Dams and their reservoirs are significant sources of heat-trapping greenhouse gases, contrary to hydro-power industry assertions, environmentalists said.
Industry groups have described dams as "climate-friendly" because they are unlike coal-powered energy plants, which emit large amounts of the air pollutants that most scientists believe cause global warming.
However, a growing body of scientific evidence refutes those assertions, said the International Rivers Network (IRN), a US-based anti-dam group, in its report "Flooding the land, warming the earth", released in Washington on Tuesday.
"In tropical areas, hydro-power reservoirs may be much worse climate polluters than even coal power plants," said Patrick McCully, IRN's campaigns director.
Scientific field studies at some 30 dam reservoirs, mostly in Canada and Brazil, have found varying amounts of the greenhouse gases carbon dioxide and methane, said the 18-page report.
David Tuft, a spokesperson for the National Hydro-power Association, said the report greatly exaggerated the relative impact dams have on climate change in comparison to emissions produced when fossil fuels such as oil, gas and coal are burned.
"There is some merit to some of the claims against dams, but we still think that by and large, hydro-power dams provide clean energy and remain part of the solution to global warming," said Tuft.
Emissions from reservoirs
The science behind calculating emissions from reservoirs is complex and relatively young, IRN acknowledged. Only a handful of researchers are working on the issue and many uncertainties about net emissions levels remain. However, according to the report, there is enough research to warrant concern.
A team of researchers headed by Vincent St Louis of the University of Alberta, Canada, estimated that the amount of methane released from reservoirs worldwide is equal to 20 percent of the total methane from all known sources tied to human activity. These include livestock, fossil fuels and landfills.
Dam reservoirs contribute an estimated four percent of all carbon dioxide emissions caused by human activity, according to the research team.
The findings on methane have worried environmentalists because although carbon dioxide is seen as the main gas responsible for global warming, methane is more powerful at trapping heat.
There are numerous uncertainties in the assumptions used to calculate these initial estimates, the researchers stressed. The amount of methane emitted by reservoirs, for example, could be either much lower or much higher depending on different assumptions for average methane emissions per unit of reservoir surface.
Emissions from dams and reservoirs vary widely according to their geographical location and other factors, the report said. Reservoir depth and shape, the local climate, and the way in which the dam is operated, could all impact emission levels, it added.
Research indicates that most of the emissions at dams in boreal and temperate regions come from the diffusion of carbon dioxide into the atmosphere from the surfaces of their reservoirs.
More methane than carbon dioxide
In the tropics, emissions vary depending on how deep a reservoir is. Shallow tropical reservoirs appear to produce much more methane than carbon dioxide. The methane is produced when bacteria break down organic matter in the water.
Some tropical reservoirs release more emissions than the dirtiest fossil fuel plants, said the report. Emissions from the 250-megawatt Balbina Dam in the middle of the Amazon basin in Brazil are exceptionally high: some 25-38 times higher than a modern coal plant of similar megawatt capacity.
The Tucurui hydro-power plant, also in the Brazilian Amazon, generates almost 20 times more electricity with a reservoir 300sq km smaller than Balbina. But Tucurui's emissions are still greater than that of a gas plant producing a similar amount of power, according to the report.
Five large hydro-power plants have been completed or are near completion in the Amazon rainforest. Plans exist for more than 70 additional plants, flooding a total area of 100,000sq km, it noted.
The various United Nations processes dealing with climate change have largely overlooked the possibility of emissions from dam reservoirs, the report added.
The UN Intergovernmental Panel on Climate Change, an international panel of hundreds of scientists, does not include reservoirs as a source of emissions in its latest assessment report on greenhouse gases. Similarly, the mandatory guidelines for producing reports of national emissions under the Kyoto Protocol on climate change do not include dam reservoirs.
"Excluding reservoirs from national emissions inventories could significantly under-represent the actual contribution to climate change of some countries, especially of those in the tropics with large reservoir areas such as Brazil and Ghana," said the report.
Accounting for reservoir emissions could increase Canada's estimated greenhouse gas output by around three percent, and the country's electric-power sector emissions by around 17 percent, according to research, cited in the report, by Eric Duchemin of the University of Quebec at Montreal.
