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Ocean Acidification


Example of an organism sensitive to ocean acidification: the pteropod Cavolinia inflexa

Since the beginning of the industrial revolution 250 years ago, the ocean has absorbed about one third of the carbon dioxide (CO2) released as a result of human activities. Without the oceans, the CO2 content in the atmosphere would have been much higher and global warming and its consequences more dramatic. But the uptake of man-made CO2 by the oceans results in ocean acidification, often referred to as "the other CO2 problem" alongside global warming.

As CO2 dissolves in the ocean it changes sea water chemistry. CO2 reacts with water molecules (H2O) and forms the weak acid H2CO3 (carbonic acid). Most of this acid dissociates into hydrogen ions (H+) and bicarbonate ions (HCO3-). The increase in H+ ions increases the acidity of the oceans and reduces their pH (measure of acidity). The average pH of today's surface waters is 8.1, which is approximately 0.1 pH units less than the estimated pre-industrial value. A change of 0.1 units might not sound much, but the pH scale is logarithmic, meaning that such a change is equivalent to a 30% increase in acidity.

Model projections demonstrate that if CO2 continues to be released at the same pace as today, ocean pH will reach 7.8 by the end of this century, corresponding to a 150% increase in acidity compared to pre-industrial levels. This level of acidity has probably not been experienced by marine ecosystems for several millions of years, and the changes are happening at a speed at least 10 times greater than has ever been observed during the geological past.

Whereas the chemical consequences of the CO2 uptake are well known the biological impacts are still poorly understood. One of the most likely consequences is the slower growth of organisms forming calcareous skeletons or shells, such as corals and mollusks.

More information: FAQ on Ocean Acidification

Use of isotopes in ocean acidification research

Nuclear and isotopic techniques are unique tools in ocean acidification research, for example to study:

  • Past changes in ocean chemistry: By studying “environmental archives”, scientists are able to look into the past and follow trends in ocean acidification several decades to millennia back. Examples of such archives are fossilized marine organisms buried in the sediments on the ocean floor or the skeleton of long-lived corals. During their lifetime, these organisms formed calcareous shells or skeletons that reflect the seawater chemistry (e.g. pH) of that time. Indeed, at the time when they built their shells or skeletons, they incorporated certain chemical compounds of which the presence and amount depended on the pH of the ocean at that time. One such compound is boron. Boron has two isotopes, 11B and 10B, and the relative amount of these isotopes incorporated into the shells is dependent on the pH (i.e. acidity) of seawater. We say that the isotopic ratio of boron is a “proxy” for seawater pH: i.e. it provides an indirect measurement of pH. The information provided by such registered data is not only useful to find out whether ocean acidification happened in the past, but also crucial to test and build models to project future acidification scenarios.
  • The impact of ocean acidification on seafood security: About one billion people rely on seafood as primary source of animal protein. Radio-tracer experiments can help us to understand how marine organisms might respond to future ocean acidification. Biological processes such as primary production, growth and calcification can be measured using isotopic techniques. The information provided can be used to estimate impacts of ocean acidification on key commercial species and help evaluating risks for the environment and for society, e.g. implications for shellfish production and ultimately seafood security.
  • The impact of ocean acidification on seafood safety: nuclear and isotopic techniques can be used to detect and monitor biotoxins produced by harmful algal blooms (via a radio-ligand receptor binding assay, RBA), accumulating in coastal areas and in seafood. It is believed that ocean acidification in combination with other environmental stressors (e.g. global warming) will increase the frequency and severity of harmful algal blooms. In addition, future changes in carbonate chemistry and pH are expected to alter the availability and uptake of trace elements in marine organisms. Radioisotopes can be used to study the toxicity of these elements (e.g. metals) in marine organisms.


IAEA-EL activities on ocean acidification