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.