The primary cause of ocean acidification is the increased absorption of carbon dioxide (CO2) by oceans. As human activities, particularly fossil fuel combustion, elevate CO2 levels in the atmosphere, a substantial portion dissolves in ocean water. This leads to the formation of carbonic acid, which lowers the pH of seawater, making it more acidic. Ocean acidification poses significant threats to marine organisms, particularly those with calcium carbonate structures, like coral reefs and shellfish, disrupting marine ecosystems and biodiversity.