Hydroelectric plants are touted for their ability to provide so-called green energy, which aims to continue to satisfy power demands without damaging the environment. In the Amazon basin, many of these hydroelectric plants have been built along the region's vast river complex, sometimes overriding the concerns of local people. These dams don't burn fossil fuels, but underwater, their reservoirs cause flooded vegetation to decay, which releases millions of tons of the greenhouse gases carbon dioxide and methane.
Run-of-river (ROR) dams, including Belo Monte along the Xingu River, were once thought to adress this issue with their smaller channels and reduced flow. But reseachers have been investigating the release of methane and carbon dioxide at Belo Monte during its first two years of use, and they found that the greenhouse gas emissions at the site have increased three-fold compared to before the reservoirs were filled. The findings have been reported in Science Advances.
Climate researcher Dailson Bertassoli's team studied methane and carbon dioxide emissions during Belo Monte's first two years of operation and compared the results to levels that existed prior to the reservoirs being filled, finding a threefold increase in greenhouse gas emissions.
"Once you have the flooding of dry land, the organic matter that was trapped in the soil starts to degrade," Bertassoli, a professor of geology and climate change at the University of Sao Paulo told AFP. Bertassoli had observed bubbling of gases at one of the reservoirs.
"Instead of a natural river, we now have a reactor that favors the production of methane," he explained.
The reservoirs that RORs rely on are still large, noted study co-author and climate researcher Henrique Sawakuchi; the largest of these reservoirs contain dead, white trees that stick out in the vast channels of stagnant green.
Study co-author and University of Sao Paulo professor Andre Sawakuchi suggested that when hydropower plants are being considered, two things should be kept in mind. The first is the impacts to local aquatic species, and the other is the societal effects on the indigenous people that rely on the river.
The efforts of environmental and indigenous groups prevented the Belo Monte project in the 1990s, but in 2011, an ROR plant was built there anyway. Forest had to be cleared for the development; land was flooded and natural river flow was manipulated. A different team of researchers has warned that the true cost of the dam has been grossly underestimated.
Andre Sawakuchi noted that the Amazon should flow without obstruction, and advised against disrupting the natural cycles with any kind of hydropower plant. The authors noted that if ROR dams are going to be built anyway, minimal vegetation should be flooded so as not to raise greenhouse gas levels.
"This is the pulse of the river," he said. "With a hydroplant, there is no more pulse."
A 2019 study by the Environmental Defense Fund indicated that hydropower plants can cause a range of environmental impacts. Some have enough organisms that absorb carbon to make them act as carbon sinks. Others, however, emit more than they take in, and could be worse than fossil fuels when it comes to climate impacts.
"There is no utopia here," Bertassoli said. "Especially for countries that look so hard at hydropower as a sustainable 'green' answer to their energy needs."