Dr. Ivan Lima and his colleagues used a theoretical model, bootstrap resampling and data provided by the International Commission On Large Dams World register of dams to demonstrate that global large dams annually release about 104 million metric tons of methane to the atmosphere through reservoir surfaces, turbines and spillways.
Methane is the principal component of natural gas. The INPE scientists say engineering technologies now in existence can be implemented to avoid these emissions, and to recover the non-emitted methane for power generation.
Under the Clean Development Mechanism of the Kyoto Protocol, they write, such technologies can be recognized as promising alternatives for human adaptations to climate change, particularly in developing nations owning a considerable number of large dams.
Methane is about 21 times more powerful at warming the atmosphere than the most abundant greenhouse gas, carbon dioxide. Methane's relatively short atmospheric lifetime of 12 years, coupled with its potency as a greenhouse gas, makes methane a candidate for mitigating global warming in the short term.
Dr. Lima and his co-authors propose capturing methane in reservoirs and using it to fuel power plants in Brazil, China and India.
'If we can generate electricity from the huge amounts of methane produced by existing tropical dams we can avoid the need to build new dams with their associated human and environmental costs,' Lima said.
The immediate benefits of recovering methane from large dams for renewable energy production is the mitigation of human impacts like the construction of new large dams, avoidance of the actual methane emissions from large dams, and the use of unsustainable fossil fuels and natural gas reserves, the authors explain in their study, published by Springer Netherlands.
Large hydroelectric dams release methane into the atmosphere because trees and other plants settle to the bottom when the reservoir is first flooded. This plant material decomposes without oxygen and dissolved methane builds up. When water passes through the dam's turbines, this methane is released.
From his office in Berkeley, California, McCully said Lima's calculations imply that the world's 52,000 large dams contribute more than four percent of the total warming impact of human activities.
They also imply that dam reservoirs are the largest single source of human-caused methane emissions, contributing around a quarter of these emissions, McCully said.
The massive amounts of methane produced by hydropower reservoirs in the tropics mean that these dams can have a much higher warming impact than even the dirtiest fossil fuel plants generating similar quantities of electricity, he said.
'It is unfortunate that Lima's study has come too late to be included in the recent reports from the Intergovernmental Panel on Climate Change, IPCC,' McCully said. 'Partly because of the influence of the hydro industry and its government backers, climate policymakers have largely overlooked the importance of dam-generated methane.'
Another Brazilian scientist has also made the connection. Philip Fearnside from Brazil's National Institute for Research in the Amazon in Manaus published a study in 2002 showing that the greenhouse effect of emissions from the Curuá-Una dam in Pará, Brazil in 1990, was more than 3.5 times what would have been produced by generating the same amount of electricity from oil.
The International Hydropower Association, IHA, says considering only total greenhouse gases measured at the surface of reservoirs can be misleading; these measurements should be considered as 'gross' emissions. 'Net' emissions for which dams are responsible must consider the emissions from ecosystems before the creation of a reservoir when the land was in its natural state.