Award: OCE-1038891

Climatological mean monthly distribution of pH and the degree of aragonite and calcite saturation have been determined for the surface water of the global oceans using a database assembled from the observations made for pCO2, alkalinity and the concentrations of total CO2 and nutrient in surface waters (depths <50 m). The mutual consistency among these quantities is demonstrated using the inorganic carbon chemistry model. The global ocean is divided into 33 regions, and the linear relationships between salinity and potential alkalinity (=alkainity + nitrate concentration) in 32 of these regions (excluding the equatorial Pacific El Nino zone) are established. Using the mean monthly pCO2 data for a reference year 2005 and the alkalinity estimated using the potential alkalinity-salinity relationships, the mean monthly distributions of pH and aragonite saturation in surface ocean waters are obtained for the year 2005. The pH in the global ocean surface water ranges from 7.9 to 8.2 in the year 2005. Lower values are located in the upwelling regions in the equatorial Pacific and in the Arabian and Bering Seas; and higher values are found in the subpolar and polar waters during the spring-summer months of intense photosynthetic production. The vast areas of subtropical oceans have seasonally varying pH values ranging from 8.05 during warmer months to 8.15 during colder months. The warm tropical and subtropical waters are supersaturated by a factor of as much as 4.2 with respect to aragonite and 6.3 for calcite, whereas the cold subpolar and polar waters are less supersaturated only by a factor of 1.2 (20% supersaturation) for aragonite and 2 (100% supersaturation) for calcite because of the lower pH values resulting from greater total CO2 concentrations. In the western Arctic Ocean, aragonite undersaturation is observed. Decadal time-series data at the Bermuda (BATS), Hawaii (HOT) and Drake Passage show that pH has been declining at a mean rate of about 0.002 pH per year, due to the absorption of CO2 from the air. This suggests that , in 20 years, the surface layer of the global ocean will become more acidic by about 10%. Impact of the acidification on marine life is not well understood and should be investigated. Last Modified: 01/10/2014 Submitted by: Taro Takahashi