Brazilian Government Severely Underestimates Dam Emissions

Back to Resources
First published on
Photo: Saoud, Flickr

Mathematical errors in the classroom or on tests usually don’t carry a whole lot of consequences besides a blot on your grade and maybe a deflated sense of self-esteem.

But what happens when the mathematical error occurs in a major government energy agency? The ramifications could be huge, especially when a country is dead set on reducing its carbon footprint through building more hydropower projects – and it’s precisely the greenhouse gas emissions from these projects that are being miscalculated.

A recent study by two renowned reservoir emissions experts revealed that official estimates by Brazil’s energy agency, Eletrobrás, for the amount of methane from the surface of dam reservoirs was way underestimated. When the scientists redid the calculations correctly, the results showed that reservoir emissions from the surface of Brazil’s dams were actually 345% higher than the official government estimate. Extrapolate to all of Brazil’s reservoirs, and the difference between Eletrobrás’s calculation and the correct calculation is almost as much as the annual fossil fuel emissions from Brazil’s largest city, São Paulo.

The finer points of math

The study, written by Salvador Pueyo and Philip Fearnside and titled “Emissions of greenhouse gases from the reservoirs of hydroelectric dams: Implications of a power law,” was recently published in the Oecologia Australis (in Portuguese). Fearnside in particular has been a leading figure in the research of methane emissions from dam reservoirs (methane or CH4 is 25 times more potent a greenhouse gas than CO2; see figure on the right).

Through his and others research, it is now an established fact that some of the methane produced by the decomposition of organic matter in reservoirs is released into the air by bubbling and diffusion (i.e. degassing) at the surface and from the water that passes through turbines and spillways. Some hydropower plants in the tropics (such as those in Brazil) contribute more to climate change than fossil fuel plants generating the same amounts of electricity (see figure on left). Even reservoirs in temperate zones can be significant sources of methane.

Pueyo and Fearnside discovered that, due to several mathematical errors, Eletrobrás has been incorrectly calculating the total methane emissions from Brazil’s 223 reservoirs. When translated into tons of CO2 carbon equivalent, the difference between the correct and incorrect values (5.2 million tons) is almost as much as the fossil fuels emissions from São Paulo. And if you include emissions from degassing at and below the dam’s turbines and spillways, as well as current estimates of the global warming potential of methane (which can be as much as 34 times greater than CO2), the number shoots up to 9 million tons, more than what the entire country of Honduras emits each year.

All this to say, politicians and policy makers need to start thinking twice – and fast – before claiming hydropower plants as clean carbon-free sources of energy. Whether it’s building massively unpopular and destructive projects like the Belo Monte Dam in the Amazon or purchasing carbon credits for carbon-emitting hydropower projects, this latest discovery shows that we need to fundamentally change the way we view hydropower and how it fits into the overall climate change debate. Dams are dirty in more ways than one, and the math is here to prove it.

Watch this slideshow to learn more about why large hydropower is neither clean nor green:

More information