Earth Sciences / 71Although we usually think of the Pleistocene as the ‘Great Ice Age’, this is misleading. There was
not a single ice age (or glaciation; the terms are synonymous) but several, and in the interglacials
separating them climates were sometimes markedly warmer than those of today. During the Ipswichian
(or Trafalgar Square) Interglacial, for example, the average summer temperature in southern Britain
was some 2–3°C warmer than today and remains of elephants, hippopotami, and rhinoceros, dated to
that time (100000–70000 years BP), have been found in what is now central London (hence the
name ‘Trafalgar Square Interglacial’). Nor is the Pleistocene the only geological period during which
glaciations are known to have occurred. In various parts of the world there may have been glaciations
some 2.3 billion years ago and between 950 and 615 million years ago, during Precambrian time, at
the end of the Ordovician Period about 440 million years ago, and in the southern hemisphere around
286 million years ago, at the end of the Carboniferous and beginning of the Permian Periods. It does
seem, however, that the Earth enjoyed an ice-free period between the Permo-Carboniferous glaciation
and the onset of the Pleistocene.
During the Pleistocene, there are now believed to have been four glacial episodes and three interglacials
in North America and five glacial and four interglacial episodes in Europe. The present epoch, the
Holocene, is also known as the Flandrian Interglacial. This makes it the fifth European interglacial,
and acceptance of the name implies that the Pleistocene glaciations have not yet ended; we are still
living in the Pleistocene and one day the glaciers will start to grow again. If and when this happens
it may do so rapidly, for glaciations can begin quickly and end even more quickly.
Even during glaciations there are periods of remission. Briefer and cooler than interglacials, these
are known as ‘interstades’ (or ‘interstadials’) and are identified by the presence of pollen from plants
known to require mild climates. During interglacials, too, temperatures fluctuate. Figure 2.25 shows
how average temperatures have varied from 18000 years ago, when the most recent glaciation was at
its most severe, to the present day. During the climatic optimum, about 5500 years ago, mid-latitude
temperatures were about 2.5°C warmer than those of today; this was the period during which
civilizations flourished in Asia Minor. A smaller optimum, peaking around AD 1000, allowed the
Vikings to colonize Greenland. Between about 1450 and 1880 temperatures fell during the ‘Little
Ice Age’. This is when fairs were held on the frozen Thames and it may be a minimum from which
our climate is still warming. As the graph indicates, the weather has been markedly warmer than it is
today during several episodes in the past.
Confusingly, glaciations and interglacials have different names in different places and it can be
difficult to match them (ALLABY, 1992a, pp. 145–148). The most recent glaciation is known
as the Devensian in Britain, the Weichselian in northern Europe, the Würm in the Alps, and
Figure 2.25 Temperature changes since the last glacial maximum (present temperature=0)