As the ocean absorbs anthropogenic CO2, its pH and carbonate ion concentration decrease, thereby decreasing the ratio of the concentration of dissolved carbonate ions in the sea water to the concentration of dissolved ions in a saturated solution of aragonite (i.e., aragonite saturation state ΩAr). If ΩAr falls below the threshold ΩAr=1, ocean acidification makes it harder for marine calcifying organisms (e.g., pteropods, corals, coccolithophores and foraminifera) to form and maintain their shells. A paper published in Nature Climate Change used large ensemble climate model simulations for the 21st century to report that a shallow (50 ~ 250 m) aragonite-undersaturation layer may emerges in many Southern Ocean locations in the future. Their analysis showed that the shallow aragonite-undersaturation layer is formed due to the slow accumulation of anthropogenic CO2 in the Southern Ocean thermocline, where the carbonate ion concentration is at its local minimum. The model results reported in this study indicate an inevitable sudden decrease in the volume of suitable habitat for aragonitic organisms in the Southern Ocean between now and the end of the century.
Negrete-García, G., Lovenduski, N. S., Hauri, C., Krumhardt, K. M., & Lauvset, S. K. (2019). Sudden emergence of a shallow aragonite saturation horizon in the Southern Ocean. Nature Climate Change, https://doi.org/10.1038/s41558-019-0418-8, https://www.nature.com/articles/s41558-019-0418-8