It is an unfortunate turn of events that most big scientific discussions (such as global warming, in this case) turn into political discussions. This could include other things such as abortion, stem cell research, and the teaching of evolution in the classroom. All of these things have a strong scientific discussion which implies that there is a truth somewhere. However, in these cases, the truth is not clear cut and the science is probably not solid. In most of these cases, there is some level of gray in the conversation and much of what is discussed as fact is simply hypothesis.
Scientists should only be using their scientific titles and credentials when they are discussing science. They should discuss this accurately with terms such as “hypothesis” and “theory” and “conjecture” used quite liberally. When they depart from the scientific conversation and into the political arena (which is certainly their right) they should be careful not to be so adamant that they give off an air of certainty that their scientific brethren would not accept.