When carbon dioxide (CO2) is released into the atmosphere, some of it dissolves in the ocean and reacts with water molecules to form carbonic acid (H2CO3). Carbonic acid then dissociates into hydrogen ions (H+) and bicarbonate ions (HCO3-). The increase in hydrogen ions leads to a decrease in the pH of seawater, making it more acidic. This process is called ocean acidification. The increase in acidity can affect the ability of marine organisms to form shells and skeletons, and can also impact the food chain and overall health of marine ecosystems.