Published data on (i) carbon dioxide emission from the US transportation sector in 2002 as well as (ii) the amount of hydrogen that would be need for the sector, if entirely fuelled by hydrogen, had been found. The (ii) data have been used for calculation of the amount of water resulting from hydrogen combustion in the sector. The case that the resulting water vapor is emitted into atmosphere has been considered. The carbon dioxide and water vapor emissions were compared using their molecular equivalents. The results show that approximately the same amount of water molecules will be emitted into atmosphere as carbon dioxide molecules eliminated by the use of hydrogen fuel in the transportation sector. Thus, the advantage of hydrogen fuel eliminating the carbon dioxid eemission and its greenhouse influence, seems to be question able since the water molecules (gas phase) possess also strong greenhouse properties.