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

Image © Steve Byland | Dreamstime.com



  • Faith Geist August 2, 2011 at 8:12 am

    Did the study look at how much CO2 the reservoirs sequester in the vegetation that grows in them.
    We need to be VERY careful not to use science to further any agenda.


  • Alice August 2, 2011 at 11:36 am

    When I think of the damage cause by dams, climate change is not the first thing that comes to mind. How about habitat damage? The interruption of many migrations and spawning habits? How many native salmon are endangered in the U.S.?How about the destruction of native(people)communities being forced to move into the modern economy? And really, do we think that hydropower is the solution to the energy crisis? Let’s think of some new ideas shall we…


  • Pam J August 11, 2011 at 12:11 am

    Are these “international team of scientists” living in the dark ages? Why do they need to attempt to justify dams and who indeed funded them? There is so much quantified research already done showing DAMS are expensive, have a long history of destroying entire ecosystems, displacing hundreds and thousands of people, eliminating species (biodiversity) and creating more problems than they solve.. as we see in China, the battles in South America, Africa and elsewhere viz:
    1. About Dams | International Rivers http://ow.ly/5UQyb
    2. AMAZON WATCH » Stop the Belo Monte Monster Dam http://ow.ly/5SO54
    3. China’s Three Gorges Dam: An Environmental Catastrophe: Scientific American http://ow.ly/5SO6h
    4. This excellent PBS documentary gives insight into the fallacy of dams using Salmon and the North West’s Columbia River as icons for why we need to Restore Our Rivers Not Dam Them Up “SALMON – RUNNING THE GAUNTLET” http://video.pbs.org/video/1891112523
    Climate Change is an outgrowth of our natural world out of balance and all the disruptions of interrelationships in our natural environmental disruptions. Let’s not create more.


  • Jennifer March 14, 2012 at 7:58 am

    Excellent article

    The World Bank estimates that forcible “development-induced displacement and resettlement” now affects 10 million people per year. According to the World Bank an estimated 33 million people have been displaced by development projects such as dams, urban development and irrigation canals in India alone.

    India is well ahead in this respect. A country with as many as over 3600 large dams within its belt can never be the exceptional case regarding displacement. The number of development induced displacement is higher than the conflict induced displacement in India. According to Bogumil Terminski an estimated more than 10 million people have been displaced by development each year.

    Athough the exact number of development-induced displaced people (DIDPs) is difficult to know, estimates are that in the last decade 90–100 million people have been displaced by urban, irrigation and power projects alone, with the number of people displaced by urban development becoming greater than those displaced by large infrastructure projects (such as dams). DIDPs outnumber refugees, with the added problem that their plight is often more concealed.

    This is what experts have termed “development-induced displacement.” According to Michael Cernea, a World Bank analyst, the causes of development-induced displacement include water supply (dams, reservoirs, irrigation); urban infrastructure; transportation (roads, highways, canals); energy (mining, power plants, oil exploration and extraction, pipelines); agricultural expansion; parks and forest reserves; and population redistribution schemes.


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