From Frum's take on the spat:

Manzi could have safely disputed Levin’s claims on global warming if he had observed a couple of conditions. First, acknowledge Liberty and Tyranny as a good and important book. Second, acknowledge Levin’s “service” (i.e., leadership) of the conservative cause. Third, isolate criticisms to one particular finite point – avoid drawing any larger conclusions – and be sure to wrap any criticisms in a blanket of compliments. Just because one particular chapter happens to be slovenly, ignorant, and hysterical should not lead you to question the intellectual merit of the book as a whole.

Manzi negligently violated the rules, and the results are as you see.