Had it been just one degree further down the thermometer, not even the polar bears could have managed what Lewis Pugh achieved in the early hours of yesterday.

Mr Pugh, a maritime lawyer and environmental campaigner from London, swam a kilometre (.62 miles) at the Geographic North Pole to highlight the effects of global warming. At -1.8C (28.76F), it is believed to be the coldest water a human has ever swum in.

Clad only in his Speedo trunks, cap and goggles as required by the rules of the Channel Swimming Association – which also forbid any buoyancy aids, swimming caps that offer any thermal protection or trunks cut above crotch level – Mr Pugh spent just under 19 agonising minutes in the melted sea ice navigating a path in a crack between broken floes.

The feat would not have been possible ten years ago, when the water was entirely frozen over, even in summer.

Mr Pugh, 37, confessed afterwards that the pain was so excruciating he almost gave up several times. At dead of night, but seeing his way in the permanent Arctic summer daylight, he entered the water at 2 am and reemerged at 2.18 and 50 seconds, perished but ecstatic.

“The water was absolutely black – it was like plunging into a dark black hole,” he said as his body temperature slowly returned to normal. “It was frightening. The pain was immediate and felt like my body was on fire. I was in excruciating pain from beginning to end and I nearly quit on a few occasions. It was without doubt the hardest swim of my life.”

He had been inspired, he said, by his friend and fellow environmentalist Jorgen Amundsen, the great-great-nephew of the first man to reach the South Pole.

Mr Pugh, who trained in a glacial lake in Norway, said: “I will never give up in front of a Norwegian, let alone a relative of Roald Amundsen.”

Because of its salinity, seawater freezes at a slightly lower temperature than fresh water. But the surface water at the North Pole is of relatively low salinity, and at -1.8C was on the verge of turning to ice that not even the bears could have swum in.

Most people who attempted such a feat would drown within minutes as the intense cold disabled their muscles. Mr Pugh believes that he can raise his normal body temperature by one degree by concentrating on raising his heart rate.

Tim Noakes, of Cape Town University, an expert on the effects of cold water on the human body, monitored the swim and found that on leaving the water Mr Pugh’s body temperature had dropped to 36.5C. Twenty minutes later it had fallen even further to a dangerously low 35C, but within an hour it had recovered to a normal 37C.

“To swim at the North Pole is an incredible achievement, and is the culmination of years of unique endeavour by an astonishing individual,” Professor Noakes said. “At the end of the swim, Lewis was showing obvious signs of distress but he never faltered and his performance was the best yet.”

Mr Pugh holds the record for the world’s most southerly swim, on the edge of the Antarctic ice sheet, and last year became the first person to swim the length of the Thames. He claims to be the only person to have completed a long-distance swim in each of the world’s five oceans.

He has already attracted the nickname “Polar Bear” for his cold-water swimming. He trained for his latest feat by eating five meals a day for three months and putting on 24 lb. Mr Pugh reached the Geographic North Pole by hitching a lift on an icebreaking ship sailing out of Murmansk in northern Russia.

The North Pole challenge was organised by the Worldwide Fund for Nature to raise awareness of environmental issues.

Scientists predict that by 2040 the Arctic could be virtually free of ice in summer. Mr Pugh said yesterday that his achievement was a bittersweet victory. “It’s a triumph and a tragedy – a triumph that I could swim in such ferocious conditions, but a tragedy that it is now possible to swim at the North Pole.”

Record breakers

Freediving Tanya Streeter: 160 metres in female category. Diver uses ballast

Freediving Stig Aavall Sverinsen: 225 metres in “dynamic with fins” category

Rowing Matthias Auer, Christian Klandt, Olaf Behrend; 163.42 miles in 24 hours

Swimming relay Most participants swimming one lap in 24 hours:2,889

Longest windsurfing journey Flavio Jardim, Diogo Guerreiro; 5,045 miles

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