section_path: Concentrations and role in the environment/Oceans/Ocean acidification
section_name: Ocean acidification
paragraph_text: Ocean acidification
  
  Carbon dioxide dissolves in the ocean to form carbonic acid (H CO), bicarbonate (HCO − 3), and carbonate (CO 2− 3). There is about fifty times as much carbon dioxide dissolved in the oceans as exists in the atmosphere. The oceans act as an enormous carbon sink, and have taken up about a third of CO emitted by human activity.
  
  Ocean acidification is the ongoing decrease in the pH of the Earth's ocean. Ocean acidification is a process that occurs when carbon dioxide (CO2) from the atmosphere is absorbed by seawater, leading to a decrease in pH levels. This results in an increase in acidity and a reduction in carbonate ions, which are crucial for marine organisms like corals, shellfish, and plankton to build their shells and skeletons. Over the past 200 years, the rapid increase in anthropogenic CO (carbon dioxide) production has led to an increase in the acidity of the Earth's oceans. Between 1950 and 2020, the average pH of the ocean surface fell from approximately 8.15 to 8.05. Carbon dioxide emissions from human activities are the primary cause of ocean acidification, with atmospheric carbon dioxide (CO) levels exceeding 410 ppm (in 2020). CO from the atmosphere is absorbed by the oceans. This chemical reaction produces carbonic acid (H CO) which dissociates into a bicarbonate ion (HCO − 3) and a hydrogen ion (H). The presence of free hydrogen ions (H) lowers the pH of the ocean, increasing acidity (this does not mean that seawater is acidic yet; it is still alkaline, with a pH higher than 8). Marine calcifying organisms, such as mollusks and corals, are especially vulnerable because they rely on calcium carbonate to build shells and skeletons.
  
  Changes in ocean chemistry can have extensive direct and indirect effects on organisms and their habitats. One of the most important repercussions of increasing ocean acidity relates to the production of shells out of calcium carbonate (CaCO). This process is called calcification and is important to the biology and survival of a wide range of marine organisms. Calcification involves the precipitation of dissolved ions into solid CaCO structures, structures for many marine organisms, such as coccolithophores, foraminifera, crustaceans, mollusks, etc. After they are formed, these CaCO structures are vulnerable to dissolution unless the surrounding seawater contains saturating concentrations of carbonate ions (CO 2− 3).
  
  Very little of the extra carbon dioxide that is added into the ocean remains as dissolved carbon dioxide. The majority dissociates into additional bicarbonate and free hydrogen ions. The increase in hydrogen is larger than the increase in bicarbonate, creating an imbalance in the reaction:
  
  To maintain chemical equilibrium, some of the carbonate ions already in the ocean combine with some of the hydrogen ions to make further bicarbonate. Thus the ocean's concentration of carbonate ions is reduced, removing an essential building block for marine organisms to build shells, or calcify: