Illustration of ocean acidification. Atmospheric carbon dioxide molecules (red and black spheres) dissolving into the ocean. This action results in the formation of carbonic acid molecules in the ocean. Some of the carbonic acid molecules dissociate into a bicarbonate ion and a hydrogen ion, the latter increasing ocean acidity. Seawater is slightly basic (meaning pH more than 7) and acidification involves a shift towards pH-neutral conditions rather than a transition to acidic conditions (pH less than 7).