When the ocean absorbs carbon dioxide from the atmosphere, fundamental changes occur in the carbonate budget of the ocean. Carbonates are consumed in the surface waters through the carbon dioxide reactions, and hydrogen cations (protons) may be released. The number of free hydrogen cations, in turn, determines the acidity of the seawater. The greater their number, the more acidic the water is.
The concentration of hydrogen cations in a solution is measured using a number known as the pH value. It indicates how acidic or basic a liquid is. The scale of the pH value ranges from 0 (very acidic) to 14 (very basic). This means that the more hydrogen cations a solution contains, the smaller its pH value is.
The average pH value at the ocean surface has decreased since the onset of industrialization from 8.2 to 8.1. This seemingly small step on the logarithmic pH scale represents a real acidity increase of about 26 per cent, a change in magnitude that has not been experienced by the world ocean or its inhabitants in millions of years. The acidification signal now reaches depths of up to 2000 metres, and even deeper in the North Atlantic and Southern Oceans. If humans continue to emit as much carbon dioxide as they have in the past, the pH value of the oceans is predicted to fall by another 0.44 units by the year 2100. This does not mean that the oceans are actually acidic technically speaking, because values of 7.6 to 7.7 are still considered to be chemically basic, but relatively speaking they are more acidic than before.
fig. 2.5 > Water chemistry changes as a result of CO2 uptake at the ocean surface. Its pH value decreases as does the aragonite saturation state. The measurement profiles show the changes in these two parameters during the period from 1800 to 2002. The black lines and numbers indicate values measured in 2002.
Together with the pH values, carbonate concentrations in the ocean are also falling with increasing carbon dioxide absorption. The saturation of seawater with carbonate ions, however, is a vital parameter for all marine organisms that construct their shells or skeletal structures with calcium carbonate. Marine organisms use carbonate primarily in the forms of aragonite and calcite, whereby aragonite is particularly susceptible to dissolution. Carbonate-saturated water masses possess a carbonate saturation state (Ω) of 1. This corresponds to a carbonate concentration of 66 micromoles per kilogram of water. If the concentration is slightly higher than this value, the seawater is considered to be supersaturated. If a water mass falls below that, however, it is referred to as undersaturated and the aragonite formed by the organisms will dissolve in the water.
Undersaturated seawater is present in all of the oceans because, due to the increasing solubility of carbonate with decreasing water temperature and increasing pressure, the deeper layers of the oceans, as a rule, are undersaturated. The boundary between the undersaturated and supersaturated water layers is called the saturation horizon. According to reports by the Intergovernmental Panel on Climate Change, the increasing inflow of carbon-rich surface waters at intermediate and greater water depths is shifting this boundary, below which the carbonate dissolves, further and further toward the ocean’s surface. In some regions of the western Atlantic Ocean, for example, the calcite saturation horizon has risen by around 300 metres since the onset of industrialization. In the Arctic Ocean, the depth of the aragonite saturation horizon has shifted upward towards the surface by 270 metres during the period from 1765 to 2005. This means that ever larger portions of the water column there are being affected by carbonate deficiency.
It is still uncertain to what extent the various marine organisms are able to adapt to ocean acidification. Single-celled algae and small zooplankton with short reproductive cycles appear to be better equipped than larger organisms with longer reproductive cycles. Additionally, researchers are becoming more convinced that increased ocean acidification combined with the declining oxygen content in the seas has a negative influence on the temperature tolerance of the individual species, especially in tropical and polar waters. This means that the temperature range in which these species can survive is shrinking with the falling pH value of the water. This development, in turn, has an effect on the geographical distribution of species and populations, and even on their basic chances of survival.
Important to know: The acidification of the seas is a development that can be attributed exclusively to the increase in carbon dioxide in the atmosphere. Other greenhouse gases are not involved in this. For an effective protection of the oceans, therefore, the prevention of anthropogenic carbon dioxide emissions and a targeted reduction of carbon dioxide concentrations in the Earth’s atmosphere are doubly important. Such action would help to limit both global warming and ocean acidification.