For polar bears survival is all about sea ice.
The largest land-based carnivore on the planet evolved to hunt seals on the pack ice. Its snow-white coat provides camouflage, its huge forepaws help it swim between ice flows and, unlike other bear species, its paw pads are covered with fur, giving it better traction on snow and ice.
It is perfectly designed for life in the Arctic – or at least it was. A number of recent studies show the environment the polar bear evolved to master is literally melting under its massive feet, forcing the predators to forage on land where they have difficulty finding prey.
Nowhere is the impact of climate change on sea ice habitat more significant than in Canada, which is home to about 16,000 of the estimated 20,000 to 25,000 polar bears in the global polar region.
Canada's polar bear population is divided into 13 management units in the Arctic, and in most of those regions the number of bears is considered stable. Environment Canada has not listed polar bears as "threatened" under the Endangered Species Act, as other nations with polar bear populations (Russia, Norway and Denmark/Greenland) have.
"The polar bear does not have a small wild population, it does not have a restricted area of distribution and no marked [population] decline has been observed," Environment Canada states on its website.
However, recent science suggests in the long run, polar bears will be threatened throughout their range because of sea ice retreat, which is happening earlier in the year and lasting longer.
A paper published last November in the journal PLOS One, warned that the sea ice habitat in the Canadian Arctic archipelago is shrinking so dramatically polar bears may soon have trouble surviving.
A computer model predicted a number of scenarios, all of which pointed to increasingly long ice-free seasons in the Arctic.
"Polar bears fare poorly when sea ice is absent for prolonged periods, losing body mass without the opportunity to hunt," states the paper, which predicted the situation could become critical for polar bears by 2075.
"Polar bears rely on sea ice as a platform for hunting, migrating, and mating, but are forced to move to land in regions where sea ice does not seasonally persist. Energetics modeling and population projections indicate that continued sea ice loss with climate warming will negatively affect polar bear survival and reproduction potentially leading to population declines," it states.
The study found that 2 to 3 per cent of adult polar bear males could starve when the ice-free period reaches 120 days, and up to 21 per cent could starve if the ice-free period lasts 180 days, as it is projected to within 60 years.
"Similarly, early break-up of sea ice could result in reproduction failure in 55 to 100 per cent of pregnant females," states the research, led by University of Alberta PhD student Stephen Hamilton.
"The speed at which ice is disappearing in the Arctic today is beyond what polar bears can adapt to," Mr. Hamilton wrote in an e-mail from Norway. "Our work suggests that if no action is taken to reduce carbon emissions even the northernmost reaches of Canada's Arctic Islands could eventually have an ice-free season that is too long to sustain a viable polar bear population, and that region has long been considered the last bastion of polar bear habitat in the face of continued warming…"
Mr. Hamilton said in the Arctic researchers are seeing drops in the number of bears and "an overall decline in body condition," clear signs the apex predators aren't getting enough to eat.
Mr. Hamilton noted that 10,000 years ago polar bears were in the Baltic Sea, when the climate was much colder and sea ice there was extensive.
"Today, the Baltic Sea still gets some sea ice – enough for the seals to remain – but not enough to maintain polar bears," he wrote in his e-mail. "This historical record gives us a pretty good idea of what happens to the bears when ice conditions diminish: namely, they can't survive and their range is constricted."
In two other recent papers, Researchers in Canada and Greenland found evidence of how the loss of sea ice is affecting polar bear behaviour.
A study led by Dr. Samuel Iverson, published in Proceedings B of The Royal Society, found that with sea ice in retreat, polar bears are increasingly foraging in bird colonies on land.
"Ice-free seasons are becoming longer in many areas, which has reduced the time available to polar bears ( Ursus maritimus) to hunt for seals," writes Dr. Iverson, who did the paper as a post-doctoral researcher in the Department of Biology at Carleton University.
He found that "there is an inverse correlation between ice season length and bear presence" in common eider and thick-billed murre nesting colonies.
In an interview, Dr. Iverson said bird eggs are a poor substitute for seals.
"Eiders have three to four eggs per nest and so 300 nests would equal what a bear might get from eating a ringed seal," he said.
The shift in behaviour by the bears, which in some colonies ate 90 per cent of the eggs, illustrates how the impact of climate change can ripple through the ecosystem. The loss of sea ice is not only making survival more difficult for polar bears, but the breeding success of bird colonies are now threatened too.
In a paper published in Ecology and Evolution, Dr. Jouke Prop of the University of Groningen, The Netherlands, also found that sea ice changes had forced bears to hunt for bird eggs in colonies in Greenland.
Looking at data over 40 years, Dr. Prop noted the "bear presence" in bird colonies has increased significantly.
"Evidence is accumulating that polar bears are suffering from a warming climate and associated loss of sea ice habitat," he and colleagues from Norway, France and Germany wrote. "It is expected that continued sea ice reductions will severely affect polar bear populations, which will force them into terrestrial ecosystems during the summer months in search of food."
Instead of doing what they have evolved for – hunting seals on the sea ice – bears are increasingly on land, scrounging for bird eggs, facing an increasingly uncertain and hungry future.