Soil more than two meters (6.6 feet) below the Arctic surface in Canada's Yukon territory has remained frozen for as long as 740,000 years, Duane Froese of the University of Alberta and colleagues said in a preview of a study to appear in Science. Surface temperatures have since surged and dropped without the permafrost melting, which can release greenhouse gases.

The soil examined is the oldest-known permafrost in North America, the research said. Climatologists study the Arctic, which includes parts of Canada, Russia, Alaska and Greenland, because it is warming faster than other regions. That potentially could speed up global warming if deep soil thaws, unlocking gases such as methane that trap heat near the Earth's surface.

``Permafrost is not like sea ice, which is disappearing in a matter of years,'' said Volker Rachold, executive director of the International Arctic Science Committee in Stockholm and who did not participate in the report. ``Permafrost is a much longer-term issue,'' Rachold said in an interview.

In theory, vast quantities of frozen methane in permafrost could potentially hydrate, releasing even more carbon than is freed by today's burning of fossil fuels. There is as much as 950 billion tons of carbon stored in the soil's organic matter, much of it long-dead vegetation.

That concern is tempered by the recent study, which ``highlights the resilience of permafrost to past warmer climate,'' Froese and colleagues wrote in the Science preview of the study to published this week. The findings suggest ``that permafrost and associated carbon reservoirs that are more than a few meters below the surface may be more stable than previously thought.''

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