Incidentally, having read both Manzi's and Levin's posts all the way through, I find nothing to choose between them when it comes to empiricism, collegiality, or goodwill. They both take derisive shots at each other's arguments. If you agree with Manzi, you'll think he won the point. If you agree with Levin, you'll put a check in his "win" column.
Is it too much to wonder whether continual and habitual assaults on the honesty, intentions, patriotism, and professionalism of scientists and intellectuals, a reflexive readiness to dispute the validity and usefulness of scientific and intellectual inquiry, in short the open adoption of anti-scientific and anti-intellectual attitudes and practices by some conservatives may also have played a role in such dramatic and long-standing trends?
Since not all scientists are left-leaning politically, would you ask a parallel question about the many who regard AGW/CC as "science that is far from settled," and who explicitly oppose political agendas that assume it?
I consider it a more balanced view of the situation to acknolwedge that racing forward in a fervor of zealotry to shut down debate on AGW/CC is pretty much exactly analogous to dismissing it, in terms of being anti-scientific and anti-intellectual.
The left has no superior claim to the skeptical empiricism inherent with a truly scientific approach. It is caught on a regular basis, rather, assuming that which has yet to be proven, and proclaiming its assumptions to be "scientific" merely because they come from theories that didn't arise from the book of Genesis.
That's a sort of narrow, tomographic definition of "scientific" that we are under no obligation to accept. It also happens to be politically convenient for the left. True empirical skepticism, however, operates under all circumstances, including when the political urge to ignore it is insistent. The left has violated that principle often in its century of proclaiming itself to be the keeper and exemplar of it. It is no form of unfairness to point that out. It's simple empiricism.