Why do we observe ocean acidification with the increase of atmospheric CO2 concentration?

The CO2 concentration is noted pCO2 (partial pressure of carbon dioxide) and expressed in ppm: part per million. This pCO2 is rapidly increasing due to anthropogenic activities. As a comparison, the atmospheric pCO2 during recent glacial interval was around 180 ppm, it was around 280 ppm during interglacial intervals before industrial times (before 1750), and it is currently around 400 ppm. In the future, it could be around 1200 pm, depending on the scenario we are following in the predictions of the models run by the climate scientists (Ipcc, 2022).

There is an equilibrium between the CO2 concentration in the atmosphere and the CO2 concentration in the ocean. The higher the CO2 concentration is in the atmosphere, the higher the CO2 concentration is in the surface ocean.

The acidity depends on the chemical properties of the surface ocean. The ocean water contains many ions, resulting of the dissolution of many elements. The measure of acidity, called pH depends on the concentration of hydrogen ions (H+) and is defined as follow:

pH = -log [H+]

The current mean ocean pH is around 8 and the acidification means that this value is decreasing through time (with geographical variation, Jiang et al., 2019).

Over the last century, more than one third of the CO2 released in the atmosphere due to anthropogenic activities has been absorbed by the ocean (Sabine et al., 2004). This led to a global ocean pH drop by 0.1. according to the CO2 emissions scenarios (Figure 1), the oceanic CO2 uptake will acidify the ocean by 0.3 to 0.4 pH units by the end of this century (Fig 2, Caldeira and Wickett, 2003; Ipcc, 2022).

When in the surface ocean, the CO2 doesn’t stay in the gas phase, it gets dissolved as follow, causing 2 steps ions dissociation, releasing bicarbonate ions (HCO3-), carbonate ions (CO32-) and hydrogen ions (H+):

CO2 (gas) + H2O <-> H2CO3

(Carbon dioxide + water <-> carbonic acid)

H2CO3 <-> H+ + HCO3-

(carbonic acid <-> hydrogen ion + bicarbonate ion)

HCO3- <-> H+ + CO32-

(bicarbonate ion <-> hydrogen ion + carbonate ion)

(see more details on the marine carbon cycle in this article)

With the increase of CO2 releases due to anthropogenic activities, there is a strong increase of the atmospheric pCO2, a strong increase of the sea surface pCO2 and therefore, a strong increase of the presence of hydrogen ions, causing the pH to decrease.

This has a very strong impact on the ocean’s chemistry, as the oceanic acidification affects marine carbon pump (Hofmann and Schellnhuber, 2009) and all the carbon cycle (DeVries, 2022) and carbonate (CaCO3) System in the Oceans (Feely et al., 2004).

Acidification of the ocean heavily affects the organisms living in it and the ecosystems (Feely et al., 2009). Marine calcifying organisms have been shown to be highly impacted by ocean acidification, as their calcite shell and structures are made of calcite, highly sensitive to acidification and getting dissolved (Beaufort et al., 2011; Hoegh-Guldberg et al., 2007b, a; Kroeker et al., 2013; Riebesell et al., 2000; Wu et al., 2010). It has also been shown that it has an impact on the organisms using light for photosynthesis, and so, removing CO2 from the surface ocean (see also this article) (Hoppe et al., 2015).

Both chemically and biologically, the ocean acidification strongly degrades the capacity of the ocean to absorb anthropogenic CO2.

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