Dissolving  in seawater increases the hydrogen ion () concentration in the ocean, and thus decreases ocean pH, as follows:
In shallow coastal and shelf regions, a number of factors interplay to affect air-ocean  exchange and resulting pH change. These include biological processes, such as photosynthesis and respiration, as well as water upwelling. Also, ecosystem metabolism in freshwater sources reaching coastal waters can lead to large, but local, pH changes.
Freshwater bodies also appear to be acidifying, although this is a more complex and less obvious phenomenon.
The absorption of CO from the atmosphere does not affect the ocean's alkalinity. This is important to know in this context as alkalinity is the capacity of water to resist acidification. Ocean alkalinity enhancement has been proposed as one option to add alkalinity to the ocean and therefore buffer against pH changes.