It was just a theory, but for years scientists believed what years of observation was telling them. As Arctic sea ice melted because of climate change, polar bears appeared to be inching their way toward a final refuge in the icy Canadian archipelago.
Now a new study of polar bear DNA backs that up. Scientists who research the animals across the Arctic teamed up to produce a paper showing that the "directional gene flow" of recent polar bear generations is "moving towards areas with more persistent year-round sea ice."
Led by scientists for the U.S. Geological Survey, the study purports to reveal three other important findings about the bears.
The study shows that the 19 recognized subpopulations of polar bears break down into four clusters that are genetically similar — the Western and Eastern polar basins, Southern Canada and the archipelago of islands and peninsulas between Canada and Greenland.
It also confirmed earlier DNA studies showing that modern polar bears likely emerged from "one or several hybridization events with brown bears" long ago, but based on nearly 3,000 samples taken from polar bears that were darted by the scientists and harvested by native Inuits, there was no evidence of current hybridization.
The study's 24 authors from the five nations where polar bears exist — Canada, the United States, Greenland, Norway and Russia — believe hybrid bears recently observed in Canada's Northern Beaufort Sea region is a local phenomenon.
"The polar bear's recent directional gene flow northward is something new," said Elizabeth Peacock, a USGS researcher and the study's lead author. "By examining the genetic makeup of polar bears, we can estimate levels and directions of gene flow, which represents the past story of mating and movement, and population expansion and contraction."
Gene flow happens over generations, and can't be detected using data from satellite-collars that stays on bears for short stretches of time. Collars don't work on male polar bears because their necks are wider than their heads.
But DNA evidence shows that the predominant gene flow was from Southern Canada and the Eastern Polar Basin towards the archipelago, "where the sea ice is more resilient to summer melt due to circulation patterns, complex geography, and cooler northern latitudes," according to a statement that announced the study, published Tuesday in the journal PLOS One.
The study's authors also said they confirmed earlier work that suggests that modern polar bears stem from one or several hybridization events with brown bears. No evidence of current polar bear-brown bear hybridization was found in the more than 2,800 samples examined in the current study.
They said hybrid bears observed in the Northern Beaufort Sea region of Canada represent a recent and localized phenomenon.
Sarah A. Sonsthagen, a researcher at USGS's Alaska Science Center in Anchorage; Andrei Bultonov, a researcher at the All-Russian Research Institute for Nature Protection and Stephen N. Atkinson, a researcher for the Department of the Environment for the government of Nunavut were among the study's two-dozen co-authors.
They said documenting the movement and behaviors of polar bears will help conservation efforts in the nations where they live, and where their sea-ice habitat is decline.
A study by the National Oceanic and Atmospheric Administration released last month also includes findings that said Arctic warming is influencing the migration of polar bears.
That study said air temperatures at the top of the world are rising twice as fast as temperatures in the rest of the world, resulting in massive ice melt on land and at sea, where polar bears mate and hunt.
As a result of the warming, Alaska recorded temperatures nearly 20 degrees higher than the January average last year as warm air flowed north, and the amount of April snow in Eurasia — the land mass that comprises Asia and Europe — hit its lowest level since satellite observations began in 1979, and the June snow in North America was the third lowest on record.
Writing for NOAA's Arctic Report Card, Geoff York, a director of conservation at Polar Bears International, wrote about the effect on the region's largest predator.
Their population declined from about 1,200 to 800 in the western Hudson Bay area of Canada over 24 years starting in 1987, largely because sea ice breaks up earlier and freezes later in a shortened season.
In the southern Beaufort Sea, the numbers of adult bears have stabilized at about 900 after a 40 percent decline from their population in 2001. In another part of the Arctic, the Chukchi Sea, bears are rebounding from a "significant harvest in the mid-1990s," York wrote.