ENVIRONMENT: Dams are major source of global warming, say scientists

By Pratap Chatterjee Reprint | | Print |

Pratap Chatterjee

WASHINGTON, Nov 29 1995 (IPS) - Dams in tropical countries may cause more global warming than coal-fired power plants, because of the carbon dioxide and methane emitted by rotting vegetable matter in flooded areas, according to a new study conducted by scientists in Brazil.

Philip Fearnside, a scientist working at the Brazilian National Institute for Research in Amazonia, calculated that in 1990 the Balbina dam in Brazil gave off 26 times as much greenhouse gases as a coal-fired power plant supplying an equivalent amount of electricity.

Energy policy makers should take note, says Fearnside. “The scale of hydroelectric development contemplated for Amazonia makes this a potentially significant source of ‘greenhouse’ gas emissions in the future,” he warns.

“The findings of these two studies refute dam industry claims that hydropower is a clean source of energy,” says Patrick McCully, campaign director of the International Rivers Network (IRN), which opposes large dam construction.

Fearnside’s research was just published in Environmental Conservation, a quarterly journal in Switzerland. It backs up previous studies of the global warming impact of dams in Canada’s temperate climate.

Fearnside says that the worst offender is the Balbina dam on the Uatuma river in the state of Amazonas. It has produced an annual average of 112 megawatts of electricity since its completion in 1989. The dam, for which the World Bank provided about 500 million dollars, flooded 3,108 square kilometres of forests.

The 4,000-megawatt Tucurui dam in the state of Para, also in the Amazon, has less of global warming impact because it only flooded 1,926 square kilometres of forests. Fearnside calculates that rotting vegetation caused by the reservoir behind the Tucurui produced 60 percent of the greenhouse gases that would be emitted from a coal-fired power plant that supplied 4,000 megawatts of power.

Emissions by both dams will decline over time, according to Fearnside. But, he points out, the total emissions during the estimated 50-year life of the Balbina dam will exceed those from a coal-fired power plant.

In total, Fearnside calculated that dam reservoirs in the Amazon produced 37 million tonnes of carbon dioxide and 260,000 tonnes of methane. He points out that another 75 dams are planned for the region which, if completed, would multiply the amount of flooded forest land 20 fold.

The environmental impact of the Balbina dam will come as little surprise to communities in Amazonas. Fofrasia Castro da Silva, a matriarch from the village of Pedras, says that the closing of the sluice gates in 1989 resulted in a massive fish kill that blanketed the river with dead fish.

The reason for the massive emission of greenhouse gases, as well as the fish kill, is the fact that the dam authorities did not clear the reservoir area of trees before flooding it. So, when the gates were closed, the dense forest rotted over time, producing carbon dioxide, hydrogen sulphide and methane gases. These turned the water acidic, killing river life.

Eyewitness accounts of the closing of the gate say that animals in the flooded forest tried to flee the waters rising behind the dam by heading for higher ground. These areas turned into islands piled high with corpses of dead animals.

Hard hit by the destruction caused by the Balbina dam and associated highway and mine development were the Waimiri-Artoari indigenous peoples who died of the new diseases introduced to the region. Their numbers declined from 3,500 in 1973 to 374 in 1986 when the dam was completed.

But the global warming data comes as a major surprise to many energy experts who have always assumed that the carbon dioxide and methane given off by trees and plants flooded by dam reservoirs were minimal.

Estimates provided as recently as 1993 in “Water Power and Dam Construction,” an industry journal, put the greenhouse impact of flooded forests at six percent of coal-fired power plants.

The first concrete evidence that these assumptions were wrong came two years ago when a team of researchers led by John Rudd of the Canadian government’s Freshwater Institute in Winnipeg studied dams on the Notigi reservoir in northern Manitoba and near James Bay in northern Quebec.

Conventional wisdom at the time suggested that lakes and peatlands are minor sources of greenhouse gas emissions because of normal decomposition.

But in the Notigi reservoir area, the Churchill/Nelson development flooded stands of white birch which rotted slowly. The researchers found that the slow decay gave off 7.4 grammes of methane per square metre of water area, every year. Because the global warming effect of methane is 60 times higher than carbon dioxide, that is equal to 444 grammes of carbon dioxide per square metre per year.

In the area around Cedar Lake near James Bay, flooded peat lands gave off even more greenhouse gases. The flooded lands emitted between 15 and 30 grammes per square meter of methane (equal to between 900 and 1800 grammes of carbon dioxide) and between 450 and 1800 grammes of carbon dioxide per year.

In both cases, Rudd concluded that the amounts of methane and carbon released by the two dams was roughly equivalent to the amount of greenhouse gases produced by a coal-fired generator of comparable capacity.

By contrast, “run-of-the river” dams, such as the Pangue dam on the Bio-Bio river in Chile, that do not impound large reservoirs of water, do not have the same greenhouse impact, although they often have other negative environmental and social impacts.

Research published in the Canadian Journal of Zoology in 1988 show that even small dams built by beavers may have an impact on global warming. T.E. Ford and R.J. Naiman say their studies show that these dams are the source of 92 percent of the methane emitted from the river catchment area. However, the total quantity of methane emitted by these areas may be quite small.

Not surprisingly, the dam industry has dismissed the Fearnside study. “Its baloney and it’s much overblown,” Karolyn Wolf, a National Hydropower Association spokeswoman, says in “Hydro- Wire,” a magazine that advertises itself as “The Newsletter of the Hydroelectric Industry.”

“As living things die and decay, they produce methane gas. Methane is produced quite substantially in the rain forest, and no one suggests cutting down the rain forest,” she added.

Officials at the World Bank, which has backed many large dams in developing countries as a relatively “clean” energy source, said the new study could not be dismissed.

“It’s possible that our assumptions about the environmental impact of dams are wrong. Nor would it be the first time that we were wrong. I would be very interested in learning more,” Richard Stern, director of industry and mining at the Bank, told IPS.