Radiative Forcing from Doubled CO2?

Water vapor and CO2 are referred to as "greenhouse gases" GHG because of their effect on the Outgoing Longwave Radiation OLR documented in the above energy spectrum for clear sky over Gulf of Mexico. The effect of CO2 is represented by the dip in the spectrum centered around a wave length of 15 microns. The "radiative forcing" from doubled CO2 of 4 W/m2 underlying the no-feedback sensitivity of IPCC of 1 C, is related to estimated changes of the dip area, with a larger dip area change connecting to larger sensitivity.

On the whole water vapor has a much bigger effect than CO2, and so may easily swamp the effect of CO2. Here is an argument showing that the estimated "radiative forcing" of 4 W/m2 from doubled CO2 may easily be too large.

Consider a a model of global climate with an equatorial region with wet air and a polar region with dry air of roughly the same area, assuming for the discussion that all OLR comes from the polar region.

Radiating 320 W/m2 from the polar region (= 2 x 160 W/m2 insolation) may require an effective temperature of about 0 C, which may be the mean surface temperature in the polar region. This means that the CO2 dip in spectrum will be counted from the lower level (0 C) than in the above figure (about 20 C), and thus have a smaller area.

In this model the effect of CO2 is totally swamped by water vapor in equatorial regions, and in polar regions the dip effect is reduced because the effective radiation temperature is lower.

This is yet another argument indicating that a no-feedback sensitivity of 1 C from "radiative forcing" of 4 W/m2, which is the basic postulate adopted by both alarmists and skeptics like Lindzen, Spencer and Monckton, may have little connection to reality.

It could very well be that the "radiative forcing" from doubled CO2 is rather 1 W/m2 than 4 W/m2, so small that it can never be identified.