Prior to the Industrial Revolution, atmospheric carbon dioxide (CO2) was 280ppm (parts per million). Burning fossil fuels has increased atmospheric CO2 until today it is 400ppm and rising. It would have been much higher, however, to achieve chemical equilibrium the ocean has absorbed 30-50% of this growing atmospheric CO2.
When CO2 combines with ocean water (H2O) it produced H2CO3, or carbonic acid, causing the oceans to become more acidic. Marine animals such microscopic plankton at the base of the food chain, shellfish, and crustaceans including krill, need biogenic calcium carbonate (CACO3) to built shells and exoskeletons. Ocean acidification dissolves these shells and prevents their growth during the planktonic stages of development. Colder waters hold more CO2 than warm waters, meaning food supply in the sub-Antarctic and Antarctic regions is affected faster than in warmer regions.
Ocean acidification is happening today faster than at anytime in the last 300 million years, making it unlikely that marine organisms will be able to adapt before becoming extinct.