A new analysis of 85 hydroelectric reservoirs distributed around the world suggests that these systems emit about 48 million metric tons of carbon annually. That figure is much lower than earlier estimates of 64 million metric tons that were based on studies relying on more limited data and which cautioned that reservoirs of all types could be a major source of greenhouse gas (GHG) emissions.

Damming rivers to make reservoirs for hydropower creates flooding that emulates lake-like conditions. Decomposing vegetation and soil organic matter in an anaerobic environment within the reservoirs—particularly when they are being constructed—have been thought to cause major emissions of heat-trapping carbon dioxide and methane. Methane is 20 times more effective in trapping heat in the atmosphere than carbon dioxide (Figure 6).

6. Hydropower emissions. Hydropower reservoirs like the UHE FURNAS one owned by Brazilian utility FURNAS may not emit as much greenhouse gas as had been previously suggested, a new analysis shows. Located in the middle of the Rio Grande, the UHE FURNAS reservoir is one of the largest in Brazil. It impounds the Grande River and has a capacity of 22,590,000,000 cubic meters (18,314,011 acre feet) and a surface area of 1,473 square kilometers (569 square miles). Courtesy: FURNAS

Studies like those conducted by Swiss scientists at Lake Wohlen near Bern, Switzerland, last year suggested that substantial amounts of methane are released not only from large tropical reservoirs but also from run-of-river reservoirs in Switzerland, especially in the summer, when water temperatures are higher.

But the analysis published in July in the journal Nature Geoscience found that hydroelectric reservoirs emit less than 16% of total carbon dioxide and methane emissions from all types of human-made reservoirs combined.

The study says that emissions decline as reservoirs age, with cold-water systems stabilizing more than warm-water systems. It also suggests, however, that impacts are not equal across all landscapes: The amount of GHGs generated by hydroelectric reservoirs depends on where they are built, and the analysis indicates that emissions are correlated with latitude and the amount of biomass in the watershed. Reservoirs in tropical locations, such as the Amazon, emit more methane and carbon throughout their lifecycles.

—Sonal Patel is POWER’s senior writer.