Maryknoll Office for Global Concerns

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Climate change: Large-scale dams problematic

With increasing pressure to lower greenhouse gas (GHG) emissions, many countries are planning to increase their reliance on hydropower, meaning more dams. The conservation group International Rivers reports that at least 3,700 hydropower projects are under construction or in the planning stage. The following article, published in the November-December 2015 issue of NewsNotes, examines the reasons why large-scale hydroelectric dams are a false solution to the climate crisis. 

Some energy experts continue to favor the creation of hydropower by large-scale hydroelectric dams.  They see large-scale dams as a clean source of energy since they do not burn fossil fuels to produce electricity. Indeed, many countries plan to increase use of hydropower in order to reduce their greenhouse gas (GHG) emissions. Unfortunately, more and more studies show that large-scale hydropower is far from being a climate-friendly source of energy.

The construction of large-scale dams for hydroelectric energy has long been criticized for creating lakes that displace communities, blocking fish migration and trapping sediments that would normally fertilize downstream areas, and changing the chemical composition and temperature of rivers. 

Few researchers have calculated the amounts of GHGs released by large-scale dams. Those who have studied the issue have found that large-scale dams actually release large amounts of GHGs. The Brazilian National Institute of Amazon Research estimated that the Tucurui dam in northern Brazil emitted an amount of GHGs “substantially greater than the fossil fuel emission of Brazil’s biggest city, São Paulo.” 

The dirtiest dam ever studied is the Balbina dam, also in Brazil, that flooded a large area of forest while producing little energy. It was estimated to release 10 times more GHGs than an equivalent coal-fired power plant. Other dams studied by the institute were two to three times dirtier. Even the Wohlensee dam in Switzerland, a small dam in the temperate region where dams are said to create fewer GHGs, creates 119 grams of CO2 per kilowatt-hour – cleaner than coal but 10 times dirtier than wind energy. 

Large-scale dams create GHGs in a variety of ways:

  • The organic material (trees, grasses, soil) of the flooded land decomposes, creating methane, nitrous oxide and carbon dioxide. Methane is a greenhouse gas estimated to be 20-25 times more potent than carbon dioxide. Nitrous oxide is almost 300 times worse. Dams built where vegetation is denser (the tropics) are the worst offenders. 
  • To build a dam often involves deforestation, which emits additional carbon into the atmosphere.
  • Dams are made with large amounts of cement and steel, which require massive amounts of energy to be made, transported and installed. 
  • Dams divert water from downstream areas drying out important wetlands that provide a variety of important ecological services. 
  • Many dams have regular drawdowns that can change water levels by many feet. Two Washington State University studies saw 20- and 36-fold increases in methane emissions during drawdowns in two dams.
  • A variety of dam-related activities also create GHGs, such as land clearing for resettlement sites, transmission lines, and access roads, and the expansion of irrigated agriculture (a major source of methane emissions).

This is in contrast with small-scale micro hydro power dams. Micro dams are seen as not quite so destructive nor contributing to GHG emissions as the large-scale dams. Indeed, they are often controlled by the local community as opposed to large-scale dams, which generate power for outside industries.

Most of the emissions are in the early years during and after construction, from dying vegetation, the cement and steel, and the loss of downstream wetlands. Philip Fearnside, a U.S. American professor at Brazil’s National Institute for Amazon Research (INPA) who studies the issue, warns “exactly as the world needs to reduce emissions quickly before the effects of global warming become even more serious, the spike [in emissions from dams] is happening right in that critical period.”

The cumulative effect of dams is significant. Brazil’s National Institute for Space Research estimated that the world’s largest dams (at least 15 meters tall) emit 104 million metric tons of methane each year, representing at least four percent of the warming impact of human activity and 23 percent of all methane emissions caused by humans. 

If we are to be serious about addressing climate change, we cannot continue to treat hydro-energy as clean. Policymakers are currently ignoring this significant source of greenhouse gases.