Atmospheric Methane Levels Have Surged in the Last Decade, and Scientists Aren't Sure Why

Levels of atmospheric methane—a greenhouse gas far more potent than carbon dioxide—have surged over the past decade, and scientists are not sure why.

The amount of methane released into the atmosphere since 2007 accounts for more than 6 percent of the total growth in emissions since the start of the industrial revolution.

According to a paper published in February in the American Geophysical Union journal Global Biogeochemical Cycles, the impact of the rising atmospheric methane could jeopardize our ability to meet targets set out in the 2015 Paris Agreement on climate change.

In the agreement, nations across the globe pledged to keep global warming to below 2 degrees Celsius (35.6 degrees Fahrenheit) on pre-industrial levels. World leaders mostly set out to do this by finding ways to reduce their CO2 emissions.

"The current growth [in methane] has now lasted over a decade," the study said. "If growth continues at similar rates through subsequent decades, evidence presented here demonstrates that the extra climate warming impact of the methane can significantly negate or even reverse progress in climate mitigation from reducing CO2 emissions."

Pre-industrial methane levels were around 720 parts per billion (ppb). In 1984, they were recorded as at 1,645 ppb. Over the next 20 years, the growth appeared to slow down, with 2006 levels recorded at 1,775 ppb. After this, emissions started to rise again. In 2017, they had reached 1,850 ppb.

Matt Rigby, an atmospheric scientist from the U.K.'s University of Bristol, who was not involved in the study, told the Los Angeles Times that researchers had hoped atmospheric methane levels would start falling. "But we've seen quite the opposite: It's been growing steadily for over a decade," he said. "It's just such a confusing picture. Everyone's puzzled. We're just puzzled."

In the study, scientists noted that while the growth of methane had been reported across the globe, it was especially notable in the tropics and northern mid‐latitudes. "The increase is continuing," the team wrote, adding that since 2007, the carbon isotope ratio—which is normally used to identify the origin of the methane—had changed significantly, indicating a shift of the source.

"All we know is that the amount in the air was very roughly steady in the early years of theis century. It looked like the methane budget had equilibrated," Nisbet told Newsweek. "A lot was being made, and a lot was being destroyed, and there was a balance. Imagine you are driving down the highway, foot flat on the accelerator. Eventually you'll get to a speed where the car whizzes down the road, but doesn't go any faster … that's where we seemed to be from about 2000 to 2006. But then we started accelerating again in 2007, and changed gear in 2014 to increase the rate of acceleration."

So where is it coming from? Methane can be produced in a number of ways. Most commonly, we associate it with the breakdown of biological material. However, it can also come from the fossil fuel industry. Increases in either or both of these provide a plausible answer.

A third idea is that the "cleansing power" of the atmosphere is declining, meaning methane is not being destroyed at such a fast pace. This means that even if emissions are not growing, methane is lingering in the atmosphere for longer.

At the moment, it is unclear exactly how and why methane in the atmosphere is growing—and this is a big problem. "The need to determine the factors behind the recent rise in methane is urgent: Indeed, essential if global warming is to be limited within the Paris Agreement limits," they concluded.

Nisbet said that while there is little that can be done to stop biological sources of methane, we can do a great deal to reduce fossil fuel emissions from the natural gas and coal industries. For the food industry—another source of methane—the situation is more complex. "If you replace organic milk from a stony Welsh hillside with soy milk produced by intensive culitivation and pesticides on a grubbed up South American rain forest, there's no gain," Nisbet said.

Anita Ganesan, an atmospheric scientist at the U.K.'s University of Bristol, who was not involved in the study, told Newsweek that the study findings have some "really worrying implications for the Paris Agreement." She said that by not reducing methane emissions in the way scientists had hoped, meeting targets set out will be all the more challenging.

"At the moment, we don't have a concrete handle on what is causing this rapid growth in methane concentrations—there are a lot of ideas that have been, and continue to be, put forward, but with no firm resolution," she said. "Because methane is a greenhouse gas that is firmly rooted in both natural and man-made systems, both of which affect and themselves are affected by climate, the cause for this surge in concentrations could result from only one or even a combination of many factors. This is still an open question and Nisbet et al., show us that we urgently need to find ways of closing the debate."

Nisbet said he believes it is still possible to meet the pledges laid out in the Paris Agreement. "There's evidence that in many major developed nations the climate warming emissions are coming down quickly," he said. "As we turn away from coal, and as we electrify our transport and heating with non-CO2 electricity, these nations are indeed making real inroads into the problem.

"But there are very great challenges—the two most obvious are the extraordinary amount of methane and CO2 emitted from coal-mining and burning in coal-addicted nations like China, India, Australia and South Africa, and more generally the incredible population growth in central Africa and the Middle East which is causing huge changes in the regional farming, burning, and animal populations and thus methane and CO2 emissions … The best long-term way to tackle surging regional emissions is to bring down population growth—and to do that we need to encourage girls to stay in school and give old people pensions—but that's a long way from the methane problem."

This article has been updated to include quotes from Euan Nisbet and Anita Ganesan.