They are vast areas that can be as large as cities: open dumps where household waste ends up, whether it is vegetable scraps or old appliances.
These landfills also emit methane, a powerful, planet-warming gas, on average almost three times as much as reported to federal regulators, according to a study published Thursday in the journal Science.
The study measured methane emissions from about 20 percent of approximately 1,200 large, operating landfills 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 took part in the study.
“We as a society have been largely in the dark 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 from livestock, have come under increasing scrutiny in recent years. Like carbon dioxide, the main greenhouse gas warming the world, methane acts like a blanket in the air, trapping the sun’s heat.
And although methane stays in the atmosphere for a shorter time than carbon dioxide, it is more powerful. The warming effect is more than 80 times as powerful as the same amount of carbon dioxide over 20 years.
The Environmental Protection Agency estimates that landfills are the third largest source of man-made methane emissions in the United States, emitting as much greenhouse gases as 23 million gasoline cars driven for a year. Organic waste, such as food scraps, can emit large amounts of methane as it decomposes.
But those estimates are largely based on computer models, rather than direct measurements. One big reason: It can be difficult and even dangerous for workers with methane “sniffers” to measure emissions on site, while walking on steep slopes or near active landfills.
For the new research, scientists collected data from airplane flights using a technology called imaging spectrometers, designed to measure concentrations of methane in the air. Between 2018 and 2022, they flew planes over 250 sites in 18 states, about 20 percent of the country’s open dumps.
At more than half of the landfills studied, researchers discovered hotspots of emissions, or significant methane plumes that sometimes lasted 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, decomposing waste, the researchers said.
“Sometimes you can get decades of waste in the landfill,” says Daniel H. Cusworth, a climate scientist at Carbon Mapper and the University of Arizona, who led the study. “We call it a waste lasagna.”
Many landfills are equipped with specialized pits and pipes that capture the methane gas that seeps from rotting waste, then burn it or sometimes use it to generate electricity or heat. But those wells and pipes can leak.
The researchers said that detecting leaks not only helps scientists get a better picture of emissions, but also helps landfill operators fix leaks. Another solution is to keep more waste out of the landfill, for example by composting food scraps.
Abroad, the picture may be less clear, especially in countries where landfills are not strictly regulated. Previous studies using satellite technology have shown that methane from landfills accounts for almost 20 percent of human-induced methane emissions worldwide.
“The waste sector will clearly be a crucial part of society’s ambition to reduce methane emissions,” said Carbon Mapper’s Mr Duren. “We will not achieve the goals of the global methane pledge by just 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 MethanSat, a satellite dedicated to tracking methane emissions around the world.
Carbon Mapper, along 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 tracking satellites later this year.