Only water above the troposphere in the stratosphere is a greenhouse gas. Water vapor under the stratosphere and near the surface is part of the water cycle and will ultimately return to it’s base level source the world ocean. Adding water vapor at ground level is like watering your yard and letting it evaporate to the sky in the summer time that no more causes greenhouse effect than letting water vapor out of a fuel cell in a vehicle sitting in the drive way next to that wet grass. Even aircraft using fuel cells would not make a difference as long as they are not above 50,000 FT or so and above the water cycle. Conventional jet engines release millions of kg of water vapor every year into the sky that’s what contrails are made of we see them every day all across the sky. More than half the combustion products of jet fuel is H2O on vapor form. Jet fuel is primarily dodecane C12H26 when burnt in a jet engine you get 12 atoms of CO2 and 13 atoms of H2O per single atom of dodecane. A fuel cell is twice as efficient as a open cycle gas turbine for equal output you will get half the water vapor anyway.
*molecules...
You've made some very good points in this thread. Thank you.