I have never been a fan of large dam projects. In the United States, dams have decimated salmon populations, reduced the Colorado River to a trickle by the time it reaches the Pacific, and otherwise transformed the ecology of our western rivers. And in many developing countries, dam projects displace hundreds of thousands of people, often without giving them adequate compensation. Large popular movements have arisen against major dam projects like the 3 Gorges Dam in China or the Narmada River dams in India.
But dam proponents have argued that dams don’t emit carbon dioxide, so they must still be better than coal, right? Well, perhaps not, at least in tropical regions. A recent article in the journal Nature (“Methane quashes green credentials of hydropower”, Nature 444(30): 524-525, 2006) highlights recent research suggesting that “the global-warming impact of hydropower plants can often outweigh that of comparable fossil-fuel power stations.” How? When land is flooded to create the reservoir, a large amount of organic matter is trapped underwater, and more organic matter flows in over time. In warm tropical waters, this organic matter decays into methane and carbon dioxide. Since methane is a 20 times more potent greenhouse gas than CO2, these reservoir emissions can be quite significant. Indeed, the article cites an example of Brazil’s Balbina Dam, whose construction caused the flooding of 2500 square kilometers of rainforest; it is now accepted that a coal plant would have been better for the climate!
The article notes that the debate over the magnitude of reservoir emissions is not yet settled, largely due to a lack of data on dam methane emissions. However, by some estimates, counting the methane emitted by dams (95-120 million tons per year) would represent a 20% increase in global methane emissions! This is a large enough number, and enough is known about dam methane emissions, to make many scientists want to start acting now on this problem.