The rise of wind and solar power, coupled with the increasing social, environmental and financial costs of hydropower projects, could spell the end of an era of big dams. But even anti-dam activists say it’s too early to declare the demise of large-scale hydro.

The International Hydropower Association (IHA)—which represents dam planners, builders, and owners in more than 100 countries—touts dams as a clean technology, but that’s not quite true: Many reservoirs emit substantial amounts of methane, a potent greenhouse gas released by decomposing vegetation and other organic matter that collect in oxygen-poor reservoirs.

John Harrison.
John Harrison

A 2016 study in BioScience found that methane emissions from reservoirs constitute 1.3 percent all of global human-caused greenhouse gas emissions, and the highest-emitting reservoirs rival coal-fired power plants⁠. It is commonly assumed that methane emissions occur chiefly in shallow, tropical reservoirs, as if it’s a problem for only a small number of dam projects. But according to John Harrison, a professor at Washington State University’s School of the Environment and one of the study’s authors, “There is strong and growing evidence⁠ that temperate reservoirs can produce methane at rates comparable to those reported from tropical reservoirs.”

Even so, the Intergovernmental Panel on Climate Change, which sets standards for measuring nations’ greenhouse gas emissions, doesn’t include reservoir emissions in its calculations; the IPCC is considering changing that policy next year. Growing understanding of the factors causing reservoir-generated methane could at least guide decisions about siting dams, avoiding places certain to produce high emissions.

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