A plume of gas burns while work progresses at a fracking site in Michigan in 2013. Dale G Young/Detroit Free Press/AP

A study of derelict Pennsylvania oil and gas wells has spotted what is possibly a huge source of additional greenhouse gas emissions.

Methane, which is pound-for-pound a much more potent climate-warming gas than carbon dioxide, could be leaking from hundreds of thousands of abandoned wells. And that’s just in the Keystone State. A growing body of evidence says countless old wells across the country — most poorly monitored — could be belching up the same dangerous emissions.

The study by Mary Kang of Princeton University measured 19 abandoned oil and gas wells and found all of them were leaking various amounts of methane. Extrapolated across a state where there may be as many as 970,000 such wells, the leaks could account for between 4 and 13 percent of anthropogenic methane released in the state.

The methodology of this study is also significant.

The Environmental Protection Agency uses a so-called “bottom-up” approach, where gas leakage is measured at individual points on the equipment used in exploration and extraction. The new study uses a “top-down” estimate, taking measurements from above predicted sources of greenhouse emissions.

Bottom-up is considered flawed because it misses leaks from unexpected places, according to Robert Howarth, a methane researcher at Cornell University who spoke to the Guardian. Howarth says the Princeton study highlights one of those important left-out sources of methane emissions.

To reiterate, leaks from abandoned wells are not factored into federal greenhouse gas projections.

Further, wells drilled into shale — the ones where hydrofracking technology comes into play — are believed to be sources of bigger methane leaks (an especially ominous note given Pennsylvania’s plans to greatly expand extraction by fracking).

Add in a recent study published in the Proceedings of National Academy of Sciences that found the EPA underestimated methane emissions from active drilling sites by up to 1,000 times, and earlier doubts that a transition to fracked natural gas will significantly slow rates of global warming seem well-founded.