For almost fifteen years now I have been (using my unfortunately trivial influence; though for some reason Fergus seems to be singlehandedly trying to change that; thanks Fergus!) pointing out that such an argument is totally wrong, pretty much exactly 180 degrees off the mark.
Here's my response, verbatim, which you can also read on RC.
My use of the word "conservative" in the concluding sentence is deliberate, of course.
Suppose we grant for the sake of argument that the total range of uncertainty (of some quantity) is a factor of 100. Does it follow that the quantity is possibly overestimated by a factor of 100? Perhaps, but surely it follows no more and no less than it follows that there is an equivalent possibility that the quantity is being underestimated by a factor of 100.
Why are people constantly harping on the risk of overestimating climate change when the risk of underestimating it has vastly greater consequences?
Rational policy under uncertainty should be risk-weighted, which implies that the less faith one has in the consensus position, the more vigorous an emissions policy one should support. It is very peculiar and striking to observe how common a position like Aam's is despite the fact that it is incoherent.
Those people who doubt the consensus in a rational way (e.g., Broecker, Lovelock) advocate for a very vigorous policy. We don't know how bad it can be, so we really ought to give considerable weight to it being very very bad. The asymmetry arises because we know how good it can be. Climate change can at best amount to a (relatively) very small net gain, if it is modest and slow enough. At worst it can quite conceivably be a threat to civilization.
Most people stressing the uncertainty, though, seem to me to deliberately strive to confuse the policy process, or to echo others who do so. It is discouraging how effective this tactic continues to be, given that it is based on a completely irrational argument. The only remotely sensible way to argue for small or no policy response is not to argue for large uncertainty. A rational argument for policy inaction requires arguing that the consensus position is certainly wrong and oversensitive. A rational, conservative response to uncertainty would be to take more effort to avoid the risk.
I always find it a tortured use of the word "conservatism" to suggest that monkeying with the biosphere (an astonishing and rare natural phenomenon) is a better idea than tuning the economy (an artifact). I can anticipate the tedious answers of course (cue Mr Duff), but I find myself wondering what, exactly, these so-called conservative people think they are conserving.