"I'll bite the bullet and say global warming is the perfect example here. It's pretty clear to me that many people hold their positions on this issue - pro and contra - for political/social reasons rather than evidential ones."

I used to think that global warming was a poor example of this because while the right wing has plenty of reasons to oppose actions to fight global warming, and thus irrational reasons to force themselves to believe that global warming does not exist, the left wing does not have any reasons to support actions to fight global warming aside from evidence that global warming is a threat. Then it occurred to me that many people on the left actually do have alternate motives for pushing anti-global warming actions: other people on the left support it too (see Eliezer's The Sky is Green/Blue parable, and this article too, I suppose). This is even more irrational, but due to the stunning level of irrationality among humans on all sides of the political spectrum, is probably a factor for some.