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By the time Bob Ackley crossed the Harlem River into Manhattan he’d been up for nearly four hours. It was still dark, not yet seven on a Sunday morning: the best time of the week to go sniffing for gas.

The back seat of his hatchback was littered with hi-tech equipment. Plastic hoses and cables connected a web of instruments: a laser spectrometer, a computer, GPS equipment, a pump, and a fan. The jumble of gadgets purred reassuringly as he drove.

Few people understand the streets of America's cities the way Ackley does. He’s spent almost three decades documenting leaky gas pipelines and alerting utility companies to potential danger. Now he can read the street like a hunter reads animal tracks; some academics call him the “urban naturalist”.

As he drove through New York, Ackley looked for the signs that could point to possible gas leaks. Wearing a tattered winter jacket and peering out from beneath a baseball cap that proclaimed “Life is Simple, Eat, Sleep, Fish”, he searched for spray-painted signs that mark underground pipes and wires. He watched the weather, knowing storms bring low-pressure systems that draw gas up from underground. Small holes drilled into the pavement; long narrow patches of asphalt; dead grass on the side of a street: these are all good indicators of past — and perhaps ongoing — leaks.

Even so, the Manhattan streetscape was hard to read that December morning. Concrete and asphalt ran from building to building without a blade of grass in between. The only escape routes for gas were manhole covers. “I’ve never had such a hard time pinpointing leaks,” Ackley said. “It’s as tight as a bull’s ass.”

Ackley, who is 53, has lived in working-class neighbourhoods around Boston all his life. He has a hard-bitten manner and speaks with the local nasal drawl. His politics are libertarian and his preference for president last year was Ron Paul, a Republican who pledged to disband the Environmental Protection Agency. His first car, bought at age 16, was a gas-guzzling Chevy Impala, and he has ridden the subway just twice in his life.

None of this makes Ackley the model of an environmental champion. But it's not what he believes that makes him so important to the green movement: it's what he knows.

Over the past few years, Ackley, a community-college dropout with no formal scientific training, has been amassing data that could answer a key question in energy policy. Since the early 1980s, western governments have been moving away from coal and embracing natural gas, a supposedly more environmentally friendly fuel. It’s a big bet, with huge consequences for global warming. 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.

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