The primary factor that contributes to ocean acidification is the absorption of atmospheric carbon dioxide (CO2) by seawater. As CO2 levels rise due to human activities like fossil fuel combustion, more CO2 dissolves in ocean water, forming carbonic acid. This process lowers the ocean's pH, making it more acidic, which can negatively affect marine organisms, especially those with calcium carbonate structures like corals and shellfish. Understanding ocean acidification is crucial for preserving marine ecosystems.