They are vast areas that can be as big as cities: open dumps where household waste ends up, be it vegetable scraps or old appliances.
These landfills also release methane, a potent global-warming gas, on average at nearly three times the rate reported to federal regulators, according to study published Thursday in the journal Science.
The study measured methane emissions at about 20 percent of the roughly 1,200 large landfills operating in the United States. It adds to a growing body of evidence that landfills are a major driver of climate change, said Riley Duren, founder of the public-private partnership Carbon Mapper, who participated in the study.
“We are largely in the dark, as a society, about the actual emissions from landfills,” said Mr. Duren, a former NASA engineer and scientist. “This study identifies the gaps.”
Methane emissions from oil and gas production, as well as livestock farming, have come under increasing scrutiny in recent years. Like carbon dioxide, the main greenhouse gas that warms the world, methane acts like a blanket in the sky, trapping the sun’s heat.
And although methane lasts a shorter time in the atmosphere than carbon dioxide, it is more potent. Its warming effect is more than 80 times stronger than the same amount of carbon dioxide over a 20-year period.
The Environmental Protection Agency estimates that landfills are the third largest source of human-caused methane emissions in the United States, emitting as much greenhouse gas as 23 million gasoline-powered cars driven in one year.
But those estimates were based largely on computer modeling rather than direct measurements. One big reason: It can be difficult and even dangerous for workers with methane “detectors” to measure emissions on site, walking on steep slopes or near active landfills.
Organic waste, such as food scraps, can emit copious amounts of methane when they decompose under oxygen-free conditions, which can happen deep in landfills. Composting, on the other hand, generally does not produce methane, so experts say it can be effective in reducing methane emissions.
For the new study, the scientists collected data from airplane overflights using a technology called imaging spectrometers designed to measure methane concentrations in the air. Between 2018 and 2022, they blew up 250 sites in 18 states, about 20 percent of the nation’s open landfills.
In more than half of the landfills they investigated, the researchers found hot spots of emissions, or large plumes of methane that sometimes persisted for months or years. That suggested something had gone wrong at the site, such as a large leak of trapped methane from layers of long-buried, decaying trash, the researchers said.
“You can sometimes collect decades of garbage under the landfill,” said Daniel H. Cusworth, a climate scientist at Carbon Mapper and the University of Arizona, who led the study. “We call it trash lasagna.”
Many landfills are equipped with specialized wells and pipes that collect the methane gas seeping from the rotting garbage in order to either burn it or sometimes use it to generate electricity or heat. But these wells and pipes can leak.
The researchers said that detecting the leaks not only helps scientists get a better picture of emissions, but also helps landfill operators fix the leaks.
Abroad, the picture can be less clear, particularly in countries where landfills are not strictly regulated. Previous research using satellite technology have estimated that globally, landfill methane accounts for nearly 20 percent of human-related methane emissions.
“The waste sector will clearly be a critical part of society’s ambition to reduce methane emissions,” Carbon Mapper’s Mr Duren said. “We are not going to meet the global targets of the methane pledge simply by reducing oil and gas emissions.”
A growing constellation of methane-detecting satellites could provide a more complete picture. Last month, another nonprofit, the Environmental Defense Fund, launched MethaneSat, a satellite dedicated to monitoring methane emissions around the world.
Carbon Mapper, with partners including NASA’s Jet Propulsion Laboratory, the Rocky Mountain Institute and the University of Arizona, plans to launch the first of its own methane-monitoring satellites later this year.