Located between Canada and Greenland, Baffin Bay is an Arctic habitat home to a unique subset of polar bears. But longer stretches without the animals' essential ice is causing heightened weight loss in the bears, resulting in fewer cubs than 30 years ago.

Polar bears depend on sea ice for almost all aspects of their life, from using it as a platform for hunting seals to traveling and creating snow dens for their young. As climate change causes longer periods of warmth, a new study published in Ecological Applications finds that sea-ice is melting earlier than it did in the 1990s, forcing polar bears to spend more time on land and off the ice.

“Climate-induced changes in the Arctic are clearly affecting polar bears,” said lead author Kristin Laidre, a UW associate professor of aquatic and fishery sciences, in a statement. “They are an icon of climate change, but they’re also an early indicator of climate change because they are so dependent on sea ice.”

An international team of researchers led by the University of Washington turned to data on 43 polar bears and sea ice collected in the 1990s, which included sea ice measurements, satellite tracking information, and data on polar bear health and litter size gathered from aerial and on-the-ground surveys. This information was compared with similar measurements collected on 38 polar bears between 2009 and 2015.

On average, polar bears spent 30 days or more on land in recent years than the 60-day average established in the 1990s. Their movements were shown to follow the annual growth and retreat of sea ice. Baffin Bay polar bears inhabit a seasonal ice zone, which means that the sea ice clears out completely in the summer. So, while the amount of ice melted in the springtime has not changed in recent years, it is melting sooner in the season, resulting in longer periods of ice-free waters.

“When the bears are on land, they don’t hunt seals and instead rely on fat stores,” said Laidre. “They have the ability to fast for extended periods, but over time they get thinner.”

But these fat reserves are also being depleted as polar bears are going longer stretches without access to previous hunting grounds. If female bears aren’t able to gain the necessary amount of fat, researchers say they are less likely to continue carrying two-cub litters. Mathematical models suggest that litter sizes will continue to decrease in the next three generations largely due to projected sea ice decline.