Globally, dams dominate the world’s clean energy production and divert water essential for irrigation. By some estimates, there are over 3,700 dams (paywall) currently in construction that will likely alter the flow of over 20% of the world’s rivers. And the projects are expensive: estimates from the World Commission on Dams suggest that building each large dam costs anywhere from $6 billion (pdf, p. 39) to $37 billion—the price tag of the Three Gorges Dam on the Yangtze River in China.
That might not be as wise an investment into environmentally-safe growth as we thought.
A team of researchers from Canada, Holland, China, the US, and Brazil will publish a study next week in BioSciences showing that globally, the reservoirs created by dams may actually contribute almost a gigaton of carbon dioxide-equivalent emissions—about 25% more than they had previously thought. This means that we’ve almost certainly been underestimating how much greenhouse gas we’ve been shooting into the atmosphere.
The concept behind hydroelectric power is simple: Rather than combusting fossil fuels, like coal, oil, or natural gas, which put over 52 million kilotons of greenhouse gases into the atmosphere each year, dams block the water flowing in rivers or lakes and use the force of the would-be flow to turn turbines, which generate electricity. Aside from the actual construction, they shouldn’t generate extra carbon emissions.
Except for one thing: When a river is blocked, water gathers behind the dam, creating an unnatural, stagnant lake that often kills off a lot of the existing ecosystem. The excess water is pushed onto the banks, which are often covered in plant life. These plants are then smothered and die. Bacteria in the water then decompose these plants, generating carbon dioxide and methane—a greenhouse gas 86 times more potent than CO2.
These gases bubble up to the surface of the reservoir and are released into the atmosphere.
Even more of these gases are produced if the upstream river water contains extra nitrogen or phosphorus as a result of fertilizer runoff. These nutrients allow naturally-occurring algae in the river to flourish, but then quickly overwhelm the water’s oxygen resources. As the algae dies, it’s broken down like other organic matter, creating even more of these gases.
For their work, the authors looked at 267 reservoirs created by dams and scaled their results up based on over 100 previous studies to account for total global emissions. John Harrison, a biochemist at Washington State University in Vancouver and co-author of the paper, told Science magazine that the results of the paper were intended to help update greenhouse gas estimates. Knowing which rivers are more likely to create high-methane reservoirs could help us stop production there in the future. “Dams provide a lot of important services,” he said. “The work that we and others are doing helps to tell the whole story.”
Correction: An earlier version of this story misstated John Harrison’s affiliation.