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Sea-level rise and vulnerable coastal populations
John A. Church, CSIRO Marine and Atmospheric Research, Antarctic Climate and Ecosystems
Cooperative Research Centre; Thorkild Aarup, Intergovernmental Oceanographic Commission, UNESCO;
W. Stanley Wilson, US National Oceanic and Atmospheric Administration and
Philip L. Woodworth, Permanent Service for Mean Sea Level, Proudman Oceanographic Laboratory
T
he coastal zone changed profoundly during the 20th
century, primarily due to growing populations and increas-
ing urbanization. In 1990, 23 per cent of the world’s
population (1.2 billion people) lived both within a 100 km
distance and 100-metre elevation of the coast at densities about
three times higher than the global average. By 2010, 20 out of 30
megacities will be on the coast, with many low-lying locations
threatened by sea-level rise. With coastal development continu-
ing at a rapid pace, society is becoming increasingly vulnerable
to sea-level rise and variability – as Hurricane Katrina recently
demonstrated in New Orleans. For example, in Europe the storm
surge of 1953 had a major impact with the loss of over 1,800 lives
in the Netherlands and 300 deaths in southeast England. In the
Bay of Bengal, there have been 23 surge events since 1737, with
over 10,000 people killed in each. The most severe impacts were
felt in 1737 (300,000 people killed), 1864 (100,000 people killed),
1876 (100,000 people killed), 1897 (175,000 people killed), 1970
(300,000 people killed) and 1991 (about 140,000 killed and ten
million made homeless).
Rising sea levels are felt most acutely through the increased frequency
and intensity of extreme storm surges and flooding, even if hurricane
intensities do not increase in response to the warming of the oceans.
For example, a 100-year coastal flooding event could become a ten-
year event at some locations before the end of the 21st century.
Increases in the frequency of extreme sea levels of a given height have
already been observed in a number of locations around the world,
particularly in the Pacific Ocean and along the east coast of North
America. Data from Australia’s east and west coasts indicates that high
sea levels of a given value occurred about three times as often in the
latter half of the 20th century compared with the first half. Many
coastal megacities are built on deltaic regions where significant sinking
is occurring, making these cities particularly vulnerable to these
extreme events. Unless such change is taken into account, design crite-
ria for existing coastal structures can become out-of-date and lead to
catastrophic flooding such as experienced in New Orleans with
Hurricane Katrina. Moreover, the possibility that severe weather events
may become more frequent and/or intense with our changing climate
can only make matters worse.
Rising sea levels will also contribute to the erosion of the world’s
sandy beaches, 70 per cent of which have been retreating over the past
century, with less than ten per cent prograding. Sandy-beach erosion
commonly occurs at tens to hundreds of times the rate of sea-level
rise and will degrade or remove protective coastal features such as
sand dunes and vegetation, further increasing the risk of
coastal flooding. Low-lying islands are particularly vulner-
able to sea-level rise.
An improved understanding of sea-level rise and vari-
ability will help reduce the uncertainties associated with
sea-level rise projections, thus contributing to more effec-
tive coastal planning and management. Adaptation
measures, including enhanced building codes, restrictions
on where to build, and developing infrastructures better
able to cope with flooding, should help to minimize the
potential losses.
Rising sea levels
Since the beginning of high-accuracy satellite altimetry
in the early 1990s, global mean sea level has been rising
at a rate of about 3.2 mm/year, compared to a rate of 1.7
mm/year over the previous century. About a third to a half
of the sea-level rise during the first decade of the altime-
ter record can be attributed to thermal expansion due to
a warming of the oceans; the other major contributions
include the combined effects of melting glaciers and ice
sheets. Changes in the storage of water on land (such as
the depletion of aquifers and increases in dams and reser-
voirs) remain very uncertain.
The Intergovernmental Panel on Climate Change
(IPCC) provides the most authoritative information on
projected sea-level change. The most recent IPCC report
projected a sea-level rise of 18 cm to 59 cm by 2095, plus
an additional 10-20 cm to allow for a potential response
of the ice sheets to global warming, making a total range
of 18 to 79 cm by 2095. The report also stated that “larger
values cannot be excluded, but understanding of these
effects is too limited to assess their likelihood or provide
a best estimate or an upper bound for sea-level rise.”
There is increasing concern about the stability of ice
sheets. For Greenland, this concern is based on measure-
ments indicating an increasing contribution from the ice
sheet, and melt water possibly finding its way to the base
of the ice sheet, facilitating rapid sliding of glaciers and
thus contributing to a more rapid sea-level rise. Much of
the West Antarctic Ice Sheet is grounded below sea level,
and the penetration of warmer water beneath the ice
shelves to the base of the ice sheet and the subsequent
dynamic response could also lead to a more rapid rate of
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