Hydroelectric reservoirs are not major contributors to the greenhouse gas problem. This statement, made by Dr Jonathan Cole, a limnologist at the Cary Institute of Ecosystem Studies[1], is in stark contrast to previous scientific thoughts on the subject. Prior studies have cautioned that all types of reservoirs could be a significant and large source of both carbon dioxide and methane to the atmosphere.

One such example was the research gathered by the World Commission on Dams, which sent shock waves through the energy sector over 11 years ago. Measurements taken at a hydropower plant in Brazil added weight to the report’s conclusions that the world’s hydropower reservoirs were responsible for between 1 and 28% of the planet’s climate gas emissions.

However Cole and his research colleagues have now downgraded these previous estimates, suggesting that they could be 16M metric tons less than previously thought, as earlier approximations were on the basis of more limited data.

Support grants from FURNAS Centrais Elétricas and the Swedish Foundation for International Cooperation in Research and Higher Education (STINT), have enabled the international team of scientists to undertake research into the subject. They gave their analysis in a paper entitled Carbon Emissions from Hydroelectric Reservoirs Linked to Reservoir Age and Latitude [2], which was published online in Nature Geoscience in August 2011. In summary their findings posit that man-made hydroelectric reservoirs emit a relatively small amount of both carbon dioxide and methane.

However Cole admits that there are some caveats. “To date, only 17% of potential hydroelectric reservoir sites have been exploited, and impacts vary based on reservoir age, size and location,” he said.

Lead author Nathan Barros, of the Federal University of Juiz de Fora in Brazil, explains further. “The bottom line is that per unit of energy, hydroelectric generation produces much less carbon dioxide and methane emissions than previously thought, but impacts are not equal across all landscapes.”

The amount of greenhouse gases generated by hydroelectric reservoirs depends on where they are built, with the team’s analysis indicating that emissions are correlated with latitude and the amount of biomass in the watershed. And as Barros adds: ‘Reservoirs in tropical locations, such as the Amazon, emit more methane and carbon throughout their lifecycles.’

Hydroelectric reservoirs cover an area of 3.4×105 km2 and comprise about 20% of all reservoirs. In addition, they contain large stores of formerly terrestrial organic carbon. Significant amounts of greenhouse gases are emitted, especially in the early years following reservoir creation but the global extent of these emissions is poorly known.

Previous estimates of emissions from all types of reservoir indicate that they emit 321M metric tons of carbon per year. In this 2011 study the research scientists assessed emissions of carbon dioxide and methane from data gathered at 85 globally distributed hydroelectric reservoirs systems. Emissions were related to reservoir age, location biome, morphometric features and chemical status. It was estimated that hydroelectric reservoirs emit about 48M metric tons as carbon dioxide and 3M metric tons as methane, corresponding to 4% of global carbon emissions from inland waters. Further putting things into perspective, hydroelectric reservoirs are responsible for less than 16% of the total carbon dioxide and methane emissions from all types of man-made reservoirs combined.

While hydro supplies an estimated 20% of the world’s electricity and accounts for more than 85% of electricity from renewable sources, future development is expected globally. In addition future emissions will be highly dependent on the geographic location of new hydroelectric reservoirs. The paper’s authors urge careful consideration of site location and design. ‘During the environmental impact phase, it should be a goal to minimise the amount of carbon dioxide and methane emitted per unit of energy generated,’ Cole says.

The authors also call for a study that assesses a site’s carbon budget before and after reservoir construction, while pre- and post flooding analysis would clarify the net carbon impact of hydroelectric reservoirs.

SINTEFF helps clean up hydro’s act

Commenting on the World Commission on Dams’ conclusions that hydropower reservoirs were responsible for between 1-28% of the planet’s greenhouse gas emissions, a senior scientist from Scandinaviaâ’s largest independent research organisation SINTEF, said that these measurements in Brazil failed to take account of the uptake of carbon dioxide by the reservoir’s ecosystem. “However,” Atle Harby said, “the results attracted much attention and were used to draw erroneous conclusions at a global level.”
In 2010 SINTEF published its own research from sub-tropical Laos and believed that its news from the field was nothing but good news. Measurements from a 30 year-old hydroelectric reservoir revealed that the organisms in the water and on the reservoir floor take up more carbon dioxide from the atmosphere than the reservoir itself releases in the form of climate gases, converted to carbon dioxide equivalents. “Our findings in Laos indicate that the true figure is much closer to 1% than 28%,” Harby added.
Greenhouse gases released by a hydroelectric reservoir originate from carbon derived from the decomposition of buried organisms and organic material brought in by streams and from human sources. However, at the same time, algae, phytoplankton, zooplankton and fish living in the reservoir will take up CO2 from the atmosphere.
“Emissions levels are greatest at the start, but decline when the buried organisms are fully decomposed. We also investigated a ten year-old reservoir in Laos, where there was a balance between gas uptake and release,” says Harby.
He explains that the vitality of the reservoir’s ecosystem and the volume of unconsolidated sediment that buries the dead organisms together determine whether or not the climate gas balance is in the red or the black over the lifetime of the reservoir in question.
The bedrock, soil and water quality in the 30 year-old Laotian reservoir have combined to promote prolific organic production, and are thus also responsible for the high uptake of carbon dioxide from the atmosphere. It is also likely that large volumes of sediment are deposited on the bottom.
SOURCE: Xergi 4-2010. www.sinteff.no