Ocean acidification is mainly caused by the absorption of carbon dioxide (CO2) from the atmosphere by seawater. The ocean absorbs about 30% of the CO2 that is released in the atmosphere, and as levels of atmospheric CO2 increase, so do the levels in the ocean. When CO2 is absorbed by seawater, a series of chemical reactions occur resulting in the increased concentration of hydrogen ions. This increase causes the seawater to become more acidic and causes carbonate ions to be relatively less abundant. Carbonate ions are an important building block of structures such as sea shells and coral skeletons. Decreases in carbonate ions can make building and maintaining shells and other calcium carbonate structures difficult for calcifying organisms such as oysters, clams, sea urchins, shallow water corals, deep sea corals, and calcareous plankton.

Other factors that can affect the acidity of the ocean include natural oceanic processes such as hydrothermal vent sites, which are naturally acidic, and ocean acidification that occurred naturally in the past but over much longer periods of time. However, the current rate of ocean acidification is occurring faster than in the last 20 million years.

In summary, ocean acidification is primarily caused by the absorption of carbon dioxide from the atmosphere by seawater, which leads to a decrease in pH and an increase in acidity. This process is mainly due to human activities such as the burning of fossil fuels and deforestation.