4% of Global Warming Due to Dams, Says New Research

Wednesday, May 9, 2007

Large dams may be one of the single most important contributors to global warming, releasing 104 million metric tonnes of methane each year. This estimate was recently published in a peer-reviewed journal by Ivan Lima and colleagues from Brazil's National Institute for Space Research (INPE).

"There is now more than enough evidence to show that large dams are a major source of climate-changing pollution," says Patrick McCully, Executive Director of International Rivers. "Climate policy makers must address this issue."

Lima’s calculations imply that the world’s 52,000 large dams contribute more than 4% 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.

Methane is a more potent heat-trapping gas than carbon dioxide, although it does not last as long in the atmosphere. One year’s large dam methane emissions, as estimated by Lima, have a global warming impact over 20 years equivalent to that of 7.5 billion tonnes of carbon dioxide – higher than annual carbon dioxide emissions from fossil fuel burning in the US.

Lima and his co-authors propose capturing methane in reservoirs and using it to fuel power plants. Lima says, "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."

"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," says Patrick McCully. "Partly because of the influence of the hydro industry and its government backers, climate policy-makers have largely overlooked the importance of dam-generated methane. The IPCC urgently needs to address this issue."

Methane is produced by the rotting of organic material in reservoirs. 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.

This is only the second estimate published in the scientific literature of global greenhouse gas emissions from dams. The previous estimate, published in 2000, which included only emissions from reservoir surfaces, estimated global releases at 70 million tonnes of methane and a billion tonnes of carbon dioxide.

Lima’s calculations take account of emissions from turbines and spillways and the rivers immediately downstream of dams, in addition to reservoir surfaces. Lima’s paper does not address dam emissions of carbon dioxide or another greenhouse gas, nitrous oxide.

Lima has also produced the first published estimates of methane emissions from dams at the national level in Brazil, China and India. These estimates show dams in Brazil and India are responsible for a fifth of these countries’ total global warming impact. Dams in China are estimated to produce 1% of the country’s climate pollution, although for methodological reasons this is likely an underestimate.

    Media contacts: 

    Patrick McCully, Executive Director, International Rivers, Berkeley, California: +1 510 213 1441 (mobile) +1 510 848 1155 (office), patrick@internationalrivers.org

    Ivan Lima, National Institute for Space Research, Brazil: + 55 67 9932-1897 (office) ivan@dsr.inpe.br

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