Carbon footprint of hydro power

//Carbon footprint of hydro power

I took a deep breath after a brisk climb up the soft hill near the border to Sweden. The sun peaked out under the clouds and the light was beautifully reflected by the water of Nesjøen, while reindeer was munching away on delicious blueberries a little further down the ridge. The reservoir below contrasted to the barren land on the other side, making it more tranquil and interesting at the same time. No wonder that hydropower has a good reputation as an environmentally benign source of electricity, renewable and carbon-free. But is it really? The answer turns out to be: it depends.

One of the more remarkable environmental impacts of hydropower is the generation of the powerful greenhouse gas methane (CH4) from the rotting of organic material; plant matter inundated when the dam was filled as in the horrible case of the Balbina power station in the Brazilian Amazon (Nature, Wikipedia), organic matter transported by tributaries, and algae and phytoplankton growing in the reservoir itself. The methane is generated when the water is depleted of oxygen, as it often happens in lower layers of still water. The amounts of methane and CO2 generated from the degradation of plant matter can be substantial; in Balbina, the amount of methane released corresponds to 2 kg of CO2 equivalents per kWh of electricity generated – twice the global warming impact as that of a coal fired power station. In addition, 8 kg of CO2 is released. The huge area of inundated dead trees – 1770 km2 for a mere 250 MW in capacity – indicates that Balbina may be an extreme, an outlier.

How serious is the issue on a global scale? The Intergovernmental Panel on Climate Change (IPCC), in its special report on renewable energy published in 2011, had investigated the issue, but not offered a conclusion. Methane emissions were presented as they were measured, in grams of methane per square meter of reservoir surface. This provides an insufficient basis for comparison with other technologies. I have hence matched published measurements to the power generation and a number of potential explanatory variables such as reservoir area, latitude, and natural biological productivity of an area and analyzed the data. The findings of my work were recently published in Environmental Science & Technology. Here are some highlights:

  1. For the global average hydro power station, methane emissions correspond to 70 gCO2 equivalent per kWh, which is more than the 2-40 gCO2/kWh from the construction of the power station. This is a lot less than the impact of fossil fuel power stations, but a lot more than the impact of wind power.
  2. The emissions rate ranges by many orders of magnitude, in a distribution we call log-normal. Such a distribution has a peak near zero and a very long tail. Most hydropower stations are really benign, while the majority of emissions is created by a few plants.
  3. The best predictor for the emissions rate is the reservoir area per unit of electricity output. If the reservoir measures below 0.1 square meter per kWh/year of electricity generation, you are on the save side.
  4. Apart from the energy density, the age of the power station and the biological productivity of the area influence the emissions. Emissions go down with time at least for the first 10 years, as the biomass inundated by the reservoir decays. Naturally, one would expect that the amount of biomass and soil organic carbon originally present at the site are also important, but I had no data on that matter. However, the natural primary productivity of the area, the amount of biomass growth per land area, also plays a role, with dams on more productive land giving rise to higher emissions.

Today, a lot of hydropower plants in developing countries receive support through the so called Clean Development Mechanism (CDM), a mechanism created through the Kyoto protocol to allow rich countries to off-set their emissions. Wisely, the CDMs are issued only to hydropower plants with a high power density. My research indicates that the threshold set for the funding is able to avoid some but not all highly emitting projects.

The Vessingfoss power station at Nesjøen generates 44 GWh per year, the energy density is 0.75 m2 per kWh/y. I am not sure I could recommend the creation of this beautiful lake today. It is borderline.


By | 2017-11-08T21:16:07+00:00 September 17th, 2013|New Research|2 Comments

About the Author:

Edgar Hertwich
I am a professor at the Yale School of Forestry and Environmental Studies and currently serve as president of the International Society for Industrial Ecology. I grew up in Braunau, Austria, studied physics at Princeton and Energy & Resources at the University of California, Berkeley. From 2003-2015, I directed the Industrial Ecology Programme of the Norwegian University of Science and Technology. My research interests cover life cycle assessment, sustainable consumption and production, trade and environment, risk analysis, and climate mitigation. I am interested in understanding how activities in our society require resources and produce environmental pressures. I would like to better understand the dynamics in our development that affect these driving forces and their resulting environmental pressures, and alternative courses of action that can reduce these pressures. What is the connection between human activities on the one hand and emissions and resource use on the other hand? What are the implications of our current development path? What do we need to change, both in terms of individual actions and policy frameworks, to achieve a more sustainable development.

Publications: See full list here (in Google Scholar)


  1. Kazimierz March 21, 2014 at 10:23 am - Reply

    The problem of dirty “clean hydropower” is known for years. Please, you can see what discovered the World Commission on Dams: “Rising a stink. Rotting vegetation in hydroelectric dams stokes global warming” – THIS WEEK. SCIENCE AND TECHOLOGY. 3 June 2000, No 2241. London.
    One excerpt from that summary: “many hydroelectric power schemes release more greenhouse gases into the atmosphere than large coal-fired stations”.

  2. Knut.alfredsen March 30, 2014 at 5:26 pm - Reply

    The calculation for Nesjøen can not be based only on Vessingfoss. Nesjøen is the main reservoir for the major power plants in the Nea cascade, and all of them must be included for a realistic number. Then comes the question if a single metric is universally applicable, and issues related to attribution of gross or net emissions to the hydropower development. How would your computation go if the reservoir is developed on an existing lake with low innundation?

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