Although water vapor is not generally mentioned in climate change discussions, it is actually the most abundant greenhouse gas in our atmosphere and is responsible for the largest fraction of the greenhouse effect. However, humans can do little to directly influence the concentration of water vapor in the atmosphere. Water vapor cycles through the atmosphere rapidly; a molecule of water vapor typically spends only 8-11 days in the atmosphere before precipitating out. Therefore, we cannot change our climate by directly affecting the concentration of water vapor in our atmosphere.

That being said, it is possible to indirectly affect the concentration of atmospheric water vapor by increasing the temperature of the Earth. Evaporation of water is a function of temperature. At higher temperatures more water vapor will be present in the atmosphere. Therefore, if humans increase the temperature of Earth by increasing the concentration of other greenhouse gases, more water vapor will evaporate, and the concentration of water vapor in the atmosphere will increase. As the concentration of water vapor in the atmosphere increases, more heating will occur, causing even more water vapor to evaporate, thereby continuing the cycle. This is an example of a positive feedback loop.