Unstable Ice Causes Polar Bears to Move Ashore

The Arctic's warming climate has prompted Alaskan polar bears to avoid using pack ice for maternal winter dens.

Female bears instead are selecting den sites on land for giving birth to their cubs.

From 1985 to 1994, most pregnant female bears, 62 percent, wintered in dens on ice floes off Alaska's north coast. A few years later, 1998 to 2004, only 37 percent of female bears were denning on pack ice, and they entirely stopped using the pack ice lying west of Barrow.

Scientists rule out the influence of hunting, or an increase in bowhead whale remains left onshore, as triggers for the polar bears altering their behaviour. Researchers believe that the shift in denning habits comes in response to changes in Arctic sea ice that were brought about by climatic warming.

For pack ice to make a good den location, it must be stable and have a snowdrift large enough to accommodate a bear den. Pack ice became less steady and carried less drifted snow over the course of the 20-year study. The ice deteriorated most rapidly in the western Beaufort Sea, where the greatest decline in denning on pack ice also occurred.

In autumn, when female bears enter dens, offshore ice floes have become thinner, smaller and more scattered. Expanses of open water between floes reduces the ice's stability during storms, putting dens at risk.

The pack ice has less snow on it for two reasons. When thinner ice floes jam together, they don't produce pressure ridges large enough to accumulate snowdrifts suitable for digging dens. The amount of snow around in autumn has also declined, as warmer temperatures have caused a greater proportion of precipitation to fall as rain.