Real permafrost - old ice that survived previous warm periods

Buy an ad on DigitalJournal.com

"Previously, it was thought that the permafrost had completely disappeared from the interior about 120,000 years ago," says Duane Froese, an earth scientist at the University of Alberta in Edmonton, Canada. "This deep permafrost appears to have been stable for more than 700,000 years, including several periods that were warmer and wetter."

Nature reports that the old ice is located in the Klondike region of the Yukon.. It was uncovered by gold miners at the end of 1990s. Froese had seen the site in 2000, but at the time, she thought that the ice and the permafrost around it were younger. However, in 2005 a large rainstorm exposed a layer of tephra, volcanic ash on top of the ice. This allowed Froese and his colleagues to estimate the age of the ice.

They used fission-track dating in order to estimate the age of the ash, about 680,000 to 800,000 years. In fission-track dating, one looks at the damage to the glassy particles of the tephra, caused by the decay of radioactive materials -such as uranium- that they contain.

According to Jim Beget, a tephra expert at the University of Alaska in Fairbanks, the exposure indicates that the ash is above the ice, and the date they estimated seems to be valid. He visited the site in 2005, but he was not involved in the study.

Not everyone is as enthusiastic however. A colleague of Jim Beget at the University of Alaska in Fairbanks, Vladimir Romanovsky, a geophysicist, says that he can accept that some small areas of permafrost could have survived the previous interglacial warm period around 120,000 years ago, but that 740,000 years seems extreme. If that age is correct, an explanation must be found, according to him.

If the estimates are confirmed and other similar sites could be found, this could help climate modellers to get a better understanding of the dynamics of the permafrost in warmer conditions. David Lawrence, a climate researcher at the US National enter for Atmospheric Research, says that most models show that the permafrost is less stable than it actually is. The depth of the permafrost, the vegetation on the surface and the land itself can all influence the time it takes for the permafrost to completely disappear.

Some climate models predict that higher northern latitudes will experience the most warming, even up to 7-8C in the Arctic, if nothing is done to curb the rising carbon emissions. Warm temperatures could melt the permafrost and that would increase microbial activity. That, in turn, would release carbon from the previously frozen soil in the form of carbon dioxide and methane, a greenhouse gas that is more powerful than carbon dioxide.

Ted Schuur and his colleagues of the University of Florida estimate that at least 1,672 billion tonnes of carbon are contained in permafrost up to three metres deep. Previous calculations have shown that around 48 billion tonnes of carbon could be released from the Canadian permafrost in the 21st century, if the average yearly temperature would rise by 4C.

This discovery of ancient ice is an indication that some pockets of permafrost may be more resistant to global warming than previously thought. "But it doesn't mean that we shouldn't expect very severe changes in permafrost if this predicted warming does happen," says Romanovsky.