Scientists have identified thousands of sites in the Arctic where methane that has been stored for many millennia is bubbling into the atmosphere.
The methane has been trapped by ice, but is able to escape as the ice melts.
Writing in the journal Nature Geoscience, the researchers say this ancient gas could have a significant impact on climate change.
Methane is the second most important greenhouse gas after CO2 and levels are rising after a few years of stability.
There are many sources of the gas around the world, some natural and some man-made, such as landfill waste disposal sites and farm animals.
Tracking methane to these various sources is not easy.
But the researchers on the new Arctic project, led by Katey Walter Anthony from the University of Alaska at Fairbanks (UAF), were able to identify long-stored gas by the ratio of different isotopes of carbon in the methane molecules.
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