The Carbon Cycle - Introduction to Carbon Capture and Sequestration - page 110

Section 3
The role of oceans
in the carbon cycle
The previous section described the interactions of atmospheric CO 2 with
the biosphere. Carbon dioxide in the air is also exchanging with CO 2
in the ocean. This exchange can only take place at the surface of the
ocean. The surface water then mixes with the deeper layers of the
ocean, but this mixing occurs much more slowly than the exchange of
atmospheric CO 2 with surface waters. Because of this difference in time
scale, we have separated the ocean into a surface reservoir and a deep
ocean reservoir.
It is the deep ocean reservoir that is responsible for the periodicity at
the 100,000 year time scale ( Figure 3.1.3 ). These fl uctuations have been
measured from air bubbles in ice cores from Antarctica ( Figures 3.3.1 )
[3.6]. The ice core data show oscillations in the CO 2 levels on a time
scale of ~100,000 years. We see that these fl uctuations change the CO 2
levels by ~100 ppm, which is about 30% of the total CO 2 concentration
in the air.
Such large changes in CO 2 levels in the atmosphere have conse-
quences for the average temperature on the earth, and this periodicity is
associated with glacial and interglacial periods on earth. The underlying
mechanism for this periodic behavior has remained an important out-
standing scientifi c question. The period that we fi nd in the ice core data
is statistically correlated to the solar cycles, and for many years the so-
called Milankovitch hypothesis has been put forward as an explanation
for the cyclic character of the glacial and interglacial periods and the
100,000 year period of CO 2 oscillations [3.7]. This theory posits that the
annual cycle of the earth around the sun is not a perfect circle, but rather
has a much more complicated pattern with characteristic periods of
roughly 100,000, 41,000, and 23,000 years. Because the earth is warmer
or cooler depending on how far it is from the sun, the oscillations in
earth's orbit lead to a warmer and colder earth, hence growing or shrink-
ing glaciers. This warming and cooling would also affect atmospheric
CO 2 concentrations, as we discuss below. The main criticism of this