Impressive technical achievements and an urgent appeal to reverse global warming distinguish this nature film from other documentaries that anthropomorphize cuddly baby animals. Husband-and-wife filmmakers Adam Ravetch and Sarah Robertson spent 15 years trekking across melting ice floes and shooting underwater in glacial temperatures to get close to polar bears and walrus herds as they struggled in a shrinking environment. Watching these endangered species evolve new approaches to hunting and shelter is fascinating, but the movie is seriously marred by a cloying screenplay and such kid-pleasing touches as shots of walruses belching and farting. Queen Latifah narrates. G, 84 min.