66 Climates ofthe past
in April 1815, which was followed in many places by two exception-
ally cold years: 1816 was described in New England and Canada as the
‘year without a summer’. Although the effect on the climate even of
an eruption of the magnitude of Tambora only lasts a few years, varia-
tions inaveragevolcanic activity have a longer term effect. Regarding
the Sun’s output, although accurate direct measurements are not avail-
able (apart from those made during the last two decades from satellite
instruments), other evidence suggests that the solar output could have
varied significantly in the past. For instance, compared with its value
today it may have been somewhat lower (by a few tenths of a watt per
square metre) during the Maunder Minimum in the seventeenth century
(a period when almost no sunspots were recorded; see also box on p. 138).
There is no need, however, to invoke volcanoes or variations in solar
output as the cause of all the climate variations over this period. As
with the shorter-term changes mentioned earlier, such variations of cli-
mate can arise naturally from internal variations within the atmosphere
and the ocean and in the two-way relationship – coupling – between
them.
The millennial record of Figure 4.3 is particularly important be-
cause it provides an indication of the range and character of climate
variability that arises from natural causes. As we shallsee in the next
chapter, climate models also provide some information on natural cli-
mate variability. Careful assessments of these observational and model
results confirm that natural variability (the combination of internal vari-
ability and naturally forced, e.g. by volcanoes or change in solar output)
is unlikely to explain the warming in the latter half of the twentieth
century.^5The past million years
To go back before recorded human history, scientists have to rely on
indirect methods to unravel much of the story of the past climate. A
particularly valuable information source is the record stored in the ice that
caps Greenland and the Antarctic continent. These ice caps are several
thousands of metres thick. Snow deposited on their surface gradually
becomes compacted as further snow falls, becoming solid ice. The ice
moves steadily downwards, eventually flowing outwards at the bottom of
the ice-sheet. Ice near the top of the layer will have been deposited fairly
recently; ice near the bottom will have fallen on the surface many tens
or hundreds of thousands of years ago. Analysis of the ice at different
levels can, therefore, provide information about the conditions prevailing
at different times in the past.