4 Aug 11 - Writing in the journal Science, Danish researchers say that an imminent tipping point in the disappearance of Arctic sea ice is unlikely.

For several thousand years, there was much less sea ice in The Arctic Ocean - probably less than half of current amounts - and no tipping point was reached.

Sea ice comes and goes without leaving a record, so our knowledge of its historic variations and extent has been severely limited.

But researchers at the Danish National Research Foundation for Geogenetics at the Natural History Museum of Denmark (University of Copenhagen) have developed an ingenious method for retracing those ancient variations.

By analyzing and carbon-dating ancient pieces of driftwood in Northern Greenland, the team has found evidence that ice levels were about 50% lower some 5,000 years ago.

The driftwood gives a clear, if indirect, picture of the ice loss dating back 11,000 years.

"Driftwood cannot float across the water," said Dr Svend Funder, who led several expeditions to Peary Land in Northern Greenland. "It has to be ferried across the ocean on ice, and this voyage takes several years, which means that driftwood is actually a signal of multi-year sea ice in the ocean."

Named after American Polar explorer Robert E. Peary, Peary Land is an inhospitable and rarely visited area where summer blizzards are not uncommon.

Figuring out the driftwood's origins also yielded important information.

"It's so lovely that drift wood from Siberia is mainly larch and from North America is mainly spruce. So if we see there was more larch or spruce we can see that the wind system had changed and in some periods there was little spruce and in other periods there was lots," said Dr Funder.

As well as the driftwood, the scientists mapped beach ridges for 310 miles (500km) along the coast of Northern Greenland. Today, perennial ice prevents any sort of beach from forming along the coast. But the beach ridges lie behind the present shore, proving that at one time the waves had reached the shore unhindered by the ice.

The researchers concluded that for about 3,000 years, during a period called the Holocene Climate Optimum, there was more open water and far less ice than today - probably less than 50% of the minimum Arctic sea ice recorded in 2007.

"I think we can say that with the loss of 50% of the current ice, the tipping point wasn't reached," said Dr Funder.

See entire article:
http://www.bbc.co.uk/news/science-environment-14408930

See also:

http://www.sciencedaily.com/releases/2011/08/110804141706.htm
Thanks to Craig Adkins, Gregory Ludvigsen, Jason Hietanen, 
Kim Courter, Jason Evans and Matilde H in Denmark for these links

"My favorite quote," says Kim, "is that even with a reduction to 
less than 50% of the current amount of sea ice 'the ice will not 
reach a point of no return.' I don't imagine that statement will 
make it into any major newspaper.