Scientists are never "sure" in the sense you mean. Even if every known scrap of evidence supports a theory, a good scientist always keeps in mind that something might be discovered tomorrow that would undermine it. The only advice any honest scientifist can ever give is "The best evidence we have is that X is the case."
Politicians understand this, and base their decisions on the probabilities. So do we all in matters of common sense. If your doctor told you the evidence was 90% that you had a life-threatening condition, you wouldn't tell him to wait till he was sure before treating it: anyone can see that it would only be 100% certain when you died.
The only people who demand 100% certainty in science are those with a financial or ideological interest in attacking a theory, like creationists and AGW deniers, because they know that that demand can never be met. And the only people who proclaim any theory to be 100% certain are journalists; the scientists who they interview will always tell them the odds, but "as certain as the evidence allows" doesn't make as good a headline as "SCIENTIST PROVES..."