Climate Sensitivity - an interesting IPCC bias

Some time ago, I noticed an interesting bias in the TAR scientific report (the third assessment report of the IPCC - the intergovernmental panel for climate change) regarding the climate sensitivity, that is by how much the average global temperature will increase if we double the amount of CO₂. The report mentions quite a few times that climate sensitivity "is likely to be in the range of 1.5 to 4.5°C". I counted (well, searched and found) a dozen occurrences, perhaps there are more I missed.

Why is this interesting? Because the actual range of global circulation models they use to predict future scenarios spans a higher range, 2.1 to 5°C. They mention this range only once explicitly (and even that occurrence is hidden in the discussion about precipitation), and implicitly when listing the global circulation models they use. In other words, their predictions are based on models which are biased towards a higher sensitivity than, and I quote the IPCC, "the likely range" for the climate sensitivity.

Thus, unless a reader is very careful to go over the report, he will get the impression that the doomsday climate predictions for say 2100AD are based on the 1.5 to 4.5°C range, not the 2.1 to 5°C actually used. This makes a big difference. By cutting out the 1.5-2.1°C range, the IPCC is significantly reducing the probability that the predicted temperature will only weakly increase, thereby throwing away the possibility of having benign scenarios.

If the IPCC uses a range which does not conform to the "likely range", they should stress that, instead of stressing the accepted range, which they don't use.

Another interesting point is that the IPCC and the climate community as a whole prefer to rely on global circulation models which cannot predict the sensitivity (to better than a factor of 3 uncertainty!), and they choose to ignore the empirical evidence which shows that the climate sensitivity is actually on the low side.

More about the sensitivity and why it is actually somewhere between 1 and 1.5°K (for doubled CO₂), can be found in this summary on the climate senstivity and its uncertainties.