The earth changes. Greenland wasn't always covered with ice. Polar bears evolved relatively recently because of the ice. If the ice goes away, bears won't need to be "polar" anymore. In a few hundred or thousand years, the ice may return and maybe polar bears will once again evolve from brown bears.

I laugh at the global warming people just like I laughed at the global cooling people in the 70s and the acid rain people in the 80s. What would they have said had they been alive during the ice age?