The primary cause of the acidification of oceans is the increase in atmospheric carbon dioxide (CO2) levels resulting from fossil fuel combustion and deforestation. When CO2 is absorbed by seawater, it reacts with water to form carbonic acid, lowering the pH of the ocean. Ocean acidification adversely affects marine life, especially organisms that rely on calcium carbonate, such as corals and shellfish, by impairing their ability to form shells and skeletons.