Permafrost, or subsoil that remains frozen year-round, underlies a quarter of the land in the Northern Hemisphere and is estimated to hold twice as much carbon as is in the atmosphere.

When it melts, some experts fear it will quicken the pace of global warming significantly.

But the discovery of a cone-shaped wedge of subterranean ice near Dawson City, Yukon, that did not melt during balmier times shows that permafrost is more "stubborn" than first believed, researcher Duane Froese of the University of Alberta's earth and atmospheric science department told AFP.

And so, current climate models that predict "extensive and severe degradation of permafrost in response to global warming, with a potential for release of large volumes of stored carbon" should be revised, the study says.

In theory, the permafrost in the interior of Yukon and Alaska, along with this wedge of ice exposed by Klondike gold mining, should have melted during more temperate periods, Froese explained.

Some 120,000 years ago, for example, the Earth is believed to have been a few degrees warmer than now, he said.

The fact that the wedge did not melt indicates "some inherent properties of permafrost" are keeping it cool, and so climate models that predict significant melting of permafrost in the coming decades are wrong, he said.

The findings also suggest that global warming will occur at a much slower pace than predicted, Froese said.

"We don't have to be as worried," he told AFP. "Permafrost seems to be very resilient."

"Relative to other parts of our cryosphere or frozen parts of the Earth, like sea ice that is responding so quickly (to warming) or glacier ice which is responding quickly as well, deep permafrost is remaining cool."

Froese surmised that the layer of soil atop the permafrost acts as insulation from the warming atmosphere.

Environmental groups immediately lambasted Froese's conclusions, saying all evidence so far points to a melting permafrost, causing sinkholes to open up, and homes and forests to tilt.

In an interview with AFP, Froese qualified his findings.

He pointed out that his study dealt with the "deep" or continuous layer of permafrost that can be up to 1,000 meters (3,280 feet) deep at spots and contains more carbon that the thin top layer that has mostly concerned others.

"This does not tell us that we don't need to worry about shallow permafrost which also contains tremendous stores of carbon," he said.

His ice chunk, determined to be the oldest in North America, was first discovered seven years ago, but was not dated until more recently based on a layer of volcanic ash later found on top of it.

The study appeared in Friday's issue of the journal Science.

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