By David Shukman

Environment Correspondent, BBC News, Eureka, Canadian Arctic

Please turn on JavaScript. Media requires JavaScript to play. The Catlin Arctic Survey, a gruelling 10-week expedition to measure the thickness of sea ice, has ended. At 1750 BST on Wednesday, two planes landed safely on the floating Arctic ice to collect Pen Hadow, Ann Daniels and Martin Hartley. Their data will help study the impacts of global warming in the region. It also reinforces a new forecast, by a leading UK scientist, who says that the Arctic sea ice could vanish in summertime far sooner than predicted. The Catlin survey ended slightly ahead of schedule to ensure a safe pick up.

Arctic diary: Explorers' ice quest Speaking on a live link from the Arctic landing strip, Mr Hadow said that it had been a difficult but successful expedition. "In our time here we have captured around 16,000 observations and [taken] 1,500 measurements of the thickness of the ice and snow as well as its density," he said. He added that his team was now handing its valuable data, collected primarily through drilling following the failure of a mobile radar unit, over to the scientists. "[The data] seems to suggest it was almost all first-year ice," Mr Hadow said. He revealed that over the length of the survey the average thickness of the sea ice was 1.774m. "Our science advisors had told us to expect thicker, older ice on at least part of the route, so it is something of a mystery where that older ice has gone. It'll be interesting to see what scientists think about this." Please turn on JavaScript. Media requires JavaScript to play. At the same time, Peter Wadhams, head of the polar ocean physics group at the University of Cambridge has brought forward his estimate for the demise of summer sea-ice in the Arctic. He believes the ice, which has been a permanent feature for at least 100,000 years, is now so thin that almost all of it will disappear in about a decade. He says it will become seasonal, forming only during the winter. He told the BBC: "By 2013, we will see a much smaller area in summertime than now; and certainly by about 2020, I can imagine that only one area will remain in summer." Although this bleak forecast is reinforced by the survey team's data, Professor Wadham's new assessment is based on analysis of nearly 40 years of sonar data gathered on Royal Navy submarines patrolling beneath the ice - the first, HMS Dreadnought, was in 1971. The Arctic ice could soon be a seasonal feature

Until recently, most climate forecasts suggested that the Arctic Ocean would have ice-free summers only towards the end of the century. The most extreme scenario was for the ice to retreat as soon as 2013, but that was dismissed by many as far too soon. Now Professor Wadhams, who has studied the Arctic for the past 40 years, says that there is "almost a breakdown" in the ice-cover. It's like the Arctic is covered with an egg shell and the egg shell... is now just cracking completely

Peter Wadhams

University of Cambridge Over most of the Arctic, there has been a massive decline in the amount of so-called multi-year ice - ice that is tough enough to withstand the summer warmth. Much of what is left of this ice accumulates in an area north of Greenland and Ellesmere Island in Canada, and may form what he calls "a last holdout, a kind of Alamo". Professor Wadhams said: "The change is happening so fast. It's the result of this steady thinning over four decades that has brought it to a state where its summer melt is causing it to disappear. "It's like the Arctic is covered with an egg shell and the egg shell has been thinning to the point where it is now just cracking completely." His prediction comes as the Canadian Ice Service prepares to issue its annual summer forecast. After a record melt in 2007, and an above-average melt last year, this coming summer is seen as crucial for determining the rate at which the sea-ice will disappear. The Catlin Arctic Survey has directly measured thickness of the ice An ice service analyst, Dr Trudi Wohlleben, said that the ice was likely to retreat as much as it had in the past two years. Typically, about 40% of the Arctic Ocean is covered with older, thicker ice, but that has been greatly reduced. Referring to the direct measurements taken by the Catlin team, Dr Wohlleben said: "It is very nice to have 'ground-truthing' of what you're interpreting from the satellite data. "So when we look at the imagery, we're expecting the first year ice to be between 1m and 2m thick and it's nice to have those numbers confirmed."



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