Dam Less?

Hydropower is often touted as a solution to climate change. Some critics have been skeptical, however, pointing to studies suggesting that the reservoirs behind dams can emit oodles of carbon dioxide and methane, particularly in tropical regions. But those concerns appear to be overblown, concludes a new study.

When rivers are dammed, the flooding often drowns tons of vegetation and organic matter in soil, an international team of scientists notes in Nature Geoscience. The conditions are perfect for the decomposition process that produce greenhouse gases, with emissions typically highest soon after reservoir construction. Studies have found that as reservoirs age, emissions decline, with cold-water rivers stabilizing more rapidly than their warm-water counterparts. As a result, past studies had warned that building new dams to produce power could end up making a significant contribution to climate change.

To size up the problem, the team analyzed emissions from 85 hydroelectric reservoirs around the world. Overall, they concluded they emitted about one-sixth of the carbon dioxide and methane that past studies had attributed to them. In total, the systems emittted 48 million metric tons of carbon annually, a downgrade from earlier estimates of 321 million metric tons. Together, the reservoirs are responsible for less than 16% of the total carbon dioxide and methane emissions from all types of human-made reservoirs combined, the authors note.

“Our analysis indicates that hydroelectric reservoirs are not major contributors to the greenhouse gas problem,” says Jonathan Cole, a limnologist at the Cary Institute of Ecosystem Studies in Millbrook, New York, and one of the paper’s authors. “But 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.”

The amount of greenhouse gases generated by hydroelectric reservoirs depends on where they are built, the team notes, with emissions correlated with latitude and the amount of biomass in the watershed. “Reservoirs in tropical locations, such as the Amazon, emit more methane and carbon throughout their lifecycles,” says Nathan Barros of Brazil’s Federal University of Juiz de Fora. “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.”

Hydropower already supplies an estimated 20% of the world’s electricity and accounts for more than 85% of electricity from renewable sources, the authors note, and many new dams are on the drawing boards. The authors urge careful consideration of site and design. “During the environmental impact phase,” Cole says, “it should be a goal to minimize the amount of carbon dioxide and methane emitted per unit of energy generated.” And to better assess the emissions generated by hydroelectricity, the authors also call for a study that assesses a site’s carbon budget before and after reservoir construction. David Malakoff | August 2, 2011

Source: Nathan Barros, Jonathan J. Cole, Lars J. Tranvik, Yves T. Prairie, David Bastviken, Vera L. M. Huszar, Paul del Giorgio & Fábio Roland. Nature Geoscience. Published online: 31 July 2011 | doi:10.1038/ngeo1211

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