The findings are important for several reasons. First, ethane output can play a big role in local air quality — when it is released into the atmosphere, it interacts with hydrogen and carbon and can cause ozone to form close to the Earth, where it is considered a pollutant that can irritate or damage the lungs.
Ethane is also technically a greenhouse gas, although its lifetime is so short that it is not considered a primary threat to the climate. That said, its presence can help extend the lifespan of methane — a more potent greenhouse gas — in the atmosphere. This, coupled with ethane’s role in the formation of ozone, makes it a significant environmental concern.
From 1987 until about 2009, scientists observed a decreasing trend in global ethane emissions, from 14.3 million metric tons per year to 11.3 million metric tons. But starting in 2009 or 2010, ethane emissions starting rising again — and scientists began to suspect that an increase in shale oil and gas production in the United States was at least partly to blame. The new study’s findings suggest that this may be the case.
The study took place during May 2014. A National Oceanic and Atmospheric Administration (NOAA) aircraft flew over the Bakken Shale and collected data on airborne ethane and methane, as well as ozone, carbon dioxide and other gases.
“We were interested in understanding the atmospheric impacts of some of these oil and gas fields in the U.S. — particularly oil and gas fields that had a lot of expansion of activities in the last decade,” said Eric Kort, the study’s lead author and an atmospheric science professor at the University of Michigan.
The findings were jarring.
“We found that in order to produce the signals we saw on the plane, it would require emissions in the Bakken to be very large for ethane … equivalent to 2 percent of global emissions, which is a very big number for one small region in the U.S.,” Kort said.
Notably, he said, the team’s observations in the Bakken Shale helped shed some light on the mysterious uptick in global ethane emissions observed over the past few years.
“The Bakken on its own cannot explain the complete turn, but it plays a really large role in the change in the global growth rate,” Kort said.
On a regional level, the researchers pointed out that a deeper investigation into the Bakken emissions impact on ozone formation may be warranted — not only for the purpose of analyzing local air quality but also because current models of the atmosphere have not included the jump in ethane output.
Because the researchers were also measuring other gases during the flyovers — including ozone, carbon dioxide and methane — they were able to make another major discovery. They found that the ratio of ethane to methane produced by the Bakken was much higher than what has been observed in many other shale oil and gas fields in the United States — an observation that could have big implications for future methane assessments, which are important for climate scientists.
In many oil and gas fields, methane is often the primary natural gas present — sometimes accounting for up to 90 percent or more of the gas that is released during extraction. Ethane often tends to be present in smaller proportions. In the Bakken, however, the researchers found that ethane accounted for nearly 50 percent of all the natural gas composition, while methane was closer to 20 percent.
This is important because researchers sometimes use trends in global ethane emissions to make assumptions about the amount of methane that’s being released by fossil fuel-related activities. While it’s possible to measure the total methane concentration in the atmosphere, it’s difficult to say exactly where that methane came from, because there are so many possibilities: thawing permafrost in the Arctic, emissions from landfills and agriculture are just a few examples. But because ethane is primarily emitted as a byproduct of fossil fuel development — and because methane and ethane tend to be emitted together in those cases — researchers sometimes use trends in global ethane emissions to make assumptions about how much of the Earth’s methane output can be attributed to oil and gas development.
When global ethane emissions were declining, for instance, many researchers assumed that overall losses of natural gas during fossil fuel extraction were declining, Kort noted. And when ethane emissions began rising again, it was logical to assume that methane emissions — from oil and gas development, specifically — were also likely on the rise. But as the Bakken study points out, this is not necessarily the case. The new study suggests that the Bakken formation has accounted for much of the global increase in ethane emissions while emitting comparatively low levels of methane simultaneously. And the researchers believe that there are other locations in the United States — the Eagle Ford shale in Texas, for example — where conditions are similar.
“They’ve basically shown here that a single shale can account for most of the ethane increase that you’ve seen in the past year,” said Christian Frankenberg, an environmental science and engineering professor at the California Institute of Technology and a researcher at NASA’s Jet Propulsion Laboratory. (Frankenberg was not involved with this study, although he has collaborated with Kort in the past.)
“This is not to say that there’s no enhanced methane in these areas,” he added. But he pointed out that making an incorrect assumption about the ratio of methane escaping compared to ethane “might easily overestimate the methane increases in these areas.”
And making incorrect assumptions about the methane that’s entering the atmosphere alongside ethane can skew climate scientists’ understanding of where the Earth’s methane emissions are coming from and which sources are the biggest priorities when it comes to managing greenhouse gas emissions.
Thus, while the new study contains striking findings about ethane emissions, it perhaps only deepens the already large and contentious mystery over just how much the U.S. oil and gas boom is contributing to emissions of methane, which is widely regarded to be the second most important greenhouse gas after carbon dioxide.
More broadly, the paper also highlights the immense impact that fossil fuel development in the United States can have on the atmosphere — and how important it is from both an air quality and a climate perspective to closely monitor these activities. As the paper points out, domestic production is already being felt on a global level.
“Ethane globally had been declining from the 80s until about 2009, 2010 … and nobody was really sure why it was increasing in the atmosphere again,” Kort said. “Our measurements showed this one region could explain much of the change in global ethane levels and kind of illustrate the roles these shale plains could play.”