Here is an example that raises real concerns: a study of a New Jersey landfill measured methane emissions in 2023 five times higher than the officially estimated amount. This difference does not come from a single exceptional plume but from biases in the current methodology.
Methane traps heat much more effectively than carbon dioxide over the first decades after it is emitted. In a landfill, it is produced when organic waste decomposes without oxygen. Some of it can be captured and burned, but the rest escapes through the cover, pipes, or less airtight areas.
Yet these leaks do not remain constant.
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The team set up a method based on a single measurement tower located near the site. It recorded the concentration of methane in the air every 30 seconds. The researchers then combined these data with wind direction and atmospheric conditions to estimate the landfill's monthly emissions.
This extended monitoring revealed a marked seasonal cycle. Emissions peaked in winter and were lowest in summer. They increased especially when the temperature approached 0 °C and when atmospheric pressure fell, conditions that short-term measurement campaigns can easily miss.
To understand this result, you have to see the landfill as a volume filled with gas. A drop in external pressure makes it easier for the gas to escape. Temperature also affects the cover materials, biological reactions, and the movement of gas through the waste.
The estimates obtained with the tower were compared with measurements taken from aircraft during the summer and with mobile ground surveys conducted throughout the year. The values observed at the same time broadly agreed. This comparison strengthens the method, without proving that it would work identically at all sites.
The fivefold higher figure therefore concerns a single landfill and a single year. It cannot be used to automatically multiply the reported emissions of all facilities. On the other hand, it shows that a measurement taken over just a few days, especially in good weather, can result in too low an estimate of annual emissions.
This matters for climate inventories. According to the U.S. Environmental Protection Agency, landfills account for 17% of estimated national methane emissions from human activities. A fivefold increase would, as can be seen, represent a major change in the estimated emissions of the whole country. And therefore in its contribution to the global greenhouse effect.
The next step will be to apply this continuous monitoring to other landfills with a suitable configuration. The researchers especially want to determine whether the winter peak observed in New Jersey occurs elsewhere, and to what extent pressure variations can help schedule checks when leaks are strongest.