Ramanthan (Journal of Geophysical Review, vol. 84, pp. 4949-4958) states:
"the direct radiative effects of doubled CO2 can cause a maximum surface warming [at the equator] of about 0.2 K, and hence roughly 90% of the 2.0-2.5 K surface warming obtained by the GCM is caused by atmospheric feedback processes described above."
A Lukewarm Greenhouse
"The average warming predicted by the six methods for a doubling of CO2, is only +0.2 degC."
Correct, but this study confirms that there is a positive water vapor feedback, and indicates that it can also be quantified. That's a major advance.
I found a really interesting figure, which it would be nice to reproduce but I can't. It's in this Powerpoint presentation:
noaasis.noaa.gov/2003/bates.ppt
Find the "Water Vapor Feedback" slide.
It confirms exactly what you say (not that I was disputing you!). Direct CO2 greenhouse heating provides 0.17 C (the slide notes that this is model dependent). Tropospheric heating by CO2 provides 0.33 C (total of 0.5 C). Water vapor feedback, and this was based on the constant relative humidity assumption now in question, was 1.7 C, for a total of 2.2 C.
It will be very interesting to find out how much weaker the researchers (or others) think that this research indicates the water feedback mechanism is, as discussed with JasonC above.