Six am on a sub-zero morning in Devon. A five-mile run in the dark, ending in a couple of hill sprints. Breakfast. Circuit training in the barn; beyond any pain threshold to physical exhaustion. Lunch. Ninety minutes dragging weighted tyres up and down a 1:6 hill. The only upside is that the mud has frozen over. It's mindless, repetitive, punishing effort, not improved by an ex-marine shouting in your ear. Tea. A three-mile run, followed by more circuits. Die.

One day of this – well, most of it. OK then, half, and I'm shattered. For Ann Daniels, Martin Hartley and Charlie Paton, the three members of the second Catlin Arctic Survey into the effects of climate change, whose latest trip to the north pole was announced yesterday, it's day six of a week-long bootcamp. I'm just thankful to have avoided the 15-mile run across Dartmoor that entailed wading waist-deep through ice-cold rivers.

You can't pull a 120kg (265lb) sled over pressure ridges for 12 hours a day for 60 days if you're not fit. And if the three weren't polar fit when they started this camp, they certainly will be by the end. Yet fitness is just a small part of the package. Anyone – even me – could probably get fit enough if we were prepared to put the hours in, but few of us would last a day out on the ice.

Daniels is one of the world's leading polar explorers, the first woman – along with teammate Caroline Hamilton – to reach both the north and south poles as part of all-women teams, and she readily admits there are many people out there who are a great deal fitter than her – "I'm 45 now, for God's sake." Yet when it comes to endurance and sheer willpower, she's in a league of her own.

"You can train all you like," she says, "but nothing prepares you for the cold. On a good day it can be minus 15, on a bad day minus 45; factor in the wind chill and it can feel more like minus 70. The cold penetrates your bones and never leaves. Even when you're in your tent at night there's no respite. It's with you the whole time; you just have to try and shut it out. You can't always do it, especially towards the end of an expedition when you're exhausted."

It's the cold Hartley and Paton fear most too. They are also polar veterans and know exactly what's coming. "I'd done a lot of climbing in the Himalayas and I thought I knew all about cold," says Hartley, the expedition photographer. "But I was hopelessly unprepared the first time I went to Resolute [the settlement in the north of Canada that is the start point for Arctic exploration]. My equipment was totally inadequate and I would have died if someone hadn't lent me some warmer clothes.

"The first few weeks are bearable but once you start to get frostbite, the cold can start to affect your judgment, especially when you're living in such close proximity to other people."

British polar exploration is sometimes seen as the stamping ground of the upper-class adventurer, an image perpetuated in recent years by the successes of Sir Ranulph Fiennes and Pen Hadow, the director of the Catlin Arctic Survey. Daniels, Hartley and Paton don't fit the stereotype.

Daniels was a bank manager's assistant until 1996 when she heard a radio advert asking for ordinary women wanting to go to the north pole; Hartley spent seven years as a studio special effects photographer before going freelance; after joining some polar trips with the marines, Paton worked out that serving in Afghanistan was a great deal riskier than a melting ice-cap. For all of them – Fiennes and Hadow included – it is a full-time, professional career.

"It's not the type of job you get told about at school," Hartley says. "I started by funding myself to join expeditions to the Himalayas and the Pamirs and selling the pictures. Pen Hadow approached me at a talk I was giving and said, 'You must come north with me one day.' I thought he was trying to pull me and invite me to his country home. Since then I've been on 22 assignments to the Arctic and Antarctic.

"You do have to be quite selfish to do this kind of work. The expedition comes first and I forget about home life and relationships for its duration. Basically, you need to be not nearly as nice as you would like to be."

Unless you're lucky enough to have the clout to attract the sponsorship money and fix up your own gigs, your average explorer has to take the jobs that are going. You might be joining a team on a new route; you might be guiding amateurs who fork out £20,000 to be airlifted 60 miles from the pole and escorted in on foot. Or you could, like the Catlin team, become scientists for the trip – something that is more controversial than it sounds.

There's a long tradition of science in the polar regions. It's the ideal environment for everything from ocean-ography, astrophysics, meteorology and glaciolology to all things climate change and the Amundsen-Scott South Pole Station has been a base for scientists for more than 50 years. But the demarcation lines have always been firmly engraved in the ice. Scientists do science: explorers explore.

Hadow crossed the divide with the first Catlin Arctic Survey last year, when his team became the first to measure ice-thickness en route to the pole – a more critical determinant than surface area of the speed at which the ice-cap is melting. Some scientists and climate change sceptics went to great lengths to rubbish their findings that the ice-cap was melting faster than previous projections had suggested, pointing to weaknesses in methodology and ridiculing equipment failures. But the survey is back again this year to continue where they left off.

It sounds as if it should be a no-brainer. If you're doing science, take a scientist. It is, until you consider the conditions up north. The south pole is on a flat, frozen land mass. You can fly in directly, snuggle down in the warmth of the base and get on with your experiments. The Arctic is rather different. It is an unmappable ocean of ice that is constantly on the move, breaking up, melting and re-forming. You can't go anywhere and expect to stay in one place. Sometimes you can only go backwards. The ice is often crushed into giant pressure ridges that take hours to cross when pulling a heavy sled and a day's hard labour can see you further away from your goal than when you started if the ice flow is against you.

It's not the sort of environment in which scientists operate. If you're measuring ice thickness and water samples, you really want to be able to go back to the same place year after year; you can't do that if the ice is constantly on the move.

"The alternative is doing nothing," says Daniels. "And that's not an option where climate change is concerned. So the expedition's goal is to take samples every day on the trek towards the pole. Charlie will be drilling through the ice to take two water samples – one at the surface and one at a depth of 10m. Some samples will be filtered for microbes, and some will be frozen to have its CO2 content analysed. It will be back-breaking work after lugging a sled for 12 hours."

Hartley will be there to record every-thing; the landscape, the water, the science. "We have a responsibility to document the Arctic ocean in summer," he says. "I've been there many times over the last few years and the ice is melting. It's a fact. It could even disappear completely in my lifetime."

The expedition comes with its own health warning. Ice floes calve, people get injured and rescue isn't always possible if a plane can't land. And then there are the polar bears.

Back at the bootcamp, two marines give us a demonstration in unarmed combat. It's all quite handy for a night out in Streatham, but not much use if a polar bear is heading your way. Even if it doesn't have a knife. So what do you do if a bear comes sneaking up on you from behind a pressure ridge?

For the first and only time, the explorers look nervous. They can see the story. Climate change explorers shoot polar bear. "Um, you would fire the gun above its head to scare it off," says Daniels eventually. And if that doesn't work? "Look," Hartley laughs. "There's no such thing as a small polar bear." Fine. So the bear gets it.