For better or worse, I think that one's stance on global warming says more about him/her than it does about the theory. Like evolution, links between cancer/smoking, WMDs, the existence of God, seatbelts save lives, etc., it becomes a convenient epistemological question which exists only to dignify one's willingness to shape one's perception of facts to one's beliefs, rather than the converse. That is, the level of complexity of certain phenomena, combined with the inherent unknowability present in any sort of intellectual endeavor (which explains the 95% or 99% level of confidence, rather than the impossible 100%), gives people plenty of room to doubt, if it is already in their interest/nature to do so.
Nobody can "prove" that smoking causes lung cancer, because the causes of cancer are so complicated and individual that proof in the traditional sense of certainty is impossible. But what would you think about somebody who vigorously doubted this link? Or with evolution - pretty much every piece of biological investigation has proven it beyond a reasonable doubt, but doubters who have a vested interest in preserving their worldview can always argue that it's untrue, because nobody alive witnessed our transition from ape to man.
Basically, you get to this point where no amount of facts can convince some people, because they will hold the threshold of belief so high that practically nothing can achieve it. What's particularly troubling is that I strongly doubt they hold everything in their lives to this level of certainty, which suggests that they pick and choose what they want to believe, regardless of the evidence. At that point you are forced to question one's reliability as a reader of facts, and suggest that perhaps at some point the views of some are simply invalid and more an artifact of poor judgment, cognitive ability and lack of introspection than a reflection of the truth.
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"They who would give up an essential liberty for temporary security, deserve neither liberty or security" - Benjamin Franklin
"Don't you see the rest of the country looks upon New York like we're left-wing, communist, Jewish, homosexual pornographers? I think of us that way sometimes and I live here." - Alvy Singer, "Annie Hall"