Geoscience Reference
In-Depth Information
Box 3.1 Future reductions in air-sea CO
2
equilibration times
It is known that air-sea equilibration requires many
months for CO
2
but only a few weeks for most
other gases. Here let us consider how air-sea CO
2
equilibration times vary regionally and with time.
For most gases such as oxygen, a sudden change in
gas concentration in the ocean mixed layer
equilibrates with the atmosphere (and vice versa)
following a simple e-folding time
CO
t
O
ratio varies regionally and
declines as atmospheric CO
2
increases. Here,
spatiotemporal differences were computed from
climatologies for the mixed layer depth and the
piston velocity (Fig. B3.1A) and evolving i elds of
carbonate chemistry variables (Orr et al. 2005).
Based on GLODAP gridded data product from the
WOCE-era carbon system measurements col-
lected in the 1990s (Key et al. 2004), the
t
Indeed the
t
2
/
CO
t
2
/
O
z
k
t
=
m
(B3.1)
ratio
(i.e. C / [CO ]) varies from 7 to 24, with
the lowest values in the Southern Ocean and the
highest in the tropics (Fig. B3.1B). Pre-industrial
values of that ratio were ~20% higher than in
1994, as computed by subtracting data-based
estimates for anthropogenic C
T
and recalculating.
In the future, the decline continues. Relative to
the modern state, at 563 ppmv (2100S)
t
∂∂
T
O
2
2
w
where k
w
is the gas transfer or 'piston' velocity (m
day
-1
) and z
m
is the mixed layer depth (m). For CO
2
though, it is more complicated, because added
anthropogenic CO
2
does not remain as dissolved
gas but reacts with carbonate ions forming
bicarbonate ions (see Box 1.1 and Eq. 3.1). As
discussed by Broecker and Peng (1974), this
reaction increases the equilibration time for CO
2
.
A rigorous development by Zeebe and Wolf-
Gladrow (2001) shows that
CO
t
O
drops by at least 40% everywhere (ranging from
3.6 to 14.2), and at 788 ppmv (2100I) it drops by
more than 60% (ranging from 2.5 to 9), based
on median changes projected by the OCMIP2
models. Scaling directly with this declining
ratio is
t
2
/
z
∂
C z
C
(B3.2)
CO
.
Pre-industrial
t
t
=
m
T
=
m
T
CO
2
k
∂
[CO ]
k
R
[CO ]
2
CO
ranged from 6 to 14 months
between the Southern Ocean and the tropics
(Fig. B3.1C). At 788 ppmv, the range drops to
1.5 to 4 months. Thus CO
2
equilibration times
will become more like those of other gases,
altering the amplitude and phasing of the annual
cycle of surface-layer carbonate system variables,
particularly for CO
2
and [CO
3
2-
]. These future
chemical reductions in
t
w
2
w
2
2
where R is the Revelle factor. A similar development
by Sarmiento and Gruber (2006) derived the
approximation
/ [CO ] [CO ] / [CO ]
C
i rst
used empirically by Broecker and Peng (1974). All
three of these studies used
∂∂
∂∂
≈
2
−
T
2
3
2
C
/[CO] 20,
≈
and
T
2
since
t
2
/
CO
t
O
has the same ratio (compare Eqs
B3.1 and B3.2), air-sea equilibration is much
longer for CO
2
than for most other gases. Thus a
typical 50-m mixed layer equilibrates with atmos-
pheric O
2
on a timescale of ~12 days, whereas it
requires ~8 months for CO
2
. But these are only
averages for today's ocean.
CO
will be reinforced by
reductions from physical changes driven by
climate change, based on projected increased
stratii cation (shallower mixed layers) and
reduced sea-ice cover, changes that will reduce
equilibration times of all gases.
2
continues
