Many observations show that the ocean has been changing over the last several decades. One aspect of these changing is the warming of ocean, resulting
in increase of ocean volume through thermal expansion. There has also been addition of water from glacier and ice sheets and changes in storage of water on
or in the land e.g. retention of water in man-made dams and extraction of water from aquifers. These together result in changes in sea level
2.4.2. Factors Contributing Sea Level Rise
Since the concern about human-induced global warming in the 1980s was emerged, sea-level rise and its impacts on the coastal areas have attracted
considerable concern. The large and growing concentration of people and assets in coastal areas imply to the height of potential impacts. It is estimated that at
least 600 million people live within 10m of sea level in 2010 , and these
populations are growing more rapidly than global trends. Populated deltaic areas and many coastal cities are highly threatened by small rises in sea level. While in
global terms relatively small in number, the very existence of small-island nation states makes them vulnerable to rises in sea level of the order of 1m Nisholls et.
al., 2011. The recent report of the Intergovernmental Panel on Climate Change IPCC
estimates that in response to rising temperatures 1.0°-3.5°C higher than 1990 levels in 2100, sea level will have risen from 15 to 95 cm by the year 2100
. IPCC AR4 Report IPCC, 2007 identifies several major factors that currently
contribute to sea level rise. These are:
1. Ocean Thermal Expansion
Instrumental records shows that the world’s oceans have warmed since 1955, accounting over this period for more than 80 of the changes in the energy
content of the Earth’s climate system. Further records also shows that during the period of 1961 to 2003, ocean layer between 0 to 3000 m. has absorbed average
heating rate of 0.2 Wattsm2 per unit area of the Earth’s surface. During 1993 to 2003, the corresponding rate of warming in the 0 to 700 m ocean layer was
higher, about 0.5 ± 0.18 Wm2. Therefore, between 1961 to 2003, the period 1993 to 2003 had much higher rates of warming, especially in the upper 700 m
of the global ocean.
2. Change in Glaciers and Icecaps
During the 20th century, glaciers and ice caps have experienced huge mass losses. These losses excluding those around the ice sheets of Greenland and
Antarctica are estimated to have contributed 0.50 ± 0.18 mmyr in sea level equivalent SLE between 1961 and 2003, and 0.77 ± 0.22 mmyr between 1991
and 2003.
3. Glacial melt from the Greenland and Antarctica ice sheets GIS
Whether the Greenland and Antarctic ice sheets had been growing or shrinking over time scales of longer than a decade is not well established from
observations. Lack of agreement standard between techniques and the small number of estimates preclude assignment of best estimates or statistically tight
error bounds for changes in ice sheet mass balances. However, acceleration of outlet glaciers draining from the interior has been observed in both the
Greenland and Antarctic ice sheets. According to the IPCC AR4 IPCC, 2007, it is very likely 90
probability that the Greenland Ice Sheet shrunk from 1993 to 2003. An assessment of the data suggests a mass balance for the Greenland Ice Sheet of –
50 to –100 Gigatonsyear a shrinkage contributing to rising global sea levels of 0.14 to 0.28 mmyr from 1993 to 2003, with even larger losses in 2005. The
estimated range in mass balance for the GIS from 1961 to 2003 is between a growth of 25 Gtyr and shrinkage of 60 Gtyr or –0.07 to +0.17 mmyr SLE.
There are even greater uncertainties for Antarctica Ice Sheet AIS. Again according to the IPCC AR4 assessment of all the data yields an estimate for the
overall AIS mass balance ranging from growth of 100 Gtyr to shrinkage of 200 Gtyr or –0.27 to +0.56 mmyr of SLE from 1961 to 2003, and from +50 to –
200 Gtyr or –0.14 to +0.55 mmyr of SLE from 1993 to 2003.
4. Smaller contribution from snow on land and permafrost
Snow cover has decreased in most regions, especially in spring. Satellite observations of the Northern Hemisphere snow cover from 1966 to 2005 show a
decrease in every month except in November and December, with a stepwise drop of 5 in the annual mean in the late 1980s. In the Southern Hemisphere,
the few long records or proxies mostly show either decreases or no changes in
