The polar bear is more vulnerable than ever before.

The total population of the iconic Ursus maritimus is currently between 20,000 and 25,000, spread across 19 subpopulations in the Arctic. Polar bears in the U.S. Arctic have been listed as "threatened" under the Endangered Species Act since 2008, and according to the International Union for Conservation of Nature and Natural Resources, at least three subpopulations of the species are in decline, with future decline predicted.

Climate change's effect on sea ice extent is a significant factor. Rapid loss of Arctic sea ice is depriving polar bears of their habitats, because they depend on that ice to hunt prey — namely seals — and access areas to build dens.

Researchers measured the fourth-lowest sea ice extent on record earlier this year, and Arctic sea ice has now been declining at a rate of 13.4% per decade relative to the 1981 to 2010 average. Several assessments show that at the current rate, the Arctic will be nearly ice-free by the middle of the century — if not sooner.

While many wildlife around the world are increasingly threatened by climate change — some species arguably more so — the polar bear and its shrinking habitat remain visible, striking reminders of global warming and the need for action.

In honor of Polar Bear Week, which runs Nov. 1 - 7 this year to coincide with the fall polar bear migration to Churchill, Manitoba, we rounded up these images that show both the magnificence and struggle of the world's polar bears, as well as resources to find out how you can help.