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.
Wouldn't denial of AGW equate to one of the following beliefs?
Anyone who believes in ~1 and ~2 must believe in some degree of AGW, even if they further believe it is trivial, or masked by natural climate variations.
Belief that sensitivity is below 2 degrees doesn't seem utterly unreasonable, given that the typical IPCC estimate is 3 degrees +/- 1 degree, the confidence interval is not more than 2 sigma either way ("likely" rather than "very likely" in IPCC parlance) and building that confidence interval involves conditioning on lots of different sorts of evidence. Belief that sensitivity is below 1 degree does seem like having an axe to grind.
All this is Charney or "fast feedback" sensitivity. The biggest concern is the growing evidence that ultimate (slow feedback) sensitivity is much bigger than Charney (at least 30% bigger, and plausibly 100% bigger). Also, that there are carbon cycle and other GHG feedbacks (like methane), so the long-run impact of AGW includes much more than our own CO2 emissions. Multiplying all the new factors together tuns a central estimate of 3 degrees into a central estimate of more than 6 degrees, and then things really do look very worrying indeed (temperatures during the last ice age were only 5-6 degrees less than today; at temperatures 6 degrees more than today there have been no polar ice caps at all).