global warming due to CO2 vs water What is the physical reason for CO2 being a vital greenhouse gas when it is present at only approx 0.03% of atmospheric composition compared to water vapour, which is present at at least 100 times greater concentration, and which is also greatly variable. 
 A: Water vapour  can be  said to be one of the  the most important contributor to the greenhouse effect. 
To  estimate its potentiality is  a bit complex exercise as   the absorption ranges of wavelengths in the infra-red region overlaps for different  green house gases.
In some of these overlap  regions , the atmosphere already absorbs 100% of radiation, meaning that adding more greenhouse gases cannot increase absorption at these  frequencies.
For other wavelength ranges , only a small proportion is currently absorbed, so higher levels of greenhouse gases do make a difference.
If it were possible to  remove all  water vapour(except clouds) from the atmosphere, only about 40% less infrared of all frequencies would be absorbed.

Take away the clouds and all other greenhouses gases, however, and the water vapour alone would still absorb about 60% of the infrared now absorbed.

By contrast, if CO2 alone was removed from the atmosphere, only 15% less infrared would be absorbed. 
If Carbon di Oxide  was the only greenhouse gas, it would absorb 26% of the infrared currently absorbed by the atmosphere.
A simple  scenario is that about 50% of the greenhouse effect is due to water , 25% due to clouds, 20% to CO2, with other gases accounting for the remainder.
One may wonder why the climate watchers are not taking water as important contributor.

The point is that  CO2 persists in the atmosphere but water vapour stays for a few days in a place and it has its own cycle of transformations.

Moreover the amount of water vapour is dependent on temperature  and its not involved in radiative forcing .
The level of CO2 gets adjusted  by the  sources and sinks, and it would take hundreds of years for it to return to pre-industrials levels even if all further emissions ceased . 
There  is no limit to how much rain can fall, but there is a limit to how much extra CO2 the oceans and other sinks can soak up.
Of course, CO2 is not the only greenhouse gas emitted by humans. And many, such as methane, are far more powerful greenhouse gases in terms of infrared absorption per molecule.
While methane persists for only about a decade before breaking down, other gases, such as the chlorofluorocarbons (CFCs), can persist in the atmosphere for hundreds or   thousands of  years. 

reference https://www.newscientist.com/article/dn11652-climate-myths-co2-isnt-the-most-important-greenhouse-gas/

