146 The impactsof climate change
sea level during the last warm interglacial period was a reduction in the
Antarctic or Greenland ice-sheets. But changes over shorter periods are
largely governed by other factors that combine to produce a significant
effect on the average sea level.
During the twentieth century observations show that the average sea
level rose by between 10 and 20 cm.^4 The largest contribution to this
rise (about one-third) is from thermal expansion of ocean water; as the
oceans warm the water expands and the sea level rises (see box below).
Other significant contributions come from the melting of glaciers and as
a result of long-term adjustments that are still occurring because of the
removal of the major ice-sheets 20 000 years or so ago. The contributions
from the ice caps of Greenland and Antarctica are believed to be small.
A further contribution to sea level change of uncertain magnitude arises
from changes in terrestrial storage of water, for instance from the growth
of reservoirs or irrigation.Thermal expansion of the oceans
A large component of sea level rise is due to thermal expansion of the
oceans. Calculation ofthe precise amount of expansionis complex be-
cause it depends critically on the water temperature. For cold water the
expansion for a given change of temperature is small. The maximum
density of sea water occurs at temperatures close to 0◦C; for a small
temperature rise at a temperature close to 0◦C, therefore, the expansion
is negligible. At 5◦C (a typical temperature at high latitudes), a rise of
1 ◦C causes an increase of water volume of about 1 part in 10 000 and
at 25◦C (typical of tropical latitudes) the same temperature rise of 1◦C
increases the volume by about 3 parts in 10 000. For instance, if the top
100 m of ocean (which is approximately the depth of what is called the
mixed layer) was at 25◦C, a rise to 26◦C would increase its depth by
about 3 cm.
A further complication is that not all the ocean changes temperature
at the same rate. The mixed layer fairly rapidly comes into equilibrium
with changes induced by changes in the atmosphere. The rest of the ocean
changes comparatively slowly (the whole of the top kilometre will, for
instance, take many decades to warm); some parts may not change at all.
Therefore, to calculate the sea level rise due to thermal expansion – its
global average and its regional variations – it is necessary to employ the
results of an ocean climate model, of the kind described in Chapter 5.Various contributions to the likely sea level rise in the twenty-first
century can be identified. Again the largest is from the thermal expansion
of ocean water. The other main contribution comes from the melting of