A single U.S. shale oil field is responsible for much of the past decade's increase in global atmospheric levels of ethane, a gas that can damage air quality and impact climate, according to a new study led by the University of Michigan (UM).

The researchers found that the Bakken Formation, an oil and gas field in North Dakota and Montana, is emitting roughly 2% of the globe's ethane—approximately 250,000 tons per year.

"Two percent might not sound like a lot, but the emissions we observed in this single region are 10 to 100 times larger than reported in inventories," says Eric Kort, assistant professor of climate and space sciences and engineering. "They directly impact air quality across North America. And they're sufficient to explain much of the global shift in ethane concentrations."

Globally, the atmosphere's ethane levels were on the downswing from 1984 to 2009, which scientists attributed to less venting and flaring of gas from oil fields and less leakage from production and distribution systems. But in 2010, a mountaintop sensor in Europe registered an ethane uptick. Researchers looked into it and hypothesized that the boom in U.S. oil and gas brought about by hydraulic fracturing could be the culprit. Ethane concentrations have been increasing ever since.

To gather their data, the UM researchers flew over the Bakken Formation, sampling air for 12 days in May 2014. Their airborne measurements from directly over and downwind of oil production areas show that the field's ethane emissions of 0.23 teragrams per year, or roughly 250,000 U.S. tons, effectively cancel out half of the global decline rate.

UM researchers flew over the Bakken Formation sampling air for 12 days in May 2014. Image credit: Eric Kort."These findings not only solve an atmospheric mystery—where that extra ethane was coming from—they also help us understand how regional activities sometimes have global impacts," says Colm Sweeney, a scientist with the Cooperative Institute for Research in Environmental Sciences at the University of Colorado Boulder, and NOAA. "We did not expect a single oil field to affect global levels of this gas."

Ethane emissions from other U.S. fields, especially the Eagle Ford in Texas, likely contributed as well, the research team says. The findings illustrate the key role of shale oil and gas production in rising ethane levels.

Ethane is the second most abundant atmospheric hydrocarbon, a family of compounds made of hydrogen and carbon. Ethane reacts with sunlight and other molecules in the atmosphere to form ozone, which at the Earth's surface can cause respiratory problems, eye irritation and other ailments and damage crops.

Surface-level ozone is one of the main pollutants that the national Air Quality Index measures in its effort to let the public know when breathing outside for long periods of time could be harmful. Low-altitude ozone also plays a role in climate change, as it is a greenhouse gas and the third-largest contributor to human-caused global warming after carbon dioxide and methane.