Not really. Let me check... according to Wikipedia, hydrogen has energy density of 142 MJ/kg. In 2014, about 136.78 billion gallons of gasoline was consumed i

Not really. Let me check... according to Wikipedia, hydrogen has energy density of 142 MJ/kg. In 2014, about 136.78 billion gallons of gasoline was consumed in the US, which is about 518 billion liters. Gasoline has energy density of 32.4 MJ/L, so this amounts to 1.68 x 10^19 J = 16.8 EJ. If all this gasoline converts to hydrogen economy, we get 118 billion kilograms of hydrogen (per year), which will produce 1.06 trillion kilograms (or 1.06 billion tons) of water. An estimate for the total amount of water vapor in the atmosphere says 12,900 cubic kilometers, or 12.9 trillion tons. So, not exactly a drop in the bucket, but more like a teaspoon in the bucket, I guess.