The scientists have credibility especially as against many naysayers, who just don't want to be bothered or whose economic interests may be adversely affected. The convenient course, because it doesn't interfere with anything, is just to ignore it as best as you can. So that initial asymmetry of motives--the scientist called by professional duty, the powers that be resisting change--gives the scientists a lot of street cred.
On the other hand, there are reasons for skepticism.
The existence of a scientific consensus does not mean that the consensus is right. There are many instances in the past in which “epistemic communities” took as an article of faith that which was subsequently shown to be “not so.” This skepticism toward the “scientific elite” can undoubtedly degenerate into a sort of know-nothing anti-intellectualism, and I am not counseling that we should all become yahoos, sublime as that prospect sometimes seems (it’s better than facing reality). But I do think the data is necessarily imperfect and scientists are sometimes fallible.
There is also the junction of scientific rationality and a sort of creedal belief in the vital necessity of addressing the threat. This, surely, is not a strange reaction, and in fact one must admire the scientists who, having reached this conclusion, have set out to spread the alarm. Indubitably, Hansen is a heroic figure, what we should all want to be when we grow up.
But I would still insist that the junction of scientific rationality and creedal belief, if that is the right way of expressing it, is a potential source of blindness. At a minimum, it creates a temptation to be clearer than truth. The ideological convictions that made the Bush administration twist the evidence about "weapons of mass destruction" in Iraq, egregious though the whole episode may have been, are human failings from which scientists are not exempt. If we really believe in something, human beings can readily overlook contrary evidence. Models can be tweaked, graphs artfully arranged, and this, too, without any conscious aim to engage in deception, but on the contrary with righteousness on behalf of the truth.
Scientists also face a dilemma in portraying their findings to the public. Too much in the way of "we're freakin' doomed" makes efforts to deal with global warming seem pointless. Antinomianism--a fancy word for sex, drugs, and rock and roll--might be the response of hearers. Or strategies to deal with mitigation rather than prevention. So there is a sort of inexorable political logic in saying: "This is very bad, but if we act quickly enough we can deal with it." Those calculations--which are different from scientific calculations--are inevitably forced on scientists when they enter the political arena with a case to make and a plan of action to encourage.
Historians are duly warned against injecting too much moral fervor into their accounts. The reason is that if you are a hanging judge it is difficult to see things clearly. The natural scientists are subject to basically the same set of temptations and admonitions.
For students, your course is clear. You are to take nothing on faith. Your obligation is to read the skeptics and consider every point fairly and dispassionately, to the best of your ability. Remember that the lone dissenter is often more penetrating than the consensus "committee" report. Never compromise on your right to independent judgment. That is the true scientific spirit.
A good book to read on this score is Karl Popper's The Open Society and Its Enemies. Popper is not the most reliable intellectual historian--he was himself a hanging judge--but the work brims with wonderful moral clarity about the scientific calling and the readiness to submit all propositions to refutation.