mheslep said:
Interesting that APS singled out greenhouse gas effects as beyond dispute and at the same time said nothing about feedbacks. The direct ~http://en.wikipedia.org/wiki/Image:Radiative-forcings.svg" greenhouse forcing alone is trivial, the IPCC gets most of its predicted warming from feedbacks fed by CO2 forcing.
If I may tune in: indeed

The passages quoted by Zz show an ambivalent stance which is the main problem of this debate:
First paragraph: catastrophic climate change is a scientific reality (?)
Second paragraph: the *physics and chemistry* of greenhouse gasses is understood (that's about what's modeled by MODTRANS), probably true.
Third paragraph: it's a complex system (the feedbacks, models and all that) which we don't understand completely yet, but some (simulation) results are so dramatic that conservative action should be undertaken... (understood: even if this has a small chance of being right, it is so dramatic that we can't take the risk).
The third paragraph almost contradicts the first: if it were a scientific reality, then the third paragraph doesn't make any sense. If it were just a matter of physics and chemistry, then the modeling wouldn't be so terribly complex (unfortunately that modeling doesn't predict anything dramatic). The complexity is indeed in the feedback mechanisms, which are the essential part of the "drama": without them, the physics and chemistry predict a modest warming, and with them, it can become indeed dramatic. However, most of them are very badly understood. If we can say that it is "a scientific reality", then we cannot at the same time hide behind "it is still complex and we didn't understand everything yet".
Does that mean that I'm against the third paragraph ? No. Does it mean I am somehow convinced that there's no climate change (in other words that I'm a sceptic ?). No. What I am against, is that one misuses words such as "scientific reality" to try to give more "truth" than one actually possesses on this issue. Yes, one should be careful. Yes, climate change is not excluded, and yes, the consequences could potentially be dramatic.
BUT: no, we don't know for sure AT ALL. It is not "a scientific reality". It is a working hypothesis, which is based partially on "well-known physics and chemistry" and also on a lot of speculation.
And I agree with Andre (who, I think, is more convinced than I am that there is *no* significant climate change - I keep the possibility totally open), that this might seriously backfire *if ever* it is proved wrong. A "scientific reality" cannot be wrong, or it means that scientists don't know how to find out anything. And *if ever* this is the case - that is, if ever it turns out that the effect of the greenhouse gasses are precisely what the *chemistry and physics* predict they are (as given by MODTRAN), then the AGW proponents amongst the scientists will have given a terrible blow to the credibility of science in general, given the enormous amount of useless expenses they will have caused with their fearmongering.
The third paragraph, by itself, would have been ok: "we don't know, but we have credible scenarios in which something terrible will happen if we don't take action. But we may be wrong too. We simply don't know. Let's be cautious."