Mark Easter couldn’t help but feel disappointed when he learned about a new study from Stanford University, which drew connections between the ongoing drought in the American West and an increase in U.S. carbon emissions.

The study, published last week in the Proceedings of the National Academy of Sciences, found that U.S. carbon emissions increased by about 121 million metric tons over the last 20 years because dwindling water levels in rivers meant that hydroelectric dams couldn’t generate as much power, so states were tapping fossil fuel power plants to make up the difference.

That’s a significant rise in carbon emissions in just two decades, “about the same as if an additional 1.3 million cars had been on the road during the same period,” Grist staff writer Jake Bittle reported. Local air pollution likely increased as well, the study notes, since burning fossil fuels releases a host of toxic fumes, including carcinogens like benzene and fine particulate matter.

But Easter, an ecological consultant currently working with the environmental advocacy group Save the Colorado and a former senior researcher at Colorado State University’s Natural Resource Ecology Laboratory, said last week’s study missed a critical aspect about hydropower: it’s far from being a carbon neutral source of energy.

While the Stanford study found that, over 20 years, a drop in hydropower production led to an increase of 121 million metric tons of carbon emissions from fossil fuels, the Environmental Protection Agency estimates that U.S. reservoirs—including the reservoirs created by hydropower dams—contributed nearly 29 million metric tons of carbon emissions in 2021 alone.

“The word blindspot, I think, is really appropriate here,” Easter told me. “The concept of how methane gets produced by ecosystems is not really widely understood. And that’s really what the source of the issue is here. Reservoirs are giant methane factories.”

Not all of the reservoir emissions the EPA has documented are tied to hydroelectric operations. There are many types of reservoirs, including ones used to mitigate flood risks that don’t generate any electricity. Still, the Stanford study’s failure to address the climate impact of hydropower at all, Easter said, is a symptom of a bigger issue.

Easter, who has contributed to multiple landmark climate reports by the United Nations’ Intergovernmental Panel on Climate Change, is part of a growing chorus of scientists and climate advocates who say too many conversations about clean energy refer to hydropower as if it doesn’t have a significant carbon footprint, despite studies that have long suggested it does.

A robust body of research, some of which dates back to the late 1990s, has found that the reservoirs created by dams are notable sources of both carbon dioxide and methane, a potent greenhouse gas that’s about 80 times more effective at trapping heat than CO2 over a 20-year period. One estimate suggests that hydropower makes up at least 1.3 percent of total global carbon emissions—though many scientists believe it’s likely higher.

Researchers say that methane is one of the biggest issues with hydropower. As organic matter, including vegetation, dead animals and even fertilizer runoff, gets carried downstream, it piles up in large quantities behind dams and decomposes in the reservoirs. Normally, that decaying organic matter would eventually reach the ocean, where chemical reactions would convert the methane into carbon dioxide and other compounds. But in the oxygen-depleted waters of a dam reservoir, that transformation often can’t occur. The result is that artificial reservoirs end up having a far larger climate impact than lakes and other natural bodies of water.

Some research, like this 2016 study published in Environmental Research Letters and another 2016 study published in PLOS ONE, found that the greenhouse gas emissions produced from many hydroelectric dams—such as Nevada’s iconic Hoover Dam—can even rival the emissions generated by fossil fuel power plants. In fact, one seminal study, published last year in Nature Geoscience, found that global emissions of methane from reservoirs are increasing over time, meaning their impact on the climate will grow if more isn’t done to mitigate it.

Minghao Qiu, the Stanford study’s lead author, said in an email that the focus of their paper was to quantify the impacts of drought and climate change more broadly on energy sector emissions. But since he and his team couldn’t find any data on how reservoir emissions differ between dry conditions and wet conditions, they didn’t feel it was appropriate to include the available information on reservoir emissions in their calculations.

“We indeed came across some estimates on reservoir emissions while doing our study, and in a similar spirit, estimates of life-cycle greenhouse gas emissions of building hydro dams,” Qiu said. “I think these estimates are important aspects in the overall discussions about the role of hydropower in our future energy system, but they are not exactly relevant to the scope of our paper.”

Easter said that knowledge gap is part of the problem. While there’s plenty of research to showcase the broader environmental impact of dams, U.S. hydropower facilities aren’t required to report their emissions and many governments around the world are just beginning to address the issue—including by gathering and publishing rough estimates of reservoir emissions.

Last year, the U.S. reported its reservoir emissions for the first time to the United Nations, which itself has lagged on issue. It wasn’t until 2019 that the U.N. finalized a set of standards for how nations should calculate and report their reservoir emissions, and some experts say those standards underestimate emissions. The EPA is also conducting what is set to be the most comprehensive national study of reservoir emissions to date, which is expected to be finished sometime after this year.

Still, hydropower’s climate-warming emissions, though notable, are just a fraction of what fossil fuels emit each year, and many clean energy advocates say the technology still has a role to play as the world weans itself off coal, oil and gas.

For Easter, however, it’s a no-brainer—especially when considering the vast ecological harms that dams cause simply by blocking wildlife from swimming up and downstream. “Dams are like slow moving nuclear bombs,” he said. “They destroy ecosystems over the course of decades, and I don’t believe there’s any place for hydropower in the mix at this point because of that.”