The Carbon Cycle - Introduction to Carbon Capture and Sequestration - page 127

0.6

0.5

0.4

Ocean Invasion

0.3

0.2

0.1

0.0

0 5000 10000 15000 20000 25000 30000 35000 40000

Year (A.D.)

Figure 3.6.2 CO 2 in the oceans
The oceans form an important buffer to absorb the extra emitted CO 2 . The equilibration
of CO 2 within the boundary layer happens relatively fast (1 year). As the capacity of this
boundary layer is limited, the equilibrium concentration of CO 2 in the atmosphere will
stay high. Mixing of surface water with the deep ocean water, in which the concentra-
tions of CO 2 are lower, and subsequent re-equilibration with the CO 2 in the atmosphere,
allows a further decrease in the CO 2 concentration. As the mixing times of the oceans
are on the order of 100 to 1,000 years, we see that it may take up to 5,000 years for
atmospheric CO 2 levels to fully equilibrate with those in the oceans. Figure adapted from
Archer and Brovkin [3.12].

fi nal reduction in atmospheric CO 2 level has to come from the inorganic
carbon cycle, i.e., through the weathering reaction of CO 2 with silicate rocks
and volcanism (see Sections 3.3, 3.4), which has a time scale on the order
of a hundred thousand years (see Figure 3.6.3 ) [3.12, 3.13].
Now let's return to the quantitative side of things. At present we are
emitting about 9 Gt of carbon per year from fossil fuels. Of these 9 Gt,
5 Gt are taken up by the biosphere and oceans in about equal amounts.
The difference, 4 Gt, is added to the atmosphere each year. In order to
employ sophisticated climate models, we have to estimate the amount of
carbon that will be emitted each year; the maximum atmospheric CO 2
concentration will depend on this rate. If we emit the same amount over
a longer period of time the biosphere and oceans can better keep up with
our emissions. The results of the different climate models for the “moder-
ate” and “large” scenarios [3.14] are shown in Figure 3.6.4 .