Why is ocean acidification happening?

Ocean acidification is a response to all the carbon dioxide absorbed in the ocean. So all our CO2 mixes with the H2O in the ocean creating H2CO3 (carbonic acid) that is lowering the Ph of the water. Originally the Ph of the ocean is 8.2 more alkaline then acidic; but now it has dropped to 8.1. Does not seem like much but is a catastrophic decent that we have reached the past 100-200 years at a faster rate than the past 300 million years.