[Re-posted with corrected formatting - please delete earlier comment]
Webby
Let’s go through that again – this time using the IPCC AR4 approach rather than your “ad hoc” method.
1.0C = No feedback 2xCO2 impact (Myhre et al.)
In AR4 WG1, Chapter 8 (p.630) IPCC states that the multi-model mean forcing and standard deviation for each feedback in W/m^2 °C is:
Water vapor +1.80 ±0.18
Lapse rate -0.84 ±0.26
Albedo +0.26 ± 0.08
Clouds +0.69 ± 0.38
On p.631 IPCC states:
“The water vapor feedback is, however, closely related to the lapse rate feedback, and the two combined result in a feedback parameter of approximately 1 W/m^2, corresponding to an amplification of the basic temperature response by approximately 50%.”
This would translate into a temperature response of 1.5*1.0°C = 1.5°C, excluding the feedbacks from clouds or surface albedo.
Including all feedbacks except clouds, IPCC estimates (p.633)
”…it can be estimated that in the presence of water vapour, lapse rate and surface albedo feedbacks, but in the absence of cloud feedbacks, current GCMs would predict a climate sensitivity (±1 standard deviation) of roughly 1.9°C ± 0.15°C (ignoring spread from radiative forcing differences). The mean and standard deviation of climate sensitivity estimates derived from current GCMs are larger (3.2°C ± 0.7°C) essentially because the GCMs all predict a positive cloud feedback but strongly disagree on its magnitude”
So, out of the mean estimate of 3.2°C, roughly 1.3°C are based on the predicted strongly positive net feedback from clouds.
IPCC does concede that ”cloud feedbacks remain the largest source of uncertainty” [AR4 WG1 SPM, p.12]
Two studies since then have cleared up some of this “uncertainty”:
Spencer & Braswell (2007) showed a strongly negative overall cloud feedback with warming over the tropics, based on CERES satellite observations.
Wyant et al. (2006) used a model study using superparameterization to better simulate the behavior of clouds; this study also showed a strongly negative net overall cloud feedback at all latitudes (about the same order of magnitude as the positive feedback predicted by the IPCC models).
Correcting the IPCC AR4 estimate for this later data on the cloud feedback would put 2xCO2 ECS at around 1.0°C to 1.5°C.
On top of this, Webby, there have been several new studies (some at least partially based on actual observations, rather than simply model predictions), all of which suggest a much lower 2xCO2 ECS than was predicted by the models cited by IPCC in AR4. The average of these is around 1.6 °C, or around half the mean value predicted in AR4.
So I think you need to rework your calculation, Webby.
Max