Alaskan polar bears have mastered the art of living in ice. Even during the frigid days of winter, these furry white creatures don’t hibernate in cozy dens, like other bears do. Instead, they go north, hunting for seals living in the ice-covered water.

Polar Bear

In recent years, however, Earth’s climate has been getting warmer, and ice near the Poles has been melting. Now, a new study shows that disappearing ice is changing polar bear behavior. For instance, more than half of pregnant polar bears used to give birth in dens on solid blocks of ice that were floating out at sea. Today, they’re digging their dens on or near land. The reason? Most of the ice has disappeared. Studies have shown that 27 percent of sea ice in the Arctic that stays frozen from year to year has disappeared during the past 30 years. So there’s now much less space for dens. Moreover, as the warming continues, the remaining ice becomes less stable. These conditions are unappealing to mother polar bears, who want a safe place to give birth. A mother polar bear often has two cubs at a time. Baby cubs stay with their mothers for more than 2 years.

For now, Alaska’s polar bears seem to prefer returning to land rather than taking the risk of giving birth on shaky ice. That solution might not work forever, however, especially if the ice keeps melting. The bottom line? Polar bears are in trouble.