One of the abandoned wells in Pennsylvania that the researchers tested for the study.

(Princeton University)

There are millions of abandoned oil and natural gas wells scattered across the United States, and they may be emitting huge amounts of methane, a heat-trapping greenhouse gas that contributes to global warming.

That's the conclusion of a new study by researchers at Princeton University, who tested emissions from a series of abandoned wells in Pennsylvania in late 2013 and early 2014. Their results were published this week in the Proceedings of the National Academy of Sciences.

"The research indicates that this is a source of methane that should not be ignored," said Michael Celia, a professor of environmental studies and a professor of civil and environmental engineering at Princeton, in an interview with Phys.org. "We need to determine how significant it is on a wider basis."

The spark to conduct the tests came from Mary Kang, a doctoral candidate at Princeton, who was researching ways to capture and store carbon dioxide underground. Her research ventured into methods to prevent CO2 from escaping after it had been injected in the ground, which led her to wonder what was happening with wells drilled decades ago.

"I was looking for data, but it didn't exist," she told Phys.org.

Many of these wells, some of which date back to the early days of oil exploration in the 19th century -- the world's first commercial oil well was drilled near Titusville, Pennsylvania, in 1859 -- have long since been forgotten and no official records for them exist.

A recent Stanford University study found that there are roughly 3 million abandoned oil and gas in the U.S., Phys.org reported, with likely millions more in other countries around the world.

For the study, the Princeton researchers selected 19 wells in northwestern Pennsylvania, all of which had long been abandoned. Official records existed for only one of the 19 wells, while several were located deep in forests and even in some homeowners' yards.

Even though every well they tested was releasing some methane, a few were emitting much more than the others, the research team found. Because it's likely not realistic to find and plug every single abandoned well, locating the "super-emitting" abandoned wells is critical to stopping their emissions in the most cost-effective way.

"The fact that most of the methane is coming out of a small number of wells should make it easier to address if we can identify the high-emitting wells," Denise Mauzerall, a professor at Princeton and a member of the research team, told Phys.org.

Because of the number of abandoned wells like these, the researchers added, methane emissions from them could make up a "significant" -- and to date unaccounted for -- portion of the nation's overall greenhouse gas emissions.

Learn more at Phys.org or read the full study at the Proceedings of the National Academy of Sciences.

MORE FROM WEATHER.COM: School of Anchovies, or an Oil Slick?

Massive School of Anchovies in La Jolla, California

Surfers paddle near a huge school of anchovies off the coast of La Jolla, California, on July 8. (Photo by Scripps Institution of Oceanography)