When there's more CO2 in the atmosphere, it makes the atmosphere warmer by trapping heat. Since humans are adding more CO2 to the atmosphere, that helps explain why temperatures are increasing around the world.
Cells are the basic unit of life. Atoms make up molecules. molecules make up cells. cells make up tissues. tissues make up organs. organs make up organ systems and organ systems make up the human body. therefore, without cells, there is no life.
A.Afarensis,A. Africans, H.Habilis, H.Erectus, H.Sapiens
If the specie can't change (using polar bears as an example) and the world gets excessively hot, they will die because they can only live in the cold because of their thick fur. The incapability of changing will cause them all to die because they can't handle the temperature. Polar bears can't just magically loose all their hair so they would probably go extinct if the temperature changed.