It should be noted that the quantum allowed orbits of electrons in a gas (Venus, 96.5% CO2 by volume, 90 atmospheres at Venus’ surface.), start to blend, at 90 atmosphere (as the gas changes phase to a liquid). Venus cannot therefore be used as an analogue to what would be the expected out come if CO2 levels were to double (from 380 parts per million C02 to 560 parts per million C02) on the earth at one atmosphere.
It should be noted that CO2 levels on the earth have been 20 times higher than current levels and the temperature was only 7C higher than current levels, so it is not clear why there would be a 3C to 4C increase in planetary temperature if CO2 were to double from current levels.
As others have noted geological evidence indicates that there is not correlation of past CO2 levels and planetary temperature. i.e. In the past there have been significant increases and decreases of CO2 and there was not an accompanying change in planetary temperature.
There is recent publish data and analysis that indicates the amount of direct forcing (without feedbacks) of an increase in CO2 is significant less than that used in the IPCC calculations.
There is recent published data and analysis that indicates the planetary response to a step increase in climate forcing is less than half what was used in the IPCC estimate. The forcing data indicates that planetary cloud cover increases when the planet warms and tropical moisture moves polar ward both of which reduce rather than amplify changes in the forcing. (This conclusion is supported by a 20 times increase in CO2 and only a 7C increase in temperature as well as significant increase and decreases of CO2 levels in the past with no accompanying change in planetary temperature.)
A comparison of tropical temperature trends with model predictions by D. Douglass, J. Christy, B. Pearson, and S. Singer
We have tested the proposition that greenhouse model simulations and trend observations can be reconciled. Our conclusion is that the present evidence, with the application of a robust statistical test, supports rejection of this proposition. (The use of tropical tropospheric temperature trends as a metric for this test is important, as this region represents the CEL and provides a clear signature of the trajectory of the climate system under enhanced greenhouse forcing.) On the whole, the evidence indicates that model trends in the troposphere are very likely inconsistent with observations that indicate that, since 1979, there is no significant long-term amplification factor relative to the surface.
If these results continue to be supported, then future projections of temperature change, as depicted in the present suite of climate models, are likely too high......In summary, the debate in this field revolves around the idea of discrepancy in surface and tropospheric trends in the tropics where vertical convection dominates heat transfer. Models are very consistent, as this article demonstrates, in showing a significant difference between surface and tropospheric trends, with tropospheric temperature trends warming faster than the surface. What is new in this article is the determination of a very robust estimate of the magnitude of the model trends at each atmospheric layer. These are compared with several equally robust updated estimates of trends from observations which disagree with trends from the models.
Does the Earth Have an Adaptive Infrared Iris? R.Lindzen, Ming-Dah Chou, Arthur Y. Hou
The calculations show that such a change in the Tropics could lead to a negative feedback in the global climate, with a feedback factor of about -1.1, which if correct, would more than cancel all the positive feedbacks in the more sensitive current climate models. Even if regions of high humidity were not coupled to cloudiness, the feedback factor due to the clouds alone would still amount to about -0.45, which would cancel model water vapour feedback in almost all models...
...This new mechanism would, in effect, constitute an adaptive infrared iris that opens and closes in order to control the Outgoing Longwave Radiation in response to changes in surface temperature in a manner similar to the way in which an eye’s iris opens and closes in response to changing light levels. Not surprisingly, for upper-level clouds, their infrared effect dominates their shortwave effect. Preliminary attempts to replicate observations with GCMs suggest that models lack such a negative cloud/moist areal feedback.