In July, UN Secretary Ban Ki-Moon highlighted the role of hydropower in boosting the use of renewable energy globally, when he visited a nonprofit institute in China that helps emerging nations develop and build hydropower plants. Many countries consider hydroelectricity a clean source of power because it doesn’t involve burning dirty fossil fuels. But that’s far from true. Hydropower is a significant source of greenhouse gas emissions: a new study shows that the world’s hydroelectric dams are responsible for as much methane emissions as Canada.

The study from Washington State University finds that methane, which is at least 34 times more potent than another greenhouse gas, carbon dioxide, makes up 80% of the emissions from water storage reservoirs created by dams. What’s more, none of these emissions are currently included in global greenhouse gas inventories. These are already revealing a worrisome future in which rising global temperatures will likely cause environmental changes such as rising sea levels and stronger hurricanes, which could uproot communities and intensify competition for food and other resources.

“I think this study shows that dams as a source of energy aren’t without their greenhouse gas costs,” says Bridget Deemer, a research ecologist at the US Geologic Survey, who led the study during her prior position as a research associate at Washington State. “Even though it’s a renewable source of energy, people should keep the greenhouse gas side of the picture in mind when making planning and policy decisions regarding dams.”

The research, which examines 100 recent studies on greenhouse gas emissions from 267 large reservoirs around the world, also calls into question the wisdom of building more hydroelectric dams as countries try to nix their dependence on coal, natural gas and oil. An estimated 3,700 new dams are proposed or under construction around the globe, the study reports. It suggests the hydropower industry will need to control its emissions.

But industry officials in the US say the study misrepresents the benefit of hydropower in the fight to rein in global warming.

“Singling out hydropower is irresponsible and inaccurate, to say the least,” says LeRoy Coleman, a spokesman for the National Hydropower Association, based in Washington DC. “Hydropower is needed to meet federal and state clean energy goals.”

Hydropower represents 6% of all energy generation in America, according to the US Energy Information Administration. For newly built power plants, hydropower’s total cost of construction and operation runs about $68 per megawat-hour, which is comparable to solar, slightly more expensive than wind, and cheaper than coal and natural gas when those sources incorporate technology to capture carbon emissions.

Hydropower is often considered a source of renewable electricity because its fuel, water, is constantly replenished by nature. By that standard, hydropower produces 52% of all renewable electricity in the US, according to the National Hydropower Association.

Only 3% of reservoirs in the US are equipped to produce hydropower, Coleman says. Others are limited to water storage and flood protection duties, and some of them are good candidates for electricity generation, he adds.

“The responsible and logical approach would be to capture the energy potential of these reservoirs and combat climate change by installing hydropower equipment,” Coleman says.

Top 10 power plants in the world/ Illustration: Ucilia Wang/The Guardian

As recently as a decade ago, little was known about emissions from water storage reservoirs. But understanding has evolved rapidly. For instance, researchers initially thought shallow reservoirs in tropical regions were the largest emitters because their stored water was more prone to heating up, which could fuel greenhouse gas production.

But analysis by Deemer and her colleagues shows that the latitude and depth of water are not leading factors. Instead, “biological activities” in a reservoir – such as decaying vegetation and nutrient runoff from watersheds upstream – are more important indicators of greenhouse gas emissions. The nutrient runoff can be from natural processes or from farming, logging and land development.

Using data from the 267 reservoirs, the authors estimate total emissions from all reservoirs worldwide and conclude that those water storage facilities account for 1.3% of all manmade greenhouse gas emissions.

Another new study published by the journal PLOS One reaches similar conclusions, although it finds carbon dioxide to be a bigger share of emissions than methane. After examining data from more than 1,400 dams worldwide, it identifies the rate of soil erosion into a reservoir as a leading predictor of carbon dioxide emissions.

Led by Laura Scherer, a research associate at the Swiss university ETH Zurich, that study concludes the “carbon footprint of hydropower is far higher than previously assumed”.

“These are massive emissions, and we can’t ignore that,” says Kate Horner, executive director of the environmental group International Rivers, based in Berkeley, California. “There are a massive number of dams that are currently proposed to be built. It would be a grave mistake to continue to finance those with the impression that they were part of the solution to the climate crisis.”

Coleman contends that it’s premature to use data from these studies to criticize hydropower because the science remains uncertain.

It is challenging to measure reservoir emissions because so many factors can play a role, Deemer acknowledges. The scientific community also doesn’t follow a standard practice for counting emissions, even though the International Hydropower Association and the United Nations Educational, Scientific and Cultural Organisation (Unesco) published a set of guidelines in 2010.

Emissions vary according to a wide variety of factors, including water and air temperatures, season and time of day, vegetation and soil types in both the reservoir and the upstream watershed, and watershed management practices.

Hydropower dams constantly manipulate water levels to make electricity, a process that also affects the amount of emissions that make their way into the atmosphere from the reservoir. As water levels decline, the hydrostatic pressure on submerged soils also lowers and allows gas bubbles to escape. Sometimes the methane in these bubbles is absorbed by the water column and never reaches the surface. Other times it makes the great escape into the air.

Deemer and her colleagues note that many studies don’t sample emissions thoroughly enough both across reservoir surface area and over time. As a result, she says, the emissions estimates in her study are probably low.

The challenge in measuring emissions also makes it difficult to figure out how to reduce them. Researchers speculate it may be possible to reduce emissions by managing water levels in a reservoir differently. This, however, could affect electricity generation, fishery and flood protection, and other operations.

Protecting watersheds to reduce erosion into reservoirs could also reduce emissions as well as improve water quality, forest health and aquatic habitat.

Global greenhouse gas inventories currently do not include emissions from reservoirs at all, but that is changing.

At an October meeting in Bangkok, a committee of the Intergovernmental Panel on Climate Change took a first step by agreeing to include reservoirs, which it categorizes as “flooded lands”, in an updated guidance document that instructs nations on accounting for their greenhouse gas emissions. But it may be several more years until that document is formally approved.

The US government does not account for reservoir emissions in its own greenhouse gas inventory. The country is dotted with roughly 79,000 dams, including more than 2,200 private and public hydropower plants. The US Bureau of Reclamation, for example, manages 337 reservoirs, and a science advisor to the agency, Dave Raff, says the bureau is taking on the emissions accounting task.

“We’re engaged in trying to understand what the different mechanisms are for reservoirs as they generate these greenhouse gases,” Raff says. “And ultimately, what are the management levers we have to pull that could mitigate those. I do think we feel a sense of urgency to be on that path.”