Ocean acidification process

Known as the "other carbon problem," ocean acidification is a critical consequence of rising atmospheric carbon dioxide levels. The ocean absorbs nearly 30% of CO2 emissions from human activity, and that absorption is what drives the chemistry below.

1. Carbon dioxide (CO₂) from the atmosphere dissolves in the ocean, creating aqueous carbon dioxide (CO₂ (aq)).
2. This dissolved CO₂ reacts with water (H₂O) to form carbonic acid (H₂CO₃).
3. Carbonic acid then breaks down into bicarbonate (HCO₃^-) and hydrogen ions (H⁺). The more CO2 absorbed, the more hydrogen ions accumulate, and the more acidic the ocean becomes.
4. Natural processes like the weathering of rocks help counteract this increase in acidity. For example, chalk (CaCO₃) from eroding cliffs is added to the ocean over time. Carbonate ions (CO₃^2-) from the chalk react with the hydrogen ions, forming more bicarbonate ions (HCO₃^-). This process, called buffering, reduces the number of carbonate and hydrogen ions in the ocean.
5. However, buffering comes at a cost. The same reaction that neutralises hydrogen ions also consumes carbonate ions, leaving less available for organisms that build shells and skeletons from calcium carbonate.

Buffering also operates far too slowly to keep pace with current emissions. It occurs over thousands of years, not the decades over which atmospheric CO₂ has risen. Since the Industrial Revolution, ocean pH has already dropped by 0.1 units. Because the pH scale is logarithmic, that represents a 30% increase in acidity.