On a cold Monday in December, one of Britain's most experienced polar explorers is sitting in an even colder room in Portsmouth, explaining his latest mission. The temperature has been turned down to a mere -20C, tropical by comparison to the almost inconceivable conditions he will have to endure during his next expedition to the Arctic. There, temperatures can drop as low as -90C; it is dark all day or the sun can blind people in minutes; the explorers will wake up, their eyelashes frozen together, in sleeping bags full of shards of ice; the ground beneath the trekkers' feet will be only inches of frozen water which can at any moment open into icy rivers which will kill almost instantly, and, apart from the odd grey seal, the only life they are likely to meet is a hungry polar bear.

Meeting Pen Hadow for the first time is something of a shock. He is the first man in history to have managed one of the ultimate feats of human endurance - to trek solo and unaided to the North Pole. But instead of the great strapping giant of a man you might expect, the 46-year-old is slightly built, and his hand, when he shakes mine hello, is almost the hand of a woman. As Hadow talks his breath frosts the air in front of his face, but he looks unperturbed while sitting still in this giant concrete freezer. Such small extremities, along with his brown eyes, olive skin and naturally low heart rate, make him ideally suited to a life of spending months at a time alone or responsible for teams of amateurs in one of the most inhospitable environments on earth.

Now, though, Hadow is about to embark on a very different expedition. In February he will leave northern Canada to trek more than 1,000km to the North Pole; what's different this time is that he is travelling with two fellow polar explorers, his friends Ann Daniels and Martin Hartley, and they will be dragging with them not just food and repair kits but 100kg sleds each, laden with equipment to take up to 12m readings of the depth and density of snow and ice beneath their feet.

The readings that Hadow and his team are taking will feed into our understanding of the Arctic's relationship with climate change. Based on occasional submarine journeys and more recently satellite data, charts of the total area of Arctic sea ice have shown a gradual decline over the past 40 years. Then, in 2007, the line on the chart appeared to drop off a cliff,

plunging below 5,000,000 sq km a full three decades ahead of forecasts. The dramatic events of two summers ago, when a Russian submarine rushed to plant a flag under the pole and Canadian and European governments tersely laid rival claims to sovereignty, led many scientists to warn that the Arctic sea ice could disappear entirely during the summer months much sooner than had been feared.

Most experts agree on the impact this will have on 5m Arctic inhabitants and the rest of the world - from the loss of the unique habitat that exists under the ice to rising global sea levels and possible changes to the ocean circulation and the weather patterns of the whole planet. Yet forecasts for when this will happen range from just four years to the end of the century. The reason is that very little is understood about the depth and density of the sea ice, and therefore the total volume of water frozen at the top of the world. This is what Hadow's Catlin Arctic Survey - appropriately sponsored by an insurance company - hopes to put right by providing the much-needed data about how much ice is left, and so help work out how much time we have to prepare for what is probably the most immediate, truly global threat of climate change. The survey is supported by the United Nations Environment Programme, the Prince of Wales and the conservation charity WWF.

"If you want to understand climate, we should invest more in making observations of climate change, and as the Arctic ocean is the amplifier of global warming, we should concentrate on the Arctic region to understand how fast the warming is taking place," says Wieslaw Maslowski, a research associate professor in oceanography at the US Naval Postgraduate School and science adviser to the Catlin survey.

Hadow puts it more chivalrously: "I see the Arctic as a maiden newly discovered on the social scene, and we're melting away her petticoats, and there are some avaricious types peering underneath, and someone needs to defend her honour."

Hadow's defining 75-day trek to the North Pole in 2003, alone and with no aeroplanes to resupply him, began with a spur-of-the-moment promise to his father on his deathbed, a promise which was to haunt him for 10 years through two earlier failed attempts and financial and health problems. So obsessed did he become that in his autobiography, Solo, Hadow wrote: "Above all other things, even the birth of my son, it seemed to be absolutely central to my being."

The roots of that trip and Hadow's long love affair with the Arctic lie deeper, though. His parents Nigel and Anne hired a nanny named Enid Wigley who had looked after Scott of the Antarctic's son Peter, and her routine involved teaching the young Pen to endure the cold by leaving him outside. She also spent years telling him stories of Antarctic explorers.

Years later, drifting in an unhappy job with sports management group IMG, Hadow found a book in the library of the Royal Geographical Society which was to bring

back those memories, lead him to both poles, and now set him on his mission to alert the world to the imminent threat to the Arctic. The book was the translated diaries of an obscure 19th-century German ornithologist called Bernhard Adolph Hantzsch who, after being shipwrecked, died trying to trek across the far north of Canada to find a ship home. Hadow was captivated and decided to finish the German's journey.

"I remember walking back to the office, thinking: 'Of all the books I have, 90% of them are written by adventurers and explorers and scientists: Francis Chichester, Jacques Cousteau, Chris Bonington, Ranulph Fiennes, [Robin] Hanbury-Tenison,'" recalls Hadow. "It never occurred to me until that moment that I could ever lead a life approximate to those. In that moment I thought: 'I'm going to start this journey.'"

The official history records that, thanks to "Nanny" Wigley and Hantzsch, Hadow advertised for a companion, made his first journey, and was hooked. Reading between the lines of his biography, though, there appears to be another crucial factor in his career choice: an extraordinarily strong need to prove himself, from hanging upside-down from trees as a child to taking up competitive gardening and school sports.

"There were lots of reasons why I did it [the solo trek] which were based around this vow I made, the main reason being that at the time it was regarded as the ultimate feat to be done," Hadow admits in conversation.

If anything, the latest expedition comes even closer to fulfilling this need. After the solo feat, Hadow was researching his book, and while in bed one night read a report by the US Navy discussing design changes to its ships undertaken to cope with changing sea ice because of global warming.

"I thought: 'Even I don't really know about this and I'm in the almost unique situation of having this relationship with the Arctic,'" he says. "I thought: 'I could be the amplifier or explainer; I might be the person to reach out to as wide an audience as possible, globally, to tell them what's going on.' That's what explorers do, classically. They discover information and then have the potential to engage audiences."

With a new reason to return to the Arctic, Hadow asked climate scientists how he could help. He discovered that measurements of sea ice began in the 1960s, but for three decades there were only annual submarine voyages, providing too little data to be sure what was happening more broadly. Since the 1990s, satellite maps have been used to calculate the height of snow and ice above the waterline, but experts have to make assumptions about the roughly five-sixths of mass underneath, and there is a "hole" in the data over the North Pole which is 1,600km across.

The satellites show that in 2007 alone, the Arctic sea ice lost an area nearly the size of Alaska, reaching an all-time low of 4,130,000sq km on 16 September. Following this and another poor year in 2008, the US National Snow and Ice Data Centre now calculates the permanent sea ice - measured in September at its nadir - is receding by 11.7% a decade, or an average area the size of Scotland every year.

Little is known about thickness, nor about the density of different layers of snow and compacted ice. Submarine data suggests a 40% thinning between the 1960s and 1990s. Last year the journal Geophysical Research Letters published a paper by three experts at University College London which calculated that the ice in the winter of 2007/8 was thinner than the previous five-year average by 26cm, plus or minus 5cm. The margin of error reflects the lack of long-term and wide-ranging data. Last September, despite a cooler summer, the sea ice only recovered to its second-lowest recorded extent, possibly because there was more thin first-year ice than usual. And some scientists think the total volume last year was even lower than 2007, says Maslowski.

Part of the wide range in estimates for when summer sea ice will disappear is due to uncertainty about how quickly the exposed darker sea will warm, triggering a cycle of more melting and warming. The models also differ in their varying assumptions about ice thickness. Maslowski, whose team has projected the most aggressive date - between 2010 and 2016, based on current trends - argues, for example, that too little is known about increasingly warmer water brought by ocean currents from the Pacific and Atlantic, and its contribution to melting sea ice.

A few scientists do venture to the far north, usually by boat or plane, to drill cores or take radar measurements, but in an area which in winter can cover up to 4% of the globe, there are only about a half a dozen such locations, says Seymour Laxon, one of UCL's experts. The problem is that few scientists have the inclination, physical endurance, time and money to do the training necessary to spend months in such harsh conditions, says Hadow, who has raised almost £3m and spent years planning the trip, including an extra delay after funding fell through for 2008.

"What captivated me more than anything was that I could do this," says Hadow. "For once in my life I was in the right place at the right time."

After the cold room, the explorers have more chilling work to do at the Institute of Naval Medicine in Portsmouth. As well as walking, the team expects to swim for up to 100 hours, dragging their specially designed sleds over "leads" of water which can open up between ice pans. While swimming they will wear bright-orange immersion suits, which they now put on, lowering themselves into the pool of icy water. Hadow says it's like being "shrink-wrapped". I tried it for myself and it was how I imagined it would feel to swim in mercury. The pool temperature is much warmer than the cold room, 4C, but because water conducts heat 26 times better than air, it "rips" the heat away from the explorers. The final test is to get back into the water in only their walking clothes to simulate what would happen if they fell through the ice. If they do, the water is likely to be even colder, probably below zero - salt water freezes at -1.8C.

Early polar explorations left a trail of graves - men killed by hypothermia, scurvy, gangrene and even poisoning after eating the livers of polar bears. Modern science has alerted those who have followed to many of these dangers, and provided remedies. But for all the advances in modern technology, many risks still remain, almost all of them bound up in the landscape over which the team will have to walk and swim.

"Your brain is so used to visual information pouring in that when you go there the instant impression is: there isn't anything up here - it's all white," says Hadow. In the first few days, the brain "retunes", and as the other senses are dulled by the cold and the heavy layers of clothing, the eyes become more alert.

"Some people talk about the Arctic as a monotonous wilderness of white, but if you open your eyes and look at the landscape, especially in spring, you realise that there are no whites whatsoever," says Hadow in Solo. "Everything is in shades and tones of pastel colours - cream, grey, blue, green, yellow, orange, pink - and only in the stark bright light at the height of the polar summer, when the sun is high in the sky, do you begin to see true whites among the other colours."

Nor is the Arctic a great flat glass to glide over. Before each trip, Hadow spends hours on Dartmoor pulling strings of tyres around tors, getting caught on and under rocks, untangling, pulling, shoving and scrabbling over cold wet granite to prepare for the huge pressure ridges he will have to clamber over: steep walls of frozen slippery ice rubble which test both his physical strength and patience. Even on the "flat" pans, the ice is "rough, cracked, pitted and pocked with holes, lumps, bumps, projections and cracks where your burden becomes wedged or threatens to topple over, spilling its load," he writes.

Then there are the wind and currents, which constantly work on the great floating, constantly changing landscape, grinding ice together, pulling pans apart, sometimes so "rapid as to equal a ship running before the wind", to cite the evocative description of the ancient Norse writer of Kongespeilet. Not infrequently travellers have to make huge detours or backtrack over a ridge or rubble field because of an impassable lead; occasionally they wake to find they have drifted south of the point they began walking the previous day. And the sounds of all this movement are amplified by the otherwise silent emptiness. The landscape is so empty that in 2003 Hadow recorded that the only life he encountered were three seals, one snow bunting and the tracks of a single polar bear.

By day, the whooshing of skis and scratching of poles and the roar of wind past their ears dominate the explorers' world. At night, however, after the cooker is turned off, they lie with their heads on the ice and listen to it. "You wouldn't conceive such random movements could produce such metronomic sounds: you get this der-der-der-der-der-errrr, der-der-der-der-der-errrr. It's disconcerting because it tends to be the ice breaking up around your tent, often literally around you. This happens three or four times in an expedition," says Hadow. "You have to take a view: will this open up and will we be falling in in the morning, or will it be little hairline cracks rather than major fractures?" Sometimes the wind also beats against the tent like a drum.

Today, the biggest threats Arctic explorers face are those things that happen quickly, before help can be summoned from a few hours' away, or possibly days if the weather is bad. There is the moment-to-moment threat of falling through the ice - a risk which rises with every year the ice recedes. There are the constant dangers of being crushed by sleds, a sudden serious illness, and always the fear of a polar bear attack. Then there's the nightly gamble with carbon monoxide poisoning from burning stoves inside tents.

And all the time, of course, there is the ever-present, grinding cold. In temperatures as low as -50C, with wind chill that ...#8594; ...#8592; can sink to -90C, cold remains a constant danger. Travellers cannot stop for more than 10 minutes to mend equipment or they start to freeze - mucus dries like gravel in the nose, contact lenses would freeze to eyeballs, unprotected parts of the body can be frostbitten before you have noticed, fillings in teeth expand and contract, sweat freezes under clothes, and as the temperature drops the human brain begins to slow, making people less responsive to problems - in extreme cases causing them to make the problem worse by acting in exactly the wrong way, such as undressing.

The constant struggle over the ice, the stress and cold are compounded by exhaustion. To keep their sled weight down, the explorers calculate they can survive on a "deficit" of about 1,500-2,000 calories a day, but after two weeks their body starts to consume muscle to keep going. And, despite their exertions, sleep is often hard.

"For the first month you're cold every night, shivering," says Hartley, "then you worry about polar bears sniffing around looking for a canapé in a sleeping bag."

To cope with such conditions, Hadow adopts an almost obsessive regime of walking, eating from his "nosebag" of chocolate and nut rations, and checking his condition and his kit regularly. In a team, some risks are mitigated by having other people to help. But this time they will carry much more weight because of the measuring work and Hartley's cameras and video equipment, and the trio has also taken advice from a psychologist about how to cope with personality problems that might arise. Despite all these reasons, getting to the North Pole is still "85% in your head", says Hadow. "Over the 70-odd days I was there last time [for the solo trip], I would only think there was less than half a day when all things were good."

"It messes with your mind," he says in another conversation. "The Arctic is a dynamic surface, and there are all sorts of things that can go against you. It feels like you're against a mightier force, which is a disastrous way to reach your goal. It's like a white crucible. You put yourself or your team in and apply a Bunsen burner to the crucible, and all the fluff and juices are evaporated off and you're just left with the essence of those people."

For these reasons and the added difficulties of dragging extra weight, having additional tasks, raising many times the usual cost of a polar expedition, and - ironically - the worsening ice conditions, Hadow admits they cannot guarantee success. His own training has also been interrupted by one of the many viruses which gripped the UK this Christmas. "We cannot know whether we can do it," he admits. "But we're not just giving it a go - we're very locked on to going the distance, to 90 degrees [north]."

All being well, on 24 or 25 February the Catlin survey team will leave the base at Resolute Bay in northern Canada, be flown up to 80°N 140°W, where the multi-year ice begins, and start walking northeast along the line of 140° longitude. There can be no maps of ephemeral sea ice, and Hadow believes that the route has not been taken for 40 years, since Sir Wally Herbert, after whose wife Hadow has named his sled.

As they travel across the ice pans, a radar specially designed for the conditions, weighing just 4kg, will take a measurement every 10cm. The team will also stop regularly to drill cores of snow and ice and take measurements of the ocean temperatures and currents below. As they travel, Hadow will dictate notes into a special voice recorder about the regularity and make-up of ridges and other features. And Hartley, a multi-award-winning photographer of difficult environments, will capture their progress and the landscape. The data will be fed back via satellites to the scientists every night, and they hope that early results will be available before a UN meeting at Copenhagen in Denmark in December, when the world's governments will be asked to agree an ambitious treaty to cut greenhouse gas emissions and so, it is hoped, reduce global warming and the resulting climate change.

"Once they have a figure for how long the Arctic sea ice will be there for, they will have to act," says Hartley.

Scientists already believe that melting ice is responsible for average air temperatures warming twice as fast in the Arctic as in the rest of the planet. So far they believe the melting of the floating ice has an undetectable effect on global sea levels and the expansion of the warming water contributes less than 1% of the annual average rise. But if the ice melts further, or disappears, that cycle of melting and warming will add noticeably to sea levels, and there are emerging concerns that the warming water temperatures are speeding up the melting of the Greenland ice cap, which could add metres to sea levels. Less certainly, the influx of fresh and warmer water could start to alter the planetary circulation of ocean currents and winds which dictate weather patterns.

At the extreme, this could trigger one of the more catastrophic "tipping points" of climate change - the switching-off of the "thermohaline circulation" which brings warm water from the Tropics to the northern Atlantic and sends cooler Arctic waters south - events which were dramatised, if somewhat fancifully, in the film The Day After Tomorrow. In the impeccably bureaucratic language required to achieve consensus among hundreds of scientists and governments, the latest UN Intergovernmental Panel on Climate Change report described such an abrupt transition as a "low-confidence" event - that is, a 2 in 10 chance.

At best, Maslowski does not believe the results will give them more than a decade beyond his 2013 projection before the end of all summer ice. What then? Is it not too late?

"Even if it's too late to do anything about sea ice, what other wildernesses are we going to let go?" adds Hadow.

• Follow the expedition team - with regular updates on the explorers' progress, physical condition and more - at www.catlinarcticsurvey.com