34 The greenhousegases
Figure 3.5 shows that these fractions may change substantially in the
future.
About ninety-five per cent of fossil fuel burning occurs in the
northern hemisphere, so there is more carbon dioxide there than in the
southern hemisphere. The difference is currently about two parts per
million and, over the years, has grown in parallel with fossil fuel emis-
sions, thus adding further compelling evidence that the atmospheric
increase in carbon dioxide levels results from these emissions.
We turn now to what happens in the oceans. We know that carbon
dioxide dissolves in water; carbonated drinks make use of that fact. Car-
bon dioxide is continually being exchanged with the air above the ocean
across the whole ocean surface (about 90 Gt per year is so exchanged –
Figure 3.1), particularly as waves break. An equilibrium is established
between the concentration of carbon dioxide dissolved in the surface
waters and the concentrationin the air above the surface. The chemical
laws governing this equilibrium are such that if the atmospheric concen-
tration changes by ten per cent the concentration in solution in the water
changes by only one-tenth of this: one per cent.
This change will occur quite rapidly in the upper waters of the ocean,
the top hundred metres or so, so enabling part of the anthropogenic (i.e.
human generated) carbondioxide added to the atmosphere (most of the
ocean’s share of the fifty-five per cent mentioned above) to be taken up
quite rapidly. Absorption in the lower levels in the ocean takes longer;
mixing of surface water with water at lower levels takes up to several
hundred years or for the deep ocean over a thousand years. This process
whereby carbon dioxide is gradually drawn from the atmosphere in the
ocean’s lower levels is sometimes known as thesolubility pump.
So the oceans do not provide as immediate a sink for increased
atmospheric carbon dioxide as might be suggested by the size of the
exchanges with the large ocean reservoir. For short-termchanges only
the surface layers of water play a large part in the carbon cycle.
Biological activity in the oceans also plays an important role. It may
not be immediately apparent, but the oceans are literally teeming with
life. Although the total mass of living matter within the oceans is not
large, it has a high rate of turnover. Living material in the oceans is
produced at some thirty to forty per cent of the rate of production on
land. Most of this production is of plant and animal plankton that go
through a rapid series of life cycles. As they die and decay some of the
carbon they contain is carried downwards into lower levels of the ocean
adding to the carbon content of those levels. Some is carried to the very
deep water or to the ocean bottom where, so far as the carbon cycle is
concerned, it is out of circulation for hundreds or thousands of years.