Pen Hadow said this expedition was about science and discovery
A British team has begun a gruelling trek to the North Pole to discover how quickly the Arctic sea-ice is melting.
Renowned Arctic explorer Pen Hadow and two companions were dropped onto the ice by plane some 668 miles (1,075km) north of Canada on Saturday night.
During their 1,000km journey, they plan to take measurements of the thickness of the ice.
It will be the most detailed survey to date and the team are expected to reach the North Pole in late May.
BBC environment correspondent David Shukman said that where there were gaps in the ice, the team would put on survival suits and swim, all the time dragging a radar system which will measure the thickness of the ice.
He said satellites had tracked how the area of Arctic ice was shrinking but this expedition should reveal how thin the ice was becoming.
The ice cap is widely believed to be melting at an increased rate because of global warming.
Only a few years ago, scientists predicted that by the end of this century the Arctic could be free of ice in summer. Some now say that could come far sooner.
Mr Hadow, 46, and the other members of the British Catlin Arctic Survey group, Ann Daniels, 44, and Martin Hartley, 40, will attempt to gather important new data about the state of the ice in winter and early spring - when the ice reaches its greatest extent.
It is intended to give scientists the very latest "ground truth", to better constrain their models and their interpretation of the observations coming from satellites.
Arctic ice modeller Professor Wieslaw Maslowski, a science adviser to the survey, hopes the data will enable him to refine his forecast of when the first ice-free summer might arrive.
Currently, he has it down for 2013 - but with an uncertainty range between 2010 and 2016.
Mr Hadow, a father-of-two from Hexworthy, Devon, became the first person to trek solo and without support from Canada to the North Pole in 2003.
He said this expedition would be about science and discovery.
"I think this time does feel different," he said. "We're only doing this because there is such an urgent need for more ground-truthing data about the permanent floating sea-ice.
"If, as scientists tell us, the ice is thinning quickly, then it should set alarm bells ringing around the world."
Both Mr Hadow and expedition photographer Mr Hartley, from ......ney, east London, will celebrate their birthdays during the trek.
Ms Daniels, a mother-of-four from Whimple, Devon, will be navigating.
They will be re-supplied every 20 to 25 days on their mission but have had to put on excess weight because, although they will be taking in 6,000 calories a day, they will be using up around 7,500.