By Kevin Bishop
BBC News, Churchill
On the shore of the Hudson Bay in Northern Canada, a huge male polar bear stretches and yawns, sniffs the air and rolls back onto his side to sleep.
It is mid-November and bitterly cold. The sea wind blows through our protective coats and our fingers start to lose circulation. But ironically, what we are witnessing is climate warming in action.
Temperatures in western Hudson Bay have been steadily rising 0.3 to 0.4 degrees every decade since 1950.
Scientists at the US space agency's (Nasa) Goddard Space Flight Center, who have been monitoring the sea-ice from satellite data, believe it could be retreating at a rate of up to 9% every 10 years.
Each autumn, polar bears in this part of Canada migrate north, heading for the sea-ice which begins to form about now and stays solid until late spring the following year.
By sniffing the air, the bears know when the temperature is dropping and the sea is beginning to freeze.
ARCTIC SEA ICE EXTENT - SEPTEMBER TREND, 1978-2005
The straight line tracks a more than 8% decline per decade
The bear, who has been named Echo by scientists, should by now be way out on the frozen waters hunting for seals.
He has not had a proper meal since the ice broke up in July. He is hungry and losing up to a kilogram in body fat every day.
For the past 30 years or so, people living in Canada's north have been noticing a phenomenon that many scientists now believe is a direct result of our planet warming up.
The waters of Hudson Bay - and many other northern seas - are beginning their annual freeze later each year.
This November, local residents are saying that the waters are up to a month late in freezing up. Similarly, in spring the ice is breaking up earlier.
Hungry bears will head into town - and conflict with humans
The net result - polar bears have less time on the solid ice to hunt.
Bears can only catch seals on ice. Seals are mammals and therefore need to breathe oxygen.
To do this they use holes or cracks in the ice to come up and gasp in some air between bouts of swimming underwater.
They also shelter in small pockets of air just under the surface. Bears anticipate this and mainly catch their prey when they are up for air.
Dr Jane Waterman is a biologist from the University of Central Florida. She has been coming to the shores of Hudson Bay in Canada for several years now, primarily to study adult males' play-fight rituals.
She has noticed worrying trends in the condition and behaviour of the bears as the freeze gets later each year.
The male bears spend some of their time play-fighting
Watching through her binoculars as bears alternately play-fight then slump down to sleep, she tells me that the long summer fast means that by now the bears are starving.
The greater period of fasting means they don't have as much energy. They are surviving by chewing on strands of kelp and seaweed to keep their juices flowing; but they need to eat soon.
We are trundling through the mud and ice of the seashore in a so-called tundra buggy.
This huge contraption that towers above the land like some lunar explorer serves not only to negotiate the harsh and slippery terrain, but also to keep the scientists safe from the bears.
At times, inquisitive and hungry males approach the buggy, standing on their hind legs to get a closer look at us.
The interest goes both ways
They appear healthy and in fact observers say that there is little to indicate that the long fast has had too detrimental an effect on them this year. But this is due to a good eating season this past spring.
The worry is whether they can adapt to successive years of shortened feeding periods on the ice.
For the residents of the nearby town of Churchill this trend raises an additional concern. Manitoba Conservation warden Sean Bobier patrols the town and its shoreline outskirts, protecting the residents from bear attacks.
As they grow hungrier, the fear is the bears become more aggressive in their search for food and so come closer and closer to town.
The local refuse dump has been closed in the last few weeks to distract the bears, but they are still a threat.
A 'mouse trap' on a scale for polar bears
Sean checks one of the "polar bear traps" his office has installed near the beach. It is a large container - shaped like a dustbin - with a door that snaps shut as soon as a bear wanders inside to the seal meat strung up in a bag.
Once caught, the bear is tranquilised and then netted to be flown away to safety by helicopter.
As yet, nobody really knows whether what we are seeing here on the shores of Hudson Bay is a direct affect of the greenhouse gases we are pumping into the atmosphere.
No-one can say either whether it is a trend that will continue to worsen, or a natural cycle that will eventually be reversed.
What is clear is that every year, as the sea-ice goes through its cycle of freeze and melt, the polar bears of Churchill will be among the first to let us know.