Carbon dioxideand the carboncycle 31timescales determined by their respective turnover times – which range
from less than a year to decades (for exchange with the top layers of
the ocean and the land biosphere) to millennia (for exchange with the
deep ocean or long-lived soil pools). These timescales are generally
much longer than the average time a particular carbon dioxide molecule
spends in the atmosphere, which is only about four years. The large range
of turnover times means that the time taken for a perturbation in the at-
mospheric carbon dioxide concentration to relax back to an equilibrium
cannot be described by a single time constant. Although a lifetime of
about a hundred years is often quoted for atmospheric carbon dioxide so
as to provide some guide, use of a single lifetime can be very misleading.
Before human activities became a significant disturbance, and over
periods short compared with geological timescales, the exchanges be-
tween the reservoirs were remarkably constant. For several thousand
years before the beginning of industrialisation around 1750,a steady
balance was maintained, such that the mixing ratio (or mole fraction, for
definition see Glossary) of carbon dioxide in the atmosphere as measured
from ice cores(see Chapter 4) kept within about ten parts per million of
a mean value of about 280 parts per million (ppm) – see Figure 3.2(a).
The Industrial Revolution disturbed this balance and since its be-
ginning in about 1700 approximately 600 thousand milliontonnes (or
gigatonnes, Gt) of carbon have been emitted into the atmosphere from
fossil fuel burning. This has resulted in a concentration of carbon diox-
ide in the atmosphere that has increased by about thirty per cent, from
280 ppm around 1700 to a value of over 370 ppm at the present day
(Figure 3.2(b)). Accurate measurements, which have been made since
1959 from an observatory near the summit of Mauna Loa in Hawaii,
show that carbon dioxide is currently increasing on average each year
by about 1.5 ppm, although there are large variations from year to year
(Figure 3.2(c)). This increase spread through the atmosphere adds about
3.3 Gt to the atmospheric carbon reservoir each year.
It is easy to establish how much coal, oil and gas is being burnt world-
wide each year. Most of it is to provide energy for human needs: for heat-
ing and domestic appliances, for industry and for transport (considered
in detail in Chapter 11). The burning of these fossil fuels has increased
rapidly since the Industrial Revolution (Figure 3.3 and Table 3.1); cur-
rently the annual total is between 6 and 7 Gt of carbon, nearly all of
which enters the atmosphere as carbon dioxide. Another contribution to
atmospheric carbon dioxide due to human activities comes from land-
use change, in particular from the burning and decay of forests balanced
in part by aforestation or forest regrowth. This contribution is not easy
to quantify but some estimates are given in Figure 3.3 and Table 3.1.
For the 1980s (see Table 3.1), annual anthropogenic emissions from