Just wait half a century now when hydrogen powered cars start emitting water vapor into the atmosphere as exhaust. Talk about a “global warming greenhouse effect”- water is 95% of the greenhouse effect and far more powerful at retaining Earth’s heat than carbon dioxide.
Yes, we would be emitting a little bit of water vapor from H2 fueled cars and portable engines.
But really, the amount of water vaporized in one hour by the oceans just around one degree of latitude makes the extra bit meaningless.
ALL of our CO2 (emitted from everything! Power plants, cars, airplanes, trucks, and burning wood, making plastics, driving ships, etc.) is still only a small part of the of the total 1% CO2 INCREASE in the atmosphere each year - and the TOTAL CO2 in the atmosphere is only 375 molecules out of every 1,000,000 molecules.
So our CO2 contribution is less than 1% of 325 molecules.