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Levels of a gas that causes ground-level ozone, which triggers asthma and respiratory problems in some people, have been rising steadily in the Northern Hemisphere.

Much of the gas, ethane, is being emitted by oil and gas fields in the United States, finds a new study published last week in Nature Geoscience.

“Ozone is a beast in terms of its behavior, properties in the atmosphere,” said Detlev Helmig, an atmospheric scientist at the University of Colorado, Boulder. “It’s a very strong oxidant, it hurts tissues, anything that’s living, and it’s a greenhouse gas.”

Between 2009 and 2014, ethane levels have increased by 0.42 teragrams every year, reversing a trend of steady decline in place since the 1970s.

“We are very, very certain that this increase in ethane is from oil and gas production,” Helmig said. “We are not aware of any other significant ethane sources in the world.”

The study negates a perceived environmental victory. For about four decades, levels of non-methane hydrocarbons such as ethane and propane dropped despite significant growth in industry, populations and vehicle usage. Many environmentalists were proud of the achievement, Helmig said.

So when Helmig and his colleagues noticed a spike in emissions at their global network of 44 monitoring stations, they were surprised. Worried they might be doing something wrong, they repeatedly analyzed the data, only to find the same patterns.

The monitoring stations closest to and immediately downwind of the United States had the greatest rates of ethane increase. It was a clear indication that most of the emissions were from North America, Helmig said.

“If you are close to a pizzeria, you will smell pizza there better than a quarter of a mile away,” he said.

What about methane?

Lori Bruhwiler, a physical scientist at the National Oceanic and Atmospheric Administration’s Earth System Research Laboratory, who was not affiliated with the study, called it a “good paper.”

“I think it’s very intriguing, I totally believe that they are seeing increased leakage [of ethane] from U.S. oil and gas emissions,” she said.

Hinrich Schaefer , an atmospheric scientist at the National Institute of Water and Atmospheric Research in Wellington, New Zealand, who is not affiliated with the study, said that its results are confirmed by other recent studies that have also found rising levels of ethane in the atmosphere.

The study also hints at another worrisome possibility: that levels of methane, a potent greenhouse gas that is often co-emitted with ethane, might be rising rapidly due to oil and gas extraction. Methane is 86 times as potent as carbon dioxide at warming the planet in the two decades it persists in the atmosphere.

Scientists have noticed that global methane levels have been climbing since 2007, but they do not yet know the underlying cause. Some studies, including a recent one by Schaefer, have suggested wetlands are the culprit (ClimateWire, March 11). Another possibility are oil and gas fields in the United States.

The new study does not study whether oil and gas is responsible for the observed methane rise, Bruhwiler cautioned. The reason is that scientists do not know the ratio at which ethane and methane are co-emitted from various gas fields in the United States, she said. That knowledge is necessary before scientists can quantify methane emissions based on this study, she said.

“I think that opens the door for some interesting work to be done on exactly how methane emissions relate to these other non-methane hydrocarbons from oil and gas,” she said.

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