Why is ocean acidification happening?

Ocean acidification is happening because of the increased level of carbon dioxide in the atmosphere resulting from the burning of fossil fuels. About half of the CO2 added to the atmosphere is absorbed by the ocean. While the chemistry of the ocean is complex the increased level of CO2 results in a lowering of the pH of the water. This in turn increases the rate at which calcium carbonate dissolves in sea water and constitutes a threat to organisms which produce shells such as corals.