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.
I appreciate someone at least providing some evidence :P
However, this article doesn't address the criticism that the temperature graph is flawed/inaccurate as I have seen persuasively argued. I don't have any resources on hand since I looked into this years ago.
If you want to make the case that this issue is a rationality "litmus test", then not only should you really be providing some evidence, but you should be showing that the arguments against the evidence are wrong, too. You should be able to make a pretty unequivocal case, right?
I'm going to take Steven's advice below and not recap climate discussion here. However, if you want to do your own research and make a large-stakes bet about persuading some designated neutral judges on the extent of warming in the last 100 years, structured to express the disagreement, I would probably be keen to take it.