For more than 200 years, or since the industrial revolution began, the concentration of carbon dioxide (CO2) in the atmosphere has increased due to the burning of fossil fuels and land use change (e.g. increased car emissions and deforestation). During this time, the pH of surface ocean waters has fallen by 0.1 pH units. The pH scale, like the Richter scale, is logarithmic, so this change represents approximately a 30 percent increase in acidity. - NOAA