52 The greenhousegases
This is done using information about how the different gases absorb
radiation in the infrared part of the spectrum, as mentioned in Chapter 2.
The radiative forcing associated with the increases in each of the gases
can then be calculated. In Figure 3.8 are brought together estimates of
global average radiative forcing for the period from 1750 to 2000 for
the different greenhouse gases and for tropospheric aerosols of different
origins which we have been considering in this chapter.
It is useful to be able to compare the radiative forcing generated by
the different greenhouse gases. Because of their different life-times, the
future profile of radiative forcing due to releases of greenhouse gases
varies very much from gas to gas. An index called the global warm-
ing potential (GWP) has been defined for greenhouse gases that takes
the ratio of the time-integrated radiative forcing from the instantaneous
release of 1 kg of a given gas to that from the release of 1 kg of car-
bon dioxide. A time horizon has also to be specified for the period over
which the integration is carried out. The GWPs of the six greenhouse
gases included in the Kyoto Protocol are listed in Table 10.2. Applying
the GWPs to the emissions from a mixture of greenhouse gases enables
the mixture to be considered in terms of an equivalent amount of car-
bon dioxide. However, because the GWPs for different time horizons are
very different, GWPs are of limited application and mustbe used with
care.
When considering radiative forcing of climate, the question is bound
to be asked as to whether variations have occurred or are likely to occur
in the amount of energy from the Sun that is incident on the Earth and
that could, therefore, affect the climate. We shall see, for instance, in the
next chapter that ice ages in the past have been triggered by variations in
the geometry of the Earth’s orbit. It is considered possible that the Sun’s
output itself could vary by small amounts over time (see box in Chapter 6
page 138). Figure 3.8 (see also Figure 6.12) indicates the range of es-
timates of solar variability that may have occurred since 1850 showing
that its influence is much less than that of the increase in greenhouse
gases.
Also included in Figure 3.8 are the possible effects of aviation on
radiative forcing and effects due to land-use changes that arise because
of changes in the albedo (see Glossary) of the surface. The effects of
aviation that are in addition to the source of carbon dioxide emissions it
provides arise from its influence on high cloud cover through its emis-
sions of water vapour. As we shall see in Chapter 5 (page 91) high
cloud providesa blanketing effect on the Earth’s surface similar to that
of greenhouse gases and therefore leads to positive radiative forcing.
Many examples exist of extensive contrail formation over regions where
many aircraft flights regularly occur. Aircraft may also influence cirrus