When you're up above the Arctic Circle, on the trail of polar bears who haven't eaten a square meal in months, it's advisable to follow a few basic rules.
Number one, as perishing cold as you may be, is don't drink too much coffee.
Unfortunately, as an incurable caffeine addict, wildlife documentary maker Nigel Marven can't adhere to this great unwritten imperative while filming his latest series out on the frozen North Canadian tundra.
As a result, I find myself peering anxiously from the safety of a frosten-crusted Jeep, wondering whether I am about to witness the moment that Nigel becomes his star performer's lunch.
Polar bears, you see, have an acute sense of smell which helps them to track down prey up to 60 miles away.
Doomed? The polar bear population today is around 25,000
Normally, they use it to sniff out seal pups or Arctic foxes, but when the call of nature forces Nigel to venture out on to the ice (coffee being a diuretic), one keen-nosed 1,200-pounder scents the unusual smells of coffee.
Unseasonably warm weather has left the huge male bear stranded for almost four months, far from his winter hunting ground on the edge of the sheet-ice - so a meaty, 6ft human looks too appetising to resist.
Nigel is just emerging from behind a snow-dusted willow bush when the great white bear comes loping towards him. His instinct is to turn and run for it back to the Jeep.
But the 46-year-old, famed for his daringly close encounters with dangerous animals, quickly remembers the rest of the bear-stalker's survival code
Realising he will never outpace a creature capable of springing across the slithery surface at 25mph by using his huge paws like snow-shoes, Nigel stands stock still.
Then, showing the bear that he isn't afraid, Nigel raises himself to his full height.
At the same time, he avoids eye contact to let it know he isn't a threat (a fact that seems rather obvious, given that the approaching beast is 3ft taller and seven times heavier).
Alarmingly, however, the bear just keeps on coming.
He is within eight or nine yards of Nigel - close enough for even a man who has swum with Great White sharks to feel concerned - when he is stopped in his tracks by two loud cracks from a pump-action rifle.
The warning shots have been fired by Dennis Compayre, a grizzled old polar bear hand hired to act as Nigel's "eyes and ears" as he films Polar Bear Week, a captivating five-part series which begins on Channel 5 next week.
Since the cameras have stopped rolling, and we are making our way back to base in the gathering gloom, viewers will not see this relatively narrow escape.
Later, however, Nigel is quick to praise his minder.
"This man is my best friend!" he grins, giving Dennis a hearty slap on the back.
Dennis, whose white-flecked woolly beard and thick grey hair make him look remarkably like the creatures he has been observing at close quarters for almost 30 years, accepts the gratitude with a "seen-it-all-before" nod.
To explain what might have happened, he recounts the chilling story of a female researcher in her 20s who was savaged near here.
The only predator that will actively stalk a human, the polar bear had hidden in wait behind the huge tyres of a tundra buggy and pounced as the woman disembarked from a helicopter and dashed to the vehicle.
"She had four huge puncture wounds in her back, and would have died if a guy hadn't jumped out of the buggy and hit the bear with a long pole," Dennis says.
"Those bears seem to love the scent after people drink coffee, and I'd hate to have to shoot one."
We are filming in Churchill, Manitoba, the so-called Polar Bear Capital of the world, where these creatures seem to have more rights than the humans - for good reason.
Not long ago, this isolated outpost on Hudson Bay was in financial trouble.
Then, wealthy tourists discovered the thrill of nature-watching breaks and Churchill, home to the most easily accessible polar bear population, became a fashionable - and newly prosperous - adventure holiday destination.
Although the town is still accessible only by train or light aircraft, its guesthouses are packed during late summer and autumn, when the vast ice-sheet over the bay melts, forcing around 1,000 bears to lollop around for months on the shore.
Lately, however, it is not only polar bear watchers who come flocking.
With the clamour over global warming, it has become a magnet for an army of environmentalists and climatologists who have given Churchill an air of impending doom.
The Arctic ice-cap is shrinking fast, is their message, and as it disappears, so too will the polar bears.
Today, the polar bear population may hover healthily around 25,000 (they live in Russia, Alaska, Greenland, Norway and Canada).
Yet, we are repeatedly warned, if the planet continues to overheat at the present rate, within four decades our biggest carnivore will be extinct, starved to death as its natural hunting grounds disappear.
"Come up and see them while you still can," is the gist of their depressing refrain.
To some Churchill residents, who base their opinions on personal experience rather than fancy charts and computer models, this is so much nonsense put about by scaremongers for their own dubious ends.
When outsiders question whether anyone would be so cynical, they are reminded of that now-famous photograph of a polar bear which appears to be teetering precariously on an Arctic ice-floe, melting faster than ice-cream, in the depths of winter.
For a while, it became a powerful symbol of the perils of global warming - until it was revealed to have been taken three years ago and during the height of summer.
And so the battle lines between Churchill's optimists and pessimists have been drawn.
Nigel Marven's new series does not pretend to answer the complexities of this increasingly heated debate.
True to his easy-going style, he prefers to glory in the natural wonders of the Arctic.
In addition to countless polar bears, he came eye-to-eye with musk ox and moose, blubbery great walruses and curious little lemmings which, we discover, aren't really suicidal after all.
He also met fluffy white seal cubs, giant owls and snow buntings, and foxes whose coats change colour from cinnamon to silver with the passing seasons.
He took an icy dip with mystical white beluga whales and marvelled at the most breathtaking light show on Earth: the Aurora Borealis.
Inevitably, after studying the bears for 80 days and speaking to the people who live among them, he formed his own view about "the disappearing polar bear" controversy.
Flying into Churchill, the weather seems cold enough.
If minus 5C means the greenhouse effect is upon us, heaven knows what it was like before.
According to my taxi driver, however, the seasons have changed, and by rights it should be a whole lot colder.
"Last week, it was minus 20C, but now it's suddenly warmed up again, and not long ago that never happened," he informs me.
In Churchill, the effects of this odd upsurge in temperature are clear.
By this time of year, Hudson Bay has usually refrozen and the bears are beginning to slide off to hunt seals on the fringe of the ice-sheet.
After freezing briefly, however, it has now melted again, and so the bears are still very much among us.
One morning, disconcertingly, I awake to learn that a family of five has been wandering around outside my hotel.
Meanwhile, at the so-called "polar bear jail" - where bears who persistently loiter around town are held after being tranquillised, pending their re-release into the wild - all the concrete cells are full.
This presents the local wildlife authorities with a major headache.
Most of these errant bears are adolescents who haven't yet learned to behave.
But you can hardly give a loutish bear an ASBO. Venturing out of town, we also find bears in abundance.
Researchers have found that their weight has dropped by up to 20 per cent because the melting ice has reduced their feeding time and forced them to swim longer distances hunting for prey. But the ones we see look healthy enough.
Filming these deceptively cuddly-looking creatures is a precarious business, but our cameraman, Peter Thorn, captures some amazing footage.
One afternoon, we watch from a few yards as two fully grown adults stand on their hind legs and box one another, in a sparring context that seems specially staged for us.
"This behaviour is unique to the Churchill bears," whispers Nigel.
"We think they do it because this is the only place they congregate.
"They're testing their mettle because, next spring, they will be fighting for real, over females."
Later, out on the tundra, we encounter a big, ten-year-old old male with distinctive scars on his nose.
"Old battle wounds," remarks Dennis Compayre knowingly.
He calls to the animal which he knows well and has nicknamed Dancer - and the bear immediately pads over to us and rises up to the viewing platform on his hind-legs, coming so close that our minder can pat him on the head.
The bond between bear and man looks uncanny until, with a wry grin, our minder explains that he used to share his breakfast with the bear - violating strictly enforced laws that forbid feeding them, for fear they may become sensitised to humans, and therefore more dangerous.
"Well, why shouldn't we feed them, if they're really so hungry?" he says, hankering for the days when he was allowed to take to the ice with a bottle of Scotch (for himself) and a tub of lard (for the bears).
"What do these do-gooders think we should do? Just let them starve?"
Born and raised in Churchill, Dennis is among those who eye the new "experts" in town with deep suspicion.
According to Polar Bears International, the most prominent and widely respected campaign organisation, the West Hudson Bay bear population has fallen by 22 pc since 1987 and its prospects are bleak.
"If we lose the sea ice, we're going to lose the bears," says Dr Andrew, who serves on the group's scientific advisory council, arguing that they will not be able to adapt quickly enough to become vegetarians if and when the ice melts, leaving them with no hunting grounds.
His world-renowned colleague, Dr Ian Sterling, who has studied the bears since the mid-1970s, says that the ice now breaks up about three weeks earlier and so the bears have a shorter time in which to store up fat.
"There's a direct relationship between the date of the ice breakup and survival.
"The health, or condition, of the bears has declined over the past 30 years."
Dr Sterling says this is the reason why more "problem bears" are appearing in Churchill - and perhaps even why one came sniffing after Nigel Marven drank all that coffee.
"A starving bear isn't going to lie down and die. It's going to look for an alternative food source.
"In West Hudson Bay, that means either garbage dumps, hunting camps or, occasionally, people."
Dennis Compayre raises bushy grey eyebrows as he listens to the environmentalists predict the polar bear's demise.
"They say the numbers are down from 1,200 to around 900, but I think I know as much about polar bears as anyone, and I tell you there are as many bears here now as there were when I was a kid," he says as the tundra buggy rattles back to town across the rutted snowscape.
"Churchill is full of these scientists going on about vanishing bears and thinner bears.
"They come here preaching doom, but I question whether some of them really have the bears' best interests at heart.
"The bear industry in Churchill is big bucks, and what better way to keep people coming than to tell them they'd better hurry to see the disappearing bears."
After almost three months of working with those who know the Arctic best - among them Inuit Indians, who are appalled at the way an animal they have lived beside for centuries has become a poster species for "misinformed" Greens - Nigel Marven finds himself in broad agreement.
"I think climate change is happening, but as far as the polar bear disappearing is concerned, I have never been more convinced that this is just scaremongering.
"People are deliberately seeking out skinny bears and filming them to show they are dying out. That's not right.
"Of course, in 30 years, if there's no ice over the North Pole, then the bear will be in trouble.
"But I've seen enough to know that polar bears are not yet on the brink of extinction."
Just then, spotting a red fox close to the ice track, Nigel calls for the driver to stop.
The timid creature makes off across the snow-blanketed scrubland as Nigel, reaching for his binoculars, dashes off in pursuit.
Within a few seconds, he has almost disappeared from view. Out in prime polar bear territory as darkness descends.
"That Nigel's a hell of a nice guy, but he gets my old blood pressure up," sighs Dennis, reaching for his rifle.
Polar Bear Week With Nigel Marven begins on Channel 5 on Monday, December 17 at 7.15pm.
