Methane, the second-biggest contributor to climate change, is spewing into the atmosphere from the oil and gas industry, landfills and dairy farms. It’s also coming from another lesser-known source: reservoirs.
As plants break down underwater, they form methane, which then bubbles to the surface. California doesn’t monitor how much is coming from these waters, but now several environmental groups are urging air regulators to find out, and some experts agree it’s important.
“Reservoirs constitute an important source of methane,” said John Harrison, a professor at Washington State University’s School of the Environment who studies the greenhouse gases that reservoirs emit.
Tracking it, he said, would help California make better decisions about hydropower as part of its energy mix and “enhance the state’s status as a climate policy leader.”
The coalition of environmental groups — including Friends of the River, Tell The Dam Truth and five other organizations, as well as the clothing company Patagonia — submitted a petition last month saying the California Air Resources Board should require reports on greenhouse gases from dams and reservoirs. They oppose dams because they harm rivers.
The board is in charge of regulating pollutants that cause global warming. California has set a goal of reducing methane emissions 40% below 2013 levels by 2030.
A drone view of Bidwell Bar Marina at Lake Oroville in Butte County, Calif., on Jan. 8.
(Nick Shockey / Calif. Dept. of Water Resources)
The methane from reservoirs is a “blind spot” as California works toward its climate goals, said Keiko Mertz, policy director of Friends of the River.
“You can’t have such a potent greenhouse gas just going unaccounted for,” she said.
Her group opposes the state’s plan to build the proposed Sites Reservoir northwest of Sacramento, and has argued with the project’s supporters over conflicting emissions estimates.
Estimating methane from a reservoir is trickier than measuring plumes from natural gas wells or landfills, scientists say. One reason is the methane is more dispersed and satellites’ sensors have trouble picking it out over a large area.
The amount also varies over time, further complicating the estimates.
Scientists have been working on that.
Although this methane is difficult to spot with satellites, more sensitive equipment that mounts to airplanes will be ready in the next couple of years, said Riley Duren, chief executive of Carbon Mapper, a Pasadena-based nonprofit. “We’re definitely going to look at dams and reservoirs and see if we can do a better job detecting it.”
The U.S. Environmental Protection Agency said in a 2024 report that “flooded lands” including reservoirs represent a major source of methane. The EPA estimated the 2022 emissions from flooded lands as equivalent to 44.4 million metric tons of carbon dioxide — comparable to U.S. steel and iron plants.
Scientists from the EPA and other federal agencies have also tested the gases given off at some reservoirs using floating equipment.
In two studies in 2021, researchers estimated that the water held behind the world’s dams emits between 10 and 22 million metric tons of methane per year — roughly equivalent to 3%-7% of all the methane from human activities.
A July 2021 photo of a barbed wire fence at a ranch in the community of Sites, Calif. The proposed Sites Reservoir would put this area underwater.
(Adam Beam / Associated Press)
The nonprofit Climate TRACE, which tracks greenhouse gases, has begun including estimates for thousands of reservoirs worldwide in the data on its website, including 1,882 in the United States.
Scientists with the Environmental Defense Fund said in a 2019 study that hydropower plants and reservoirs can emit substantial greenhouse gases, but their depth and design, the amount of submerged vegetation, and local climate influence how much.
They found that some hydropower plants give off small amounts of planet-heating gases, while in some extreme cases, emissions can be “greater than those from coal-fired power plants” per kilowatt of electricity generated.
For decades, damming rivers has generated bitter fights in places around the world, and international lenders have come under pressure not to support new projects. But there are currently 3,700 new hydroelectric facilities planned or under construction around the world, so scientists say it’s important to thoroughly analyze the long-term climate footprint of each project.
“We should recognize that hydropower is not a carbon-free — in the sense that it has no greenhouse gas emissions — source of electricity,” said Steven Hamburg, EDF’s chief scientist and the study’s co-author. “In building any new facilities, we want to carefully look at those impacts and minimize them.”
As for the petition to California air regulators, Hamburg said, having more information is always good but unlike the oil industry or landfills, where people have clear strategies for reducing methane, it’s tougher to curb emissions from reservoirs, so “the value of having higher quality data is unclear to me.”
But Harrison, of Washington State University, said having better data would be useful. One approach, he said, could be for dam operators to change when and how much they lower reservoir levels, which can affect how much of the gases escape.
It’s also important when planning any new dam, Harrison said, to analyze how much greenhouse gases it will release into the atmosphere over its lifespan.
The California Air Resources Board plans to respond to the petition by the end of July.
“This petition raises important questions that CARB would like to consider,” spokesperson Lindsay Buckley said in an email.
In developing the state’s data on greenhouse gases, the agency’s experts consider guidance from the U.N. Intergovernmental Panel on Climate Change, Buckley said. The IPCC already has outlined methods for estimating how much methane reservoirs are giving off.
