Theism is often a default test of irrationality on Less Wrong, but I propose that global warming denial would make a much better candidate.
Theism is a symptom of excess compartmentalisation, of not realising that absence of evidence is evidence of absence, of belief in belief, of privileging the hypothesis, and similar failings. But these are not intrinsically huge problems. Indeed, someone with a mild case of theism can have the same anticipations as someone without, and update their evidence in the same way. If they have moved their belief beyond refutation, in theory it thus fails to constrain their anticipations at all; and often this is the case in practice.
Contrast that with someone who denies the existence of anthropogenic global warming (AGW). This has all the signs of hypothesis privileging, but also reeks of fake justification, motivated skepticism, massive overconfidence (if they are truly ignorant of the facts of the debate), and simply the raising of politics above rationality. If I knew someone was a global warming skeptic, then I would expect them to be wrong in their beliefs and their anticipations, and to refuse to update when evidence worked against them. I would expect their judgement to be much more impaired than a theist's.
Of course, reverse stupidity isn't intelligence: simply because one accepts AGW, doesn't make one more rational. I work in England, in a university environment, so my acceptance of AGW is the default position and not a sign of rationality. But if someone is in a milieu that discouraged belief in AGW (one stereotype being heavily Republican areas of the US) and has risen above this, then kudos to them: their acceptance of AGW is indeed a sign of rationality.
"They are not idiots" and "they are experts" does suffice.
Well then look up the information for yourself (it'll probably be an interesting process). But why in particular are you interested in this? Are you for instance, interested in the details as to how the absorption spectrum of CO2 is calculated, the details of the machines and the computer programs used? Why not - errors could creep in from that front just as much as from your example.
You seem to be saying "in this field, of which I know little, this thing seems superficially to be a problem. Instead of learning about it in detail, I will assume that there is an actual problem and that the specialists are either not aware of the problem or are dishonest about it. I will then challenge other non-specialists to bring me all the data I haven't bothered to look up for myself." This is precisely why I was talking about motivated skepticism.
This is a good point, and something that should be talked about more. Too many people who think they're clever find a sort of obvious challenge to a claim they don't like, and don't investigate further because of motivated skepticism. Also, then they look stupid if they have to admit their objection was ignorant and lazy later.
Note: Obviously just because an objection is addressed doesn't mean I think it's addressed sufficiently well. Religious people always have responses to objections to religion, many of which utterly fail to even properly address the objection. But I still think there's a huge problem, especially in in-person debates, with people who never investigate their own objections.