Mercedes Benz F-CELL cars
It all depends on how the hydrogen is obtained. If you are obtaining the hydrogen from water molecules, then the net sum is zero impact.
That said, the amount of added water vapor from cars would be mostly negligible. Although water vapor adds to global warming it takes an incredible amount of it.
Mercedes Benz F-CELL Hydrogen Electric Drive ZERO emissions car - boasts a driving range of 240 miles on a single tank of hydrogen and produces no emissions other than water vapor .
How could they call this enviromentally friendly when natural water vapor is one of the* MAJOR* causes of global warming?