Not really.  Let me check... according to Wikipedia, hydrogen has energy density of 142 MJ&#x2F;kg.<p>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&#x2F;L, so this amounts to 1.68 x 10^19 J = 16.8 EJ.<p>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.<p>An estimate for the total amount of water vapor in the atmosphere says 12,900 cubic kilometers, or 12.9 trillion tons.<p>So, not exactly a drop in the bucket, but more like a teaspoon in the bucket, I guess.