Southeast Asian Dam Still a Carbon Source Ten Years Later
The international team of researchers spent two years measuring the greenhouse gas emissions from two sub-tropical reservoirs in Laos, the Nam Ngum and the Nam Leuk reservoirs (the latter of which diverts water from the Nam Leuk River to the Nam Ngum Reservoir). What they found was that at Nam Leuk, "GHG emissions are still significant 10 years after impoundment" and that the emissions values were comparable to other tropical reservoirs. The annual carbon export (including both diffusion into the atmosphere from the reservoir and from downstream) amounted to about 2.2 ±1.0 gigagrams of carbon per year. While much less than a coal-fired power plant, this is still roughly equal to the emissions from the electricity use of over a thousand US homes, which is far from insignificant.
The other reservoir, Nam Ngum, was a carbon sink as a result of its age, which is consistent with existing data (it was impounded in 1971). Biomass removal did not reduce the amount of greenhouse gas emitted from the reservoir, as may have been previously assumed. Nam Leuk Reservoir was impounded in 1999 after partial vegetation clearance and burning, while Nam Ngum Reservoir was filled without any significant biomass removal.
So why is this study important?
(1) Increasing the data pool: Studies of reservoir emissions thus far have largely been conducted in Brazil, with a few in Panama and French Guiana, and even fewer in temperate zones. This is the first study done in Southeast Asia, which is important not only because there is a dirth of info on reservoir emission from that region, but also because Southeast Asia is undergoing a dam building boom without adequate consideration for the potential carbon footprint of new reservoirs.
(2) Tropical reservoirs can be significant carbon sources decades later: The study supports existing research that tropical reservoirs are a significant source of greenhouse gases, and that age is a key factor in determining the amount of carbon emitted. Prior to this study, it was assumed that carbon emissions from reservoirs fell away a few years after a reservoir is filled. However, Nam Leuk showed that even dams a decade old can still be a significant carbon source.
|Major recent CDM projects in Southeast Asia include:|
(3) Implications for "clean development": Southeast Asia is experiencing a rise in the number of projects entering the Clean Development Mechanism (CDM), particularly in Vietnam, but increasingly more in Cambodia and Laos (see box).
CDM projects are meant to help reduce global emissions, but if projects in the tropics are a potential carbon source the first ten years of their operation, this raises a number of important questions including how many carbon credits they are eligible to receive and whether more stringent guidelines on reservoir emissions should be included in the CDM. Currently, the CDM deals with greenhouse gas emissions by calculating the power density of dams (which is the installed capacity divided by the reservoir area) – any project with a power density over 10 W/m2 is considered to have negligible emissions. While this has done a fair job so far of excluding tropical dams with high emissions rates from the CDM, it has also led project developers to assert that their projects have zero emissions, which reinforces the myth that hydro is a clean pollution-free energy source.
Nam Leuk's emissions are not insignificant, and the research on reservoir emissions has steadily improved and increased over the years. It's time that global institutions like the IPCC and UNFCCC , which help countries monitor their emissions, started treating tropical reservoirs seriously by requiring that countries include them in their national greenhouse gas inventories.
- Major reservoir emissions studies
- Stay tuned for an NGO briefing on the latest reservoir emissions science.