What happens to CO2 in oceans and why is this important for ocean chemistry?

• A. CO2 dissolves in seawater forming carbonic acid, lowering pH and decreasing carbonate ion availability, affecting calcifying organisms.
• B. CO2 dissolves in seawater and increases carbonate ion availability.
• C. CO2 does not affect ocean pH.
• D. CO2 increases salinity.

Answer: (A)

CO2 from the air dissolves in seawater and forms carbonic acid, which then dissociates to release hydrogen ions. The extra H+ lowers the ocean’s pH (ocean acidification) and shifts the carbonate system so that carbonate ions become less available. That matters because many marine organisms rely on carbonate ions to build calcium carbonate shells and skeletons. When carbonate is scarcer and the water is more acidic, calcification slows, affecting growth and survival of these organisms. The other statements don’t fit: CO2 does affect pH; it generally reduces, not increases, carbonate ion availability; and CO2 does not raise salinity.