ABSTRACT

Ocean chemistry-including carbonate content and pH-has remained constant for hundreds of thousands of years, with typical surface ocean variation between ice ages and warming periods of less than 0.2 pH units. Since the late 1790s, however, anthropogenic combustion of fossil fuels has resulted in an unprecedented increase of atmospheric CO2 of about 100 ppm to 350 ppm CO2. The oceans have taken up about 30% of the CO2 from the atmosphere where it forms carbonic acid and this is projected to lower surface ocean water pH an estimated 0.3 units by the year 2100. Such a rapid change in ocean pH has very likely not happened in 65 million years. Dissolution of oceanic carbonate sediments will eventually restore oceanic pH to its pre-industrial level, a process estimated to take more than a million years. During the last glacial period, about 8,000 years ago, global atmospheric CO2 concentrations were reduced by 80 ppm; however, dissolution of foraminiferan tests in the western equatorial Atlantic Ocean was documented at about the same time. The reasons for these phenomena are unknown, again illustrating the uncertainties associated with oceanic acidifi cation.