The oceans are becoming more acidic because of the rapid release of carbon dioxide (CO2) caused by anthropogenic (human) activities, such as burning of fossil fuels. So far, the oceans have taken up around 30% of all anthropogenic CO2 released to the atmosphere. The continuous increase of CO2 has a substantial effect on ocean chemistry because CO2 reacts with water and carbonate molecules. This process, called 'ocean acidification,' lowers pH, and calcium carbonate becomes less available. This is a problem for calcifying organisms, such as corals and molluscs, that use calcium carbonate as the main building blocks of their exoskeleton.