Projections ofglobal averagetemperature 123for climate change that has been widely used is the change in global
average temperature.
The projected rise in global average temperature due to the increase
in greenhouse gases and aerosols from pre-industrial times is illustrated
in Figure 6.4(b). It shows an increase of about 0.6◦C up to the year 2000
and an increase ranging from about 2◦C to about 6◦C by 2100 – that
wide range resulting from the very large uncertainty regarding future
emissions and also from the uncertainty that remains regarding the feed-
backs associated with the climate response to the changing atmospheric
composition (as described in Chapter 5).^10
Compared with the temperature changes normally experienced from
day to day and throughout the year, changes of between 2◦C and 6◦C
may not seem very large. But, as was pointed out in Chapter 1, it is in
fact a large amountwhen consideringglobally averagedtemperature.
Compare it with the 5◦Cor6◦C change in global average temperature
that occurs between the middle of an ice age and the warm period in
between ice ages. The changes projected for the twenty-first century
are from one-third to a whole ice age in terms of the degree of climate
change!
The rate of change of global average temperature projected for the
twenty-first century is in the range of 0.15◦Cto0.6◦C per decade.
These might seem small rates of change; most people would find it
hard to detect a change in temperature of a fraction of a degree. But
remembering again that these are global averages, such rates of change
become very large. Indeed, they are much larger than any rates of change
the climate has experienced for at least the past 10 000 years as inferred
from paleoclimate data. As we shall see in the next chapter, the ability
of both humans and ecosystems to adapt to climate change depends
critically on the rate of change.
The changes in global average temperature shown in Figure 6.4(b)
from the IPCC 2001 Report are substantially greater than those shown
in the IPCC 1995 Report. The main reason for the difference is the much
smaller aerosol emissions in the SRES scenarios compared with the
IS 92 scenarios. This is illustrated by the temperature in 2100 shown
in Figure 6.4(b) for the IS 92a scenario which is similar to that for the
SRES B2 scenario even though the carbon dioxide emissions at that date
for IS 92a are fifty per cent greater than those for B2.
In many of the modelling studies of climate change, the situation
of doubled pre-industrial atmospheric carbon dioxide has often been in-
troduced as a benchmark especially to assist in comparisons between
different model projections and their possible impacts. Since the pre-
industrial concentration was about 280 ppm, doubled carbon dioxide is
about 560 ppm. From the curves in Figure 6.2 this is likely to occur