How do we know what ocean pH was in the past even though the pH scale was not introduced until 1909?
When ice sheets build up into glaciers, air bubbles become trapped in the freezing ice. Scientists have analyzed the CO2 concentration of air in these bubbles and have developed a record of the atmospheric CO2 concentration in the recent past. Because large parts of the surface ocean CO2 concentration remains roughly in equilibrium with the atmospheric CO2 concentration, the ocean CO2 content can be calculated from these air bubbles, and ocean pH can also be calculated. In fact, the ice core record shows that the atmospheric CO2 concentration has never been higher than about 280 ppm during the last 800,000 years, creating conditions leading to an average preindustrial surface ocean pH of ca. 8.2.
