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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, CO
molecules
are present in three major forms: the undissociated species in water, [CO]aq,
and two ionic species, [HCO
]
and [CO
]
(Figure 1). The concentration of [CO]aq depends
upon the temperature and chemical composition of seawater. The amount of [CO]aq is
proportional to the partial pressure of CO exerted
by seawater. The difference between the pCO in
surface seawater and that in the overlying air represents the thermodynamic
driving potential for the CO transfer
across the sea surface. The pCO 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 pCO seasonal
variability of about 20 µatm in remote uncontaminated marine air, the
direction and magnitude of the sea-air CO transfer
flux are regulated primarily by changes in the oceanic pCO.
The average pCO 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 pCO in
mixed-layer waters that exchange CO directly
with the atmosphere is affected primarily by temperature, DIC levels and A
.
While the water temperature is regulated by physical processes, including solar
energy input, sea-air heat exchanges and mixed-layer thickness, the DIC and
A are primarily
controlled by the biological processes of photosynthesis and respiration and
by upwelling of subsurface waters rich in respired CO and
nutrients. In a parcel of seawater with constant chemical composition, pCO 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 kg
in
polar regions to 1850 µmol kg in
the tropics as a result of biological processes. This change should reduce pCO by
a factor of 4. On a global scale, therefore, the magnitude of the effect of
biological drawdown on surface water pCO is
similar in magnitude to the effect of temperature, but the two effects are
often compensating. Accordingly, the distribution of pCO in
surface waters in space and time, and therefore the oceanic uptake and release
of CO , is governed
by a balance between the changes in seawater temperature, net biological utilization
of CO and the
upwelling flux of subsurface waters rich in CO.
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