figure 6. Observations show that
the global average sea level has
risen by about 20 cm (8 inches)
since the late 19th century. Sea level
is rising faster in recent decades;
measurements from tide gauges
(blue) and satellites (red) indicate
that the best estimate for the
average sea level rise over the last
two decades is centred on 3.2 mm
per year (0.12 inches per year). The
shaded area represents the sea level
uncertainty, which has decreased as
the number of gauge sites used in
the global averages and the number
of data points have increased.
Source: Shum and Kuo (2011) | Long-term measurements of tide gauges and recent satellite data show that global sea
level is rising, with best estimates of the global-average rise over the last two decades
centred on 3.2 mm per year (0.12 inches per year). The overall observed rise since 1901 is
about 20 cm (8 inches) This sea-level rise has been driven by (in order of importance): expansion of water volume as the ocean
warms, melting of mountain glaciers in most regions of the world, and losses from the Greenland and
Antarctic ice sheets. All of these result from a warming climate. Fluctuations in sea level also occur due to
changes in the amounts of water stored on land. The amount of sea level change experienced at any given
location also depends on a variety of other factors, including whether regional geological processes and
rebound of the land weighted down by previous ice sheets are causing the land itself to rise or sink, and
whether changes in winds and currents are piling ocean water against some coasts or moving water away.
The effects of rising sea level are felt most acutely in the increased frequency and intensity of occasional
storm surges. If CO2 and other greenhouse gases continue to increase on their current trajectories, it is
projected that sea level may rise by a further 0.5 to 1 m (1.5 to 3 feet) by 2100. But rising sea levels will
not stop in 2100; sea levels will be much higher in the following centuries as the sea continues to take up
heat and glaciers continue to retreat. It remains difficult to predict the details of how the Greenland and
Antarctic Ice Sheets will respond to continued warming, but it is thought that Greenland and perhaps West
Antarctica will continue to lose mass, whereas the colder parts of Antarctica could start to gain mass as
they receive more snowfall from warmer air that contains more moisture. Sea level in the last interglacial
(warm) period around 125,000 years ago peaked at probably 5 to 10 m above the present level. During this
period, the polar regions were warmer than they are today. This suggests that, over millennia, long periods
of increased warmth will lead to very significant loss of parts of the Greenland and Antarctic Ice Sheets and
to consequent sea level rise. |