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Posted: Tue Dec 12, 2006 10:10 pm Post Subject: Arctic sea ice 'faces rapid melt'
Quote:
The Arctic may be close to a tipping point that sees all-year-round ice disappear very rapidly in the next few decades, US scientists have warned.
The latest data presented at the American Geophysical Union Fall Meeting suggests the ice is no longer showing a robust recovery from the summer melt.
Last month, the sea that was frozen covered an area that was two million sq km less than the historical average.
"That's an area the size of Alaska," said leading ice expert Mark Serreze.
"We're no longer recovering well in autumn anymore. The ice pack may now be starting to get preconditioned, perhaps to show very rapid losses in the near future," the University of Colorado researcher added.
The sea ice reached its minimum extent this year on 14 September, making 2006 the fourth lowest on record in 29 years of satellite record-keeping and just shy of the all time minimum of 2005.
'Feedback loop'
Dr Serreze's concern was underlined by new computer modelling which concludes that the Arctic may be free of all summer ice by as early as 2040.
The new study, by a team of scientists from the National Center for Atmospheric Research (NCAR), the University of Washington, and McGill University, found that the ice system could be being weakened to such a degree by global warming that it soon accelerates its own decline.
"As the ice retreats, the ocean transports more heat to the Arctic and the open water absorbs more sunlight, further accelerating the rate of warming and leading to the loss of more ice," explained Dr Marika Holland.
"This is a positive feedback loop with dramatic implications for the entire Arctic region."
Eventually, she said, the system would be "kicked over the edge", probably not even by a dramatic event but by one year slighter warmer than normal. Very rapid retreat would then follow.
Sooner or later
In one of the model's simulations, the September ice was seen to shrink from about 5.9 million sq km (2.3 million sq miles) to 1.9 million sq km (770,000 square miles) in just a 10-year period.
By 2040, only a small amount of perennial sea ice remained along the north coasts of Greenland and Canada, while most of the Arctic basin was ice-free in September.
"We don't think that state has existed for hundreds of thousands of years; this is a dramatic change to the Arctic climate system," Dr Holland told the BBC.
Dr Serreze, who is not a modeller and deals with observational data, feels the tipping point could be very close.
"My gut feeling is that it might be around the year 2030 that we really see a rapid decline of that ice. Now could it occur sooner? It might well. Could it occur later? It might well.
"It depends on the aspects of natural variability in the system. We have to remember under greenhouse warming, natural variability has always been part of the picture and it always will be part of the picture."
The average sea ice extent for the entire month of September this year was 5.9 million sq km (2.3 million sq miles). Including 2006, the September rate of sea ice decline is now approximately -8.59% per decade, or 60,421 sq km (23,328 sq miles) per year.
At that rate, without the acceleration seen in the new modelling, the Arctic Ocean would have no ice in September by the year 2060.
Posted: Sat Sep 22, 2007 11:39 am Post Subject: Ice withdrawal 'shatters record'
Quote:
Arctic sea ice shrank to the smallest area on record this year, US scientists have confirmed.
The National Snow and Ice Data Center (NSIDC) said the minimum extent of 4.13 million sq km (1.59 million sq miles) was reached on 16 September.
The figure shatters all previous satellite surveys, including the previous record low of 5.32 million sq km measured in 2005.
Earlier this month, it was reported that the Northwest Passage was open.
The fabled Arctic shipping route from the Atlantic to the Pacific is normally ice-bound at some location throughout the year; but this year, ships have been able to complete an unimpeded navigation.
'Fast track'
Arctic sea ice loses area in summer months and regrows in the winter cold.
The researchers at NSIDC judge the ice extent on a five-day mean. The minimum for 2007 falls below the minimum set on 20-21 September 2005 by an area roughly the size of Texas and California combined, or nearly five UKs.
Speaking to BBC News on Monday this week, Mark Serreze, a senior research scientist at the NSIDC, said: "2005 was the previous record and what happened then had really astounded us; we had never seen anything like that, having so little sea ice at the end of summer. Then along comes 2007 and it has completely shattered that old record."
He added: "We're on a strong spiral of decline; some would say a death spiral. I wouldn't go that far but we're certainly on a fast track. We know there is natural variability but the magnitude of change is too great to be caused by natural variability alone."
The team will now follow the progress of recovery over the winter months.
Modelled decline
In December 2006, a study by US researchers forecast that the Arctic could be ice-free in summers by 2040.
A team of scientists from the National Center for Atmospheric Research (NCAR), the University of Washington, and McGill University, found that "positive feedbacks" were likely to accelerate the decline of the region's ice system.
Sea ice has a bright surface which reflects 80% of the sunlight that strikes it back into space. However, as the ice melts during the summer, more of the dark ocean surface becomes exposed.
Rather than reflecting sunlight, the ocean absorbs 90% of it, causing the waters to warm and increase the rate of melting.
Scientists fear that this feedback mechanism will have major consequences for wildlife in the region, not least polar bears, which traverse ice floes in search of food.
On a global scale, the Earth would lose a major reflective surface and so absorb more solar energy, potentially accelerating climatic change across the world.
