In 1956, Roger Revelle and Hans Guess, two visionary geochemists, warned of a possible increase in carbon dioxide in the atmosphere. We all know that this has happened, yet they saw beyond the rise in temperatures, because this and other gases that are responsible for the so-called greenhouse effect were forming higher and higher concentrations due to industry, transport, agricultural uses, and so on. The increase in this gas could lead, among other things, to acidification of both inland and marine waters. The issue is complex, but it arises in the following way: the carbon dioxide entering the sea combines with water and gives rise to another compound, weak carbonic acid (H2CO3), capable of releasing hydrogen ions (H+) with ease. Having lost this hydrogen, the carbonic acid remains as a bicarbonate ion, HCO3. When this happens, hydrogen ions remain in the water, acidifying the liquid medium.
Rossi S., 2019. More acidic seas. In: Oceans in Decline, pp 227-239. Chapter (subscription required).