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.
We need to think about what AGW actually "predicts".
In detail, it depends on how big climate sensitivity is, and also what time period we are looking at.
Remember that if small enough, the AGW impact will clearly be trivial compared to natural climate variations and weather. Also, even if the overall impact is big, the delta impact between two very close dates (AGW impact at 2012 minus AGW impact at 2011) is always going to be trivial compared to natural variations.
Qualitatively, over short enough time periods, the AGW signal will always be masked by the natural climate /weather noise; whereas over longer time periods, the signal will be evident as a statistically significant trend imposed on top of the noise.
Questions:
Do you disagree with this qualitative prediction, or do you really think that AGW predicts that every year should be hotter than the previous one (so each year is a record high)? Do you think it even predicts that every day should be hotter than the previous one (so each day is a record high)? Clearly if your mental model of AGW is "AGW predicts year on year increasing temperatures; we don't observe that, therefore AGW is falsified" then this is a rational deduction, but from false premises.
Would you like to make a bet on the average global temperature in the decade 2010-2019 versus average global temperature in the decade 2000-2009? If you really believe there is no global warming trend, you should be happy to take an even money bet on this decade being cooler than the last one. Fancy the bet? (Hint, you would have lost a similar bet made in any previous decade since the 70s).
If your answer to question 2 is "Ahh, I think I'd bet on warming after all, but I don't think it is anthropogenic" then you're acknowledging that the argument about year to year temperatures (less than decade averages) is basically irrelevant. So why raise it? If this were a political forum, and you were trying to score points in a debate, I'd understand it.... But this is Less Wrong.
If your answer to question 2 is "Climate scientists/meteorologists might end up reporting that each recent decade is hotter than the previous one, but I won't believe them because I think they're lying" then again you're acknowledging that an argument over year to tear temperatures is irrelevant (fictional/made-up evidence is irrelevant to choosing between hypotheses). So again, why raise it except for point-scoring?