Climatological distributions of pH, pCO2, total CO2, alkalinity, and CaCO3 saturation in the global surface ocean, and temporal changes at selected locations

• Based on observations, pH and CaCO3 saturation in global surface ocean is computed.
• pH in the global ocean surface waters ranges from 7.9 to 8.2 in the year 2005.
• Warm subtropical oceans have seasonally varying pH ranging from 8.05 to 8.15.
• Time-series data show that pH has declined at a mean rate of about -0.002 pH per year.
• Warm waters are supersaturated by aragonite by 4.2, whereas cold waters only by 1.2.

Climatological mean monthly distributions of pH in the total H+ scale, total CO2 concentration (TCO2), and the degree of CaCO3 saturation for the global surface ocean waters (excluding coastal areas) are calculated using a data set for pCO2, alkalinity and nutrient concentrations in surface waters (depths < 50 m), which is built upon the GLODAP, CARINA and LDEO databases. The mutual consistency among these measured parameters is demonstrated using the inorganic carbon chemistry model with the dissociation constants for carbonic acid by Lueker et al. (2000) and for boric acid by Dickson (1990). Linear potential alkalinity-salinity relationships are established for 24 regions of the global ocean. The mean monthly distributions of pH and carbon chemistry parameters for the reference year 2005 are computed using the climatological mean monthly pCO2 data adjusted to a reference year 2005 and the alkalinity estimated from the potential alkalinity-salinity relationships. The equatorial zone (4°N-4°S) of the Pacific is excluded from the analysis because of the large interannual changes associated with ENSO events. The pH thus calculated ranges from 7.9 to 8.2. Lower values are located in the upwelling regions in the tropical Pacific and in the Arabian and Bering Seas; 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 supersaturated by 1.2 for aragonite and 2.0 for calcite because of the lower pH values resulting from greater TCO2 concentrations. In the western Arctic Ocean, aragonite undersaturation is observed. The time-series data from the Bermuda (BATS), Hawaii (HOT), Canary (ESTOC) and the Drake Passage show that pH has been declining at a mean rate of about -0.02 pH per decade, and that pCO2 has been increasing at about 19 μatm per decade tracking the atmospheric pCO2 increase rate. This suggests that the ocean acidification is caused primarily by the uptake of atmospheric CO2. The relative importance of the four environmental drivers (temperature, salinity, alkalinity and total CO2 concentration) controlling the seasonal variability of carbonate chemistry at these sites is quantitatively assessed. The ocean carbon chemistry is governed sensitively by the TA/TCO2 ratio, and the rate of change in TA is equally important for the future ocean environment as is the TCO2 in ocean waters increases in the future.