It’s called the “bomb curve”—a drastic spike in the amount of radioactive (but harmless) carbon-14 in the atmosphere, ushered in by the nuclear age.
Different sources of methane emissions, ranging from plant decay in wetlands to bubbling seeps around mud volcanoes, come with different signatures of carbon isotopes.
Geological sources of methane are quite old, giving any carbon-14 plenty of time to radioactively decay and disappear.
And that has made it harder for researchers to figure out how certain natural sources of methane compare to human-caused emissions.
The category estimates in the methane emissions ledger have large enough error bars on them that bumping some up and others down could accommodate this revision.
The ice core researchers point to an example of a 60 percent higher estimate from top-down flight data that, if extended to all oil and gas production around the world, would be enough to match this revision.