WMOD00435

• Title: Limits on Global Warming
• Date: 5/9/1991
• Weather Modification - Doc Type: Report

moves westward and warms under the tropical sun (fig. 3.4
of Brewer 1983). However, the tendency toward degassing
from seawater that is being warmed can be reversed by
rapid growth of plankton in nutrient-rich water.

Short-term exchanges of CO2 between the ocean and
the atmosphere are almost in balance, as figure 1 indicates.
The apparent net absorption by the ocean over the past few
decades has been driven by the rapid increase in the
atmospheric concentration, which either speeds CO2
absorption or suppresses its escape depending on local
conditions. If the ocean warms as a result of global
climate change, its ability to absorb atmospheric CO2 will
be diminished slightly. Keeling et al. (1989) have
calculated that a slight observed warming of the sea surface
from 1978 to about 1988 reduced the total COz uptake over
the decade by roughly 3 Gt.
Short-term exchanges of CO2 between the ocean and
the overlying atmosphere involve only the ocean's mixed
layer. Convection and turbulence distribute dissolved CO2
in its various forms rather uniformly throughout the mixed
layer in a few months. The depth of the mixed layer
varies, depending mainly on wind speed and vertical
density gradients, but averages around 75 m (Keeling et al.
1989). The mixed layer reaches its maximum depth in
areas where loss of heat from the sea surface to overlying
cold airmasses leads to convective overturning within it.
At high latitudes during winter, convective currents of
chilled surface water can penetrate downward as much as 1
km, but mixing to that depth is incomplete.

2.3 Carbon in Seawater

In order to discuss CO2 in seawater quantitatively it
is necessary to consider the various forms that it assumes.
The CO2 dissolved in seawater is hydrated to form a weak
solution of carbonic acid (H2C03), which dissociates in two
stages to yield bicarbonate (HC03') and then carbonate
(C03 =) ions. The relative abundance of the various ions is
controlled by a number of buffering reactions, of which the
most important is

2 HC03' +- -+ H2C03 + C03 =

The equilibrium constants for the various reactions
vary with temperature and the pH value (acidity).
According to a review by Archer (1990), only 1 percent of
the carbon in seawater remains as CO2 or undissociated
H2C03, 88 percent of it exists as bicarbonate ions, and the
other 11 percent as carbonate ions. Some writers use the
expression "total CO2'' to mean the total of dissolved CO2
gas, H2C03, and bicarbonate and carbonate ions.

The fact that CO2 forms H2C03, which then
dissociates, makes it possible for seawater to hold about 30
times more CO2 under present conditions than it could
otherwise. Although oxygen is about 600 times as plentiful
as CO2 in the atmosphere, it is only about 20 times as
plentiful in the ocean as the CO2 equivalent of the carbon
in the ocean. However, there is resistance to further
additions of CO2 to seawater. This resistance is expressed
as a buffer factor, which is sometimes called the Revelle
factor (Brewer 1983). The Revelle factor is a function of
temperature, alkalinity and salinity, and so varies from

place to place. Its present values are around 10, which
means that changes in the total concentration of carbon in
seawater, expressed as a percentage of the existing
concentration, are only 0.1 times the corresponding
changes in the partial pressure of CO2 gas in the overlying
atmosphere (Brewer 1983). Even so, seawater still is
taking up about 3 times more CO2 than it would if CO2 did
not dissociate.

Numbers quoted in table 8 of Keeling et al. (1989)
can be combined to show that the ocean's mixed layer now
holds roughly the same amount of carbon as does the
atmosphere. Using that result and the current Revelle
factor of 10, one can estimate the final apportionment
between the atmosphere and the mixed layer of incremental
amounts of atmospheric CO2, As an increase of 10 percent
in the atmospheric concentration of CO2 will produce only
a 1 percent increase in the total carbon content of the
mixed layer, equilibrium is reached once the mixed layer
takes up 0.09 Gt of each additional gigaton of carbon,
leaving the remaining 0.91 Gt in the atmosphere.

2.4 Intermediate Cycles: Carbon in the Deep Ocean

It has been estimated that the CO2 released from
fossil fuels since the beginning of the Industrial Revolution
has been divided about equally between the ocean and the
atmosphere; yet the result just quoted in section 2.3 shows
that the mixed layer can not absorb and hold more than
one-tenth of the excess carbon being released to the
atmosphere each year. The mixed layer's ability to
continue to draw down atmospheric CO2 depends on
exchanges with the deep ocean, which holds about 98
percent of the carbon listed for the oceanic reservoir in
table 1. Exchanges of carbon between the mixed layer and
the ocean depths are brought about by ocean currents,
large-scale turbulence and biological processes.

(1)

All of the oceans share a common circulation. It is
characterized by regions where cold water sinks into the
depths, especially in the Weddell Sea near Antarctica and
between Greenland and Norway in the North Atlantic, and
by regions where cold water from the depths reappears at
the surface. Upwelling is marked along the west coast of
South America, for example. The deep-ocean circulation
proceeds very slowly, with complete circuits taking
hundreds or thousands of years. Tritium released by
nuclear bomb testing in 1962 and carried downward in the
North Atlantic circulation still had not penetrated south of
300 N in the deep Atlantic by 1981 (Brewer 1983).

The cold water that sinks into the OCf<'Ul depths
tends to be rich in CO2 and associated forms of carbon
simply because it is cold. Its carbon content is enhanced
further by biological processes. Organisms growing in the
mixed layer deplete its CO2 significantly, thereby making
possible the uptake of more atmospheric CO2. and ensuring
that the sinking water carries significant amounts of carbon
downward in organic as well as inorganic forms. On its
long circuit in the deep ocean, the water is further enriched
by organic waste falling from the mixed layer.
Decomposition of this waste gives off CO2, which leads to
an increase of some 10 percent in the concentration of CO2
and associated ions over the first kilometer downward from

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