evidence: Scientists say some populations have already reached their survival limits as the Arctic sea ice shrinks. [...] Studies show that declining sea ice is likely to decrease polar bear numbers, perhaps substantially. The new study, published in Nature Climate Change, puts a timeline on when that might happen. [...] Sea ice is frozen seawater that floats on the ocean surface, forming and melting with the polar seasons. Some persists year after year in the Arctic, providing vital habitat for wildlife such as polar bears, seals, and walruses.