animal life, and decaying materials). Soils and vegetation together contain
about 3700 Gt. Accessible fossil fuels – mainly coal – contain about 1600 Gt.
Finally, the atmosphere contains about 600 Gt of carbon.

Until recently, all these pools of carbon were roughly in balance: all
flows of carbon out of a pool (say, soils, vegetation, or atmosphere) were
balanced by equal flows into that pool. The flows into and out of the fossil
fuel pool were both negligible. Then humans started burning fossil fuels.
This added two extra unbalanced flows, as shown in figure 31.3.

The rate of fossil fuel burning was roughly 1 Gt C/y in 1920, 2 Gt C/y
in 1955, and 8.4 Gt C in 2006. (These figures include a small contribution
from cement production, which releases CO2 from limestone.)

How has this significant extra flow of carbon modified the picture
shown in figure 31.2? Well, it’s not exactly known. Figure 31.3 shows
the key things that are known. Much of the extra 8.4 Gt C per year that
we’re putting into the atmosphere stays in the atmosphere, raising the at-
mospheric concentration of carbon-dioxide. The atmosphere equilibrates
fairly rapidly with the surface waters of the oceans (this equilibration takes
only five or ten years), and there is a net flow of CO2 from the atmosphere
into the surface waters of the oceans, amounting to 2 Gt C per year. (Recent
research indicates this rate of carbon-uptake by the oceans may be reducing,
however.) This unbalanced flow into the surface waters causes ocean
acidification, which is bad news for coral. Some extra carbon is moving
into vegetation and soil too, perhaps about 1.5 Gt C per year, but these
flows are less well measured. Because roughly half of the carbon emissions
are staying in the atmosphere, continued carbon pollution at a rate
of 8.4 Gt C per year will continue to increase CO2 levels in the atmosphere,
and in the surface waters.

What is the long-term destination of the extra CO2? Well, since the
amount in fossil fuels is so much smaller than the total in the oceans, “in
the long term” the extra carbon will make its way into the ocean, and the
amounts of carbon in the atmosphere, vegetation, and soil will return to
normal. However, “the long term” means thousands of years. Equilibration
between atmosphere and the surface waters is rapid, as I said, but
figures 31.2 and 31.3 show a dashed line separating the surface waters of
the ocean from the rest of the ocean. On a time-scale of 50 years, this
boundary is virtually a solid wall. Radioactive carbon dispersed across the
globe by the atomic bomb tests of the 1960s and 70s has penetrated the
oceans to a depth of only about 400 m. In contrast the average depth of the
oceans is about 4000 m.

The oceans circulate slowly: a chunk of deep-ocean water takes about
1000 years to roll up to the surface and down again. The circulation of
the deep waters is driven by a combination of temperature gradients and
salinity gradients, so it’s called the thermohaline circulation (in contrast to
the circulations of the surface waters, which are wind-driven).

This slow turn-over of the oceans has a crucial consequence: we have

Figure 31.3. The arrows show two extra carbon flows produced by burning fossil fuels. There is an imbalance between the 8.4 Gt C/y emissions into the atmosphere from burning fossil fuels and the 2 Gt C/y take-up of CO2 by the oceans. This cartoon omits the less-well quantified flows between atmosphere, soil, vegetation, and so forth.