Questions 51to the increase in greenhouse gases of about 2.6 W m−^2. Comparing
global average forcings, however, is not the whole story. Because of
the large regional variation of particles in the atmosphere (Figure 3.7),
any effect they have on the climate can be expected to be substantially
different from the effect of increases in greenhouse gases, which is much
more uniform over the globe. This will be illustrated in Chapter 5 where
we consider the likely pattern of anthropogenic climate change to date.
More consideration of it will also be given in Chapter 6 when discussing
projections of climate change on regional scales that can depend critically
on assumptions about the likely concentrations of atmospheric particles
in the future.
An important factor that will influence the future concentrations of
sulphate particles is ‘acid rain’ pollution, caused mainly by sulphur diox-
ide emissions. This leads to the degradation of forests and fish stocks in
lakes especially in regions downwind of major industrial areas. Serious
efforts are therefore under way, especially in Europe and North America,
to curb these emissions to a substantial degree. Although the amount
of sulphur-rich coal being burnt elsewhere in the world, for instance
in Asia, is increasing rapidly, the damaging effects of sulphur pollution
are such that tight controls on sulphur emissions are being extended to
these regions also. For the globe as a whole therefore, sulphur emissions
are likely to rise much less rapidly than emissions of carbon dioxide;
the IPCC SRES scenarios of future emissions as presented in Chapter 6
anticipate these changes. The climate change resulting from an increase
in sulphate particles therefore will become increasingly less by compar-
ison with that from the likely increase of greenhouse gases.
Estimates of radiative forcing
This chapter has summarised current scientific knowledge about the
sources and sinks ofthe main greenhouse gases and the exchanges which
occur between the components of the climate system – the atmosphere,
the ocean and the land surface – including the close balances that are
maintained between the different components and the way in which these
balances are being disturbed by human-generated emissions. Different
assumptions about future emissions have been used to generate emission
scenarios. From these scenarios estimates have been made (for carbon
dioxide, for instance, using a computer model of the carbon cycle) of
likely increases in greenhouse gas concentrations in the future.
Given information about the possible increases in greenhouse gases,
the next step is to calculate the effect of these increases on the amounts
of thermal (infrared) radiation absorbed and emitted by the atmosphere.