[Lewis Thomson (OP)]

John wants help finding the answers to this question.

"If we all start to drive Hydrogen powered cars, where will the H2O exhaust fumes go? Will H2O be the next greenhouse gas?"

Leave your answers in the comments below...

[Bored chemist]

where will the H2O exhaust fumes go?

Rain.

[alancalverd]

It is already the principal greenhouse gas. Since gasoline and diesel are essentially ...(CH₂)_n..... chains of various lengths, about 60% of vehicle emissions is already H₂O so it won't make much difference.