One possible reason why polar bears might not be able to survive if the environment they live in changes is because

Polar bears are animals that live in cold environments. Their environment should be a place conducive for breeding and propagating their species. If ice caps are melting and the length of the cold season is declining because of the effects of climate change, polar bears will have less time hunting for food. Generally, the coldest season is the best hunting time for polar bears. It is easier to catch seals and fish due to the thickness of the ice and the rise of water in their environment. This time to hunt is enough to restore the polar bears’ body fat and fitness in preparation for the warm season where there is less ice and food. As the warm season lengthens, the condition of the polar bears’ bodies decline resulting to effects that are serious for pregnant polar bears who are nursing and cubs who need their mothers’ energies to survive the harsh environment. 

The answer is ozone layer.