Ocean surface pH, 1900. Global map showing the surface pH of the world's oceans in 1900. pH is the measure of the amount of hydrogen ions in a solution, and is used to measure its alkalinity or acidity. The oceans are slightly alkaline, with an average pH of around 8.1 (7 is neutral). More than a quarter of the carbon dioxide released into the atmosphere is absorbed by the oceans, causing them to become more acidic. Data based on a computer model that makes predictions of pH change based on estimates of carbon dioxide emissions. For a sequence showing the changes in ocean surface pH between 1861 and 2100, see images C047/5620 to C047/5626.