Thanks to Inel for finding this; look there for the links. So Schwartz (yes that Schwartz) said: “The latest report from the Intergovernmental Panel on Climate Change assesses the skill of climate models by their ability to reproduce warming over the twentieth century, but in doing so may give a false sense of their predictive capability”. Exactly why they think this gets quite messy but seems to amount to not knowing the aerosol forcing too well; and they go on to link this to AR4 estimates for climate sensitivity.
This reads wrong to me; and Forster et al reply “However, they have misinterpreted the use of AR4 Fig. SPM-4 (shown here as Fig. 1) and, as such, their criticisms are misplaced” which seems right: “the AR4 assessment did not derive estimates of climate sensitivity, of future warming, or of past warming attributable to greenhouse gases directly from these simulations”, which I don’t think S et al have fully grokked.
S et all reply that F et al “mainly criticize us for points which we did not make regarding climate sensitivity and future global warming and fail to come to grips with our central point, namely that in assessing the skill of climate models by their ability to reproduce warming over the twentieth century, the latest report from the IPCC3 may give a false sense of their predictive capability”, but I think they have misunderstood again (or are perhaps redefining what their main point was). I took their main point to be the climate sensitivity estimates; they don’t seem to appreciate F et al’s point about where these come from.
[Update: JA doesn't think much of it either]