I don't know how it happens. Just ask the government. They seem to have figured out how a gallon of gas can make 20 pounds of CO2 out of thin air!
http://www.fueleconomy.gov/feg/co2.shtml
It could be magic, or, perhaps you just slept through high school chemistry class. But there it is.
Fact.
When you start your car, it's not just the gas you are burning, it's also that high SCF carburetor sucking air in too. Bringing in oxygen from the atmosphere to combine with the carbon in gasoline to pump CO2 out the exhuast. Unlike the space shuttle, Your car does not carry it's own oxygen supply in the fuel tank.
It uses the air we breath.
And thus, The Ocean gets more Acidic from CO2.