Already on UN’s list of endangered species, polar bears are starting to feel the effects of the global warming, as more than 50 square kilometers of ice are melting every single day at the North Pole.

Pregnant polar bear females are starting to build dens more and more on land rather than ice, because of the latter’s scarcity and fragileness.

According to a 20-year study by the U.S. Geological Survey, the proportion of dens built on ice fell to only 37% between 1998 and 2004, compared to 62% from 1985 to 1994. Researchers tracked the habits of the bears in northern Alaska using satellite telemetry.

"Right now, pregnant females foraging offshore in summer must wait up to a month longer than they did even 10 years ago for new sea ice to form so they can travel to denning areas on land," USGS researcher Steve Amstrup said in a statement.

"Alternatively, they must swim ever greater expanses of open water to reach suitable land denning habitat or they must den on ice that may not be stable enough to survive the winter."

The reason for the polar bears’ changed breeding habits is the rise in global temperatures, which makes ice less durable for building dens (where cubs are being sheltered from the freezing winds blowing over the surface). At the same time, ice forms slower and lasts less.

"In recent years Arctic pack ice has formed progressively later, melted earlier and lost much of its older and thicker multi-year component," another researcher, Anthony Fischbach, said.