Ocean Acidification

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•  Threats to coral reefs from increasing concentrations of dissolved carbon dioxide

The oceans are a major carbon sink and absorb roughly a third of all human produced (anthropomorphic) CO2 emissions

  • CO2 solubility is temperature dependent (more soluble when cooler), so less CO2 will be absorbed as temperatures rise

When oceans absorb atmospheric CO2, some of it will remain dissolved in a gaseous state but most will be chemically modified:

  • Carbon dioxide will combine with water to form carbonic acid, which dissociates into hydrogen ions and hydrogen carbonate
  • H+ ions will lower the ocean pH (acidification) and will also combine with free carbonate ions to form more hydrogen carbonate
  • With less free carbonate ions in the water, marine organisms are less able to produce calcium carbonate (via calcification)
  • Calcium carbonate is used to form the hard exoskeleton of coral and is also present in the shells of certain molluscs
  • Hence increasing concentrations of dissolved carbon dioxide threatens the viability of coral reefs and certain molluscs

Ocean–Atmosphere Carbon Dioxide Exchange

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  Click on the diagram to cycle between normal versus elevated carbon dioxide levels

CO2 Emissions and Ocean Acidification

Rising levels of atmospheric carbon dioxide are causing a decrease in the pH of ocean water (ocean acidification)

  • Since the start of the industrial revolution ocean pH has dropped from ~8.2 to ~8.1 (roughly a 30% increase in acidity) 
  • It is predicted that if current conditions continue, oceanic pH could fall to roughly 7.8 by the turn of the century (2100)

The decrease in ocean pH is predicted to threaten the survival of marine organisms that require calcium carbonate

  • An increase in the concentration of H+ ions means there are less free carbonate ions available for calcification
  • Shells and coral exoskeletons are also likely to begin to dissolve when ocean conditions are more acidic
  • Experiments have shown that increasing water acidity correlates with the significant thinning of shells over several weeks
  • Corals, sea urchins and shelled molluscs do not exist in regions with high levels of dissolved CO2 (e.g. near hydrothermal vents)

Relationship between Carbon Dioxide Levels, Ocean Acidity and Shell Thickness

ph vs shells

Consequences of Ocean Acidification

An increase in ocean acidification as a result of elevated anthropomorphic CO2 emissions could have several consequences:

  • The disappearance of coral reefs could result in a loss of shoreline protection and habitat, altering coastal ecosystems
  • The loss in revenue from tourism and food industries is predicted to cost economies upwards of $1 trillion by 2100
  • Increasing the dissolved CO2 levels in oceans would cause invasive species of algae to flourish (more photosynthesis)

Summary of Ocean Acidification