Transfer of CO2 between the atmosphere and the oceans is a critical process of the global carbon cycle and is important for the future of the earth's climate. During the past decade, about 6 Pg-C yr-1 [1 Pg (peta grams) = 1015 grams = 1 Giga ton] has been emitted into the atmosphere from various anthropogenic sources including the combustion of fossil fuels, cement production, deforestation, land use changes, and others. On the basis of various independent scientific methods including changes in oxygen and CO2 concentrations in the atmosphere (Keeling et al. 1996, Bender et al. 2005, Garcia and Keeling 2001), sea-air differences in CO2 partial pressure (Takahashi et al. 2002), distribution of carbon isotopes (12C, 13C, and 14C) (Quay et al. 2003), inversion of atmospheric CO2 distribution data using atmospheric circulation models (Patra et al. 2005), and various global carbon cycle models (Sarmiento et al. 2000, Gruber and Sarmiento 2002), the annual uptake rate of CO2 by the oceans has been estimated to be about 2 Pg-C yr-1 for the past decade. Thus, ~30% of the anthropogenic CO2 emissions are absorbed annually by the oceans, and ~50% remain in the atmosphere. As a result, the atmospheric CO2 concentration is increasing at a mean rate of about 1.5 ppm yr-1 (or 0.4% per year), and the concentration of CO2 dissolved in surface ocean waters is also increasing, causing the acidification of ocean waters. Accurately documenting changes in the CO2 chemistry in ocean waters over time is therefore important for understanding the fate of anthropogenic CO2 released into the atmosphere as well as charting the future course of atmospheric CO2 levels that would affect the earth's climate.
Carbon dioxide molecules react chemically with water to form bicarbonate (HCO3-) and carbonate (CO3=) ions, neither of which communicate with the overlying air. Only about 0.5% of the total CO2 molecules dissolved in seawater communicate with air via gas exchange across the sea surface. This quantity is called the partial pressure of CO2 (pCO2), which represents the CO2 vapor pressure. The seawater pCO2 depends on the temperature, the total amount of CO2 dissolved in seawater, and seawater pH. Over the global ocean, it varies from about 100 µatm to 1000 µatm (1 µatm = 10-6 atm). When seawater pCO2 is less than the atmospheric pCO2 (presently about 370 µatm), seawater takes up CO2 from the overlying air; when it is greater than the atmospheric pCO2, it emits CO2 to the air. The rate of transfer of CO2 across the sea surface is estimated by: (sea-air CO2 flux) = (transfer coefficient) x (sea-air pCO2 difference). The transfer coefficient depends primarily on the degree of turbulence near the interface, and is commonly expressed as a function of wind speed. Since the time-space variation for atmospheric pCO2 is much smaller than that for surface ocean water pCO2, the magnitude of sea-air CO2 flux is governed primarily by seawater pCO2. Therefore, the inter-annual and seasonal variability for surface water pCO2 is of particular interest.
The atmospheric CO2 concentrations observed at a number of locations over the globe is summarized in GLOBALVIEW-CO2 (2006) and TRENDS ON LINE (2006). On the other hand, no single data file for surface water pCO2 that includes long-term and global coverage has been made accessible to the general public. About 20 years ago, we started to assemble a global surface water pCO2 data for time-space variability studies for the global oceans using the observations made by the Lamont-Doherty Earth Observatory (LDEO) group. Many investigators from other institutions contributed data to the database for the first publication on the global ocean pCO2 and sea-air CO2 flux (Takahashi et al. 1997), which was based on about 0.25 million pCO2 measurements; the second publication (Takahashi et al. 2002) was based on about a million measurements. The LDEO database now consists of more than 3 million pCO2 measurements (Fig. 1) plus a number of other measured supporting parameters. A paper summarizing these observations has been submitted for publication in the Deep Sea Research (Takahashi et al. in review). A subset of this database is used for estimating the sea-air CO2 flux from coastal waters surrounding North America (Chavez and Takahashi 2007). Mindful of the increasing importance of CO2 studies for future global welfare, LDEO is sharing this data file widely with the global research communities and the public through the Carbon Dioxide Information Analysis Center (CDIAC) data archive.
Fig. 1. Location of LDEO master database of sea surface pCO2 observations.