After all hydro may not be green

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After all, hydro may not be emissions free! Surprised? Some scientists are suggesting that hydropower reservoirs may in fact be producing greenhouse gases.

Scientists are suggesting that reservoirs around the world could be emitting as much as 70 millions tons of methane and 1 billion tons of carbon dioxide annually- equivalent to carbon dioxide generated by more than 12 typical coal fired plants operating 24 hours a day for 365 days. This discovery was made long ago by a scientist at University of Alberta and published in a peer reviewed article in 2000. According to the article,

...hydropower can actually emit more greenhouse gases per kilowatt-hour than fossil fuels, including dirty coal...

 

Another scientist in Brazil has also warned about the greenhouse gas contribution of hydro reservoirs in Brazil. According to him, for every kilowatt-hour of electricity produced from hydro reservoirs in Brazil, 2154 grams of carbon dioxide equivalent is emitted. On the contrary, for a typical modern coal plant, that figure stands between 790 and 1200.

This San Francisco Chronicle op-ed that appeared in November 2006 warns that one Brazilian hydro reservoir alone emits the greenhouse gases equivalent of 54 natural gas plants, for the same amount of power.

As explained by scientists, decaying of organic matter in the reservoir is the primary culprit for production of greenhouse gases. The reservoirs created by hydro dams flood a large area thus submerging large amounts of carbon-rich vegetation and soil. Over time the organic matter decays producing carbon dioxide and potentially methane in some cases, which has much more potential than carbon dioxide to cause global warming. Although such emission is higher during the first few years (up to a decade in some cases) the reservoir will continue to emit green house gases as long as the river feeding the reservoir continues to carry along plants and other organic matter which act as fuel for such emissions. At the same time, it has also been found that when methane-rich water bursts out of turbines and spillways, it releases most of the methane- just like when you open a bottle of soda.

Here is another article by Forbes that cautions about the greenhouse gas contribution of hydropower generation. The article states:

The biggest kicker of all is this: Big hydro may not be so green after all. Philip Fearnside, an ecology professor at Brazil's National Institute for Research in the Amazon, has published groundbreaking studies indicating that large dam reservoirs in tropical climates can produce significant amounts of methane, a greenhouse gas more than 20 times as potent as carbon dioxide. (Decomposing vegetation produces methane, which is released slowly by bubbling up in the reservoir, or quickly when water is run through the turbines and released downstream.

In the United States, there has been limited research to estimate greenhouse gas contribution of reservoirs. So far only five lakes in Wisconsin and one in Colorado have been studied for emissions. The average annual emission of carbon dioxide and methane from the five lakes in Wisconsin was calculated to be 242 and 12 grams per square meter respectively. On the contrary, an average tropical forest absorbs 260 grams of carbon dioxide and 0.07 grams of methane per square meter annually. These figures suggest that lakes in Wisconsin, on an average produce more greenhouse gases than tropical forests, traditionally known to be carbon sinks, absorb. Similarly, Dillon Lake in Colorado emits between 5 and 622 grams of methane per square meter.

What is clear from the science available so far is that contrary to the popular belief, hydropower reservoirs cannot be assumed to be emissions free. The amount of emissions varies depending on a number of factors- nature of the reservoir (size and depth), climate type, organic content and other hydrological conditions. More research on this topic is needed to better understand the impacts of reservoir emissions and hydropower on global warming.