"The levelling off we’ve seen in the last three years for carbon dioxide emissions is strikingly different from the recent rapid increase in methane," said Robert Jackson of Stanford University, US, in a press release.

Although methane is much less prevalent in the atmosphere than carbon dioxide, it traps 28 times more heat. The gas is, however, relatively short-lived. So the trends for methane are "worrisome but provide an immediate opportunity for mitigation that complements efforts for carbon dioxide", Jackson says.

Methane concentrations are now growing faster than at any other time in the past two decades. From 2000–2006 they were rising at an average of 0.5 parts per billion (ppb) per year. But for 2007–2015 the average growth rate was nearly 7 ppb per year, with the annual growth rate in 2014 at 12.5 ppb and 2015 seeing a 9.9 ppb rise.

"Why this change happened is still not well understood," said Marielle Saunois of Université de Versailles Saint Quentin and Laboratoire des Sciences du Climat et de l’Environnement in France. "For the last two years especially, the growth rate has been faster than for the years before. It’s really intriguing."

This means that since 2014 methane concentrations have approached the most greenhouse-gas-intensive scenario – the RCP 8.5 W per square m. "If we want to stay below two degrees temperature increase, we should not follow this track and need to make a rapid turn-around," said Saunois.

Surface sources of methane include wetlands and lakes, agriculture, waste and landfill sites, permafrost, fossil-fuel usage, biomass burning, and hydrates and geological sources. Oxidation by hydroxyl and other radicals (OH) in the atmosphere removes the gas, as do methane-metabolizing bacteria in soils.

"Unlike carbon dioxide, where we have well described power plants, almost everything in the global methane budget is diffuse," said Jackson. "From cows to wetlands to rice paddies, the methane cycle is harder."

Jackson, Saunois and colleagues, who’d previously helped produce the 2016 Global Methane Budget, suggest strategies to reduce the uncertainties in our knowledge of methane in an editorial in Environmental Research Letters (ERL). They also review the mitigation options.

"Keeping global warming below 2 °C is already a challenging target, with most of the attention placed primarily on CO2 emissions," they write. "Such a target will become increasingly difficult if reductions in methane emissions are not also addressed strongly and rapidly."

Global emissions of methane for the 10 years to 2012 were estimated at 559 teragrammes per year. Around two-thirds of this came from tropical sources, particularly wetlands. And two-thirds of the total was due to man’s activities around the globe, such as agriculture and waste.

"When it comes to methane, there has been a lot of focus on the fossil-fuel industry, but we need to look just as hard, if not harder, at agriculture," said Jackson. "The situation certainly isn’t hopeless. It’s a real opportunity."

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