Geoscience Reference
In-Depth Information
Relative global sea level
The IPCC has also compiled all the current data on global sea level. It shows that between
1901 and 2010, the global sea level has risen by about 17 to 21 centimetres (cm) (see
Fig-
ure 9
). Sea-level change is difficult to measure, as relative sea-level changes have been de-
rived from two very different data sets—tide-gauges and satellites. In the conventional tide-
gauge system, the sea level is measured relative to a land-based tide-gauge benchmark. The
major problem is that the land surface is much more dynamic that one would expect, with a
lot of vertical movements, and that these become incorporated into the measurements. Ver-
tical movements can occur as a result of normal geological compaction of delta sediments,
the withdrawal of groundwater from coastal aquifers, uplift associated with colliding tec-
tonic plates (the most extreme of which is mountain-building such as in the Himalayas), or
ongoing postglacial rebound and compensation elsewhere associated with the end of the
last ice age. The latter is caused by the rapid removal of weight when the giant ice sheets
melted, so that the land that has been weighed down slowly rebounds back to its original
position. An example of this is Scotland, which is rising at a rate of 3 millimetres (mm) per
year, while England is still sinking at a rate of 2 mm per year, despite the Scottish ice sheet
having melted 10,000 years ago. In comparison, the simple problem with the satellite data
is that it is too short, with the best data starting in January 1993. This means it has to be
combined with the tide-gauge data to look at long-term trends. However, the 1993 to 2010
data clearly show more than a 60 mm rise in global sea level.
