Prime Minister Anders Fogh Rasmussen of Denmark explains to the scientific community what he needs from them leading up to the Copenhagen climate conference later this year (PDF):

But understand me correctly; at the end of the day, here in Copenhagen, we have—as politicians—to make the final decision, and to decide on exact figures, I hope. And this is a reason why I would give you the piece of advice, not to provide us with too many moving targets, because it is already a very, very complicated process. And I need your assistance to push this process in the right direction, and in that respect, I need fixed targets and certain figures, and not too many considerations on uncertainty and risk and things like that.

The problem, of course, is not that a politician would offer such advice, rather it is that many scientists have decided to follow it.