Global Warming

42 The greenhousegases

unlikely to be fulfilled accurately in practice, it is necessary to make a

variety of different assumptions, so that we can get some idea of the

range of possibilities. Such possible futures are calledscenarios.

In Chapter 6 are presented two sets of emission scenarios as devel-

oped respectively by the Intergovernmental Panel on Climate Change

(IPCC) and the World Energy Council (WEC). These emission scenar-

ios are then turned into future projections of atmospheric carbon dioxide

concentrations through the application of a computer model of the carbon

cycle that includes descriptions of all the exchanges already mentioned.

Further in that chapter, through the application of computer models of

the climate (see Chapter 5), projections of the resulting climate change

from different scenarios are also presented.

Other greenhouse gases

Methane

Methane is the main component of naturalgas. Its common name used

to be marsh gas because it can be seen bubbling up from marshy

areas where organic material is decomposing. Data from ice cores show

that for at least two thousand years before 1800 its concentration in the

atmosphere was about 700 ppb. Since then its concentration has more

than doubled (Figure 3.6). During the 1980s it was increasing at about

10 ppb year−^1 but during the 1990s the average rate of increase fell to

around 5 ppb year−^1.^9 Although the concentration of methane in the

atmosphere is much less than that of carbon dioxide (less than 2 ppm

compared with about 370 ppm for carbon dioxide), its greenhouse effect

is far from negligible. That is because the enhanced greenhouse effect

caused by a molecule of methane is about eight times that of a molecule

of carbon dioxide.^10

Figure 3.6Change in
methane concentration (mole
fraction in ppb) over the last
1000 years determined from
ice cores, glacier ice and
whole air samples. Radiative
forcing since the pre-industrial
era due to the methane
increase is plotted on the right
axis.