In this case, a natural gas water heater is the least costly option, followed by a passive solar system with a natural gas backup. (peakoil.net)
CO2 and methane are gases water vapour probably exists in two states (gas and suspended liquid). (johnquiggin.com)
In addition, the atmosphere contains the greenhouse gas water vapor, whose amounts are highly variable but typically around 1 % volume mixing ratio. (climatehotmap.org)