Hydrogen not the answer.

We have all heard that hydrogen is the fuel of the future. It's clean and its only exhaust is water. But could that be another climate problem in the making? What would happen if every car on earth was changed to run on hydrogen? What would the impact on the climate be? Would it impact the climate? Would it be a good impact or a bad impact?
Just think about it? This would raise the amount of water vapor in the air significantly. More humidity in the air. Would that be good or bad for the climate? In some places it might be good, and others it might be bad. We really don't know.