When Costa Rica brags about running on 100% renewable electricity for months at a time, it's because the country relies on hydropower for a big chunk of its grid. But dams are not quite the clean energy source they seem to be. In a new study, researchers calculated that the dams around the world—used for hydropower, flood control, water supply, and other human needs—emit more greenhouse gases than the entire country of Canada.
As the world tries to figure out how to drastically cut emissions, it turns out we haven't been counting dams as a major source of those emissions. Every year, the world's million reservoirs emit roughly a gigaton of greenhouse gases; that's 1.3% of global emissions, or about the same percentage emitted by coal mining.
Right now, countries adding up all of their emissions sources don't have to include dams in their official inventories.
Researchers have known that dams were a problem for more than a decade, but the new study explains that the problem is quite a bit worse than previously thought. The biggest impact comes from methane, a gas that has 34 times the global warming impact as carbon dioxide.
When acres of land are flooded to build a reservoir, the trees and plants that end up underwater can start to break down and release methane. Fertilizers like nitrogen and phosphorus, flowing into dams from farms and waste, can make algae quickly grow—another source of methane as it breaks down.
Past studies have undercounted methane because it can't be quantified as easily as carbon dioxide or nitrous oxide, another pollutant from dams. Methane doesn't dissolve in water, and the emissions tend to happen in hard-to-measure bubbles. Many studies have also measured emissions only in certain parts of a dam or for short periods of time, missing the bigger picture.
The good news: better design might help a little. "Because reservoirs are human designed and human operated, there may be an opportunity for greenhouse gas mitigation at both the planning and the operation stages," says Bridget Deemer, research associate at Washington State University and lead author of the paper.
The study suggests that reservoirs downstream of "nutrient" inputs—nitrogen and phosphorus—produce more methane. It's possible that reducing those inputs, or locating some dams in different areas, could help.
In the meantime, as governments plan to build hundreds of new reservoirs around the world, they'll need to start adding up how that will affect the global carbon budget.
"I think that understanding the sources and controls on anthropogenic methane emissions is really critical given how much more powerful methane is than carbon dioxide over the shorter time periods relevant to climate policy," says Deemer.
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