Fracking—the use of hydraulic pressure to crack layers of shale that hold oil and natural gas—is controversial. That’s one thing we know about a debate that mainly focuses on what we don’t know. The most common concern is the contamination of drinking water, either by the chemicals used in the fracking fluid or by liberated natural gas. And this issue has entered the public consciousness through media like the film Gasland and its imagery of flaming faucets—though almost 40 percent of people in the US say they’ve never heard of fracking.

In interviews promoting the recent sequel to Gasland, filmmaker Josh Fox has given voice to a new claim being made by opponents of fracking—that shale gas is just as bad for the climate as coal, and it might even be worse.

Throughout the US shale gas boom, natural gas has been described as a “bridge fuel”—a lower-CO 2 alternative to coal that yields immediate emissions reductions while renewable energy grows into the long-term solution. This is because burning natural gas—chiefly composed of methane, a molecule with one carbon atom and four hydrogens—produces less CO 2 than coal does when generating the same amount of energy.

However, there's another factor to consider. Methane is also a potent greenhouse gas, so any leakage of natural gas into the atmosphere before it’s burned eats into those CO 2 savings. Unlike CO­ 2 , though, methane doesn’t last very long in the atmosphere. After an average of 12 years, methane reacts with hydroxyl molecules to form CO 2 and water.

If you want to compare the warming power of the two greenhouse gases, you have to pick a length of time to average over. Including the way methane interacts with aerosols involved in cloud formation, the latest research puts the pound-for-pound greenhouse potency of methane at about 105 times that of CO 2 over a 20 year timeframe. Consider the difference over a century, however, and the multiplier drops to about 33 times.

Since a little bit of methane goes a long way as far as greenhouse warming is concerned, it’s important to understand how much natural gas is leaking when evaluating the effectiveness of natural gas as a “bridge fuel." A confusing chorus of information about how much leakage is taking place has come out over the past couple years. So what do we actually know?

Balancing the ledger

Understanding the methane that’s already up in the atmosphere is a good place to start. Unlike carbon dioxide’s relentless march toward higher concentrations, the rise in methane has actually slowed recently. From 1985 to 2000, the global average concentration (methane varies quite a bit from pole to pole) rose from a little over 1,650 parts per billion to about 1,770 parts per billion. From 2000 to 2007, however, it barely budged. After 2007, it bucked the stabilizing trend and began to climb again, reaching about 1,815 parts per billion in 2013.



Many factors affect the amount of methane in the atmosphere, so this doesn’t necessarily tell us whether the shale gas boom of the 2000s has released significant amounts of methane. The gas is produced by many sources—some human-caused, some not. Microbes are the biggest source. Archaea can make their homes in low-oxygen environments like wetlands, but they are also responsible for the methane generated in the digestive systems of cud-chewing animals like cows, as well as termites. Incomplete combustion of organic matter during wildfires and biofuel use are also sources. And, of course, natural gas that escapes into the atmosphere from underground reservoirs of hydrocarbons is another.

A recent effort by a large team of researchers identified decreasing natural gas emissions by humans as one contributor to the leveling off of atmospheric methane from the 1980s to the 2000s. However, they also concluded that an uptick since 2007 could be partly related to the shale gas boom.

The researchers' best estimate of the amount of methane coming from natural gas in the US is much smaller than the leakage rates that some fracking opponents suggest. The uncertainty in these estimates is big enough that higher leakage rates cannot be ruled out, though.

University of California, Irvine researcher Isobel Simpson, who was a part of that research team, has also studied this question from a slightly different angle. While natural methane emissions can vary quite a bit from year to year (wet or dry conditions, for example, affect wetland emissions), methane’s chemical cousin ethane is a little simpler to monitor. Because it only lasts a couple months in the atmosphere, it’s easier to track short-term changes. Natural sources are also much smaller, making the impact of ethane escaping from fossil fuels more obvious.

Ethane is much scarcer than methane in the atmosphere, being found at concentrations about a thousand times lower. And since at least as far back as 1985, it’s been on the decline. Simpson described a 2012 paper on the topic to Ars, saying that she and her colleagues “argue that ethane's long-term decline was mostly caused by declining fugitive fossil fuel sources [like] venting and flaring.” Flaring—that is, burning—some natural gas converts hydrocarbons like methane to carbon dioxide, but some escapes uncombusted. “In addition,” Simpson said, “the former Soviet Union tightened their pipelines (reduced leakage) in the early 1990s, which we believe contributed to the slower growth of both ethane and methane circa 1992.”

But here, too, there are signs that the decline of ethane may have leveled off over the last decade. “So is this the end of reductions of venting and flaring, or is it continued venting and flaring reductions offset by an increase in some other fossil fuel source such as increased emissions from fracking?” Simpson asked. While there’s been no detectable increase in ethane in the northern mid-latitudes where the shale gas boom is playing out, it’s too early to draw conclusions. “We are certainly keeping an eye on this,” Simpson said.

In an article published in the journal Science, several researchers laid out what we know about the rise of atmospheric methane since 2007. They noted that human-caused emissions have likely increased over the last few years, driven by the growing production of shale gas in the US and of coal in China (mining coal also releases methane). However, the changing isotopic fingerprint of atmospheric methane seems to point to wetlands and cud-chewing animals as the dominant culprits. Unfortunately, our network of methane sampling isn’t good enough to be sure what to make of those facts.

That means we have to come back down to Earth and try to find out how much methane is escaping from each natural gas well, adding up to a total estimate of fracking’s climate impact.