The last thousandyears 651000 1200 1400 1600 1800 2000
Year−1.0−0.50.00.51.0relative to 1961 to 1990
Northern Hemisphere anomaly (°C)1998 instrumental valueInstrumental data (AD 1902 to 1999)
Reconstruction (AD 1000 to 1980)
Reconstruction (40 year smoothed)Figure 4.3Millennial Northern Hemisphere temperature reconstruction from
tree rings, corals, ice cores and historical records (for the period 1000–1980) and
from instrumental data (for 1902–1999). A smoothed version of the proxy
record is also shown, as are the 95% confidence limits (grey shaded).
period the ‘Little Ice Age’ associated with the fifteenth to nineteenth
centuries. These only affected part of the Northern Hemisphere and
are therefore more prominent in local records, for instance those from
central England. The increase in temperature over the twentieth century
is particularly striking. The 1990s are likely to have been the warmest
decade of the millennium in the Northern Hemisphere and 1998 is likely
to have been the warmest year.
Although there is as yet no certain explanation for the variations that
occurred between 1000 and 1900, it is clear that greenhouse gases such
as carbon dioxide and methane cannot have been the cause ofchange.
For the millennium before 1800 their concentration in the atmosphere
was rather stable, the carbon dioxide concentration, for instance, vary-
ing by less than three per cent. However, the combined influences of
variations in volcanic activity and variations in the output of energy
from the Sun can provide some part of the explanation.^4 The effect of
individual volcanic eruptions can be very noticeable. For instance, one of
the largest eruptions during the period was that of Tambora in Indonesia