Scientists link world's big dams to methane and global warming
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Shaun McKinnonThe Arizona RepublicMay. 19, 2007 12:00 AM
Brazilian scientists say they have found evidence that the planet's large dams emit nearly 115 million tons of methane every year, a figure that would put the water-control structures among the top contributors of human-caused greenhouse gases.In a study released earlier this month, the scientists claim the world's 52,000 dams contribute more than 4 percent of the warming impact linked to human activities. The study even suggests that dams and reservoirs are the single largest source of human-cased methane, a gas that traps heat in the atmosphere.Environmental groups have rallied against dams for years, arguing that they destroy rivers and riparian habitat and wastewater through evaporation. The claim that the structures also cause global warming is new and is certain to be disputed by the hydropower industry as well as water supply managers.

"There is now more than enough evidence to show that large dams are a major source of climate-changing pollution," says Patrick McCully, executive director of International Rivers Network. "Climate policy makers must address this issue."The findings would prove troublesome for Western states that rely on dams and reservoirs for a significant share of their water and power. Industry groups have already dismissed the report as an exaggeration of the truth.Dams don't seem a likely source of pollutants, but decomposing organic materials in the reservoirs produce methane. Older and larger dams can produce even more of the gas as the materials accumulate in silt.The rivers network, in its analysis of the Brazilian study, suggested that hydroelectric dams in warmer, equatorial climates compound the methane problem:"The massive amounts of methane produced by hydropower reservoirs in the tropics mean that these dams can have a much higher warming impact than even the dirtiest fossil fuel plants generating similar quantities of electricity," according to the analysis.The Brazilian study measured methane emissions from reservoir surfaces, turbines, spillways and rivers immediately downstream from the dams. The study suggests that dams in Brazil and India are responsible for one-fifth of those countries' total global warming impact.Ivan Lima, one of the study's co-authors, said dam operators should harness the methane to fuel power plants instead of allowing it to escape:"If we can generate electricity from the huge amounts of methane produced by existing tropical dams we can avoid the need to build new dams with their associated human and environmental costs," he said.