How does hydropower affect global carbon emissions?

Hydropower isn't 100% environmentally friendly source of energy, in fact such energy source does not exist. Can hydropower contribute to increase in greenhouse gas emissions that cause climate change and global warming? Yes, but not as much as fossil fuels such as coal do.

In some rare cases, the effects on greenhouse gas emissions locally can even offset the benefits of hydro energy production. Steven Bouillon, a carbon cycles researcher at the University of Leuven in Belgium argues that the main factor contributing to the total level of emissions connected with the hydropower depends on the design of the reservoir.

The emissions connected with hydropower reservoirs originate from the grass, vegetation and trees submerged underwater which begin to slowly decompose and in the process releasing the carbon dioxide they had been storing through photosynthesis for hundreds of years.

There are several possible ways of emissions in this case; carbon dioxide can bubble to the surface of the reservoir and escape, it can be released as the water makes its way through the turbines; and it can be also released farther downstream.

Bouillon says that hydropower developers should avoid building shallow reservoirs with a wide surface area as they can emit more since they flooded more carbon-rich land, which means that gas can easily escape out of the shallow water. On the other hand, deep dams with a small surface area connected with significantly lower emissions.

There is also the methane issue. Methane is greenhouse gas that  is much more potent than carbon dioxide (CO2). The major fears originate from the fact that the stagnant reservoir water is able to deplete oxygen out of water at the bottom of the lake, creating oxygen-free environment that can turn some of the decomposing carbon into methane instead of CO2, thus creating even bigger level of environmental damage.

It has been also said that the large number of hydroelectric power plants draw water from the bottom layers of reservoirs thus creating the conditions for a portion of the methane gas to be emitted downstream or as it passes through hydroelectric turbines.

The opposite opinions claim that these reports about significant emissions are exaggerated, and that this is basically a "natural system going on, unrelated anthropogenic activities going on", as according to words by Richard Taylor, executive director of the International Hydropower Association.

The final truth still remains unknown. There is definitely a need for new comprehensive studies because there are many factors contributing to the natural carbon cycle.

This, however, does not mean that the developers of hydropower projects should wait for new studies before taking the necessary steps to reduce the total amount of emissions. Taylor suggested the best practice would be to avoid building dams near major carbon sinks and the installation of an off-take system that draws water from the upper levels of the reservoir, not from the bottom where there are  methane-rich lower levels.