Clean Power

Published on May 31st, 2012 | by Katy Yan


Tropical Dams Dispel Clean Energy Myth

May 31st, 2012 by  


Reservoirs from hydroelectric dams emit greenhouse gases

Dead trees in the Petit Saut reservoir, French Guiana

Tropical reservoirs are a “methane factory, continuously removing carbon from the atmosphere as carbon dioxide and returning it as methane, with a much greater impact on global warming.”

Thus argue the scientists Philip Fearnside and Salvador Pueyo, whose latest article just appeared in the journal Nature Climate ChangeIn their commentary, Fearnside and Pueyo dispel the myth that dams are clean by illustrating the different pathways of methane release from both upstream and downstream of a dam – pathways that current methods of reservoir emissions measurement mostly ignore. (Methane is 25 times more potent a greenhouse gas than carbon dioxide over 100 years, and 72 times more potent over 20 years.)

In addition, they focus on Brazil, where the largest reservoir emitters reside, and where errors in calculations by energy companies have underestimated reservoir emissions by as much as 345 percent. The authors single out Electrobrás, one of the leading Brazilian energy giants in the Amazonia dam-building spree. “Various mathematical errors have resulted in Brazil’s electrical authorities estimating the magnitude of emissions from reservoir surfaces at a level of only one-fourth what it should be,” write Fearnside and Pueyo.

Setting aside such sloppy math, researchers have shown for years that tropical dams are far less green than most people think (not to mention the loss of river biodiversity, forests, farms and fields). Dams in the tropics have two principle greenhouse gas emissions sources:

  1. One-time emissions of “fixed sources” of carbon, which are released from soil carbon stocks and dying vegetation when the reservoir is flooded; and
  2. Methane formed when “renewable sources” of carbon originating from various inputs of organic matter end up in reservoirs and then decay at the bottom. Hence the “methane factory.”

So why are these hydroelectric dams in the Amazon being promoted as clean energy, despite the overwhelming evidence? According to the authors, the Brazilian government is not only promoting them for their supposed benefits in mitigating global warming, but also to take advantage of mitigation funds such as the Kyoto Protocol’s Clean Development Mechanism (check out the Global CDM Hydro Hall of Shame for the worst of these projects). Unfortunately, these tropical dams are expected to have cumulative emissions greater than those of fossil-fuel plants for several decades to come, “making them indefensible on the basis of global warming mitigation,” says Fearnside and Pueyo.

Ahead of the Rio+20 conference, which is in less than a month, dispelling this myth is more critical than ever. This is especially true for Brazil, which has the dirtiest dams and plans to add 30 more to the Amazon within its borders by 2020. Among them is the controversial Belo Monte Dam, which will flood tens of thousands of hectares and displace more than 20,000 people, including indigenous communities. Electrobrás also plans to build 18 more dams in Peru and other Amazonian countries. All told, this is equivalent to building one dam every four months in Amazonia.

Photo courtesy of the Bruno Manser Fund

Bakun Dam in Malaysia

Of course Brazil is not the only country with tropical dams. Countries in the Mekong region are building dozens of dams – many of which are also seeking to benefit from limited mitigation funding mechanisms like the CDM. Sarawak in Malaysian Borneo aims to build a series of tropical dams on rainforest lands that support thousands of indigenous peoples. A dam can’t go up without trees being felled or flooded, so displacing forests with dams means we lose these critical carbon regulators and do irreparable damage to our planet’s lungs.

And of course, the picture is not complete without seeing the human story behind many of these dams. Mongabay puts it well when it states:

“It is possible to reduce the climate impact of a tropical dam by minimizing the size of its reservoir and capturing methane emissions. Yet neither of these fixes address [the] social conflict that often arises from forced displacement in dam catchments.”

Clearly, it’s time to do away with the belief that tropical dams produce clean energy. Not doing so would lead to countries under-reporting their emissions and allowing global temperatures to continue to increase. It would also allow decision-makers to continue greenwashing hydropower while overlooking the hidden costs of hydroelectric dams on rivers and riverine communities.

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About the Author

I am the China Program Coordinator at International Rivers, an environmental and human rights NGO that seeks to protect rivers and defend the rights of communities that depend on them. I coordinate our activities around capacity-building of Chinese grassroots activists, manage our Intern and Volunteer Program, provide advocacy and analytical support to groups fighting destructive carbon-financed dams, and support our education and outreach efforts. On CleanTechnica, my blog explores dam-building in China and compliance carbon offsets.

  • Bob_Wallace

    I’d suggest some caution here prior to rushing into bloomer bunching.

    The first study that made this claim studied a uniquely shallow lake formed by a dam. It covered a very high amount of land and flooded far more vegetation than dams normally flood. Looking at the top picture that’s a very shallow lake. Of course something like that is going to give off more methane than a much deeper lake.

    Clearly, it’s not time to do away with the belief that tropical dams produce clean energy. It’s time to make a distinction between shallow lakes that flood a lot of vegetation and deeper lakes.

    Second, there’s only so much methane that is going to be given off. There’s only so much vegetation that gets covered. After a few years that process is finished but the dam may well keep producing power for a hundred years or more. So how about we make sure we’re making the correct comparison? Divide the methane emission by the useful lifetime of the dam. Compare that to the output of CO2 that is avoided.

    An approach that might make more sense? Make sure the lake has minimal surface area and is deep. Perhaps even remove a lot of the vegetation prior to flooding and let it compost.

    • katyyan

      Thanks Bob_Wallace for drawing this important distinction. Very true that a variety of factors will influence how much methane is emitted, such as reservoir depth, surface size, biological inputs, and age. However, the picture is often murkier than just a clean comparison between shallow vs deep, old vs new, etc etc. Some reservoirs in Laos have been show to continue emitting ten years after they were filled – and vegetation clearing prior to flooding had no impact (

      The more important take-away from this research for me at least is that we need to stop stating point blank that all dams are clean. We need more global estimates and direct in situ measurements, and until we do, we cannot keep pretending that these hydropower projects are carbon neutral – and rewarding them as such with limited mitigation funding and finance mechanisms when there are other more deserving clean technology projects.

      Another important take-away is that focusing on just the carbon footprint of a dam ignores the other social and environmental costs that often attend such large high-impact projects. There’s an environmental and social cost to most every renewable energy project. However, large reservoir hydropower projects have been shown to proportionally exhibit a greater impact in terms of people affected (40-80 million have been displaced, 472+ million people negatively affected downstream, human rights abuses associated with land grabs), freshwater habitats harmed, key fisheries and agricultural deltas threatened, etc.

      The approach that makes most sense to me is to do a full carbon accounting of tropical dams including the carbon sinks lost through deforestation, and a full cost-benefit analysis that includes these “externalities.” We may need more research and data in order to achieve this, but the time to have this big picture discussion is now.

      • Bob_Wallace

        I really don’t want to spend $42 to read the study you reference. But you must have a copy since you wrote an article about it. So, let me ask you a couple of things…

        1) Some dams continuing emitting after ten years. At what sorts of rates? Is the rate holding steady over those ten years or declining as vegetative matter slowly decomposes?

        “Significant” can simply mean measurable above baseline levels.

        2) Removing vegetation had no impact? Doensn’t that seem a bit weird? Lower the input and the output stays the same?

        3) What’s the lifetime GHG footprint vs. the first year or first ten years?

        I totally agree that we need to thorough assessments rather than acting on dogma. But dogma cuts both directions.

        And sometimes the only choice we are presented with is evil vs. not-quite-as-evil.

        (I’m hoping we see large scale battery storage become affordable soon so that we can forget about dams.)

        • katyyan

          Haha, yes, better battery tech would help, but it reminds me a bit of the movie The Graduate where the uncle(?) says “One word: Plastics.”

          There won’t be a silver bullet, all energy sources will have their impacts, and we won’t be forgetting about dams anytime soon – partly because there’s a lot of political and industry interest in it but also because some dams do play an important role in a country’s overall energy portfolio.

          I’d be happy to send you the study (perhaps via Zachary?). The reservoirs in Laos were sampled in 2009/2010 in Jan, April and Oct (there’s definitely a seasonal range). At Nam Leuk, which was impounded in 1999 after forest clearing, the reservoir continues to be a carbon source comparable to other tropical reservoirs ten years later because the high initial carbon stock (I assume they mean in the soil) and continuous carbon inputs. I don’t think they were able to assess the reservoir continuously over the entire decade (though such research would definitely help).

          Finally, when we are presented with two options, it’s always good to ask who’s presenting those options, and what other options we’ve overlooked. Here’s a start:

  • derekbolton

    If you were to underestimate emissions by more than 100% you’d be estimating a negative value. Perhaps the 345% figure means emissions are 4.45 times the estimate. That would be an underestimation by 77.5%.

    • katyyan

      Thanks derekbolton, you’re right. It should have said Electrobrás’ estimates should be 345 percent higher. Great catch of an erroneous mathematical statement about a mathematical error!

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