This image shows how carbon dioxide and other "non-condensing" greenhouse gases sustain the Earth's greenhouse effect. From the public-domain source: "Attribution of individual atmospheric component contributions to the terrestrial greenhouse effect, separated into feedback and forcing categories. Dotted and dashed lines depict the fractional response for single-addition and single-subtraction of individual gases to either an empty or full-component reference atmosphere, respectively. Solid black lines are the scaled averages of the dashed and dotted line fractional response results. The sum of the fractional responses must add up to the total greenhouse effect. The reference model atmosphere is for 1980 conditions.

(...) Radiative modeling analyses of the terrestrial greenhouse structure described in a parallel study in the Journal of Geophysical Research (Schmidt et al., 2010) found that water vapor accounts for about 50% of the Earth's greenhouse effect, with clouds contributing 25%, carbon dioxide 20%, and the minor greenhouse gases (GHGs) and aerosols accounting for the remaining 5%, as shown in (this image).

The following paragraph is not contained in the paper but was attached to the one above as if the author had included it: see http://pubs.giss.nasa.gov/docs/2010/2010_Schmidt_etal_1.pdf and http://pubs.giss.nasa.gov/abs/sc05400j.html

Thus, while the non-condensing greenhouse gases account for only 25% of the total greenhouse effect, it is these non-condensing GHGs that actually control the strength of the terrestrial greenhouse effect since the water vapor and cloud feedback contributions are not self-sustaining and as such, only provide amplification. Because carbon dioxide accounts for 80% of the non-condensing GHG forcing in the current climate atmosphere, atmospheric carbon dioxide therefore qualifies as the principal control knob that governs the temperature of Earth."

References:

• Schmidt, G.A., R. Ruedy, R.L. Miller, and A.A. Lacis (2010), “The attribution of the present-day total greenhouse effect”, in J. Geophys. Res.^[1], volume 115, DOI:10.1029/2010JD014287, archived from the original on 2011-10-22, D20106. Web page for paper.