Aside from from carbon dioxide, one of the other main greenhouse gases is methane. While hydro-electric dams are considered by most people to be a clean form of energy, it is far from perfect. One of the problems with dams is that organic material that would normally be washed down a river gets trapped at the bottom of a reservoir. The anaerobic conditions in the relatively still water in a reservoir means that the organic material is broken down by bacteria which then give off methane which dissolves into the water. Then the methane-rich water is drawn into the turbines where it's stirred up and the methane is released into the atmosphere.

Brazilian scientists have estimated that methane emissions from dams worldwide could be equivalent to 800 to 900 million tonnes of CO2 annually. They have devised a means of capturing the methane from dams and using it to fuel extra power generators to supplement the output from the dam. The amount of methane from any given dam varies but some dams in the Amazon region of Brazil could increase their power output by up to fifty percent.

While burning the methane would produce C02, because the original source is decaying plant material, the net carbon effect would be neutral. The CO2 would also be replacing methane which has about twenty times the greenhouse effect of CO2, so the net benefit would be huge.

[Source: BBC via TreeHugger]

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