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: http://www.internationalrivers.org/solutions

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.)

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.

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.