Ocean Carbon Chemistry
The concentration of atmospheric carbon dioxide (CO2) has increased dramatically since the Industrial Revolution (from around 280 parts per million [ppm] in preindustrial times to 409 ppm in 2019), primarily due to human activities such as the burning of fossil fuels and changes in land-use.ref The ocean plays an important role in reducing atmospheric CO2 by absorbing about 1/4 of CO2 that has been released each year into the atmosphere. ref This process helps to reduce the impacts of global warming associated with increasing emissions, but it has come at a cost: ocean acidification.
Changes in Ocean Chemistry
When CO2 is absorbed by the ocean, chemical reactions occur. In particular, carbonic acid is formed and hydrogen ions are released, and as a result the pH of the ocean surface waters decrease (making them more acidic). When hydrogen ions are released in seawater, they combine with carbonate ions to form bicarbonate. This process lowers the carbonate ion concentration. The reduction of available carbonate ions is a problem for marine calcifiers (corals, crustaceans, and mollusks) who need the carbonate ions to build their shells and skeletons.
Changes in the carbonate ion concentration in seawater can affect the saturation state (and hence biological availability) of several types of calcium carbonate (e.g., calcite, aragonite, or high-magnesian calcite.) ref In many parts of the ocean, the seawater is supersaturated with respect to these calcium carbonate minerals, meaning that there is enough calcium carbonate for calcifying organisms to build their skeletons and shells. However, continued ocean acidification is causing many parts of the ocean to become undersaturated with these types of calcium carbonate, thus adversely affecting the ability of some organisms to produce and maintain their shells.
The pH of the surface ocean has fallen by 0.1 pH units since the beginning of the Industrial Revolution. ref While this may not sound like a lot, the pH scale is logarithmic, and this change represents a 30% increase in acidity. As the oceans continue to absorb CO2 , they will become increasingly more acidic. Ocean pH is projected to drop an additional 0.4 pH units by 2100 under a high CO2 emission scenario, ref with carbonate saturation levels potentially falling below those required to sustain coral reef accretion. ref Such changes in the carbon chemistry of the open ocean probably have not occurred for more than 20 million years. ref
While anthropogenic CO2 emissions are driving acidification at global scales, processes occurring at local scales can also affect ocean chemistry. For example, freshwater inputs, pollutants (e.g., acidic fertilizers, chemicals discharged from water treatment and power plants), and soil erosion have the potential to acidify coastal waters at substantially higher rates than atmospheric CO2 alone. ref