"For clear skies, water vapor is the most important greenhouse gas, accounting for 60% of the total. The second most important greenhouse gas is CO2, which contributes 32 W m−2"
See "Earth’s Annual Global Mean Energy Budget", page 203 (7th page of PDF): http://www.atmo.arizona.edu/students/courselinks/spring04/at...
From the DOE...
"Given the present composition of the atmosphere, the contribution to the total heating rate in the troposphere is around 5 percent from carbon dioxide and around 95 percent from water vapor."
It is important to note the next sentence, however:
"In the stratosphere, the contribution is about 80 percent from carbon dioxide and about 20 percent from water vapor."
http://www.eia.doe.gov/cneaf/alternate/page/environment/appd...
(end of last paragraph)
As is always the case in these matters, conflicting sources
Not being an atmospheric scientist I cannot be sure but I feel like the vast majority of the atmosphere's density being in the troposphere temperature effects from the stratosphere would be negligible.