Large dams produce hydroelectricity, but at what cost? Megahydro provides of lots of power with relatively little expense, but environment advocates often oppose projects because they drown significant amounts of otherwise valuable land, erasing cultural resources and drowning entire ecosystems. Now there’s another reason to question this source of power.
In 2007, researchers at Brazil’s National Institute for Space Research calculated that the world’s largest dams emitted 104 million tons of methane annually and were responsible for 4% of the human contribution to climate change. We now have more data that supports the belief that in some locales, greenhouse emissions of methane from hydroelectric dams may cause more harm than benefit.
A French team from the National Centre for Scientific Research, studying methane emissions at the largest reservoir in Southeast Asia (Nam Theun 2, in Laos) has found that big dams, dubiously renewable and often more costly than other measures, can add to global warming rater than reducing it.
Paul Brown reports on ClimateNewsNetwork:
“In many rocky regions low on vegetation and population, such as in Iceland and other northern mountainous regions, the production of electricity from hydropower is clearly a net gain in the battle against climate change.”
The story seems to be different in the case of tropical forests, however. When we drown tropical forests in Asia, Africa, and South America for large dams, bacteria feed on and rot the submerged plant material, and then upstream rivers and rains add more organics to the mix.
The combination results in deluging the atmosphere with methane discharges much greater than previously calculated. Since methane’s impact on the atmospheric greenhouse is over 80 times higher during the first 20 years than that of carbon dioxide, building large dams in these locations may be doing more harm than good, at least for the next two decades, because it puts us even closer to the climate change threshold of 2˚C .
Some of the gas diffuses into the atmosphere, some passes through the turbines to be released downstream, some bubbles up directly to the water’s surface and exits into the atmosphere (ebullition). The researchers say that the majority of total emissions (between 60-80%) come from immediate bubbling at the dam. The ebullition is not constant: it varies with season, temperature, water level, and atmospheric pressure.
Large dam projects thus may not translate to clean energy for two reasons: the painful land use tradeoff and the greenhouse gas potential. Research will proceed at this site in quantifying nonebullient emissions and the overall contribution of this type of facility to the global greenhouse trend.
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