Card #24: Ocean Acidification
When CO2 dissolves in the ocean, it turns into acid ions (H2CO3 and HCO3-). This makes the oceans more acidic and the pH drops.
Ocean acidification is sometimes referred to as "the other carbon problem". It is not strictly speaking a consequence of climate change, but another consequence of CO2 emissions.
The card Concentration of CO2 (ppm) is often identified by players as the precursor of this card. However, not every molecule that is found in the ocean was previously in the atmosphere, and therefore the most logical link is with the carbon sinks card.