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Introduction

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 (HCO_3^-) and carbonate (CO_3⁼) 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.

akozyr 09/28/2007