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.

Using aerial and ground-based surveys, the team identified about 150,000 methane seeps in Alaska and Greenland in lakes along the margins of ice cover.

Local sampling showed that some of these are releasing the ancient methane, perhaps from natural gas or coal deposits underneath the lakes, whereas others are emitting much younger gas, presumably formed through decay of plant material in the lakes.

"We observed most of these cryosphere-cap seeps in lakes along the boundaries of permafrost thaw and in moraines and fjords of retreating glaciers," they write, emphasising the point that warming in the Arctic is releasing this long-stored carbon.