Ocean Acidification

The ocean is the largest active carbon sink on Earth, responsible for absorbing roughly a third of all human-produced CO2

  • Some CO2 may remain as dissolved gas within the water, however the majority will combine with water to form carbonic acid
  • The solubility of CO2 in sea water is inversely proportional to oceanic temperature (i.e. more soluble in cooler temperatures)
  • It is therefore a concern that global warming could limit carbon storage in oceans, exacerbating climate change

Ocean-Atmosphere Exchange

  • Typically, when carbon dioxide is dissolved in the ocean, it combines with water molecules to form carbonic acid (H2CO3)
  • The carbonic acid then dissociates to form bicarbonate ions (HCO3) and hydrogen ions (H+)
  • The ocean also contains carbonate ions (CO3), which are absorbed by coral and molluscs to form calcium carbonate (CaCO3)
  • Calcium carbonate is used to form the hard exoskeletons of reef-building corals and is used by molluscs to develop shells
  • H+ ions can reduce the stock of carbonate ions in the ocean by combining with it to form bicarbonate
  • Hence, the levels of H+ ions must be kept low to ensure that their is sufficient stock of carbonate ions for aquatic organisms

acidification before

Ocean Acidification

  • As a result of deforestation and the increased burning of fossil fuels, atmospheric carbon dioxide concentrations have increased
  • With more CO2 being absorbed by the oceans, there is an associated increase in the production of H+ ions
  • These H+ ions lower the pH of the ocean, causing acidification (ocean pH has dropped ~0.2 since the industrial revolution)
  • The H+ ions will also combine with carbonate ions, reducing the amounts available to marine organisms
  • This will result in the formation of thinner, deformed shells and reduce the population numbers of reef-building corals
  • The reduction in pH will also dissolve calcium carbonate structures, enhancing the damage to shells and corals

acidification after

Acid Test – The Global Challenge of Ocean Acidification