In our quest to overcome dependency on greenhouse-gas-emitting fossil fuels, we have been searching for creative ways to provide energy for growing populations. Along with solar, wind, geothermal, and nuclear power, hydroelectric dams show promise in curtailing our need for continuous use of energy sources which emit carbon into our atmosphere. Use of these alternative fuel sources is in keeping with the goals of the Paris Agreement whose signatories pledged to maintain temperature change well below 2 degrees Celsius above the global pre-industrial average.
Close to 500 dam projects are either operational, under construction, or planned for the Amazon region. These dams are often portrayed as green alternatives, though over two decades of research indicates that tropical dams, in particular, are big emitters of greenhouse gasses. Phillip Fearnside, in a 2016 paper published in Environmental Research Letters, discusses the merits of these green energy claims surrounding tropical dams in the Brazilian Amazon. Citing a study conducted by Faria et al (2015), Fearnside argues that dams may emit even more greenhouse gasses than their “dirty” fossil fuel counterparts. This is, in part, due to CO2 (carbon dioxide) emissions from trees flooded through creation of reservoirs as well as the release of soil carbon.
Tropical dams also emit the greenhouse gas methane (CH4) because of temperature differentials in the reservoirs. Cold water at the bottom of the reservoir is quickly depleted of oxygen, and as organic material (fish, algae, etc.) dies off and decomposes, methane is released into the atmosphere. Methane has a much higher impact on global warming than does its more infamous counterpart, carbon dioxide (the Intergovernmental Panel on Climate Change estimates an impact 34 times higher than that of CO2 in a 100-year time frame). Methane is also comparatively short-lived in the atmosphere, lasting only about a tenth of the time carbon dioxide lasts. This makes controlling methane emissions critical to short-term climate change mitigation as the IPCC estimates the impacts of one ton of methane in the atmosphere to be 86 times as great as one ton of CO2 in the atmosphere in a 20-year-time frame. When the greenhouse gas emissions of tropical dams are considered at the 20-year time horizon to account for the relatively-short-lived methane emissions, the impacts are much greater.
Though tropical hydroelectric dams are still being sold as green energy alternatives, Fearnside’s review of Faria’s paper shows that they are actually much worse climate polluters than fossil fuels. The good news is tropical countries have many green energy sources to choose from, including solar and wind, so this need not be a choice between the lesser of two polluters. Investing into these clean energy alternatives will be essential to meet the goals of the Paris Agreement.