A comparison between how long a CO2 molecule remains in the atmosphere compared to a water molecule is kind of irrelevant, as the water vapour molecule is replaced by another one. This is the beauty of the system. I personally think it is highly arrogant of mankind to believe that it will ultimately win against nature. Worrying about CO2, as MAC says, is not the answer here. In nature, population sizes are controlled. We have used our technology to fight nature, and have artificially enhanced the size of human populations, and indeed the animal populations we feed upon, or keep as pets. At some point, nature insists we will come up against a population size barrier which cannot be broken using our science.
CO2 and the greenhouse effect is a red herring. The real problem is over population, but that is a far more difficult problem to address.