Scenario #1:

Polar bear lives on the ice. The ice is mostly anchored to a stable landmass or large body of other ice. The polar bear goes swimming to look for food. When the polar bear gets tired of swimming, polar bear goes back to his ice bound home.

Scenario #2:

Polar bear lives on the ice. The ice is melting and breaking up, so for many polar bears, "home" is now a floating/melting small floe. Polar bear goes swimming to look for food. When the polar bear gets tired of swimming, polar bear heads back for his home ... but it isn't there anymore. Polar bear drowns.

Scenario #2 is happening.

One recent survey in a couple of days counted approximately a dozen dead polar bears¹, because the breakup of the ice left them no place to return home. They are good, but not particularly strong swimmers. They died.

¹ A dozen dead bears found extrapolates (by that survey's estimate) to about 40 bears dead, since only a few are likely to be seen. That's a "whole digit" percentage of all the polar bears there are.

Any clearer?

John