Current daily oil production: 81.7 Million barrels of oil; that gives 2.98*10^10 barrels of oil per year.
Let's calculate the total amount of energy we get if we burn all of it:
2.98*10^10 * 159 liters/barrel * 41.8 M J/liter = 1.98*10^20 J.
If we replaced ALL of it with hydrogen (with combustion heat of 141.8MJ/kG), we would need 1.4*10^12 kG of H2.
Each kilogram of H2 produces 9 kG of H2O, so having burned 1.4*10^12 kG of H2, we would emit 1.26*10^13 kG of H2O.
That is for a year, remember. Is it a lot or a little?
To put things in prospective, a few numbers:
- Total current amount of water vapor in the Earth's atmosphere: 1.3*10^16 kG.
That means if we burned the entire energy equivalent of H2, we would contribute 0.1% of current atmosphere's volume of water vapor. But the current atmosphere's volume of water vapor is like a holding tank, which receives water vapor and releases it. Another number for comparison is annual worldwide rainfall, known to be about 5*10^17 kG. That means the human contribution to it would be in our case 0.0025 of ONE PERCENT.
More numbers to compare: a cumulus cloud contains between 500 and 5000 metric tons of water. One average hurricane contains 1.2*10^13 kG of water vapor, about equal to our calculated human annual contribution.
So water vapor is something I just wouldn't worry about yet.
Some sources:
http://www.netl.doe.gov/energy-analyses/pubs/Water%20vapor%20impacts_final2.pdf
http://www.geocraft.com/WVFossils/greenhouse_data.html
192.211.16.13/curricular/sailpower/hurricansSoln.pdf