Can The "Greenhouse Effect" Be Detected?

The recent exhange with Roy Spencer and Fred Singer concerning the "greenhouse effect" and "backradiation" identifies three groups in the climate debate with the following standpoints:

1. Alarmists: There is a greenhouse effect and it threatens to overheat the globe.
2. Skeptics: There is a greenhouse effect, but it is so small that it cannot be detected.
3. Deniers: As long as no greenhouse effect has been identified, one can act as if there is no greenhouse effect.

Roy and Fred belong to 2. and myself to 3. The discussion gets complicated by the fact that "the greenhouse effect" is not clearly described in the literature, but it is somehow connected to the radiative properties of the atmosphere:

Both alarmists and skeptics assume as a starting point that doubled CO2 will cause global warming by 1 C, which is referred to as no-feedback climate sensitivity. Alarmists then inflate it to 3 C with positive feedback and skeptics to about 0.5 C with negative feedback.

Both alarmists and skeptics thus firmly believe that science says that increasing the absorptivity of the atmosphere a little by doubling CO2, will cause warming.

Deniers are not so sure about that referring to the following argument:

• Earth surface temperature is determined by the lapse rate.
• Lapse rate is determined by thermodynamics with radiative forcing.
• Fully transparent atmosphere may be isothermal with zero lapse rate.
• Fully opaque atmosphere may be isothermal with zero lapse rate.
• Reality is somewhere in between with a certain lapse rate.
• It is not clear if more CO2 will increase or decrease the lapse rate.
• It is thus not clear if more CO2 will cause warming or cooling.
• There is no convincing evidence that any effect of doubled CO2 can be detected.

The dividing line goes between alarmists on one hand and skeptics/deniers on the other hand.

The quarrel between skeptics and deniers, about the existence of an effect which cannot be detected, seems to be of minor scientific importance compared to the main question of significant global warming or not.