may not, once again, be too critical because
the lack of lateral transport implies that preindustrial net fluxes should have been close to zero on
some long-term average everywhere. (However,
Sarmiento and Sundquist (1992) point out that
fluxes associated with weathering, and carbon
transported via rivers and groundwater, should be
considered.) Hence it should now be possible to
equate long-term measurements of the net CO 2
flux into or out of the terrestrial biosphere with
the flux of Excess CO 2 .
This is certainly not the case in the ocean where
spatial variation in surface heating, biological productivity and respiration are convoluted by lateral
and vertical circulation to produce natural spatial
variability of annually averaged net air–sea CO 2
fluxes. The net CO 2 fluxes associated with these
natural processes occurred throughout the preindustrial era, and should not be confused with the
uptake of Excess CO 2 , which refers only to the
change of the air–sea flux since 1750. Although
we can estimate the net air–sea flux of CO 2 from
measurements of the air–sea pCO 2 difference
(e.g. Takahashi et al., 1997) there is no easy way to
separate the natural component of this flux from
its anthropogenic or Excess component. The net
flux across the air–sea interface is therefore a
(spatially and temporally variable) mixture of both
natural and Excess (or anthropogenic) components.
6.3.2.2 What controls Excess CO 2 uptake?
The elevation of atmospheric pCO 2 over preindustrial levels means that natural areas of CO 2 outgassing from the ocean now release less CO 2 to the
atmosphere than during preindustrial times. On
the other hand, natural areas of CO 2 uptake have
become correspondingly larger sinks for CO 2 , as
have regions that were naturally regions of zero
net transfer. An interesting combination of the
complex chemistry of CO 2 in seawater and ocean
circulation does allow for the counterintuitive case
where, locally, the air-to-sea transfer of CO 2 can
theoretically be lower than that of the preindustrial ocean (see Section 6.3.5.3).
Given about 1 year, surface ocean waters will
re-equilibrate with the ‘new’ atmospheric pCO 2
and the further uptake of Excess CO 2 will become
negligible. This implies that the major regions for
Excess CO 2 uptake from the atmosphere are regions
where ‘old’, poorly ventilated ocean waters regain
close contact with the atmosphere (and where, in
addition, gas exchange is efficient in transferring
CO 2 across the air–sea interface). These regions
may, or may not, be coincident with regions of
deep water formation: the Excess CO 2 uptake from
the atmosphere will take place close to where these
poorly ventilated waters are returned to the sea
surface, and not necessarily where the newly ventilated surface waters are transported back into the
deep ocean again. On the other hand, the storage
of Excess CO 2 within the ocean water column
will be largest in regions where recently ventilated
waters accumulate or converge.
6.3.2.3 Why does Excess CO 2 uptake matter?
CO 2 that is stored in the ocean does not affect
the earth’s radiation balance, so oceanic uptake of
Excess CO 2 mitigates the potential for global
warming. In order to predict the magnitude of
future climate change resulting from greenhouse
gas emissions, a requirement is the prediction of
future atmospheric CO 2 levels for given emissions
scenarios. There is also an immediate sociopolitical
requirement for better understanding of the global
carbon cycle as a consequence of the endorsement
of the Kyoto Protocol in 1997. Attempts to limit
the future growth of atmospheric CO 2 concentration, however modest, will involve major, and
potentially costly, changes in energy and technology policy. The inclusion of certain terrestrial carbon sinks in carbon emission budgeting increases
the need to better define global carbon sinks and
sources. Furthermore, future assessment of the
effectiveness of measures taken to reduce emissions
will ultimately be judged by their long-term effect
on atmospheric CO 2 levels, which in turn requires
an understanding of long-term storage changes in
the other key carbon reservoirs (oceans and the terrestrial biosphere).
There are therefore at least three key scientific
questions of relevance to global carbon cycle science arising from current policy-related issues:
1 How large are present-day oceanic and terrestrial carbon sources and sinks, where do they
operate, and what processes are responsible?
2 How will terrestrial and, particularly, oceanic
carbon sources and sinks behave in the future
under higher CO 2 and a possibly altered climate
and ocean circulation?
3 How will we monitor and assess the effectiveness
of emissions controls and sequestration activities
6.3 Storage and Transport of Excess CO 2 in the Oceans
491
Wallace
the lack of lateral transport implies that preindustrial net fluxes should have been close to zero on
some long-term average everywhere. (However,
Sarmiento and Sundquist (1992) point out that
fluxes associated with weathering, and carbon
transported via rivers and groundwater, should be
considered.) Hence it should now be possible to
equate long-term measurements of the net CO 2
flux into or out of the terrestrial biosphere with
the flux of Excess CO 2 .
This is certainly not the case in the ocean where
spatial variation in surface heating, biological productivity and respiration are convoluted by lateral
and vertical circulation to produce natural spatial
variability of annually averaged net air–sea CO 2
fluxes. The net CO 2 fluxes associated with these
natural processes occurred throughout the preindustrial era, and should not be confused with the
uptake of Excess CO 2 , which refers only to the
change of the air–sea flux since 1750. Although
we can estimate the net air–sea flux of CO 2 from
measurements of the air–sea pCO 2 difference
(e.g. Takahashi et al., 1997) there is no easy way to
separate the natural component of this flux from
its anthropogenic or Excess component. The net
flux across the air–sea interface is therefore a
(spatially and temporally variable) mixture of both
natural and Excess (or anthropogenic) components.
6.3.2.2 What controls Excess CO 2 uptake?
The elevation of atmospheric pCO 2 over preindustrial levels means that natural areas of CO 2 outgassing from the ocean now release less CO 2 to the
atmosphere than during preindustrial times. On
the other hand, natural areas of CO 2 uptake have
become correspondingly larger sinks for CO 2 , as
have regions that were naturally regions of zero
net transfer. An interesting combination of the
complex chemistry of CO 2 in seawater and ocean
circulation does allow for the counterintuitive case
where, locally, the air-to-sea transfer of CO 2 can
theoretically be lower than that of the preindustrial ocean (see Section 6.3.5.3).
Given about 1 year, surface ocean waters will
re-equilibrate with the ‘new’ atmospheric pCO 2
and the further uptake of Excess CO 2 will become
negligible. This implies that the major regions for
Excess CO 2 uptake from the atmosphere are regions
where ‘old’, poorly ventilated ocean waters regain
close contact with the atmosphere (and where, in
addition, gas exchange is efficient in transferring
CO 2 across the air–sea interface). These regions
may, or may not, be coincident with regions of
deep water formation: the Excess CO 2 uptake from
the atmosphere will take place close to where these
poorly ventilated waters are returned to the sea
surface, and not necessarily where the newly ventilated surface waters are transported back into the
deep ocean again. On the other hand, the storage
of Excess CO 2 within the ocean water column
will be largest in regions where recently ventilated
waters accumulate or converge.
6.3.2.3 Why does Excess CO 2 uptake matter?
CO 2 that is stored in the ocean does not affect
the earth’s radiation balance, so oceanic uptake of
Excess CO 2 mitigates the potential for global
warming. In order to predict the magnitude of
future climate change resulting from greenhouse
gas emissions, a requirement is the prediction of
future atmospheric CO 2 levels for given emissions
scenarios. There is also an immediate sociopolitical
requirement for better understanding of the global
carbon cycle as a consequence of the endorsement
of the Kyoto Protocol in 1997. Attempts to limit
the future growth of atmospheric CO 2 concentration, however modest, will involve major, and
potentially costly, changes in energy and technology policy. The inclusion of certain terrestrial carbon sinks in carbon emission budgeting increases
the need to better define global carbon sinks and
sources. Furthermore, future assessment of the
effectiveness of measures taken to reduce emissions
will ultimately be judged by their long-term effect
on atmospheric CO 2 levels, which in turn requires
an understanding of long-term storage changes in
the other key carbon reservoirs (oceans and the terrestrial biosphere).
There are therefore at least three key scientific
questions of relevance to global carbon cycle science arising from current policy-related issues:
1 How large are present-day oceanic and terrestrial carbon sources and sinks, where do they
operate, and what processes are responsible?
2 How will terrestrial and, particularly, oceanic
carbon sources and sinks behave in the future
under higher CO 2 and a possibly altered climate
and ocean circulation?
3 How will we monitor and assess the effectiveness
of emissions controls and sequestration activities
6.3 Storage and Transport of Excess CO 2 in the Oceans
491
Wallace