ALT Chemical representation of the problems caused by carbon dioxide in the ocean: Carbon dioxide (CO2) and water (H2O) react to form carbonic acid (H2CO3). The carbonic acid further splits to form hydrogen carbonate (HCO3-) and positively charged hydrogen ions (H+). More hydrogen ions lead to a decrease in pH. This means increasing acidification of the ocean. At the same time, hydrogen ions combine with carbonate ions (CO32-) to form more hydrogen carbonate. This leads to a reduction of carbonate ions in the ocean. However, calcifying organisms such as calcareous algae, mussels or corals need the carbonate ions to build up their skeleton with.