The pH scale is a measure of how acidic or basic a solution is. It is a measurement that compares the proportion of hydrogen (H⁺) or hydroxide (OH⁻) ions in a solution (ions are charged atoms or molecules). Water (H₂O) can dissociate into hydroxide ions (OH⁻) and hydrogen ions (H⁺). In pure water, there is always one H+ for every OH⁻. These solutions are neutral, with a pH of 7.0. Acids have a pH lower than 7.0 and have extra H⁺ ions in the solution. Bases (alkaline solutions) have a pH higher than 7.0 and have extra OH⁻ ions in the solution.

The increase in ocean acidity correlates well with the increases in CO₂ dissolved in seawater and atmospheric CO₂ concentrations.

Ocean pH will continue to drop significantly if we do not curtail fossil fuel use and our release of extra CO₂.