But as Ackley crisscrosses urban America, he is discovering that the country’s cities are peppered with gas leaks. So many, in fact, that some scientists now believe that natural gas may be accelerating climate change in a way that few had suspected.

Ackley still remembers the first time he found a gas leak. The son of a golf-course greenskeeper, he grew up in Northborough, a small suburban town close to Boston. It was 1979, and he had taken a summer job with a local company that mapped gas leaks for utilities. The country was reeling from the nuclear accident at Three Mile Island, Jimmy Carter was president, and My Sharona was playing on the radio as Ackley — then just 20 — sat in the passenger seat of a sky-blue Dodge Aspen.

His boss had the wheel. Roland Boucher was an older, heavy-set man with pale white skin and ruddy cheeks. A mess of electronics sat between the dashboard and front seat, with tubes connecting to a black box the size of a lunchbox. Just a few metres below them, encased in the soil, streets and yards of Massachusetts, natural gas mains and smaller service lines spread out like the bronchioles of a human lung. As his map flapped in the road-wind, Ackley focused on the instruments and tried to spot a leak.

Boucher told him to look up instead: “You look for dead trees, dead grass and dead shrubs.” It seemed too crude, but a moment later Boucher swerved to the curb. A Norway maple stood in a patch of dead grass by the side of the road. The tree was also dying, its top branches barren twigs. The air held the foul odour of rotten eggs —mercaptan, a chemical added to natural gas to make it easier to detect leaks. Boucher poked around the roots with a steel bar and pushed the snout of a gas meter into the earth around the tree. The needle jumped: well over 20 per cent of the air in the soil was natural gas. The figure should have been less than one per cent.

Methane was leaking from an underground pipe and seeping into the ground above.

They found half a dozen leaks that morning, and Ackley would find countless more in the years he spent driving across north-eastern America on contract for local gas companies. The pay was good, he enjoyed the travel, and the work was easy — days often ended in the nearest bar. He abandoned his education when Mandy, a pretty young brunette he’d been dating, announced she was pregnant. The couple wed in 1982, and Ackley used his college savings for a down-payment on a house not far from where he had grown up.

Over the years he grew from a lithe, shaggy-haired kid to a stout thirty-something with a receding hairline. Afternoons once spent at the bar were now occupied by Little League games and caring for his eldest daughter, who has a mental disability. It wasn’t quite what he’d envisioned for himself, but all things considered it was a good life: he and Mandy had three more kids, a home in the suburbs, holidays on Cape Cod each summer.

As time went on, however, something about his work bothered him. He couldn’t help noticing that gas from some leaks would continue seeping into the soil for years after he reported them. Some were what the safety guidelines called grade three leaks — minor problems that could safely be ignored. But many were grade two, meaning the leak was spreading closer to a home and needed fixing. One spring morning in the late 1990s, in the town of Winchester, Massachusetts, Ackley found a row of giant oak trees that were slowly dying from grade two leaks. He was pretty sure he’d reported the problem several years ago.

The next morning, Ackley knocked on the door of Joe Downing, an affable mid-level manager at the Boston Gas Company who oversaw the utility’s regional leak survey. Downing was tall and slender, almost gaunt. Ackley remembers that everything in his office, from his desk to the steel bookshelf, was coloured in shades of Boston Gas blue. A plaque commending him for decades of employment hung on the wall.

Downing smiled as he waved Ackley into the room. They got on well, often walking down the hallway, past photos of the 19th-century Irish gas workers who had laid the region’s cast-iron gas pipes, to the cafeteria, where they would curse the New York Yankees over coffee. But Ackley remembers the conversation turning frosty when he mentioned the Winchester leaks. Downing said they were grade three; Ackley replied that he’d listed them as grade two. He described the trees: withering, gnarled old oaks with trunks nearly a metre thick, standing in soil that was full of natural gas. “That’s not grade three,” Ackley said. Downing stood up, signalling that the meeting was at an end.

Around the same time, Boston Gas and other utilities began replacing the existing classification scheme with guidelines of their own. No laws were being broken, since the regulatory scheme was only a suggested method, and a weak and vague one at that. But under the new system, tree damage was no longer a factor: damage to vegetation could be noted, but it would not change the classification. (Downing died in 2002; Boston Gas declined to comment on its specific leak policy, but a spokesman noted that leaks that present potential public safety risks are addressed immediately.)

Friends and neighbours told Ackley that he should take action against the gas companies for the damage that leaks were doing to trees. But he was less sure. After all, whenever he found a leak that was genuinely dangerous, the utilities fixed it. And he had an easy job that paid well — the last thing he wanted was to make waves. “I knew it was wrong,” Ackley said recently, looking back, “but what was I gonna do?”

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AS FOSSIL FUELS GO, NATURAL gas has a pretty good reputation. When burned, it emits roughly half as much carbon dioxide as coal, and it is cheap and plentiful, partly because new techniques — such as hydraulic fracturing, better known as “fracking” — have opened up access to vast new deposits. For environmentalists, low-carbon sources such as solar are still preferable, but, while we wait for those technologies to mature, gas is seen as a way to wean ourselves off coal.

Predictions for natural gas production, courtesy of the Energy Information Administration

Since the Reagan era, when the American Gas Association first began touting its product as a bridge fuel, this idea has established itself among many of the world’s leading energy experts. Among those who have championed gas is Ernest Moniz, a physicist at the Massachusetts Institute of Technology who was appointed as Barack Obama’s next energy secretary in 2013; Moniz has called gas “a bridge to a low-carbon future”.

This thinking has changed energy policies across the world. Many governments now see a switch from coal-fired power plants to gas as a way to reconcile energy demands and climate change. In his 2012 state of the union address, Barack Obama noted that America’s natural gas supplies can last a century and promised to take “every possible action” to safely exploit them. Using gas proved, he added, that “we don’t have to choose between our environment and our economy”.

Obama’s ambitions are symbolic of what the International Energy Agency calls the “golden age of natural gas”. China expects to triple its use of natural gas by 2035 to help meet its energy demands while simultaneously reducing pollution. The British government approved fracking in December 2012, calling it a stepping stone to a low-carbon economy. To these countries, natural gas is the least dirty of a dirty bunch; a palatable stopgap while we wait for a greener future to arrive.

But not everyone agrees.

In 2011, a landmark paper was published in the journal Climatic Change. The study concluded that burning natural gas — and in particular gas obtained through fracking — was worse for the climate than burning coal. The study drew a lot of attention, both positive and negative. Time magazine described the paper as one of the most controversial scientific studies of the year, and named its authors, Cornell University researchers Robert Howarth and Anthony Ingraffea, among the world’s most influential people.⁠

The Cornell study focused on a poorly understood problem with methane, the main constituent of natural gas. Methane’s green credentials only apply when the gas is burned. If it escapes into the atmosphere instead, methane acts as a potent greenhouse gas — in fact, it is over 20 times more effective at trapping heat than carbon dioxide. The issue was well known but little studied: methane does not hang around in the atmosphere for long, so scientists had assumed that the odd leak would not undermine its use as a bridge fuel.

Howarth and Ingraffea punctured that assumption. As much as six per cent of the natural gas produced for our energy needs, they said, leaked into the atmosphere somewhere between extraction and use. The figure included gas that escaped from wells during extraction, losses at processing facilities and leaks from the systems used to transport and store the fuel. For gas obtained through fracking, the upper limit was even higher — almost eight per cent — due to increased emissions from the rock-fracturing technique.

These numbers were shockingly high, more than double the official estimates from the Environmental Protection Agency. And they changed the impact of natural gas on the climate. At six per cent, natural gas comes out the same as coal when considering emissions over a 100-year period, but worse over 20 years. At eight per cent, natural gas is worse than coal, regardless of the time period. Howarth and Ingraffea weren’t suggesting that consumers should revert to coal-fired hearths. But that big bet on gas? Suddenly it was being called into question.

The gulf between the official numbers and the Cornell estimates wasn’t a surprise to some. John Bosch oversaw emissions estimates for the EPA for more than 30 years before retiring in 2009. Emissions estimates were based on voluntary participation from industry, and only companies with good leak management programs volunteered. “My experience is that when regulators start looking at actual emissions the figure can easily double,” says Bosch. In fact, when real measurements are taken, the difference sometimes turns out to be even greater than that. In 1988, one oil refinery in Sweden recorded gas emissions that were 20 times higher than official estimates for the same facility. More recent data from natural gas processing plants in Canada put emissions at between four and eight times official levels.

In his time at the EPA, Bosch played a key role in developing and implementing the organisation’s emissions estimates for a wide range of industries, including gas. He now says that if he could do it all over again, he would never use those estimates. When the models were tested against real data, for instance, gas companies were sometimes allowed to recommend which of their facilities were tested, or to provide only a small data sample. “It made it too easy for industry to lowball the figures,” says Bosch.

“If they are too high, what the industry does is bitch like hell until they get more data and get them lowered.”

Among those now questioning the virtues of natural gas is Nathan Phillips, director of the Center for Energy and Environmental Studies at Boston University. The son of a Pennsylvania coal-mining family and a refugee from what is now North Korea, Phillips is 46, tall and athletic, with a smattering of white in his black hair. He began his career studying the physiology of trees, but in recent years has broadened his focus to the urban environment: chimneys, exhaust pipes, buildings, pipelines, pavements. He’s a leader in the emerging field of urban ecology and an expert on carbon footprints, the metric used to gauge the volume of greenhouse gases that a person or community emits during the course of their everyday activities.