It's never simple

Just when you thought it was safe to bet on hydropower for clean energy, there’s new evidence that the reservoirs created behind dams in tropical countries may emit as much – or more – greenhouse gasses than their fossil-fuel counterparts.

In a report in the September 30th issue of Nature, Jim Giles reports on a growing body of research that shows that both carbon dioxide and methane are emitted in large quantities from the water trapped behind, and flowing from, dams in tropical countries. Studies at one site in particular, the Balbina Dam in Brazil, have concluded that the environment would in fact have been better off if they’d built a conventional fossil fuel plant instead.

The problem is organic matter in the reservoir behind the dam: plants submerged as the reservoir is formed and plant matter that gets washed in and trapped later. As the organic matter decays – rapidly in those warm tropical waters – carbon dioxide and methane are produced. There is still disagreement on a formula for calculating how much of these gases are released, and by what mechanism: more research is definitely needed, and urgently.

But the evidence we do have is enough that future construction of hydroelectric facilities in tropical countries needs to be approached with caution. Giles notes that the Clean Development Mechanism, which allows developed nations to fund clean-energy projects in developing countries as a way to earn carbon credits under the Kyoto Protocol, may need to be revised as well.

Even more scary, though, is that greenhouse emissions from tropical dams are not included in current climate models, and projections using existing evidence mean we are pumping around 20% more greenhouse gasses into the atmosphere than we thought, bringing the day of reckoning that much closer.