46 / Basics of Environmental Science
atmospheric greenhouse effect is real and important, and the gases which cause it are justly known
as ‘greenhouse gases’.
Both the global climate and atmospheric concentrations of greenhouse gases vary from time to time.
Studies of air trapped in bubbles inside ice cores from Greenland and from the Russian Vostok
station in Antarctica have revealed a clear and direct relationship between these variations and air
temperature, in the case of the Vostok cores back to about 160 000 years ago. The correlation is
convincing, although it is possible that the fluctuating greenhouse-gas concentration is an effect of
temperature change rather than the cause of it. As temperatures rose at the end of the last ice age, the
increase in the atmospheric concentration of carbon dioxide lagged behind the temperature (CALDER,
1999) and so carbon dioxide cannot have been the cause of the warming. There is also evidence that
the carbon dioxide concentration was far from constant prior to the start of the Industrial Revolution
(WAGNER ET AL., 1999). Carbon dioxide measurements taken from air bubbles trapped in ice
cores are unreliable, because carbon dioxide is soluble in solid ice.
Nor has the temperature always been linked to the concentration of carbon dioxide. The two were
uncoupled between about 17 and 43 million years ago. The air then contained less than two-thirds of
the present concentration of carbon dioxide (180–240 μmol mol-1 compared with 360 μmol mol-1
today), but the climate was up to 6°C warmer than it is today (COWLING, 1999).
Nevertheless, it is estimated that the atmospheric carbon dioxide concentration immediately prior to
the Industrial Revolution was about 280 μmol mol-1 and that the increase since then has been due
entirely to emissions from the burning of fossil fuels. This may not be the case. The solubility of
gases, including carbon dioxide, is inversely proportional to the temperature. A rise in temperature,
therefore, will cause dissolved carbon dioxide to bubble out of the oceans. This is called the ‘warm
champagne’ effect. Rising temperature will also stimulate aerobic bacteria. Their respiration will
release carbon dioxide. This is called the ‘warm beer’ effect (CALDER, 1999).
Carbon dioxide is the best-known greenhouse gas, because it is the most abundant of those over
which we can exert some control, but it is not the only one. Methane, produced naturally, for example
by termites, but also by farmed livestock and from wet-rice farming (present concentration about 1.7
ppm), nitrous oxide (0.31 ppm) and tropospheric ozone (0.06 ppm), products from the burning of
fuels in furnaces and car engines, and the industrially manufactured compounds CFC-11 (0.00026
ppm) and CFC-12 (0.00044 ppm) are also important. The most important of all, however, is water
vapour. This enters into the calculations only indirectly, because its concentration varies greatly
from place to place and from day to day and because it is strongly affected by temperature. Its
influence, therefore, tends to add to those of the other gases and generally varies as they do. Figure
2.13 shows the anticipated changes in concentration for carbon dioxide, methane, and CFC-12,
which is one of the family of CFC compounds. These increases are based on the (uncertain) assumption
that industrial and vehicle emissions are the only source of carbon dioxide.
All greenhouse-gas effects are usually expressed as ‘global warming potentials’ (GWPs) which
relate them to carbon dioxide. GWPs take account of the wavelengths at which particular molecules
absorb, some of which overlap, and the length of time they remain in the atmosphere before
decomposing or being deposited at the surface. On this basis, over a 100-year period, with carbon
dioxide given a value of 1, methane has a value of 11 (i.e. it is 11 times more effective than
carbon dioxide, molecule for molecule), nitrous oxide 270, CFC-11 3400, and CFC-12 7100.
The estimates of future climatic warming are based on the consequences calculated for a doubling
of the carbon dioxide concentration, which includes the GWPs for all the relevant gases. Figure
2.14 shows that, depending on the sensitivity of the atmosphere to greenhouse forcing, a doubling
of carbon dioxide would raise the average global temperature by 1.5–4.5°C, with a ‘best estimate’