Ocean acidification

Ocean acidification refers to decrease of the pH of the oceans, due to the increased intake of carbon dioxide from the atmosphere. Due to the release of carbon dioxide by human activity, the increased acidity of the oceans could cause some marine life, such as coral and plankton, to lose their external calcium carbonate skeletons. The ocean surface pH is believed to have been reduced from 8.25 to about 8.14 between 1751 and 2004. Studies indicate the pH could drop by a further 0.14 to 0.35 units by 2100.

Carbonate buffering system

Carbon dioxide (CO₂) combines with water (H₂O) to form carbonic acid (H₂CO₃). Carbonic acid can then lose a hydrogen ion (H⁺) to form the negatively charged bicarbonate ion (HCO₃^–). The bicarbonate can lose its hydrogen ion as well, though it does so less readily than carbonic acid. When the bicarbonate ion loses its hydrogen ion, it forms the double-charged negative carbonate ion (CO₃^2–), some of which combines with calcium ions to form calcium carbonate (CaCO₃). Some of the calcium carbonate is precipitated and deposited on the seafloor, which can cycle back into the ocean by dissolving at depth.

In a process called buffering, chemical reactions involving carbonate changes the pH of the ocean. Buffering protects the ocean from getting too basic. If the pH of the ocean increases (becomes too basic), it causes H₂CO₃ to release H⁺ and the pH drops. If the pH of the ocean decreases (becomes too acidic), HCO₃^– combines with H⁺ to remove it, causing the pH to rise. Buffering maintains the pH of the ocean at an average of 8.1 with little variation over time.

Studies indicate the slow process cannot offset the rapid acidification from carbon dioxide absorption.