Current affairs: Over to our man in the Thames ...

The extreme sportsman known as the Human Polar Bear has now set his sights on swimming the 203 miles from the Cotswolds to Southend. Could Jonathan Brown keep up?

When T S Eliot mused in The Waste Land on the pollution hazards lurking beneath the murky waters of the "sweet Thames", he envisaged nothing worse than "empty bottles, sandwich papers, silk handkerchiefs, cardboard boxes and cigarette ends". And that was a long time ago when, truth be told, the mighty river was an altogether more fetid proposition.

But as I strike out tentatively into the rolling deep at Temple Lock, near Marlow in Buckinghamshire, alongside Lewis Gordon Pugh, my concerns are of a more biological nature, namely diphtheria, tetanus and hepatitis A and B, not to mention Weil's disease, the life-threatening illness passed on through rats' urine. Apparently, though, there is nothing to worry about. "Just keep your mouth closed and don't swallow," Pugh reassures me, just after I have gulped down a couple of mouthfuls of murky water. "I keep all my jabs up to date but I did get a hepatitis booster before I started this," he explains. I rack my brains to remember the last time I had a vaccination.

To compound my paranoia we are just a few miles from the spot where a million cubic litres of raw sewage spilt into the river last year when drains overflowed during a storm. Only this week Thames Water topped the list of Britain's most fined polluters.

But, surveying the scene in Buckinghamshire, there is nothing more sinister to behold than children leaping joyfully from bridges and boaters getting stuck into their first gin and tonic of the day. What could be wrong with the world?

For Pugh, a 36-year-old City lawyer, the answer is quite a lot actually.

He is 13 days into a feat that will make him the first person ever to swim from the source of the Thames in a Cotswolds field at Mill Farm at Ewen (he actually had to run the first 20 miles until it was deep enough) to the point where it meets the North Sea at the end of the pier at Southend-on-Sea, Essex. In other words all 203 miles of its winding, wending way. However, it is not the everyday problems of water pollution that are on his mind. In fact, he says, the Thames is actually rather clean here, pointing out a dragonfly buzzing lazily through the reeds. His concerns are of a more global nature; global warming to be precise. "This is my first political swim," he tells me as we prepare to take to the water. "What convinced me to do this was time spent up in the Arctic. For the past couple of years I've been returning there to train and race, to become the first person to complete long distance swims in all five of the world's oceans," he says.

It is a goal he achieved earlier this year and the reason why the barrel-chested, Speedo-sporting figure beside me is now known as the Human Polar Bear, the ice king of cold water swimming. For as well as becoming the first man to conquer all the oceans, he has bamboozled the sporting and medical worlds by completing feats of endurance in water once thought too cold for humans. And not for Pugh, the relative comforts of a wetsuit, or even a good smothering of lard. His swims have been completed wearing just a swimming hat, a pair of goggles and, of course, those famous Speedos.

To date, he holds the record for the most northerly and southerly swims completed. On the premise that each new feat must be harder and more challenging than the last, in 2005 he methodically carved his way through a kilometre of icy water off Spitsbergen followed four months later by a similar distance in the Antarctic. He has traversed fjords, rounded rugged capes and naturally enough for a true Brit adventurer, kicked the whole thing off by crossing the Channel while still a teenager (although his relatively pedestrian 14-hour time in rough conditions still rankles).

Next week he hopes to have reached Westminster, where he will clamber ashore and present Tony Blair with a letter asking him to take global warming seriously.

It will be the centrepiece of a campaign being masterminded by WWF. But why do it here in the well-heeled splendour of the Royal Borough of Windsor and Maidenhead? According to some of the gloomier climate change forecasts, if present trends persist, the entire Thames Valley, including London and swaths of the South-east could be under water in 100 years as sea levels surge and the Thames floods. The Thames Barrier, which has been raised on just 55 occasions since it was completed in 1983, would be hoisted 325 times every year by 2100, it is predicted. A single flood of London could cost £30bn - 3 per cent of Britain's GDP.

In Pugh's beloved polar regions the effect is even more pronounced. In the past half-century, temperatures at the icecaps have risen by 4C, while an area of sea ice five times bigger than Britain has been lost in the past four years. Soon there will be no sea ice at all in the summer, lethal news for the swimmer's namesake, the polar bears that are rapidly becoming the symbol of the battle to avert catastrophic climate change.

"The jury is not out on climate change," he says. "It's happening and we need to take action. People have accepted the idea and now it's time to move to the next stage - changing our behaviour. Everyone including myself needs to think about the things that we do on a daily basis and how this is affecting the climate."

The world of long-distance swimming is a puzzling one for most people whose idea of endurance is a couple of lengths floundering up and down the local swimming baths. For Pugh, it offered the excitement and adventure enjoyed by mountaineers in the 1950s when few of what have now become the great climbs had been achieved. His father, a Royal Navy surgeon and historian, had introduced him to the tales of real-life derring-do. After moving to South Africa as a boy he first came to public attention when he swam through the shark-infested waters between Cape Town and Robben Island. But it was his "Pavlovian" ability to perform something called "anticipatory thermo-genesis" that has seen him conquer the world's coldest waters. The mysterious process allows him to mentally raise his core body temperature from 37C to a simmering 38.4C. This increase would take a typical Tour de France cyclist 30 minutes of hard pedalling to achieve. It is a profound advantage when trying to cope in an extreme environment allowing him to stay in the water for an additional 15 minutes compared to "normal" athletes.

But in the Thames, it has not been the cold but the heat that has proved his biggest foe. A two-week heatwave has seen temperatures soar to 37C and progress has been delayed by incessant stops to apply sunblock and rehydrate. The current has slowed to a trickle and he has fallen behind schedule. The daily routine is gruelling - one hour in the water, one hour out, with the aim of completing 13 miles a day - equivalent to half a Channel swim. "There is not a single morning I have woken up that I have not thought, 'I can't go another metre'," he says. But what keeps him going is not just his back-up team on the barge Thames Crusader, led by Major-General Tim Toyne Sewell, a former Sandhurst commandant, but also the extraordinary support he has received from the general public. As we make our way down stream passers-by call out support and river traffic fastidiously gives him a wide berth.

Labelled "hunky but chunky" by one report, he says he has lost nearly a stone since stepping into the water two weeks ago. Standing next to him one feels a little like a sapling beside a mighty oak. But, he tells me, the secret of his success is all about having the right attitude. "The major limiting factor to all human performance is in the mind," he says. "I get my inspiration from concentrating on the things I believe in - like averting climate change."

Steadily, he pulls away from me, with powerful, rhythmic strokes, desperate to make up time. More children shout from the bank. His next challenge, he says, will be to return to the Arctic where he will race "against animals". He won't reveal any more, but I wouldn't bet against him beating them.

Remarkable feats of strength and endurance

By Ed Caesar

* WALK AROUND THE WORLD

Although there is a story that the American George Schilling walked around the world in 1904, the first person recorded to have done so was Schilling's compatriot David Kunst. Kunst's feat, which took four years and three months from his start date of 20 June 1970, was not without mishap. He set out with his brother John from Waseca, Minnesota, but, in Afghanistan in 1972, the brothers were set upon by bandits. Only David survived the attack, and he finished his journey alone.

* ROWING ACROSS THE PACIFIC SINGLE-HANDED

The sport of ocean rowing is one of the more extreme of extreme sports. To row the Pacific single-handed was considered impossible until a British man, Peter Bird, did just that in 1983. Bird started his journey in San Fransisco, and, 294 days later, arrived in Australia. Sadly, he died attempting the return journey.

* MOST SIT-UPS IN 24 HOURS

Skip Chase not only possesses an all-American name, but the world sit-up record too. Two years ago, the 52-year-old fitness manager managed to knock out 110,915 crunches in a 24-hour period. "I'd never attempted to do more than, say, 50 sit-ups at a time," he said. There's hope for all of us yet.

* ULTIMATE FITNESS CHALLENGE

Paddy Doyle from Solihull completed a 13-discipline event to secure the title of world's fittest athlete, including a 12-mile run; a two-mile swim; a 110-mile cycle ride; 3,250 abdominal crunches and lifting a total of 300,000lb on weight machines.