Chapter 4: ocean chemistry – Irish Ocean Climate And Ecosystem Status Report 2023

Carbon dioxide (CO2) emissions to the atmosphere have increased inexorably since the industrial revolution, due to fossil fuel combustion, cement production and land-use change. This has resulted in an average global atmospheric partial pressure of CO2 of ~ 415.7 +/- 0.2 ppm in 2021, which is 149% of preindustrial levels (WMO 2022). Today’s atmospheric CO2 levels would likely be much higher, had the oceans not absorbed about one quarter to one third of the total anthropogenic CO2 emissions (IPCC, 2019; Bindoff et al., 2019). Increasing carbon dioxide in the Earth’s atmosphere results in changes to ocean chemistry, which impact marine life. The uptake of CO2 in the oceans has caused the ocean to become more acidic due to the increase of protons (H+ ions) as a result of reactions of CO2 with the surrounding seawater. This change is measured using the (logarithmic) pH scale and the process is known as ocean acidification (OA).

Not only is the pH of seawater decreasing with ocean acidification, the carbonate ion concentration (CO3 2-) is decreasing at the same time, which particularly affects calcifying organisms. The two most common forms of calcium carbonate used by calcifying organisms to produce their shells and skeletons are aragonite and calcite. Calcifying organisms such as bivalves and corals will find it particularly difficult to build their protective hard parts when CO3 2- is diminishing. In addition, the ocean depths below which aragonite and calcite tend to dissolve is getting shallower. The aragonite saturation horizon (ASH) has already shoaled by 80–400 m in the North Atlantic since pre-industrial times (Feely et al., 2004; Tanhua et al., 2007) and is projected to rise further from 2600 m to as shallow as 200 m depth by the end of the century (Orr et al., 2005). Benthic deep-sea ecosystems such as cold-water coral reefs that currently live in supersaturated waters with respect to aragonite are projected to be exposed to aragonite undersaturation by the end of the century due to OA (about 70% of known habitats, Guinotte et al., 2006; Zheng and Cao, 2015).

Büscher J. V., Cave R. R., O’Donnell G., Cronin M. & McGovern E., 2023. Chapter 4: Ocean chemistry. In: Nolan G., Cusack C. & Fitzhenry D. (Eds.), Irish Ocean Climate & Ecosystem Status Report. Marine Institute, Galway, Ireland. Report.

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