Why are polar bears going Extinct?

OK so i have a project for school which i have almost finished, all i need now is a full explanation of how it started. eg; from us polluting the earth, to now, explain the ozone situation etc, i will put it in my own words, mention you in the bibliography and we so grateful. i am doing this from my dads house/home office and he needs the computer so i cannot research myself, thank you very much