SEA LEVEL RISE AND GLOBAL
WARMING
As with many aspects of global warming, there
is debate over its effects in connection with rising sea levels.
Sea levels are definitely rising and global warming is at least
part of the cause. However the level of future rises is under debate,
as is the degree of threat faced by low-lying islands and coastal
areas. Relatively small rises in sea level would make some densely
settled coastal plains uninhabitable and create a significant refugee
problem. If the sea level were to rise in excess of 4 meters almost
every coastal city in the world would be severely affected, with
the potential for major damage to world-wide trade and economy.
Future sea level rise
Presently, the IPCC predicts sea level rise is
most probable to be just short of half a meter, and at least between
9 and 88 cm through 2100 - but they also warn that global warming
during that time may lead to irreversible changes in the Earth’s
glacial system and ultimately melt enough ice to raise sea level
many meters over the next millennia.
It is estimated that around 200 million people
could be affected by sea level rise, especially in Vietnam, Bangladesh,
China, India, Thailand, Philippines, Indonesia, Nigeria and Egypt.
London and New Orleans already need storm-surge defenses, and would
need more if sea level rose, though they also face issues such as
sinking land.
Future sea level rise, like the recent rise, is
not expected to be globally uniform. Some regions show a sea level
rise substantially more than the global average (in many cases of
more than twice the average), and others a sea level fall. However,
models disagree as to the likely pattern of sea level change.
Environmental refugees due to global warming?
An example of the ambiguity of the concept of
environmental refugees is the emigration from the island nation
of Tuvalu, which has an average elevation of approximately one meter
above sea level. Tuvalu already has an ad hoc agreement with New
Zealand to allow phased relocation and many residents have been
leaving the islands. However, it is far from clear that rising sea
levels from global warming are a substantial factor - best estimates
are that sea level has been rising there at approximately 1–2 millimeters
per year, but that shorter timescale factors such as tides have
far larger temporary effects.
Short-term causes of rising sea levels
There are many factors which can produce short-term
(a few minutes to 14 months) changes in sea level. Some of these
include tides, variations in the Earth’s rotation, winds, seasons,
floods, earthquakes and tsunamis.
Long-term causes of rising sea levels
Long-term changes are mainly caused by temperature
(because the volume of water depends on temperature), and the mass
of water locked up on land and sea as fresh water in rivers, lakes,
glaciers, polar ice caps, and sea ice. Over much longer timescales,
changes in the shape of the ocean basins and in land/sea distribution
will affect sea level.
Ice Shelves float on the surface of the sea and, if they melt, to
first order they do not change sea level. Likewise, the melting
of the northern polar ice cap which is composed of floating pack
ice would not significantly contribute to rising sea levels. Because
they are fresh, however, their melting would cause a very small
increase in sea levels, so small that it is generally neglected.
It can however be argued that if ice shelves melt it is a precursor
to the melting of ice sheets on Greenland and Antarctica.
If all glaciers and ice caps melt, the projected rise in sea level
will be around 0.5 m. If the melting includes the Greenland and
Antarctic ice sheets (both of which contain ice above sea level),
then the rise is a more drastic 68.8 m. The collapse of the grounded
interior reservoir of the West Antarctic ice sheet would raise sea
level by 5-6 m.
Debate over effects on low lying islands
To date, sea level changes have not been implicated
in any substantial environmental, humanitarian, or economic losses.
Previous claims have been made that parts of the island nations
of Tuvalu were “sinking” as a result of sea level rise. However,
subsequent reviews have suggested that the loss of land area was
the result of erosion during and following the actions of 1997 cyclones
Gavin, Hina, and Keli. The islands in questions were not populated.
Reuters has reported other Pacific islands are
facing a severe risk including Tegua island in Vanuatu, Fiji. There
are claims that Vanuatu data shows no net sea level rise. These
claims are not substantiated by tide gauge data and are reminiscent
of claims made in Michael Crichton’s State of Fear that there is
no threat to this island chain. Vanuatu tide gauge data show a net
rise of ~50 mm from 1994-2004. Linear regression of this short time
series suggests a rate of rise of ~7 mm per year, though there is
considerable variability and the exact threat to the islands is
difficult to assess using such a short time series.
According to Patrick J. Michaels, “In fact, areas to the west such
as [the island of] Tuvalu show substantial declines in sea level
over that period.” Despite President Gayoom speaking in the past
about the impending dangers to his country, the Maldives, research
found that the people of the Maldives have in the past survived
a higher sea level about 50-60 cm and there is evidence of a significant
sea level fall in the last 30 years in that Indian Ocean area (20-30
cm).
A much overlooked fact about coral islands is
that they exist above sea level today only because sea level was
once high enough that these currently dry areas were underwater.
Corals and other reef-building organisms cannot survive prolonged
exposure to air, so the corals from which the islands are formed
could have grown only during interglacial periods when sea level
was higher than today (e.g. 120,000 years ago).
Changes in sea level over history
The sea level has risen more than 120 meters since
the peak of the last ice age about 18,000 years ago. The bulk of
that occurred before 6,000 years ago. From 3,000 years ago to the
start of the 19th century sea level was almost constant, rising
at 0.1 to 0.2 mm/yr. Since 1900 the level has risen at 1 to 3 mm/yr.
Since 1992 satellite altimetry indicates a rate of about 3 mm/yr.
This change may be the first sign of the effect of global warming
on sea level. Global warming is predicted to cause significant rises
in sea level over the course of the twenty-first century.
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