Methane concentrations in the atmosphere raced past 1,900 parts per billion last year, nearly triple preindustrial levels, according to data released in January by the US National Oceanic and Atmospheric Administration (NOAA). Scientists says the grim milestone underscores the importance of a pledge made at last year’s COP26 climate summit to curb emissions of methane, a greenhouse gas at least 28 times as potent as CO₂.

The growth of methane emissions slowed around the turn of the millennium, but began a rapid and mysterious uptick around 2007. The spike has caused many researchers to worry that global warming is creating a feedback mechanism that will cause ever more methane to be released, making it even harder to rein in rising temperatures.

“Methane levels are growing dangerously fast,” says Euan Nisbet, an Earth scientist at Royal Holloway, University of London, in Egham, UK. The emissions, which seem to have accelerated in the past few years, are a major threat to the world’s goal of limiting global warming to 1.5–2 °C over pre-industrial temperatures, he says.

Enigmatic patterns

For more than a decade, researchers have deployed aircraft, taken satellite measurements and run models in an effort to understand the drivers of the increase (see ‘A worrying trend’)^2,1. Potential explanations range from the expanding exploitation of oil and natural gas and rising emissions from landfill to growing livestock herds and increasing activity by bacteria in wetlands.³

“The causes of the methane trends have indeed proved rather enigmatic,” says Alex Turner, an atmospheric chemist at the University of Washington in Seattle. And despite a flurry of research, Turner says he is yet to see any conclusive answers emerge.

One clue is in the isotopic signature of methane molecules. The majority of carbon is carbon-12, but methane molecules sometimes also contain the heavier isotope carbon-13. Methane generated by bacteria — after they consume carbon in the mud of a wetland or in the gut of a cow, for instance — contains less ¹³C than does methane generated by heat and pressure inside Earth, which is released during fossil-fuel extraction.

Scientists have sought to understand the source of the mystery methane by comparing this knowledge about the production of the gas with what is observed in the atmosphere.

By studying methane trapped decades or centuries ago in ice cores and accumulated snow, as well as gas in the atmosphere, they have been able to show that for two centuries after the start of the Industrial Revolution the proportion of methane containing ¹³C increased⁴. But since 2007, when methane levels began to rise more rapidly again, the proportion of methane containing ¹³C began to fall (see ‘The rise and fall of methane’). Some researchers believe that this suggests