What substance is primarily responsible for ocean acidification?

Ocean acidification is primarily caused by an increase in carbon dioxide (CO2) levels in the atmosphere. When CO2 is absorbed by seawater, it undergoes a series of chemical reactions that form carbonic acid. This process decreases the pH of the ocean, leading to a more acidic environment. The higher acidity negatively impacts marine life, particularly organisms that rely on calcium carbonate for their shells and skeletons, such as corals and some shellfish.

While other substances like methane, sulfur dioxide, and nitrogen oxides contribute to various forms of atmospheric pollution and can indirectly affect ocean chemistry and health, they are not the primary drivers of ocean acidification. Methane contributes to climate change, while sulfur dioxide and nitrogen oxides are mainly associated with acid rain and air pollution. Thus, the correct answer identifies carbon dioxide as the key substance responsible for the ongoing changes in ocean chemistry that characterize ocean acidification.