Definition: the ongoing decrease in the pH of the Earth’s oceans
(and therefore, increase in acidity) of the Earth’s oceans, caused by the uptake and storage of carbon dioxide (CO2) from the atmosphere.
To address a basic misconception: pH is a measure of acidity or alkalinity of water soluble substances (pH stands for ‘potential of hydrogen’). A pH value is a number from 0 to 14, with 7 as the middle (neutral) point. Values below 7 indicate acidity which increases as the number decreases, 1 being the most acidic. The pH scale is logarithmic. This means a change factor of 10-fold is behind each whole number jump.
Oceanic pH has moved in the last two hundred and fifty years from a value of around 8.25 to a current value of around 8.15. It is therefore true to say that ocean acidification could also be equally expressed as a move towards pH neutrality. That of course does not garner headlines. The disturbing fact is that whatever the appropriate label is, marine ecosystems are showing significant signs of stress as a result of increasing concentrations of carbonic acid caused by carbon dioxide storage.