Polar bears need a platform of sea ice to reach their prey: ringed and bearded seals. But not all sea ice is equal: some sea ice lies over more productive hunting areas—and some ice regions will melt sooner than others in a warming Arctic.
Scientists have identified 19 populations of polar bears living in four different sea ice regions in the Arctic.
Why does it help to divide the Arctic into ice regions? For those polar bear populations that are little-studied, scientists can make informed estimates on how they're faring based on the health and condition of other populations in the same region.
Four Sea Ice Ecoregions:
1. Seasonal Ice – Seasonal ice areas occur at the southern extreme of the polar bear's range and include places like Canada's Hudson Bay, where the ice melts each summer and the bears must wait for freeze-up in the fall until they can hunt again. Polar bears in these seasonal areas are the most endangered, with longer and longer ice-free seasons testing the limits of their fat reserves.
2. Polar Basin Divergent Ice – In these areas, sea ice forms along the shore but then retreats, especially in summer. As the sea retreats farther and farther from shore in a warming Arctic, these polar bears are faced with a choice of coming ashore—fasting until the ice returns in the fall—or swimming long, exhausting distances to reach the remaining pack ice. Ice that's located far offshore, however, often covers unproductive parts of the sea, so bears in these areas may successfully complete a marathon swim, but still not find any seals to hunt. Polar bears that live in these areas are at great risk: from longer and longer swims, prolonged fasting periods, and encounters with humans on shore.
3. Polar Basin Convergent Ice – Sea ice formed in other parts of the Arctic collects along the shore of these habitats, providing polar bears with access to seals. Polar bears in these areas are faring well now, but scientists predict that ice in these areas will disappear within 75 years—and, with it, resident polar bear populations—unless action is taken to reduce CO2.
4. Archipelago Ice – Islands in the Canadian High Arctic and Greenland are far enough north that sea ice remains along the coast even in summer, providing hunting for the bears. This eco-region is predicted to be the last stronghold for polar bears, but it, too, is expected to melt within 100 years unless greenhouse gas emissions are greatly reduced.
Polar Bear Population Distribution by Sea Ice Ecoregion
Ecoregion 1: Baffin Bay, Davis Strait, Foxe Basin, Southern Hudson Bay, Western Hudson Bay
Ecoregion 2: Barents Sea, Chukchi Sea, Kara Sea, Laptev Sea, Southern Beaufort Sea
Ecoregion 3: Eastern Greenland, Northern Beaufort Sea, Queen Elizabeth Islands
Ecoregion 4: Gulf of Boothia, Kane Basin, Lancaster Sound, M'Clintock Channel, Norwegian Bay, Viscount Melville Sound
Citation: Amstrup, S.C., Marcot, B. G., and Douglas, David C. 2008. A Bayesian Network Modeling Approach to Forecasting the 21st Century Worldwide Status of Polar Bears. Pages 213-268. In Eric T. DeWeaver, Cecilia M. Bitz, and L.-Bruno Tremblay Eds. Arctic Sea Ice Decline: Observations, Projections, Mechanisms, and Implications. Geophysical Monograph 180. American Geophysical Union, Washington, DC.