What is the effect of increased atmospheric CO2 on the calcium carbonate saturation state in ocean water?

Increased atmospheric carbon dioxide (CO2) lowers the calcium carbonate (CaCO3) saturation state in ocean water. This occurs because as atmospheric CO2 concentrations rise, more CO2 dissolves into the ocean. When CO2 dissolves in seawater, it reacts with water molecules (H2O) to form carbonic acid (H2CO3). Carbonic acid then dissociates, releasing hydrogen ions (H+), which increases the acidity of the ocean; this process is known as ocean acidification. The increased concentration of hydrogen ions (H+) reacts with carbonate ions (CO3^2-), which are a key building block for marine organisms that form shells and skeletons made of calcium carbonate, such as corals and shellfish. The reaction between hydrogen ions and carbonate ions reduces the availability of carbonate ions in the seawater. The saturation state of calcium carbonate (Ω) is a measure of how easily calcium carbonate minerals will form or dissolve in seawater. It is determined by the concentrations of calcium ions (Ca^2+) and carbonate ions (CO3^2-) in the water. A lower concentration of carbonate ions decreases the calcium carbonate saturation state, making it more difficult for marine organisms to build and maintain their shells and skeletons. If the saturation state drops below 1, the water becomes undersaturated, and existing calcium carbonate structures can begin to dissolve. This can have severe consequences for marine ecosystems, particularly coral reefs and shellfish populations.