Carbonate equilibrium
Dissolved CO2 reacts with water to form carbonic acid, which dissociates into hydrogen and bicarbonate ions. The extra hydrogen ions combine with carbonate ions, so carbonate ion concentration falls.
- CO2 + H2O ⇌ H2CO3 ⇌ H+ + HCO3-
- H+ + CO3 2- ⇌ HCO3-
How much has changed
The ocean has absorbed roughly a quarter of human CO2 emissions. Surface pH has fallen from about 8.2 to about 8.1, roughly a 30% increase in hydrogen ion concentration, as long-term records such as the Hawaii Ocean Time-series show.
Ecological effects
- Calcifiers: corals, oysters, mussels and pteropods build calcium carbonate shells; lower saturation slows growth or dissolves shells.
- Food webs: losses of plankton such as pteropods affect fish and fisheries.
- Combined with warming, it weakens coral reefs already stressed by bleaching.
Responses
The fundamental remedy is cutting CO2 emissions, alongside monitoring networks, water management at shellfish farms and marine protected areas.
References
- Doney, S. C., Fabry, V. J., Feely, R. A., & Kleypas, J. A. (2009). Ocean acidification: The other CO2 problem. Annual Review of Marine Science, 1, 169-192.
- Caldeira, K., & Wickett, M. E. (2003). Anthropogenic carbon and ocean pH. Nature, 425, 365.
- Orr, J. C., Fabry, V. J., Aumont, O., et al. (2005). Anthropogenic ocean acidification over the twenty-first century and its impact on calcifying organisms. Nature, 437, 681-686.
- IPCC (2019). Special Report on the Ocean and Cryosphere in a Changing Climate. Cambridge University Press.