Sure, the link gives a brief explanation of how the increasing CO2 in the atmosphere increases acidity of the ocean. The basis is of this is what is called the partial pressure of a gas. ie even at low concentrations some is dissolved in the ocean then the chemistry follows. There is nothing controversial about this.
The other point that is missed in the comments above is that no one is claiming the ocean as acidic ie PH < 7.0 , it's just less alkaline or more acidic this was used in a relative sense rather than absolute.
