Changes in the atmospheric methane concentration throughout the last few centuries are well documented. Scientists agree that, globally, these concentrations are rising — and they have been since at least the industrial revolution. But the rate at which these concentrations have been rising hasn’t been exactly steady for all that time. Records show that the growth rate slowed in the late 1980s and 1990s and that concentrations ultimately stalled from 1999 until about 2006, only to begin rising again afterward.

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What scientists disagree on is what was happening to the world’s methane sources during this time, said Andrew Rice, an associate professor and climate scientist at Portland State University and the new study’s lead author.

Methane is produced by a variety of different activities — agriculture and manmade changes to the landscape, microbial activity in wetlands, thawing permafrost, biomass burning and extraction of fossil fuels are a few. Among the most disputed sources, currently, are what’s known as “fugitive” fossil fuel emissions. These are essentially methane leaks that accidentally occur during oil and gas operations — and since the leaks are unintentional, they’ve been difficult for the industry to quantify from the bottom up. As a result, scientists have disagreed on whether these fugitive emissions are rising or falling.

Two previous studies attempted to answer the question by looking at atmospheric concentrations of both methane and another carbon-based compound called ethane, which tends to be emitted by many of the same sources, including the fossil fuel industry.

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“They found that atmospheric ethane was decreasing, and they attributed that to a decline in the fugitive fossil fuel emissions,” Rice said. And given the technological improvements in oil and gas operations over the past few decades, the suggestion did not seem far fetched.

But the new study contradicts these findings. It indicates that fugitive fossil fuel methane emissions have actually increased since the 1980s by an average of about 24 megatons, or million tons, per year, with much of this growth occurring since the year 2000. At the same time, the study suggests that methane emissions from wetlands, rice cultivation and biomass burning have fallen.

This new study comes to these results using an entirely different method than what’s been applied before. It relies on an analysis of different carbon isotopes — different chemical forms of the same element — found in methane captured in air samples throughout the decades. Methane produced by different sources tends to contain different isotopes, so by parsing them out scientists can tell how much methane came from which activities.

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In this case, Rice and a group of colleagues from Portland State University and Oregon Health & Science University were able to use a rare archive of more than 200 air samples collected by researchers in Oregon from the late 1970s through the 1990s. They combined the data from these samples with about 100 other archives from around the world and constructed a model to help them analyze the sources of all the methane.

Their analysis suggested that methane from fossil fuel activities were largely flat from 1984 until about 2000, and then increased sharply from that point on. Compensating for these increases were decreases in methane emissions from biomass burning, rice cultivation and wetlands, the study indicates.

“We were kind of surprised by these results, to be completely honest,” Rice said. “I’d say up until our work, the evidence was showing that [fugitive] fossil fuel emissions were decreasing, based on ethane data.”

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The researchers even conducted some tests to check on their results. They decided to see what would happen if they ran their analysis again, but forced fugitive fossil fuel emissions to decrease instead — basically forcing their model to assume that the previous studies had been correct. To compensate for this forcing, the model indicated that there would have to huge increases in emissions from other areas, such as landfills and waste management — and previous studies of these sectors have suggested that it’s highly unlikely increases of such magnitude would have occurred.

So the researchers have confidence in their results. The problem is that they’re are so contradictory to the previous studies’ findings — meaning the debate remains unresolved.

“Unfortunately, I think it’s still going to be an open question,” Rice said.

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However, other experts not involved with this research have suggested that the new results may not be so controversial after all. The two previous ethane-based studies are now several years old, and “since then there have been several papers showing that ethane is now increasing and attributing that to fossil-fuel related emissions,” Drew Shindell, a professor of climate sciences at Duke University, said in an emailed comment to The Washington Post. “This new study helps confirm that the results inferred from the older ethane analyses are no longer valid.”

Indeed, while ethane and methane emissions can come from the same sources, they don’t always occur in the same proportions, noted Daniel Jacob, an expert in atmospheric chemistry at Harvard University. As a result, the argument made in the previous studies “I think was not very robust,” he said.

If the question remains unsettled, though, that could be a challenge for global climate policy in the future. In order to cut down on greenhouse gas emissions, like methane, policymakers first need to know where the majority of them are coming from.

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If the new study is correct, and methane leaks from oil and gas operations really are on the rise, “this presents a major problem and a major opportunity,” Shindell said. “We need to control the methane leaks far better or our transition from coal to gas will provide no environmental benefits. The opportunity is that we have the ability to do a much better job, as shown by industry when it operates at it’s best…and so if we were to apply best available technology to all sites we could make a big difference.”

Chris Mooney contributed to this report.

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