What is a major oceanic consequence of increased atmospheric CO2?

• A. Ocean acidification due to CO2 dissolving in seawater forming carbonic acid.
• B. Oceans become more basic with higher CO2.
• C. CO2 has no effect on ocean chemistry.
• D. Ocean temperatures decrease as CO2 rises.

Answer: (A)

The key idea is that dissolving atmospheric CO2 changes seawater chemistry, causing ocean acidification. When CO2 enters seawater, it reacts to form carbonic acid, which quickly dissociates to release hydrogen ions. These extra H+ ions lower the water’s pH, making the ocean more acidic. This shift in acidity alters the carbonate balance that calcifying organisms rely on, reducing available carbonate ions and making it harder for shells and skeletons to form. That chemical change in the water is the major oceanic consequence because it directly affects marine organisms and ecosystem processes, independent of temperature effects.

Options claiming the ocean becomes more basic are inconsistent with the chemistry—CO2 raises acidity, not basicity. Saying CO2 has no effect on ocean chemistry is incorrect because CO2 directly alters carbonate chemistry and pH. Finally, the idea that ocean temperatures decrease as CO2 rises runs against the well-established warming effect of CO2, which typically increases ocean temperatures.