system
Given the following post, respond as if you're commenting on Reddit:
human
If every vehicle on the road were a hydrogen-fueled vehicle, would the climate impact of those water vapor emissions be significant?

This is obviously assuming the water was released as vapor and not cooled and disposed of as a liquid or something like that. I've heard that water vapor is a much stronger greenhouse gas than carbon dioxide, but a lot of information out there acts as though hydrogen vehicle emissions are better for the environment because they're water vapor and not CO2.
gpt
To answer your question, you must first understand that most combustion reactions produce water. For an example, let’s take a look at one of the most basic combustion reactions, the combustion of methane to water and CO2. We’re going to assume that we have complete combustion, and that we don’t make any carbon monoxide.

CH4 + 2O2 = CO2 + 2H2O

As you can see, for every mole of methane we burn, we produce 2 moles of water. We can take this 1 step further and look at the combustion of Octane, which is an acceptable gasoline substitute.

C8H18 + (12.5)O2 = 8CO2 + 9H2O

So for every mole of octane we burn, we produce 9 moles of H2O. The density of octane is roughly 0.38 lbmole/ft3, or 0.051 lbmole/gallon, so 1 gallon of octane produces roughly 0.46 moles of water (MW 18 lbs/lbmole) which is (drumroll) 8.3 lbs of water.

So every gallon of gas you burn produces 8.3 pounds of water.

To answer your question: If adding this much water to the atmosphere was a problem, then we have the problem with or without hydrogen powered cars.

I have a feeling a few of you are going to jump in and say that a gallon of octane only weights 5.83 lb, so how are we producing 8.3 pounds of water per gallon? The answer is that all of the mass comes from the oxygen we're adding to the reaction. We're burning roughly 20 lb of oxygen with every gallon of octane that we burn. The remainder goes directly into carbon dioxide.

[Fun fact] Air is only 20% oxygen. So when you burn 1 gallon of gasoline using 20-lb of oxygen, you're really pulling 100 pounds of air through your engine, of which 80% passes through mostly unchanged (Shakes old man fist at nitrogen).[/Fun fact]