Ocean acidification

Roughly one third of anthropogenically emitted CO2 has been taken up by the oceans. When this CO2 combines with water to form H2CO3, a weak acid, water acidity increases in a process referred to as ocean acidification (OA). From the preindustrial era until present time the average pH has decreased by 0.08 units on average and it is projected to decrease a further 0.15 to 0.50 until year 2100 (IPCC RCP2.6 and RCP8.5 projections). Increased acidity hampers calcification in shell forming invertebrates, but OA also acts on a wider range of physiological processes, especially those related to cellular ion regulation, and most often non-calcifying species are equally affected. Meta-analyses show severe effects on many species of corals, echinoderms, molluscs, crustaceans, and fish at levels predicted for year 2100. Nevertheless, generalizations are presently hampered by our lack of knowledge on the variability of effects among life cycle stages, variability among taxa, how evolutionary adaptation and transgenerational effects may alleviate OA effects, and effects of OA on entire communities. Even closely related species react differently, and differences among populations of the same species separated geographically have been recorded. Also, specific life cycle stages seem to be more sensitive. In general, planktonic larvae and juveniles seem more affected than adults. Knowledge on evolutionary adaptation to OA is scarce, but the few studies that do exist indicate possible fast adaptation and buffering of OA effects by transgenerational exposure. Studies show that future OA may shift the biodiversity of entire communities. Two marine communities are of particular concern. Model studies indicate that coral reefs could be pushed beyond sustainability be the end of the century, and OA is progressing fast in the Arctic where many species are physiologically lesser capable of countering OA. OA works in concert with many other environmental stressors and knowledge on OA should be incorporated into decisions on suitable areas to protect so as to minimise effects of other stressors in habitats most vulnerable to OA.

Thor P. & Dupont S., 2018. Ocean acidification. In: Salomon M. & Markus T. (Eds.), Handbook on Marine Environment Protection, pp 375-394. Springer, Cham. Chapter (restricted access).


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Ocean acidification in the IPCC AR5 WG II

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