Human activities are responsible for a 36% increase in atmospheric CO2 since the beginning of the industrial era (1800). This increase is due to the CO2 emissions from a variety of sources like the combustion of fossil fuels (coal, oil, and natural gas), or from industrial processes such as the production of iron, steel, and cement. Mass deforestation also contributes by reducing the amount of CO2 that is captured. The atmosphere’s level of CO2, measured as the partial pressure (pCO2), rose from 280 ppm (part per million) before the start of the industrial era to 385 ppm in 2008. Atmospheric CO2 will continue to increase in coming decades, as predicted under all the Intergovernmental Panel on Climate Change (IPCC) fossil-fuel emission scenarios, with levels that may well reach 700 ppm or more by 2100.
The figure below shows the changes in atmospheric pCO2 (red dotted line) and surface ocean pH over time (source: modified from Körtzinger in IMBER, 2005)
Modifications of oceanic carbonate chemistry
Changes in atmospheric Carbon content also change the Carbon in the Ocean. Over much of the ocean, the aqueous CO2 in the upper ocean is nearly in equilibrium with atmospheric CO2 (the gas phase). This equilibrium follows Henry’s Law. Thus an increase in atmospheric pCO2 also increases pCO2 in surface-ocean waters.
Ocean acidification is the term used to describe the decrease in seawater pH (unit to qualify the acidity of a liquid) due to ocean’s absorption of anthropogenic carbon dioxide (CO2) from the atmosphere. The average surface-ocean pH, which is currently hovering around 8.1, has already fallen by 0.1 unit since the beginning of the industrial era, and it is likely to decline by another 0.2 to 0.4 unit by the end of this century. By limiting the accumulation of CO2 in the atmosphere, and therefore climate change, the ocean CO2 uptake has a beneficial environmental effect. However, this CO2 dissolves in the surface water and reacts with the water molecules (H2O), forming carbonic acid (H2CO3). Most of this acid dissociates into hydrogen ions (H+) ions and bicarbonate ions (HCO3–). The increase in the concentration of H+ ions reduces pH (pH = -log10[H+]) as well as the carbonate ion concentration (CO32-), which join with H+ ions to form HCO3– via the reaction CO2 + H2O + CO32- → 2HCO3–.
The figure describe the different chemicals forms of the Carbon in the Ocean and the production of H+.
Absorption of CO2 by the ocean
Effect of ocean acidification on marine organisms
Ocean acidification affects marine organisms through changes in pH as well as through changes in other carbonate system variables.
On one hand, the decreased availability of CO32- as induced by ocean acidification, has an impact of many species that make use of these CO32- ions to build calcareous shells and skeletons. The phenomenon of calcification occurs in a large number of marine species, such as algae, corals, mollusks, foraminifera, echinoderms, crustaceans, and bryozoans.
On the other hand, increasing seawater CO2 concentrations could affect carbon fixation by photosynthesis. Both benthic and pelagic photosynthetic organisms have major biogeochemical and ecological roles. They provide more than 99% of the organic material used in marine food webs. In the terrestrial environment, increasing atmospheric CO2 generally has a beneficial effect on plant photosynthesis, which is often limited by the atmospheric CO2 concentration. In the ocean, increasing CO2 also appears to be beneficial to the growth of certain species of marine seagrasses and seaweeds.