Polar bears, whose global population is estimated at nearly 25,000, are increasingly losing life-sustaining sea ice crucial for their hunting, resting as well as for breeding in the Arctic region, a study has found.
According to scientists, the Arctic is warming at nearly double the global rate as a result of climate change fuelled by mankind's burning of fossil fuels, a process that emits heat-trapping greenhouse gases.
The rising temperatures have resulted in ice melting earlier in spring and re-freezing later in the autumn.
Satellite data revealed that the total number of ice-covered days across the 19 regions declined at a rate of seven to 19-days per decade from 1979 to 2014, the researchers said.
"Polar bears' dependence on sea ice means that climate warming poses the single most important threat to their persistence," said Harry L. Stern from the Polar Science Centre at University of Washington.
The bears, which have become emblematic of the ravages of global warming, spend most of their time on sheets of frozen ocean water, which melt and recede in warmer months, and then reform in the winter.
When the ice melts, the animals come ashore and survive on stored fat until it refreezes -- a period that for some has become longer and longer.
With longer iceless periods, polar bears have to swim further and further to find solid ground, the study noted.
The bears need sea ice for hunting as they cannot outswim seals, their preferred prey. They get around these by waiting near holes in the ice and ambushing the seals as they come up for air.
Further, the loss of ice may also affect their breeding, as male and female bears meet on ice sheets to mate.
"Changes in sea ice have been shown to impact polar bear abundance, productivity, body condition, and distribution," Stern observed.
In addition, the retreating sea ice is more frequently bringing polar bears into confrontation with humans who live on land, according to World Wildlife Fund -- a Switzerland-based organisation for wildlife conservation and endangered species.
With more water to navigate, Arctic shipping activities have increased, as well as opportunities for oil and gas development, further threatening the animals' habitat.
Arctic sea ice melt will in turn also contribute to the sea level rise, and feed back into global warming -- ice reflects warming sunlight away, while water absorbs the heat, the researchers explained, in the journal The Cryosphere.
According to a report from the International Union for the Conservation of Nature (IUCN) in 2015, the polar bears' numbers may dwindle by nearly a third by the mid-century. The status of these animals have been currently listed as "vulnerable" on the IUCN's Red List of endangered species.