128 Climate changein the twenty-firstcenturyand beyond
page 62–3), with the warming in the eastern tropical Pacific more than
that in the western tropical Pacific and with a corresponding eastward
shift of precipitation, are projected to continue by many models. There
is also evidence that warming associated with increasing greenhouse gas
concentrations will cause an intensification of the Asian summer mon-
soon and an increase of variability in its precipitation. The influence of
increased greenhouse gases on these major climate regimes, especially
the El Ni ̃no, is an important and urgent area of research.
A complication in the interpretation of patterns of climate change
arises because of the differing influence of atmospheric aerosols as com-
pared with that of greenhouse gases. Although in the projections based
on the SRES scenarios the influence of aerosols is much less than in those
based on the IS 92 scenarios published by the IPCC in its 1995 Report,^15
their projected radiative forcing is still significant. When considering
global average temperature and its impact on, for instance, sea level rise
(see Chapter 7) it is appropriate in the projections to use the values of
globally averaged radiative forcing. The negative radiative forcing from
sulphate aerosol, for instance, then becomes an offset to the positive
forcing from the increase in greenhouse gases. However, because the
effects of aerosol forcing are far from uniform over the globe (Figure
3.7), it is not correct, when considering climate change and its regional
characteristics, to considerthe effects of increasing aerosol as a simple
offset to those of the increase in greenhouse gases. The large variations in
regional forcing due to aerosols produce substantial regional variations
in the climate response. Detailed regional information from the best cli-
mate models needs to be employed to assess the climate change under
different assumptions about the increases in both greenhouse gases and
aerosols.Changes in climate extremes
The last section looked at the likely regional patterns of climate change.
Can anything be said about likely changes in the frequency or intensity of
climate extremes in the future? It is, after all, not the changes in average
climate that are generally noticeable, but the extremes of climate – the
droughts, the floods, the storms and the extremes of temperature in very
cold or very warm periods – which provide the largest impact on our
lives (see Chapter 1).
The most obvious change we can expect in extremes is a large
increase in the number of extremely warm days (Figure 6.6) coupled
with a decrease in the number of extremely cold days. A number of
model projections show a generally decreased daily variability of surface
air temperature in winter and increased daily variability in summer in