I was trying to get you to clear up what you were saying not ask a question literally. Also we already have cars that put off only water vapor (ULEVs). The problem is that (except in California) we dont deny any vehicle to remain on the roads and the older vehicles are the worst polluters (not sure even California succeeds). But that doesnt really matter. All the gases these vehicles have ever put off or ever will put off cant effect our atmosphere. Hydrocarbons are too heavy as gases to ever get high enough in the atmosphere to do any damage. The reports that say othewise are bad science and hand-picked because of political agendas. Los Angeles problem with smog has never influenced the air in the Sierras and never will.
I think I did clear that up for you.
I would also like to note that ULEVs do not have water only as exhaust. ULEV stands for ultra low emission vehicle which emits 50% less emssions. found info here
http://rds.yahoo.com/_ylt=A0geu_H0OZNIfowAqfxXNyoA;_ylu=X3oDMTBybnZlZnRlBHNlYwNzcgRwb3MDMQRjb2xvA2FjMgR2dGlkAw--/SIG=11la1t4pt/EXP=1217694580/**http%3a//en.wikipedia.org/wiki/ULEVTo add to that carbon dioxide is heavy than air and will tend to lay close to ground and may not travel to upper atmosphere, however carbon monoxide is also a green house gas emitted when burning hydrocarbons. This gas has a relative density to air of 1 which means equal to air and will mix easily and yes in time will begin to effect not only air quality but possibly the greenhouse gas problem.
Now I will lean a little in your direction here. Even some of the most known scientist are not 100% sure if we are causing the ever increasing average temperature of the Earth or if it is part of a cycle. What I am pretty sure is happening though is that we are even in the smallest part making it worse by adding more carbon to the atmosphere. These gases are known as greenhouse gases and that is fact that is agreed upon by most all scientist and all most of them also agree that the carbon content in our atmosphere has been high enough in Earth's past that temps. did indeed get higher than they are now.
Now back to what I was saying before I think it a bad idea even if it does become economically feasible to bring hydrocarbons from other planets to this one until we figure out how to manage the carbon we have already been putting into the atmosphere.