A few words on Evolution. I hope it helps anyone that's confused...
a) Populations tend to increase indefinitely in a geometric ratio (e.g. 2, 4, 8, 16...)
b) In a natural environment, population numbers must stabilize at a certain level. The population cannot increase to infinity. The earth does not have sufficient resources for infinite expansion.
c) There must, therefore, be a "struggle for existence". Not all the organisms produced can survive.
d) There is variation in every species.
e) Those individuals with variations that are better adapted to the environment leave behind more offspring than do individuals that are less well adapted. This is referred to as "differential reproductive success."
The process of natural selection carried out over uncounted generations leads species to change enough to develop into new species. The polar bear is very different from other bears and is a simple example of how a bear is changed by adaptation to its environment. Long ago, the lightest-colored brown bears were more successful hunters upon the snow and ice (and survived the winters in larger numbers) than darker bears. This bias was so marked, through successive generations, the ice-dwelling bears became nearly white, and no longer interbreed with other bears, and became a new species. As global warming eliminates the icy environment, polar bears less suited to the warmer climate (the poorer swimmers perhaps) will fail to breed as effectively (by drowning in the open sea). White bears, in a world without ice, have no camouflage advantage. Those however-best-suited to create offspring in this new environment will carry polar bears to whatever future awaits, or if there is not enough variety, or by being out-competed by more successful species of bears, will become extinct altogether
If the climate change were slow enough, there would be enough time to allow enough variety for polar bears to “adapt” to that environment. That said, they wouldn’t really be polar bears by then, they might look more like leopard seals, or of course, something entirely unpredictable.