Water vapor has a rather large natural source and sink--the oceans. The amount of vapor evaporating and condensing off the oceans daily is much greater than any direct anthropogenic emissions (which come mainly from combustion of organic compounds, such as fossil fuels and biomass; also from operations where water is boiled and the steam released into the atmosphere). Even the anthropogenic water vapor gets removed readily from the atmosphere through condensation onto the oceans or into clouds, where the water eventually precipitates back to the Earth's surface.

While there does not appear to be any harm from anthropogenic water vapor emissions, it is possible for water vapor to play a role in global warming, given that the amount of water vapor in the atmosphere is temperature-dependent. If warming occurs, it is possible for the atmospheric water vapor content to increase, which can lead to further warming effects.