By burning fossil fuels, humans produce carbon dioxide gas which then enters the atmosphere. Next, as the concentration of atmospheric carbon dioxide increases, more carbon dioxide dissolves into the ocean. Carbon dioxide then reacts to produce more carbonic acid and more hydrogen carbonate ions. This causes the following reactions to shift to the products side and produce more hydronium ions.

However, increasing concentrations of atmospheric carbon dioxide also causes the concentration of carbonate ions to change. Several reactions are involved in this process, but one important reaction is:

As greater amounts of carbon dioxide in the ocean increase the concentration of carbonic acid, the above reaction shifts to the left and reduces the concentration of carbonate ions.

As the concentration of carbonate ions decreases, the reaction below is affected.

Decreased concentrations of carbonate ions cause the reaction shown above to shift to the right, increasing the solubility of calcium carbonate.

These changes affect marine ecosystems because many marine animals use calcium carbonate to build their external shells. Because concentrations of carbonate ions are decreasing, it is becoming more difficult for marine animals to take up the carbonate ions they require. Also, the increasing solubility of calcium carbonate may even cause the shells of marine animals to dissolve. Many of the marine animals that will have their shell production affected by these changes form the basis of marine food webs and corals even provide the habitat necessary for the survival of creatures without shells.