Hydropower can emit more carbon than coal plants

Hydroelectricity is often posed as a carbon-free energy source. But get this. Some hydroelectric dams – particularly in the tropics – are even worse than fossil fuel power plants.

A study published in the Journal of Geophysical Research examined greenhouse gas emissions coming from a large hydroelectic plant in Brazil: the Balbina Dam along the Uatuṃ River in the central Amazon basin. This dam flooded out more than 900 square miles of lush, tropical rainforest when it was built in 1987 Рthe first knock to its green credentials.

Photo: EletroNorte

On the whole, dams throughout the world contribute about 4 percent of anthropogenic sources of CO2. But according to the study, led by the National Institute of Amazonian Research in Brazil, very few tropical reservoirs were included in that assessment, making dams a potentially worse source of power than has been previously estimated.

What makes Balbino bad is not only that tropical rainforest was destroyed to build it. But also this hydro-dam is adding CO2 and methane to the atmosphere on a daily basis. The research team measured gas emissions from the reservoir and at discharge points near the turbines, the turbine outflow, and downstream in the river below the dam.

They found that the total emissions from this one reservoir is emitting 2.9 tons of carbon per megawatt hour of electricity, a multitude higher than than that of a coal‐fired thermoelectric power plant (0.3 tons).

Where is it all coming from? Dams trap organic matter when they are first flooded and over time as vegetation gets flushed in or grows in the system. In tropical areas, such as Brazil, the matter tends to decay quickly in an underwater anaerobic environment to form methane and CO2. Carbon emissions come out when water is discharged under pressure in a process known in the hydroelectric industry as “degassing.”

The carbon from the vegetation would have eventually gone into the atmosphere under non-dam conditions. But there is greater damage from this anaeronic decay than if the forests had decayed naturally.

The results are especially troubling in that dams are a growing source of energy for the Amazon region.