New Scientist reports that the IPCC is considering changes to the calculations of "climate budgets" to include greenhouse gas emissions from hydroelectric dams. It turns out that hydro power can produce significant amounts of both CO2 and methane, sometimes even more than fossil fuel-using generators.
This is because large amounts of carbon tied up in trees and other plants are released when the reservoir is initially flooded and the plants rot. Then after this first pulse of decay, plant matter settling on the reservoir's bottom decomposes without oxygen, resulting in a build-up of dissolved methane. This is released into the atmosphere when water passes through the dam's turbines.
The IPCC changes would increase greenhouse inventories in countries with lots of hydro power by up to 7%. New Scientist has a map of which countries would be most affected by the proposed changes. Unsurprisingly, they are generally the nations with the largest land area, largest populations, or both.
There is ongoing debate over whether to retain dams or remove them. One of the strongest arguments for retaining hydroelectric dams has been how "clean" the power generation is. If the research into dam greenhouse emissions is correct, that rationale could be seriously undercut.
(Via Warren Ellis)