pH

pH is the negative base-10 logarithm of hydrogen-ion activity and the master variable of the seawater carbonate system. Open-ocean surface pH sits near 8.1 on the total scale, mildly alkaline, and is buffered by the bicarbonate-carbonate equilibrium. Because the ocean takes up about a quarter of human CO2 emissions, surface pH has dropped about 0.1 unit since the 1800s, the central metric of ocean acidification.