Last summer I wrote about a study of the greenhouse gas impact of large hydropower dams, how it's lower than often assumed but still varies widely based on where the dams are built. Now there's some more research on how when hydropower dams are built in the tropics, the emissions are much, much higher than their counterparts in temperate regions—so much so that they really can't be considered a clean energy source, a solution to climate change.
Mongabay reports on a study in Nature Climate Change, which has determined that "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. [...] The myth can no longer be sustained that tropical dams produce clean energy."
Currently Brazil plans 30 more dams in the Amazon by 2020, including the hotly protested Belo Monte project.
There are a couple of ways large hydropower dams release greenhouse gases, all of which are only exacerbated in the tropics. Briefly: When you clear forest to make the reservoir you've eliminated the carbon storage potential of that land and possibly started releasing and carbon stored in the soil. Once the reservoir is flooded, methane is formed when any plant matter that remains starts decaying. This can bubble up for years, facilitated by the turbines of the dam, which can draw it out. So, while no emissions are created directly by the electricity, only half a step removed from that considerable emissions can occur, sometimes for years. The reason this is so much greater in the tropics than in temperate areas is that the latter generally store less carbon in the forest and soil, and in some cases no land has to be cleared at all for the reservoir.
If you need to get up to speed on hydropower's eco-impact as well as the social implications of it all, plus some of the more prominent projects in the works, check out the links to the left.