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Uptake and Storage of Carbon Dioxide in the Ocean: The Global CO2 Survey

Richard A. Feely1, Christopher L. Sabine2, Taro Takahashi3, and Rik Wanninkhof4

1Pacific Marine Environmental Laboratory, National Oceanic and Atmospheric Administration, Seattle, Washington, 98115
2Joint Institute for the Study of the Atmosphere and Ocean, University of Washington, Seattle, Washington, 98195
3Lamont-Doherty Earth Observatory, Palisades, New York
4Atlantic Oceanographic and Meteorological Laboratory National Oceanic and Atmospheric Administration, Miami, Florida

Oceanography, 14(4), 18–32 (2001).
Copyright ©2001 by The Oceanography Society. Further electronic distribution is not allowed.

CO2 Exchange Across the Air-Sea Interface

In seawater, CO2 molecules are present in three major forms: the undissociated species in water, [CO2]aq, and two ionic species, [HCO3minus] and [CO32-] (Figure 1). The concentration of [CO2]aq depends upon the temperature and chemical composition of seawater. The amount of [CO2]aq is proportional to the partial pressure of CO2 exerted by seawater. The difference between the pCO2 in surface seawater and that in the overlying air represents the thermodynamic driving potential for the CO2 transfer across the sea surface. The pCO2 in surface seawater is known to vary geographically and seasonally over a range between about 150 µatm and 750 µatm, or about 60% below and 100% above the current atmospheric pCO2 level of about 370 µatm. Since the variation of pCO2 in the surface ocean is much greater than the atmospheric pCO2 seasonal variability of about 20 µatm in remote uncontaminated marine air, the direction and magnitude of the sea-air CO2 transfer flux are regulated primarily by changes in the oceanic pCO2. The average pCO2 of the global ocean is about 7 µatm lower than the atmosphere, which is the primary driving force for uptake by the ocean (see Figure 6 in Karl et al., this issue).

The pCO2 in mixed-layer waters that exchange CO2 directly with the atmosphere is affected primarily by temperature, DIC levels and AT. While the water temperature is regulated by physical processes, including solar energy input, sea-air heat exchanges and mixed-layer thickness, the DIC and AT are primarily controlled by the biological processes of photosynthesis and respiration and by upwelling of subsurface waters rich in respired CO2 and nutrients. In a parcel of seawater with constant chemical composition, pCO2 would increase by a factor of 4 when the water is warmed from polar temperatures of about –1.9°C to equatorial temperatures of about 30°C. On the other hand, the DIC in the surface ocean varies from an average value of 2150 µmol kgto the minus 1 in polar regions to 1850 µmol kgto the minus 1 in the tropics as a result of biological processes. This change should reduce pCO2 by a factor of 4. On a global scale, therefore, the magnitude of the effect of biological drawdown on surface water pCO2 is similar in magnitude to the effect of temperature, but the two effects are often compensating. Accordingly, the distribution of pCO2 in surface waters in space and time, and therefore the oceanic uptake and release of CO2 , is governed by a balance between the changes in seawater temperature, net biological utilization of CO2 and the upwelling flux of subsurface waters rich in CO2.

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