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.