They are a majestic icon of Canada's North, but polar bears have also come to symbolize climate change. And scientists say the future for one particular population of polar bears, in northern Manitoba, is dire.

Biologist Nick Lunn has been studying polar bears for more than 30 years. He says the evidence is clear: the northern Manitoba polar bear population is in steady decline.

(Alex Beatty/Churchill Northern Studies Centre) (Alex Beatty/Churchill Northern Studies Centre)

Polar bears have long gathered in October and November near Churchill, Man., as they begin to move from their summer habitat on the tundra back to seal-hunting territory: the pack ice of Hudson Bay.

Duncan McCue/CBC

More than 10,000 visitors flock here every year, to see bears in the wild from the safety of tundra buggies. Polar bear tourism injects several million dollars annually into the economy of the town some call the "Polar Bear Capital of the World."

Duncan McCue/CBC

The bears' proximity to Churchill also makes them the most studied polar bears in the world. Scientists from the Wildlife Research Division of Environment and Climate Change Canada have been compiling data about this population of polar bears since the 1970s, based out of the Churchill Northern Studies Centre.

Duncan McCue/CBC

That invaluable database provides scientists with hard evidence of changes in climatic or ecological conditions. It's why researchers at the Churchill Northern Studies Centre know something is wrong.

Duncan McCue/CBC

The polar bears' annual migration ashore to the Churchill area makes them relatively easy to count in the spring and fall. From the air, the scientific team can spot a white bear on the tundra more easily than on sea ice.

Duncan McCue/CBC

Biologist Nick Lunn says the polar bear population in the region isn't what it used to be.

"There were 1,200 bears in the late 1980s here in Western Hudson Bay. The most recent estimate is down to about 800 or so," says Lunn. "That is cause for concern, but it's also a warning bell."

Alex Beatty/Churchill Northern Studies Centre

From the chopper, Lunn's team tranquilizes the bears. They then quickly tag them and take key measurements that allow the scientists to track the health of the bears over time.

Amy Johnson/Churchill Northern Studies Centre (Amy Johnson/Churchill Northern Studies Centre)

The story of the polar bear decline is in their fat. The bears of Hudson Bay now spend an average of 30 days longer on land than they did three decades ago, because warming global temperatures have resulted in shorter periods of sea ice. That means less time for the polar bears to eat their preferred food: ringed seals.

"The models all predict continued loss of sea ice, so that will mean for bears longer and longer and longer periods with no sea ice ... and that will not be good for polar bears," Lunn says.

A pair of cubs born earlier this year in Northern Manitoba is part of a declining population of polar bears being studied by Environment Canada researchers 0:09

In this video filmed by Amy Johnson of the Churchill Northern Studies Centre, cubs watch as samples are taken from their mother. Since the scientists began tracking the polar bears, fewer cubs are being born and they are less likely to survive. Lunn has not seen any females with triplets since 1996.

"Pregnant females are not as heavy as they used to be. Cubs' survival is down, weights are down," Lunn says.

Alex Beatty/Churchill Northern Studies Centre

The scientists cover the bear's eyes to keep them calm. The research team measures teeth, and also take blood samples.

Alex Beatty/Churchill Northern Studies Centre

Lunn says teenage bears are the group feeling the most impact from climate change. Since they are still growing, they need more energy reserves. Yet in fights over seals they often get pushed away by adult males, which means when they arrive in Churchill in October-November they are the hungriest. These often become "problem bears," coming close to people in their search for food.

The number of polar bear encounters in the town of Churchill reached record-levels last year: 351. Of those, 65 bears - the highest number ever - needed to be tranquilized and housed in the town's holding facility, known as the polar bear jail, before being released into the wild.

Dave Rae/CBC

Back in the laboratory at the Churchill Northern Studies Centre, Nick Lunn has another big batch of samples to analyze. He's not optimistic about what it will show.

"I expect a gradual decline in the size of this sub-population," says Lunn. "At some point down the road, if it continues, it won't be a viable population. They'll be gone."

Dave Rae/CBC

Lunn has no firm predictions about when that tipping point will come. But unless the planet begins to cool, polar bears are likely to disappear from the Churchill area within 20 to 40 years, he says.

Lunn also emphasizes that there may be other places in the high Arctic that have enough sea ice to sustain polar bears.