Just to mix the issue up again, it appears that IPCC is moving to lowering its estimate of climate sensitivity for the GCMs.
Climate Change 2001
The Scientific Basis
IPCC
6.2 Forcing-Response Relationship
6.2.1 Characteristics
As discussed in the SAR, the change in the net irradiance at the tropopause, as defined in Section 6.1.1, is, to a first order, a good indicator of the equilibrium global mean (understood to be globally and annually averaged) surface temperature change. The climate sensitivity parameter (global mean surface temperature response DTs to the radiative forcing DF) is defined as:
DTs / DF = l(6.1)
(Dickinson, 1982; WMO, 1986; Cess et al., 1993). Equation (6.1) is defined for the transition of the surface-troposphere system from one equilibrium state to another in response to an externally imposed radiative perturbation. In the one-dimensional radiative-convective models, wherein the concept was first initiated, l is a nearly invariant parameter (typically, about 0.5 K/(Wm-2); Ramanathan et al., 1985) for a variety of radiative forcings, thus introducing the notion of a possible universality of the relationship between forcing and response. It is this feature which has enabled the radiative forcing to be perceived as a useful tool for obtaining first-order estimates of the relative climate impacts of different imposed radiative perturbations. Although the value of the parameter “l” can vary from one model to another, within each model it is found to be remarkably constant for a wide range of radiative perturbations (WMO, 1986). The invariance of l has made the radiative forcing concept appealing as a convenient measure to estimate the global, annual mean surface temperature response, without taking the recourse to actually run and analyse, say, a three-dimensional atmosphere-ocean general circulation model (AOGCM) simulation.
The often quoted IPCC range of 0.75-1.1oC/Wm-2 is a measure derived from and applicable to their models which, for one thing, do not recognize water vapor as a component of GHGs.
Water vapor for one thing is treated as "feedback" not a "forcing" by IPCC in the GCMs and is not included in their measure of DF of the climate sensitivity parameter l. Thus their model l cannot be reflective of the observational and real world parameter based on total forcing from all factors, to which I refer.
The IPCC baseline assumption(a political decision rather than one rooted in science) is that mankind is the primary causative agent of warming through introduction of additional GHGs into the atmosphere, and that temperature and everything else derives from changes in those GHG additions that can be attributed to mankind.
The position that I and many others take is that climate temperature can vary for a multitude of reasons(variation in solar irradiation, astronomical primary and secondary effects, ...) and the increase in CO2(and other GHG's) arise from changes in temperature as well as contributions by man. The contributions of CO2 and other GHGs that are recognised by IPCC, do not have the degree of effect on the global climate temperatue the IPCC attribute to them in their Global Climate Models(GCMs).
That alternative position makes for a substantially different view of the world and conforms more readily to observational data. It also allows us to examine the real physical phenomena for true causitive factors rather than staying with models based in an apriori underlying premise and agenda to perceive mankind as the primary causitive agent of climate change.