Cooked Up Brit:
I agree with you that scientists are not qualified to ask the political, moral, or economic questions. But I have 3 problems with your analysis:
First, before we can start performing any cost-benefit analysis of future actions, we need to pass the threshold question of "whether man has any part in global warming to begin with." There's no need to reach question 2 without first answering question 1 in the affirmative.
Second, we need to realize that while we waste time answering question 1, the strategy choices for question 2 might be changing (if we answer question 1 in the affirmative). In other words, if we wait 30 years before we decide upon question 1, the costs might be much greater or it might be too late. That is why I think we don't necessarily have to be CONVINCED that global warming is occuring, we just need a strong enough reason to start taking precautions. In the same way, we don't need to be CONVINCED some foreign nation might bomb us before we take precautions (I'm using this example to illustrate the principle that you don't need certainty, not to compare the effect of global warming with a bomb).
Third, you portray the costs of taking precautions in absolute terms. It's not like we need to cut all CO2 emissiosn and return to the stone age. It's more just reducing emissions and finding alternatives.