Chapter 4
Climates of the past
To obtainsome perspective against which to view future climate
change, it is helpful to look at some of the climate changes that have
occurred in the past. This chapter will briefly consider climatic records
and climate changes in three periods: the last hundred years, then the
last thousand years and finally the last million years. At the end of the
chapter some interesting recent evidence for the existence of relatively
rapid climate change at various times during the past one or two hundred
thousand yearswill be presented.
The last hundred years
The 1980s and early 1990s have brought unusually warm years for the
globe asa whole (see Chapter 1) as is illustrated in Figure 4.1, which
shows the global average temperature since 1860, the period for which
the instrumental record is available with good accuracy and coverage.
An increaseover this period has taken place of about 0.6◦C (ninety-five
per cent confidence limits of 0.4 to 0.8◦C). The year 1998 is very likely^1
to have been the warmest year during this period. An even more striking
statistic is that each of the first eight months of 1998 was very likely the
warmest of those months in the record. Although there is a distinct trend
in the record, the increase is by no means a uniform one. In fact, some
periods of cooling as well as warming have occurred and an obvious
feature of the record is the degree of variability from year to year and
from decade to decade.
A sceptic may wonder how a diagram like Figure 4.1 can be prepared
and whether any reliance can be placed upon it. After all, temperature
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