pH is a measure of the ocean acidity. Low pH values indicate an acid environment whereas high pH values show basic environment.
Large quantities of CO2 are absorbed by seawater. During this process, chemical reactions occur resulting in a diminishing of pH, and so an increase in acidity. Water pH is important for marine organisms and, more particularly, for the building and the maintaining of the skeleton of seashells and corals, which are calcifying organisms. Thus, changes in pH, such as acidification, affect these organisms and all the ensued food chain.
pH can be measured from water sampling or estimated through remote sensing and modelling.