How do dams emit greenhouse gases (GHGs)?
The “fuel” for the methane, carbon dioxide and nitrous oxide emitted by dams is the rotting of the vegetation and soils flooded by reservoirs, and of the organic matter (plants, plankton, algae, etc.) that flows into, and is produced in, reservoirs over their lifespan. The gases are released at the reservoir surface, at turbines and spillways, and downstream of the dam. Greenhouse gases are also produced by various other dam-related impacts including the fossil fuels and building materials used during dam construction; land clearing for resettlement sites, transmission lines and access roads; and the expansion of irrigated agriculture (an important cause of methane emissions). The trapping of sediments in reservoirs may act as a carbon sink; it may also indirectly increase the concentration of carbon dioxide in the atmosphere by reducing the amount of river-borne sediments available to fertilize oceanic plankton, which are important consumers of carbon dioxide.
