If scientists in the 70s were predicting a new Ice Age, the question you should be asking is, "what made them change their minds so fast?" Really you're talking about two different things; one a natural event that would take thousands of years to become reality and the other the result of human activity predicted to become reality in less than a 100. Why are these two concepts even in the same conversation, except as fodder for the deniers to confuse the debate?