Ocean acidification

Ocean Acidification is often referred to as the ‘other carbon problem’ and corresponds to the modification of the chemistry of the ocean as a response to the increase in atmospheric concentration in CO₂, resulting from human activities. 

When dissolved in the ocean, CO₂ molecules react with water molecules (H₂O) to produce carbonic acid (H₂CO₃) which is unstable and, in sea water, quickly dissociates into bicarbonate ions (HCO₃^–) and H⁺ ions: 

CO₂ + H₂O →  H₂CO₃                                        (1)

H₂CO₃ → HCO₃^– + H⁺                                         (2)

The resulting excess in H+ ions increases the acidity of sea water (thus lowering its pH) and also decreases the concentration in carbonate ions (CO₃^2-):

CO₃^2- + H⁺ → HCO₃^–                                           (3)

At the pH of sea water, which is slightly alkaline (pH~8.2), the concentration in bicarbonate ions is usually roughly 10 times larger than that of carbonate which itself, is also roughly 10 times larger than that of dissolved CO₂. 

Ocean acidification has been reported worldwide and quantified in the last IPCC synthesis by an average global decrease of ~0.1 pH unit since the beginning of the industrial revolution. This seemingly small change actually corresponds to an increase in acidity of ~30 % because pH is expressed as a logarithmic function of the concentration in H⁺ ions in water.

pH = -log₁₀[H⁺]                                                    (4)

The change in pH already observed in global oceanic waters impacts a large number of marine organisms, profoundly modifying the structure of marine ecosystems. Particularly impacted are organisms producing shelves or skeletons made of calcium carbonate (CaCO₃). Indeed, the decrease in pH induced by ocean acidification modifies the solubility of calcium carbonate, thus inhibiting the growth of calcifying organisms. These include corals, several groups of phytoplanktonic organisms as well as microscopic crustacean species and numerous molluscs such as oysters or scallops. Many other marine organisms ranging from microalgae to fish species are also affected by ocean acidification but responses significantly vary from a group of organisms to the next. As a consequence, the overall impact of ocean acidification on marine communities is complex and still not fully understood but often results in a loss of biodiversity and biological productivity. The death of corals (also called coral whitening) being maybe the most sticking example: healthy coral reefs are typically very biologically productive and diverse ecosystems which lose most of their biodiversity when the coral dies. 

Suggested further reading

Scott C. Doney, D. Shallin Busch, Sarah R. Cooley, Kristy J. Kroeker The Impacts of Ocean Acidification on Marine Ecosystems and Reliant Human Communities Annual Review of Environment and Resources 2020 45:1, 83-112 https://doi.org/10.1146/annurev-environ-012320-083019