Ocean acidification

Ocean acidification is the decrease in the pH (scale of acidity and alkalinity) and increase in acidity of the Earth's oceans. It is caused by the increase of carbon dioxide (CO2) that humans have put into the atmosphere. More than 30% of the carbon dioxide in the air goes into the oceans. Therefore, when carbon dioxide in the atmosphere increases, the carbon dioxide in the ocean increases, too.
When carbon dioxide enters the oceans, it makes it more acidic.
Problems.
The skeletons and shells of many sea animals need what is known as calcium carbonate minerals. Due to ocean acidification, there are fewer of these minerals and there are species that could no longer produce or keep their shells. With more carbon dioxide in the ocean, this problem becomes worse and many species will be at risk. As a result, if humans continue to emit greenhouse gases into the atmosphere, ocean life will be in severe danger. Also, coral is at risk, as it is being eroded by the acid in the ocean.
Between 1751 CE and 1994 CE the ocean's pH on the surface went from 8.25 to 8.1.