| Endangered Islands |
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“Many
low-lying coastal areas are going to disappear. In fifty years’ time
the geography of the Pacific region will be quite different to the way
it is today.” Prof Nunn
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The
western and central Pacific Ocean is dotted with 25,000 islands and
islets; many of which are eroded volcanic islands, coral reefs or
atolls with a maximum land elevation of 5m above mean sea level.
The 25 Pacific
countries have contributed just 0.06% to global greenhouse gas
emissions, but some of them could be simply wiped off the map
by the rising seas brought about by global warming. Yet now, changing
climate and sea levels are affecting their water supply, food
production, fisheries and coastlines.
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Just south of Funafuti Airport, sea water seepage flooding across
Funafuti Road in February 2006 - Source
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« Mean sea level » at the coast is
defined as the height of the sea with respect to a local land
benchmark, averaged over a period of time, such as a month or a year,
long enough that fluctuations caused by waves and tides are largely
removed.
Changes in mean sea level as measured
by coastal tide gauges are called “relative sea level changes”, because
they can come about either by movement of the land on which the tide
gauge is situated or by changes in the height of the adjacent sea
surface (both considered with respect to the centre of the Earth as a
fixed reference).
These two terms can have similar rates
(several mm/yr) on time-scales greater than decades. To detect sea
level changes arising from changes in the ocean, the movement of the
land needs to be subtracted from the records of tide gauges and
geological indicators of past sea level.
Widespread land movements are caused
by the isostatic adjustment resulting from the slow viscous response of
the mantle to the melting of large ice sheets and the addition of their
mass to the oceans since the end of the most recent glacial period
(“Ice Age”). Tectonic land movements, atoll decay, rapid displacements
(earthquakes) and slow movements (associated with mantle convection and
sediment transport) can also have an important effect on local relative
sea level.
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Why does sea level rise?
Sea level is affected by the combination of tidal, weather, climate and
oceanographic conditions as well as geodynamic processes. So, in
addition to long-term climate change, sea level is influenced by quasi
periodic large natural climate fluctuations or by short term extreme
events (tsunamis or tropical cyclones).
Global warming
Sea level change is an important consequence of climate change. The two
major causes of global sea level rise are thermal
expansion of the oceans (warm water occupies more volume
than cold water) and the increased melting of land ice,
glaciers and ice-shelves (only loss of land-based ice
contribute to sea level rise). If the Greenland ice
sheet was to melt, it would raise sea level by 7 m.
Melting of the West Antarctic Ice Sheet would raise sea level by 5 m.
But even just partial melting of these ice sheets will have a dramatic
effect on sea level rise.
Climate variability
Variations in sea level and climate are inextricably linked, and both
undergo interrelated seasonal, interannual and interdecadal
fluctuations. Quasi-periodic fluctuations associated with natural
phenomena such as El Niño or the Pacific Decadal Oscillation can be
large and cause significant impacts. Particularly noisy or low
frequency variations can conceal the underlying long-term trend over
several decades.
Extreme events
Extreme sea levels arise when reinforcing combinations of tides,
short-term weather effects, tsunamis or climate conditions occur.
Abnormally high sea levels can cause flooding, coastal erosion and
property damage. Abnormally low sea levels can be hazardous for
navigation and reduce under-keel clearances for shipping operations in
ports.
In recent years, El Niño events and extreme climatic events like severe
storms have been more frequent, more intense and lead to more severe
impacts.
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How does sea
level rise?
Satellite
altimeters and tide gauges are complementary datasets used to determine
sea level. Satellite altimetry provides measurements of absolute sea
level against which tide gauge measurements of relative sea level can
be compared.
Sea level
data collected since 1992 by satellite altimeters, namely
TOPEX/Poseidon and Jason-1, show that Global Mean Sea
Level (GMSL) has risen at a rate of
around 3.3 mm/year. This is more than 50% larger than
the average value over the 20th century.
However
global mean sea level change during this time has not been
geographically uniform. Increasing sea levels in the western Pacific
and falling sea levels in the eastern Pacific during this period are
related to decadal fluctuations. Sea level trends in the
southwest Pacific region are almost twice higher than
the global average (around 6 mm/year). The largest rates in the
southwest Pacific region are being observed along the South Pacific
Convergence Zone and along the equator.
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In the future
The
Intergovernmental Panel on Climate Change in 2007 predicted that global
warming would cause mean sea level to rise by 18 cm to 59 cm by 2100
(depending on a range of greenhouse-gas emission scenarios), with a
possible additional rise of 10 to 20 cm due to continued melting of
polar ice sheets. A recent study suggests a new projection of up
to 1.4 m in sea-level rise, caused by accelerating rates
of loss from ice sheets in Greenland and Antarctica on account of
higher global temperatures. |
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Impacts
The most serious impacts are caused not only by changes in mean sea
level but by changes to extreme sea levels, especially storm surges and
exceptionally high waves, which are forced by meteorological
conditions.
Worldwide
The International Institute for Environment and Development (IIED) has
analyzed the effect of a 10-meter rise in sea level. The study points
out that 634 million people live along coasts at or
below 10 m above sea level, in what is called the Low
Elevation Coastal Zone. This massive vulnerable group includes one
eighth of the world’s urban population. One of the countries most
vulnerable is China, with 144 million potential climate refugees. India
and Bangladesh are next, with 63 and 62 million respectively. Vietnam
has 43 million vulnerable people, and Indonesia, 42 million. Others in
the top 10 include Japan with 30 million, Egypt with 26 million, and
the United States with 23 million.
A 50-cm rise in sea level would lead to
the displacement of about 200 million people.
For low-lying
islands
Low-lying islands, especially those lacking a surrounding shallow
shelf, are particularly vulnerable to changes in sea level and storm
patterns.
Any rise in sea level increases the frequency of flooding and this
would be exacerbated by increased wave action. Saltwater intrusion is
adversely affecting drinking water and food production and coastal
erosion is arising. Higher temperatures also lead to more destructive
storms; higher surface water temperatures in the tropics and subtropics
mean more energy radiating into the atmosphere to drive storm systems. It is estimated that a sea level rise of
« only » 20-40 cm could make an island such as Tuvalu (which has a mean
elevation of 1.5 m above mean sea level and his highest point
protruding at 5 m) uninhabitable. If rising sea-level predictions come
true, it means that the entire 11,800-strong population would have to
be evacuated within the next 50 years. The ocean could swallow the
whole island, making Tuvalu the first country to be wiped off the map
by global warming (already at least two small atolls in Kiribati have
sunk beneath the Pacific).
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Sources
Alofa Tuvalu
Altimeters chart sea level - Aviso
(Archiving, Validation and Interpretation of Satellite Oceanographic
data), CLS, CNES
Climate Change and Low-lying Pacific Islands
by Philip Hall, Faerber Hall
Global Sea Levels, Past, Present and Future
- UNESCO Intergovernmental Oceanographic Commission
Melting Ice and Rising Seas - Earth Policy
Institute
Pacific Country Report, Sea Level &
Climate - Commonwealth of Australia 2009, Bureau of Meteorology
Pacific Islands Ocean Observing System
Petites îles grands enjeux - Le courrier de
l'UNESCO
Rising Sea Level Forcing Evacuation of
Island Country - Earth Policy Institute
Sea level rise - CSIRO Marine and
Atmospheric Research
Small Island developing states - UNEP
Vanishing Islands - UNEP
The Australian Baseline Sea Level Monitoring
Project
The IPCC Explains... Sea Level Rise
The South Pacific Sea Level &
Climate Monitoring Project
Links
Sea level rise of
one meter within 100 years - EO Portal
Tuvalu - Global
Education
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