Reservoirs may add to global warming

December 15, 2006|By Patrick McCully

Except for a handful of fossil-fuel-funded lobbyists who deny the reality of global warming, we all agree that we urgently need to "green" our energy sources. Investors in forms of electricity generation with low greenhouse-gas emissions stand to make a lot of money in the coming green economy. The big-hydro industry - under fire for harm to river ecology and the eviction of communities in the way of its reservoirs - has seized the opportunity to reposition itself as climate-friendly.

The problem is, big hydro is nowhere near as climate-friendly as the industry claims.

Although few people are aware of it, the reservoirs behind the world's dams are likely a major source of global-warming pollution. In the case of big reservoirs in the tropics - where most new dams are proposed - hydropower can emit more greenhouse gases per kilowatt-hour than fossil fuels, including the dirtiest coal plants.

Eminent climate-change scientist Philip Fearnside estimates that hydro projects in the Brazilian Amazon emit at least twice as much as coal plants. The worst example studied, Balbina dam, had a climate effect in 1990 equal to an astonishing 54 natural gas plants generating the same amount of power.

How is this possible? When a big dam is built, its reservoir floods vegetation and soils that contain vast amounts of carbon. This organic matter rots underwater, creating carbon dioxide, methane and, in at least some cases, the extremely potent global warming gas nitrous oxide. While emissions are particularly high in the first few years after a reservoir is created, they can remain significant for many decades. This is because the river that feeds the reservoir, and the plants and plankton that grow in it, will continue to provide more organic matter to fuel greenhouse gas production.

Some of the emissions bubble up from the reservoir surface. The rest occur at the dam: When methane-rich water jets out from turbines and spillways, it releases most of its methane, just like the fizz from a newly opened bottle of soda. Although all the scientists working in the field agree that emissions are released from reservoir surfaces, there is a heated dispute between industry-backed and independent researchers on the amount of gases released at dams. Accounting for these "fizz" emissions greatly increases estimates of the global warming impact of hydropower. (Research published in November by a team of French scientists indicates that conventional estimates of "fizz" emissions understate the problem.)

Although reservoirs in all climate zones emit greenhouse gases, it is only in the tropics that these emissions are likely to often be worse than fossil-fuel pollution. No comprehensive studies have been done on this issue in the United States, but it is likely that the many thousands of U.S. reservoirs cumulatively emit significant greenhouse gases.

Given the amounts of money at stake in carbon-trading schemes and other measures to address global warming, it is not surprising that the hydropower industry is alarmed that it would be considered another global warming culprit. Canadian and Brazilian hydro interests dominate funding for reservoir emission science and have tried hard to control the interpretation of the results. In Canada, industry giant Hydro-Quebec has cut funding to scientists whose work was leading to conclusions the utility considered inconvenient. Hydro-Quebec also tried, unsuccessfully, to pressure the scientific journal Lakes & Reservoirs: Research and Management into not publishing an article by these scientists, according to one of the authors.

The industry-backed scientists accuse Mr. Fearnside (who has worked on projects with the International Rivers Network), a rigorously independent researcher, of being seduced by the "lures" of the fossil fuel and nuclear lobbies.

Mr. Fearnside's findings, however, were supported in a recent editorial in the respected journal Climatic Change written by Danny Cullenward and David Victor from Stanford University. The authors criticized the hydro industry's control of the reservoir emissions research agenda and called for an independent analysis of the data and their interpretation by the United Nations' Intergovernmental Panel on Climate Change.

Officials from the panel agreed last month at the latest round of U.N. climate negotiations held in Nairobi, Kenya, that they should look at the reservoir emissions issue after finishing work on their fourth major assessment of the state of climate science, which is being readied for release next year.

Given the high stakes - the billions that will be directed to reducing climate change and the importance of these investments being effective - it is vital that decisions on climate policy are not made based on evidence produced by self-interested industry lobby groups. An independent review of reservoir emission science is essential.

Patrick McCully is executive director of the International Rivers Network. His e-mail is

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