Fun fact: Water vapor makes up 98% of the greenhouse effect.
Funner fact: Where the effect happens is more important. The troposphere is close to IR opaque, with gas and black body temperatures closely coupled. The black body temperature of the earth, and hence the amount of IR radiation emitted into deep space, is the deep cold of the upper atmosphere. Clouds and sulfate particulates determine the amount of light reaching the surface (mostly ocean), where almost all is turned into heat. In the longer term, that heat is equal to the IR black body radiation, with whole system temperatures adjusting until they do.
Atmospheric temperature decreases about 6.5C per kilometer altitude, and density decreases by about a factor of two every 7 kilometers. Water vapor pressure drops rapidly with temperature, and the water freezes out around 0C forming clouds and precipitation. There is very little water vapor above the high cloud tops, and that region is ruled by CO and methane, which do not freeze out at atmospheric temperatures. At some even higher altitude, the remaining amount of CO is transparent to space. Double the CO, and the transparency altitude goes up around 7km, to a region 45C colder than 7km below it.
If you could see the earth in the infrared, from space, it would appear colored by high altitude haze from the CO and methane, with lower altitude islands of cloud floating in an opaque sea of water vapor. The land and oceans are be invisible - and irrelevant to black body radiation. If you double the CO or methane, the long term effect is to increase the coloring, raise the water vapor "sea level" altitude somewhat. The temperature at this level stays close to constant (remember, that is the temperature where water freezes out), but at a higher altitude, there is more 6.5C/km air beneath. A very small change in atmospheric properties, raising the top of the troposphere a few percent, can easily result in an average 5C change down here on the surface.
I started out as a climate change sceptic, horrified by the pseudo-scientific and pseudo-technical nonsense spewed by the media. As a responsible technologist, I studied primary sources and ran the numbers myself, and changed my mind. Atmospheric scientists know a lot, have a lot more to learn, and cannot produce a definite prediction of exactly when our fiendishly complex, multi-billion cubic kilometer atmosphere will be broken beyond repair. The fate of civilization is dependent on the slow integration of effects that depend on other integrals of integrals; when the first pebbles of the landslide reach us, the unstoppable wall of rock will follow soon after, far too late to stabilize the slope.
Nature could soak up much of the excess CO, turning it into climax forest, plankton sea floor sediment, and perennial-plant root-mediated carbonate rock, but we are destroying the absorbers with annual-crop agriculture, especially atrocities such as "biofuel". We could reduce our methane use; instead we invest in erratic sources of "energy" such as wind turbines and grid-scale solar, which must be backed kilowatt-for-kilowatt with fast turn-on natural gas turbines instead of slow-response base-load hydro and thermal generation.
The natgas pipelines are overcapacity, leaking methane, bursting, even catching fire and exploding, compounding the problem of leaky production fields and leaky aging cities. If this is a problem in Europe and the US, imagine how bad it could get if impoverished India and China tried to copy our example.
No responsible technologist should blindly repeat numbers without checking them out, whether those numbers are ideologically comforting or not. Better to shut the hell up than to drown out the few responsible people who, whatever conclusions they come to, at least try to build those conclusions from direct observation, primary data, and replicable calculation.
Self-education and calculation is not only responsible, it is lucrative. Instead of the highly touted, vastly expensive "alternative energy" proposals touted by idealogues, there are many possibilities that are inexpensive, highly profitable, and a lot of fun to work on. I'm working long hours on a few, as are a few colleagues, and after stage 3 ("..."), we might even get paid for it. Help is welcome, competition is welcome, mindless slogans are not.