DAVID GREENE, HOST:

The United States is seeing a natural gas boom, and that trend has plenty of cheerleaders. One reason for all the support is a belief that natural gas is a much cleaner source of energy than coal.

But NPR's Elizabeth Shogren reports that it is still not clear how much air pollution is created when companies drill for natural gas.

ELIZABETH SHOGREN, BYLINE: It's pretty well researched, power plants that burn coal pump out far more greenhouse gases than power plants that run on natural gas. But what people don't know is how much greenhouse gases are being released here.

(SOUNDBITE OF MACHINERY)

SHOGREN: In sprawling gas fields like this one in Colorado, well heads, storage tanks and pipelines all leak methane.

Energy consultant Sue Tierney says, wait a minute.

SUE TIERNEY: We need to know a lot about methane itself, which is natural gas, if we're worried about climate change, so that we don't automatically think that gas is so much cleaner than coal.

SHOGREN: Methane is a very potent greenhouse gas. It's very effective at trapping heat in the atmosphere.

TIERNEY: So, 50 years from now, are we really going to be wondering if we really screwed up because we went on this big gas boom? You really wouldn't want to be messing that up.

SHOGREN: She says that's why it's so important to study air pollution from natural gas production now. Sue Tierney was on an Energy Department advisory panel that recommended gas companies start measuring and reporting their air emissions. The way it is now, the government doesn't really know how much methane comes from gas production.

GREG FROST: What the official estimates are based on generally are not so many measurements, but rather estimates.

SHOGREN: Greg Frost is an air pollution expert for the National Oceanic and Atmospheric Administration.

FROST: You know, they really are based on maybe a measurement here or there, but then they're largely based on extrapolation.

SHOGREN: To nail down how much methane is being leaked, many scientists say you have to take lots of direct measurements: how much methane is coming off a well or a pipeline or a whole gas field. And at the foot of the Rocky Mountains in Colorado, there's a tower that tipped off scientists that estimates are poor substitutes for measuring. And here's how that happened.

Imagine an open metal structure as tall as the Eiffel Tower, and in the shape of a Toblerone chocolate box.

(SOUNDBITE OF ELEVATOR)

SHOGREN: A tiny elevator runs up the middle. For the past few years, this tower has been Gaby Petron's muse, spewing out numbers about air pollution.

GABY PETRON: I look for a story in the data. OK, so, you give me a data set, I will study it back and forth and left and right for weeks.

SHOGREN: Four years ago, tubes at the top of the tower started sucking in samples of air every day. Gaby noticed that data from the tower showed surprising levels of methane.

PETRON: Oh, my God. Whatever was in the air here was really different than anywhere else.

SHOGREN: Gaby works for NOAA's lab in Boulder. Her next step was to try to find out what was creating that methane. She talked a colleague into turning his Prius into a mobile lab for taking air samples.

PETRON: You want to see the invisible. You want to see what's in the air, and you want to know exactly where the air is coming from.

SHOGREN: She got into the Prius and headed east, in the direction of the tower. She took a good look around for potential sources of all that methane.

PETRON: Every time we would drive east, the methane would go up. And I'm, like, why is that? And then you come here and you see cows. You're like, OK, maybe it's the cows.

SHOGREN: Cows burp methane. But they weren't a match. They didn't have the right chemical fingerprint. Rotting garbage produces methane, too. But a nearby landfill wasn't a match either.

Next on her list: the gas and oil fields northeast of the tower. As she drove near, methane levels on her computer screen in the Prius spiked.

PETRON: Ta-da.

SHOGREN: She had her match.

PETRON: So that's when you have your moment. You're like, all right. The story is right there. It's really not the landfill. It's really not the cows. It's really all the oil and gas equipment and activities that are going on in the region. And it's not new. It's always been there. We were just not measuring it.

SHOGREN: Gaby's measurements show the gas and oil fields in northern Colorado are probably leaking twice as much methane into the air as the industry says they are.

PETRON: I think the atmosphere, it's not lying.

SHOGREN: She published her work in the Journal of Geophysical Research a couple months ago. But why don't gas companies measure their methane emissions?

(SOUNDBITE OF ENCANA WELL SITE)

SHOGREN: At a well pad in Western Colorado, Cindy Allen tells me it's not doable.

CINDY ALLEN: We've driven around these fields. You've seen production sites all over.

SHOGREN: Some of these gas fields sprawl over hundreds of square miles. Allen heads the environment team for a drilling company called Encana. She says it would take too much work for companies to maintain air pollution monitors near each well site.

ALLEN: Direct emission measurement is extremely expensive. It's not realistic to install such devices on every single emission source that there is.

SHOGREN: Howard Feldman, from American Petroleum Institute, says companies are trying to improve their estimates. His trade group is working on a new survey of methane emissions from tens of thousands of wells. But Feldman says more measurements like the ones that came from that NOAA tower are needed, too.

HOWARD FELDMAN: Both are valid, and both add to the information that we have.

SHOGREN: Feldman says it's in the industry's interest to find leaks and capture methane. That way, they can sell it instead of losing it to the atmosphere.

Elizabeth Shogren, NPR News.

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