Originally Posted by GGDub
Yup.
http://en.wikipedia.org/wiki/Greenho...of_water_vapor
Its the most significant greenhouse gas. Ever wonder why cloudy winter nights are warmer than clear ones?
edit: The source for the hydrogen for a fuel cell economy is still unknown. Right now thoughts are to either get it from water in the first place or get it from hydrocarbons. Either way the potential exists there to add more water to the natural water cycle or change the balance of gaseous water and liquid water, which could lead to increased concentrations of H2O in the atmosphere, which might cause more global warming.
Well....
Originally Posted by Wikipedia
Water vapor concentrations fluctuate regionally, but human activity does not directly affect water vapor concentrations except at very local scales.
This is a pretty good read on the subject.
Water vapour is a "reactive" GHG with a short atmospheric lifetime of about 1 week. If you pump out a whole load of extra water vapour it won't stay in the atmosphere; it would condense as rain/snow and we'd be back to where we started. If you sucked the atmosphere dry of moisture, more would evaporate from the oceans. The balance is dynamic of course: humidity of the air varies by place and time, but its a stable balance.
So, while water vapor is a GHG because it absorbs IR radiation, the amount of it in the atmosphere, and amount of heat it retains, depends on how much energy/heat is in the atmosphere. More heat means more water vapor, etc... But, since it's reactive, it won't stay in the atmosphere long enough to significantly increase the amount of heat trapped, so we can't have a runaway greenhouse effect from water vapor alone. Otoh, CO2 stays up there for decades, if not centuries, and the additional heat trapped will result in more vaporization of water, but simply vaporizing water will not lead to more heat and more vaporized water, etc... The amount of gaseous water depends on the balance between condensation and evaporation, and this depends on temperature and pressure. Any water vapor in the atmosphere will simply return to liquid if the conditions are suitable.
I don't say this to be an a$$hole, it's just that imo, too many interests propagate the water vapor is a GHG line to downplay the significant threat posed carbon dioxide to our climate. As of now, the Hydrogen economy would be completely dependent on fossil fuels to produce the Hydrogen, so yeah, in that context it's just robbing Paul to pay Peter, still plenty of C)2 production. EV technology is suitable, but not very appealing to big business because you can easily generate electricity at your own home. Hydrogen otoh, can be made in bulk from any kind of power generation, fission, wind, etc... And would be cheaper for the large company to make, insuring there is a product to sell.