much more potent greenhouse gas. Over a hundred years, his figures suggest, these reservoirs will contribute about 7 per cent of the global warming impact of all human activities. The hydropower industry disputes the impact of emissions from its reservoirs and argues that the results vary hugely from place to place.

The U.N.'s initiative follows a summit meeting last year that brought industry experts, independent scientists, and government officials together in an attempt to settle the arguments. The two-day meeting in Paris, organised by the Unesco International Hydrological Programme, concluded that methane emissions from existing reservoirs need to be urgently assessed. A joint statement released after the meeting said: "A better understanding of the methane footprint of tropical reservoirs is required. From the point of view of power generation, the majority of the world's hydropower potential remains in the tropics. It is essential to close the knowledge gaps and establish best practice."

The participants called for "a focussed programme of field measurements that clearly define the methane footprint of existing and new reservoirs," and said that developers must learn from past mistakes and find new ways to reduce the emissions.

Typical of the problem is the 250 MW Balbina dam in Brazil, created to feed energy to the city of Manaus in the 1980s by flooding 2,500 sq km of Amazonian rainforest. According to the International Rivers Network campaign group, greenhouse emissions from Balbina's shallow and carbon-rich reservoir are 25-38 per cent higher than from a comparable modern coal-fired power plant. A more efficient Amazonian hydropower plant at Tucurui, which generates almost 20 times as much electricity with a reservoir 300 sq km smaller than Balbina, still has a greater global warming impact than a comparable gas power plant.

In 2000, the World Commission on Dams concluded: "In some cases, the gross emissions [from hydropower schemes] can be considerable, and possibly greater than the thermal [fossil fuel] alternatives."

Philip Fearnside, a conservation biologist at the National Institute for Amazon Research, in Manaus, has studied the Balbina and Tucurui dams and says the problem could be worse than many scientists think. He calculates that a typical tropical hydropower plant, over the first 10 years of its life, emits four times as much carbon as a comparable fossil fuel station. A separate study of Balbina found that such downstream emissions alone have the same greenhouse warming potential as 6 per cent of the fossil fuels consumed by Sao Paulo, a city of more than 11 million people. But other scientists have accused Professor Fearnside and colleagues of exaggerating.

Underlying vegetation

Bob Watson, chief scientist at the World Bank, says: "There are some very poorly designed systems, but the emissions depend on the area and depth of the reservoir and the nature of the underlying vegetation. There's no question that we need to look at it carefully, but it needs to be on a case-by-case basis. Basically, the deeper the reservoir, the better."

The problem is not so great from hydroelectric schemes in temperate regions of the world such as the U.K., because the cooler temperatures stop bacteria producing as much greenhouse gas.

The U.N.'s Intergovernmental Panel on Climate Change is taking steps to force countries to include emissions from artificially flooded land in their national inventories of greenhouse gas pollution.

The Kyoto protocol the international agreement to regulate greenhouse gases offers lucrative carbon credits for hydropower schemes, depending on how much methane they produce.

One problem, according to Professor Acreman, is that the true climate impact of hydroelectric power is difficult to calculate. "The big issue is what would have happened if the reservoir hadn't been there," he says. "You couldn't go to one and measure the methane coming off the surface and say that that was definitely caused by the hydropower scheme."

Guardian Newspapers Limited 2006