An Introduction to Environmental Chemistry

Section 7.2.2. The reservoir of carbon in fossil fuels and mudrocks is also sub-

stantial and a major portion of the latter is thought to be recoverable and thus

available for burning. The smallest reservoirs are the land biosphere (2000 GtC)

and the atmosphere (749 GtC, equivalent to an atmospheric concentration of

about 354 ppm). It is the small size of the latter which makes it sensitive to even

small percentage changes in the other larger reservoirs, where these changes

result in emissions to the atmosphere, as, for example, in the burning of fossil

fuels.

Natural fluxes

It is often assumed that natural flows between the major reservoirs are balanced

two-way fluxes when averaged over the whole year and the total surface of the

reservoir. For example, the land biosphere and the oceans exchange approxi-

mately 120 and 90 GtC yr-^1 respectively in both directions with the atmosphere.

There is, however, some uncertainty about this assumption over periods of years

and it is surely wrong on longer timescales. Evidence for short-term imbalance

comes from careful inspection of the atmospheric record (as shown in Fig. 7.8).

At the end of the record (in the early 1990s) the rate of increase of atmospheric

CO 2 is significantly smaller than for previous years. The explanation for this

decrease in the rate of change is very unlikely to be alterations in anthropogenic

inputs to the atmosphere, since there is no evidence that fossil fuel burning or

Global Change 253

Atmosphere

749 (354 ppm CO 2 ) Annual increase 3.3 ± 0.1

Fossil fuels

and shales

(7500 recoverable)

12 000

1020

38 100

Oceans

Sediments

20 000 000

2000

Land biosphere

Reservoirs

Natural fluxes

Anthropogenic fluxes

Key:

5.4

± 0.3

0.2

(^120) ± 0.7
1.9
(^90) ± 0.6
Fig. 7.9A simplified version of the global carbon cycle for the 1980s. The numbers in boxes indicate the
reservoir size in GtC. Arrows represent fluxes and the associated numbers indicate the magnitude of the flux
in GtC yr-^1. After IPCC (2001). With permission of the Intergovernmental Panel on Climate Change.