WMOD00435

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

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the surface. Farther down, the concentration does not vary
much with depth (Brewer 1983).

Consideration of equation 1 shows that the presence
of dissolved carbonates enhances the ability of seawater to
handle more CO2, because any increase in carbonate
concentration drives the equation to the left, that is, it
converts CO2 already present into bicarbonate. Surface
seawater is supersaturated with respect to calcium
carbonate (CaC03); many marine creatures take advantage
of this fact and precipitate it to form their shells. The
precipitation of CaC03, proceeds mainly according to the
following equation:

2HC03- + Ca + + -+ CaC03 + CO2 + H20 (2)

Although this process results in a release of CO2, its
ultimate effect is to remove carbon from seawater. The
most spectacular product of this process is coral reefs, but
it has other results of much greater importance. Over
millions of years, seashells and other organic debris have
accumulated on the seafloor, creating the vast reservoir of
carbon in sedimentary rocks (table 1).

The solubility of carbonates in water varies directly
with pressure and inversely with temperature and pH value.
In the depths, the temperature is always near 2 DC,
pressures run to thousands of bars, and the pH is lowered,
that is, the acidity increased, by the CO2 released by
decaying organic matter. Below some level, the water is
no longer supersaturated with respect to CaC03, which
begins to dissolve and thereby increases the ability of the
water to hold COz' This changeover occurs at a great
depth, which has been estimated at 5 km for the North
Atlantic Ocean (Brewer 1983), and does not prevent the
accumulation of carbonates on much of the earth's seafloor.

2.5 Geochemical Carbon Cycles

Carbonates do not build up on the seafloor
indefinitely; seashells, like almost everything in nature, are
recycled. If the shells are converted to sedimentary rock
(limestone) and elevated above sea level, the limestone is
eventually attacked by HZC03 and carried away to the
ocean as calcium and bicarbonate ions. The H2C03 can be
formed as atmospheric COz dissolves in rainwater or in
surface water, or as CO2 released by decaying organic
matter in the soil enters groundwater. The simplified
chemical equation for weathering of CaC03,

CaC03 + HzO + CO2 -+ CaC03 + HZC03
-+ Ca + + + 2HC03' (3)

is the converse of equation 1, which describes the
precipitation of CaC03 as seashells.

It has been known for over a century (e.g.,
Arrhenius 1896) that volcanos release COz to the
atmosphere and that the weathering of rocks removes it.
Thanks to the development of the theory of plate tectonics,
it has been recognized in the last decade that the two
processes just named and the carbonates on the seafloor are
linked in a grand geochemical carbon cycle. According to
Ruddiman and Kutzbach (1991), "Chemical weathering of

continental rocks removes carbon dioxide from the
atmosphere and carries it in dissolved chemical form to the
ocean, where it is taken in by marine biota and deposited
in sediments on the seafloor. Tectonic activity eventually
frees this trapped carbon dioxide in the following manner.
The motion of the earth's lithospheric plates transports the
seafloor to ocean trenches, where subduction carries old
crust and sediments down toward the earth's hot interior.
At great depths, the sediments melt, releasing carbon
dioxide, which emerges from the volcanic islands that
overlie the buried crust and rejoins the atmosphere,
completing the cycle." Additional information is given by
Berner and Lasaga (1989), who emphasize that the
weathering of silicates followed by precipitation of CaC03
in the sea produces a net loss of CO2 from the atmosphere.
The governing equations for the conversion of silicates to
CaC03 are:

2C02 + HzO + CaSi03 -+ CaH + 2HC03' + Si02 (4)

2HC03' + CaH -+ CaC03 + CO2 + H20

(5)

In the absence of offsetting processes, notably
emissions from volcanos and ocean-atmosphere exchanges,
the weathering of rocks could deplete the atmospheric CO2
reservoir completely in about 10,000 years. Berner and
Lasaga (1989) note that the processes in the magma under
the earth's crust that break down carbonates result not only
in the expulsion of CO2 but in the recombination of
calcium and silicon into silicate rocks, which also is
required to close the geochemical carbon cycle.

2.6 CO2 in the Atmosphere

Summarizing to this point, the concentration of CO2
in the atmosphere is affected by many factors, including
the growth and decay of plants and trees, the deposition
and burning of fossil fuels, sea surface temperatures,
volcanic activity, and topography. Topography enters in
because the rise and fall of mountains and plateaus affect
sea level and the rate of rock weathering (Ruddiman and
Kutzbach 1991). An incr.eased concentration of
atmospheric CO2 hastens weathering of silicates, which
removes COz from the atmosphere and deposits it in the
ocean. This negative feedback effect may have set the
outer limits on the CO2 concentration over geologic time,
but did not control the concentration very closely.
Experimental data indicate wide ranges in the atmospheric
CO2 concentration in the distant past, as noted in a review
article by Larius et al. (1990). Air bubbles trapped in the
famous Vostok ice core from Antarctica have been
analyzed to estimate the atmospheric concentration back for
some 150,000 years. The concentration during that time
varied from 175 to 300 ppm by volume (fig. 2).

Estimates of the concentration of COz in the
atmosphere at the start of the Industrial Revolution, say in
1815, cluster around 270 ppm. These estimates are based
on ice-core samples and on sporadic direct measurements
during the nineteenth century. The concentration
approached 300 ppm by 1900.

The concentration of CO2 in the atmosphere has
been measured systematically since 1958, which was the

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