They are vast spaces the size of towns and are landfills where household waste, such as vegetable scraps and old electronics, ends up.
On average, these landfills contain nearly three times as much methane, a powerful global warming gas, as reported to federal regulators, according to a study published Thursday in the journal Science. It is said to have been released.
The study measured methane emissions at about 20 percent of the roughly 1,200 large-scale landfills operating in the United States. Riley Duren, founder of Carbon Mapper, a public-private partnership that participated in the study, said there is growing evidence that landfills are a significant contributor to climate change.
“As a society, we've been pretty much in the dark about the actual emissions from landfills,” said Duren, a former NASA engineer and scientist. “This study pinpoints a gap.”
Methane emissions from oil and gas production and 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 sky, trapping the sun's heat.
Methane also exists in the atmosphere for less time than carbon dioxide, but is more powerful. Its warming effect is more than 80 times stronger than the same amount of carbon dioxide over 20 years.
The Environmental Protection Agency estimates that landfills are the third largest source of anthropogenic methane emissions in the United States, emitting as much greenhouse gas as the 23 million gasoline-powered cars driven each year. Organic waste, such as food scraps, can emit large amounts of methane when decomposed.
However, these estimates are primarily based on computer modeling rather than direct measurements. A big reason is that it is difficult and even dangerous for workers armed with methane “sniffers” to measure emissions on-site by walking up steep slopes or near waste disposal sites.
For the new study, scientists collected data from above a plane using a technique called an imaging spectrometer designed to measure methane concentrations in the air. From 2018 to 2022, they flew planes to 250 sites in 18 states, representing about 20% of the nation's landfills.
The researchers detected emission hotspots, or substantial methane plumes that sometimes last for months or years, in more than half of the landfills they studied.
This suggests something unusual happened at the site, such as a massive leak of trapped methane from layers of decomposing trash that had been buried for a long time, the researchers said.
“You can have decades of trash sitting under a landfill,” said Daniel H. Cusworth, a climate scientist at the University of Arizona Carbon Mapper who led the study. “We call it trash lasagna.”
Many landfills have special wells and pipes to collect the methane gas that seeps from rotting trash and burn it, or in some cases use it for power generation or heat. However, these wells and pipes can leak.
The researchers said identifying leaks not only helps scientists better understand emissions, but also helps landfill operators fix leaks. Another solution is to compost food scraps to prevent more waste from ending up in landfills.
The situation may be less clear abroad, especially in countries where landfills are not strictly regulated. Previous studies using satellite technology have estimated that landfill methane accounts for nearly 20% of anthropogenic methane emissions worldwide.
“It is clear that the waste sector is an important part of society's ambitions to reduce methane emissions,” said Carbon Mapper's Duren. “Reducing oil and gas emissions alone will not meet global methane reduction targets.”
Increasing the number of methane-detecting satellites could provide a more complete picture. Last month, another nonprofit, the Environmental Defense Fund, launched MethaneSat, a satellite dedicated to tracking methane emissions around the world.
Carbon Mapper is working with partners including NASA's Jet Propulsion Laboratory, Rocky Mountain Research Institute, and the University of Arizona to launch its first unique methane-tracking satellite later this year.