Ocean Acidification

Ocean acidification is defined as a decrease in ocean pH over decades or more that is caused primarily by uptake of carbon dioxide (CO₂) from the atmosphere. The concentration of atmospheric CO₂ has increased dramatically since the Industrial Revolution, from around 280 parts per million (ppm) in preindustrial times to 419.05 ppm as of April 2021. This increase in atmospheric carbon dioxide (CO₂) is absorbed by the ocean and leads to changes in the ocean’s carbonate chemistry, commonly referred to as 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; as a result, the pH of the ocean surface waters decreases, 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, such as corals, crustaceans, and mollusks, who need the carbonate ions to build their shells and skeletons.

Example of the effects of ocean acidification on shells. The healthy shell on the left is transparent with smooth ridges; by contrast, the shell exposed to more acidic, corrosive water is cloudy, ragged, and marked with weak spots. Photo © National Oceanic and Atmospheric Administration

Biological and Ecological Impacts

A growing number of studies have demonstrated adverse impacts on marine organisms as a result of ocean acidification, including the following: ref

• Skeletal Growth: Decreased rate of skeletal growth in reef-building corals
• Protective Shell: Reduced ability to maintain a protective shell among free-swimming zooplankton (zooplankton include “animal plankton”, mainly small crustaceans and fish larvae, and form the base of most marine food webs)
• Calcium Carbonate: Reduced rate of calcium carbonate production in marine algae (crustose coralline and green algae)
• Larval Marine Species: Reduced survival of larval marine species, including commercial fish and shellfish
• Developmental Stages: Impaired developmental stages of invertebrates (fertilization, egg cleavage, larva, settlement, and reproduction)
• CO₂ Toxicity: CO₂ at toxic concentrations in the blood of fish and cephalopods
• Growth and Fecundity: Significantly reduced growth and fecundity in some invertebrate species

Impacts of ocean acidification are particularly worrisome for reef building corals that need carbonate to build their skeletons. Decreasing carbonate ions will likely lead to weaker, more brittle coral skeletons and slower coral growth rates. This may cause coral reefs to erode faster than they can calcify, thus decreasing the ability of coral species to compete for space. A study of brain corals in Bermuda found that calcification rates have declined by 25% over the past 50 years, and ocean acidification is a likely contributing factor. ref

Socio-economic Impacts

Because acidification affects fundamental processes related to the overall structure and function of marine ecosystems, any significant change could have far-reaching consequences for the oceans of the future and the billions of people that depend on marine resources for their food and livelihoods.

In particular, ocean acidification will likely affect commercial and recreational fisheries by:

• Decreasing the abundance of commercially important shellfish species, such as clams, oysters, and sea urchins
• Disrupting marine food webs due to changes in the structure and productivity of primary and secondary benthic and planktonic production

Such impacts may threaten the protein supply and food security of millions of people, as well as the multi-billion dollar fishing industry. ref  By impacting coral reef health and structure, ocean acidification also threatens millions of dollars in tourism revenues, the protection of shorelines from erosion and flooding, and the foundation for coral reef and ocean biodiversity.

Vulnerability and sensitivity of commercially and ecologically important species to ocean acidification. Source: IGBP, IOC, SCOR 2013