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OCR for page 90
8
Coastlines and Rising Seas
Rising sea level may be one of the most widely felt and eas-
ily recognized consequences of a warmer global climate, but the
inundation it would bring is already familiar to the millions of
people who live and work in the worlcl's coastal areas. Over the
centuries, engineers have struggled to protect low-lying coasts
from advancing waters. Their techniques range from straight-
forward measures such as pumping sand onto eroded beaches,
building levees, and relocating ports, to complex, multibillion
dollar systems of carefully maintained dikes and sand dunes,
such as the one that protects more than half of The Netherlands
from inundation by the North Sea. if the earth's surface tem-
perature grows warmer, as many researchers predict, the task
of keeping the sea at bay will become more difficult, and more
urgent.
Scientists expect global sea level to rise for two reasons.
First, as greenhouse gases accumulate in the atmosphere and
eventually raise the earth's surface temperature, glaciers and
land ice around the world will melt more rapidly, releasing
water that will raise average sea level. Second, as the ocean
90
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COASTLINES AND RISING SEAS
9
absorbs additional heat from the air above it, the water will
expand.
Like climate, sea levels have fluctuated throughout the
earth's history. Eighteen thousand years ago during the last
ice age, mean sea level was 100 to 150 meters (about 330 to
500 feet) lower than it is now. About 100,000 years ago during
a period when the global average temperature was I° to 2°C
(about 2° to 3.5°F) warmer than today and the glaciers rececled,
sea level was about 5 to 7 meters higher. Even during the past
6000 years, sea level in some places has varied by a meter or
more.
Currently, sea level is rising. Scientists estimate that in this
century, sea level has risen by about 0.15 meter or more. Though
it is tempting to correlate this rise with the coincident buildup
in atmospheric carbon dioxide, a cause-and-effect relationship
has not been established. The reason for the rise is poorly
understood and exceeclingly difficult to determine. Mark Meter,
a glaciologist at the University of Colorado in Boulder, cautions
that any projections for future sea level rise must be "tempered
by the uncertainty of our understanding of the present."
Estimates made in the last decade projected that we could
expect global average sea level to rise from about 0.5 to 1.5
meters, with some estimates as high as 3 meters, in response
to a 3° to 5°C warming that some estimate would be expected
by 2050 with an effective doubling of carbon dioxide. Recent
estimates have revised this figure downward to about a third of
a meter, primarily because of new calculations on how snowfall
will increase on the polar ice sheets (warmer air can transmit
more moisture, resulting in more snowfall) and thus bind up
the water that would otherwise go into the oceans and on how
long it will take meltwater to escape from subfreezing glaciers.
Scientists are still working to resolve the many uncertainties
surrounding these projections.
The earlier high estimates considered the possibility that the
unstable West Antarctic Ice Sheet might disintegrate, releasing
about 2 million cubic kilometers of ice into the ocean. Scientists
increasingly believe that such a collapse, if it occurs, would be
slow and that little effect would be seen during the coming cen
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THE FACES OF GLOBAL ENVIRONMENTAL CHANGE
fury. On the other hand, as concentrations of greenhouse gases
build up past the effective carbon dioxide doubling benchmark,
researchers believe that global surface temperature will continue
to increase. As it does, Meter explains, "Our feeling is that sea
level rise will continue. In fact, it will accelerate. The problem
is that we don't yet know how to estimate very well what is
going to happen."
About some processes and conditions related to sea level
rise, scientists are confident. Worldwide average sea level de-
pends simply on the volume of water resting in the ocean basins.
Locally, at a particular coast or island, sea level is influenced by
the characteristics of the locale, including its geology and the
way humans manage the land. For this reason, global sea level
may rise a little in some places and a lot in others. In some
places it may even drop. The human and economic costs also
would vary from nation to nation.
Some parts of the world, such as Canada, Scandinavia, and
Scotland, are rising, springing up in response to the removal of
glacial ice that began with the end of the last ice age. In such
places rates of uplift exceed past rates of sea level rise, so that the
sea level appears to be falling. In other areas, such as Bangkok
and most of the U.S. Atlantic coast, sea level appears to be
rising more rapidly than the global, or eustatic, change. In fact,
the land itself is subsiding in response to downwarping of the
earth's crust from slow geologic processes, sudden earthquakes,
or compaction of soft underlying sediments such as mud or
peat. Human activities such as pumping of groundwater, oil,
and gas from below the land surface can be major factors in
subsidence. Because sea level in any one location is subject to
these many factors, it is measured in relative terms, namely the
difference between the eustatic change and any local change in
land elevation.
THE HUMAN COSTS OF RISING SEA LEVEL
As many as one billion people, or 20 percent of the worId's
population, live on lands likely to be inundated or dramatically
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COASTLINES AND RISING SEAS
93
changed by rising waters. Many buildings are at elevations
low enough that even at current sea levels, the safety of people
during storms cannot be assumed. On the basis of current pre-
dictions of sea level rise, a 1989 Resources for the Future report
explains that all land currently up to 5 meters (about 16.5 feet)
above mean sea level is potentially vulnerable in the next one
to three centuries to upstream effects of flooding, particularly
storm surges walls of water pushed by heavy winds and to
intrusion of saltwater into groundwater and estuaries.
The effects of sea level rise will vary with conditions along
the coastlines. Sandy areas and unconsolidated cliffs of sedi-
mentary material will erode quickly, whereas cliffs of resistant
rock will not. Steep-sloped areas will be affected by each in-
crement of rising water but less so than gently sloping coastal
terrain, which would be flooded by the same sea level rise. An
extreme example is the Republic of Maldives, a nation of Il90
small islands, most of which are barely higher than 2 meters in
elevation. If mean sea level rises 2 meters, the entire country
would be submerged. Even if mean sea level rises ~ meter, a
storm surge would be, as Maldives president Maumoon Abdul
Gayoom told the U.N. General Assembly in 1987, "catastrophic
and possibly fatal to the nation."
Some of the most visible effects of predicted changes in
climate will be along the ocean coastlines, where rising seas
would flood wetlands and lowlands and hasten coastal erosion.
Water tables near the coast would rise, and saltwater would in-
trude into rivers, bays, and the underground aquifers that sup-
ply water for drinking, industry, and irrigation. In developed
areas, buildings, roads, and sewage systems would be threat-
ened. Gjerrit P. Hekstra, of the Ministry of Housing, Physical
Planning, and Environment in The Netherlands, reports that
if maximum storm surges and the upstream effects of flood-
ing are taken into account, about 3 percent of the land area
worldwide would be subject to inundation or made vulnerable
by intrusion of saltwater. This area includes one third of the
world's cropland because productive soils commonly occur in
delta areas. Estuaries, where freshwater rivers flow into the sea
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THE FACES OF GLOBAL ENVIRONMENTAL CHANGE
7200
7100
7000
6900
-
J 1 900
>
CO
LU
7300
7200
7100
7000
6900
6800
6700
BALTIMORE
1920 1940 1960 1980
YEAR
- STOCKHOLM
' ~ Vet
1 1 1 1
1910 1930 1950
YEAR
1970 1 990
(a)
(b)
Tide-gauge records from the early twentieth century indicating (a) rising local relative
sea level at Baltimore on the North American East Coast and (b) falling local relative
sea level at Stockholm in Scandinavia, as the land rises in "rebound" from the
weight of receding glaciers. (Courtesy D. G. Aubrey and A. R. Solow, Woods Hole
Oceanographic Institution.)
and mix with the incoming tides, and groundwater aquifers are
most vulnerable to intrusion. According to lames Titus, of the
EPA, increased salinity already is considered a cause of reduced
oyster harvests in the Chesapeake Bay and has hastened the
conversion of cypress swamps in Louisiana to open water.
Perhaps no other impact of climate change would displace
more people from their homes than rising seas, swelling a grow-
ing class of "environmental refugees." Not surprisingly, most
of these refugees would be in developing countries, where scarce
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COASTLINES AND RISING SEAS
95
economic resources cannot accommodate the costly remedies
required to keep the sea at bay.
The system of wetlands that grace many stretches of the
worIct's coastlines may be a prominent victim of sea level rise.
Coastal wetlands inclucle marshes, swamps, and salt-tolerant
mangrove swamps in the tropics. Wetlands are nurseries for
many terrestrial animals and commercial fish species, and the
ecosystems they support are among the most productive on
earth. They help maintain water quality and protect the shore-
lines from erosion and the full brunt of coastal storms.
For centuries people have drained and filled wetlands, con-
verting them to cropland or urban areas. Where navigational
channels transect wetlands, or where flood control structures
have been installed, wetlands have been converted to open wa-
ter. In the 1960s, however, environmental awareness began to
increase, and many governments have taken steps to curtail
wetlands destruction. Titus explains that in the United States
these restrictions have substantially reduced the conversion of
coastal wetlands to drylands but that the conversion of wet-
lands to open water continues. Louisiana, for example, loses 50
square miles per year to open water.
How the wetlands respond to sea level rise will depend in
part on the degree to which they have been developed. To pro-
tect their property from the sea, people in many areas have built
bulkheads just above the wetlands. Many coastal ecologists fear
that these obstacles squeeze the wetlands against the advancing
sea.
In the absence of human activity, coastal ecosystems are pre-
served by a highly effective mechanism that helps compensate
for fluctuating sea levels. The rising waters flood areas that are
now dry land, creating new wetlands, which can grow upward
as sediment and organic material accumulate. Where coastlines
are heavily populated and developed, this mechanism may have
only limited potential to prevent a major loss of wetlands in the
coming century.
Titus cautions that the area of wetlands today is far greater
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THE FACES OF GLOBAL ENVIRONMENTAL CHANGE
than the area that would be available for new wetlands as sea
level rises. In its report to Congress on the Potential Effects of
Global Climate Change on the United States, the EPA reports that
a 1-meter sea level rise by 2100 could drown 25 to 80 percent of
U.S. coastal wetlands. Their survival depends on whether they
can migrate inland, or if levees and bulkheads built to protect
coastal properties block their path.
Nor is the future bright for the string of barrier islands that
adorns the eastern United States and Gulf coasts. Barrier islands
such as Hatteras Island, North Carolina, and Long Beach Island,
New Jersey, are long, narrow islands and peninsulas only a few
feet above sea level in elevation, with one side facing the ocean
and the other side facing a bay that separates the island from the
mainland. Left on their own, barrier islands keep pace with sea
level rise by migrating, rolling landward as storms wash sand
from the oceanside beach to the bayside marsh. Even with-
out the obstacles presented by human-built structures, barrier
islands may disintegrate as sea level rise speeds up. Coastal
Louisiana, for instance, is subsiding so rapidly that relative sea
level is rising by ~ centimeter (about 0.4 inch) per year, and its
barrier islands are breaking up. (This already serious situation
will worsen if the rate of sea level rise accelerates in the next
century.)
While rising waters may inundate the baysides of the barrier
islands, the oceansides of the islands ant! the mainland coastal
beaches are subject to scouring wave action and are particularly
at risk for runaway erosion. Surveys indicate that during the
past century, less than 10 percent of the length of the worId's
sandy shorelines advanced seaward, while more than 60 percent
retreated. In the United States, a 1-foot rise in sea level could
cause beaches from the Northeast to the mid-AtIantic to erode
50 to 100 feet. Along the Carolinas, the sea would advance 200
feet; In Florida, where shoreline protection already costs millions
yearly, the shoreline would retreat from 100 to 1000 feet; and in
Louisiana, several miles. EPA notes that at high tide, most U.S.
recreational beaches are less than 100 feet wide.
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COASTLINES AND RISING SEAS
97
THE PLIGHT OF THE DELTAS
A rise in global sea level would be most deeply felt in river
deltas, the wetlands that form when a river carries more sedi-
ment into a body of water than can be carried away by currents
and waves. This sediment builds up to form the delta, which
acts as a barrier between the lanct and the sea. Under natural
conditions, deltas form and break down continuously as they
accrete and subside. But when humans intervene by damming,
diverting, or channeling rivers, the amount of sediment reach-
ing the delta is reduced. The delta is thrown out of equilibrium,
with the accumulation of sediment no longer offsetting the sub-
sidence.
The Mississippi River delta in Louisiana is the most vul-
nerable area to relative sea level rise in the continental United
States and provides a ready example of the changes uncler way
in many of the world's deltas. The fine-grained sediment car-
riect by the river to the sea tends to compact and subside in the
manner typical of deltas. Normally this effect is counteracted
by annual flooding, cluring which the Mississippi overflows its
banks and supplies sediment to the wetlands. But measures
to trap sediment upstream in order to reduce dredging costs
at the mouth of the river, and levees built for flood control,
prevent sediment, fresh water, and nutrients from reaching the
wetlands. Titus estimates that if current management practices
continue and sea level rises as projected, most of Louisiana's
wetlands will be destroyed in the next century.
Low-lying countries in the developing world such as Egypt
and Bangladesh, where rivers are large and the deltas extensive
and densely populated, will be hardest hit by sea level rise.
Where the rivers are dammed, as in the case of the Nile, the ef-
fects of inundation and coastal erosion will be especially severe.
John Milliman, lames Broadus, and colleagues at the Woods
Hole Oceanographic Institution studied the economic and hu-
man consequences of sea level rise for Egypt and Bangladesh.
They estimate that the combined effects of sea level rise anct sub-
sidence caused by extraction of groundwater could threaten the
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THE FACES OF GLOBAL ENVIRONMENTAL CHANGE
~ ~ , ~
, ~ =
\~:
I At' Aft _: hi' ~- ' ~ `_'
"Lamentable News out of Monmouthshire": Old woodblock illustration of fourteenth-
century coastal flooding in England. (Courtesy of University of East Anglia, Norwich,
United Kingdom.)
homes and livelihoods of 46 million people in these two coun-
tries alone. in their analysis, the researchers used two estimates
for sea level rise: a minimum of 13 centimeters by 2050 and 28
centimeters by 2100, and a maximum rise of 79 centimeters by
2050 and 217 centimeters by 2100. They then developed three
possible scenarios. The "best case" assumes that the minimum
rise in global sea level occurs and that the delta is in equilibrium.
The "worst case" assumes the maximum rise in sea level and the
complete damming or diversion of the sediment carried by the
river ciraining into the delta (in this case the sediment would
not be able to replenish the delta so that natural subsidence
would increase the relative rate of sea level rise). The "really
worst case" assumes that the effects of natural subsidence are
compounded by excessive removal of groundwater as well as
by the maximum increase in global sea level.
Almost all of Bangladesh lies in the massive Bengal delta
formed where the Ganges, Brahmaputra, and Meghna rivers
converge after traversing India, Nepal, Bhutan, Tibet, and China.
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99
Half of the country is at elevations of less than 5 meters. The
Woods Hole researchers find that in the best case, sea level
would rise about 13 centimeters by 2050 and less than ~ percent
of the nation's total land area wouIc! be lost to advancing seas. In
the worst case, 18 percent of the land, which currently supports
about 15 percent of the nation's people, would be lost. In the
really worst case, by 2100 waters would cover an area currently
supporting 35 percent of the nation's population. The land
area that supports nearly one third of the nation's current gross
national product would vanish into the sea.
Global warming could further compound the miseries of
Bangladesh if the hypothesized possibility of more frequent
storms occurs. Currently, an average of one and one half severe
cyclones hit Bangladesh each year, and the storm surges act-
vance up the rivers as far as 200 kilometers inland. If the storms
increase in frequency and sea level rises, the storm surges will
reach even further toward the main centers of population.
Egypt too would experience massive change with global
sea level rise. Though Egypt is much larger than Bangladesh,
its people live on only about 3.5 percent of the land. Within
this area, population density IS00 people for every square
kilometer is twice that of Bangladesh. Much of Egypt's pro-
ductive agricultural land is within the Nile delta.
The Nile River has been completely dammed since comple-
tion of the High Dam at Aswan in 1964. For this reason the Nile
delivers no sediment and little fresh water to the Mediterranean,
where the delta fans out into the sea. This loss of sediment com
bines with subsidence to cause dramatic erosion of the delta
shores. Between 1966 and 1974, many areas eroded more than
~ meter per year, and some locations lost more than 100 meters
per year.
Egypt is precluded from the best case scenario because the
Aswan Dam already traps the sediment. The Woods Hole re-
searchers report that the worst and really worst cases are more
likely. In the worst case, local sea level would rise between 13
and 133 centimeters by 2050, which would affect as much as 19
percent of the currently habitable land. In the really worst case,
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THE FACES OF GLOBAL ENVIRONMENTAL CHANGE
sea level would rise between 28 and 332 centimeters by 2100
and would affect up to 26 percent of the habitable lanct. Several
brackish lakes behind the coastal dunes supply approximately
half of the nation's fish catch and would be enciangered.
Other areas with low-lying deltas and large rivers could
experience similar fates. The researchers suggest that although
there may be little that individual developing countries can do
to control global climate change, they can control groundwater
extraction and the trapping of sediments in dams. These factors
are perhaps more responsible than global climate change for
local changes in relative sea level.
PREPARING FOR RISING SEAS
In many cases, decisions about how to prepare for rising sea
levels come down to stark economic facts that will vary with
specific countries and localities. A 1987 report from the National
Research Council on the engineering implications of sea level
rise suggests that "holding back the sea as water levels rise will
almost always be feasible," but that in some cases it may not
be economically or environmentally sound. Responses fall into
three main categories. Humans can retreat from the shoreline,
use structures that prevent flooding and shoreline recession, or
raise the land.
For some cities the choice whether to protect is obvious:
Manhattan will never surrender to rising seas. In fact, most well-
cleveloped coastal communities will probably decide that protec-
tive measures such as bulkheads, levees, and pumping systems,
and the high costs of maintaining them, are worthwhile, whereas
sparsely developed areas may continue to adapt as the shoreline
retreats. The EPA estimates that low-lying coastal cities in the
United States could be protected from even a 2-meter (about 6.6
feet) sea level rise. A conservative cost estimate ranges from
$30 billion to $100 billion, but these sums would still be a small
fraction of the values of the coastal cities. Cost estimates for
raising the nation's recreational barrier islands, which cover a
combined area of 100 to 150 square miles, range from $50 billion
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COASTLINES AND RISING SEAS
101
to $100 billion for a I-meter rise and from $135 billion to $215
billion for a 2-meter rise. The trade-offs between protection from
rising seas and economic costs of protection will be difficult to
balance. In addition, levees and seawalIs hold back the sea but
also block waterfront views, cause the beach to erode, and pre-
vent the migration and preservation of wetlands. The choice of
response should be based on a sound understanding of coastal
processes. If people opt to stem destruction of wetlands, they
may have to surrender control of these areas to natural forces.
This would mean removing artificial structures that thwart sed-
imentation and migration of wetlands, or moving them back
from the shore as the waters advance. Coastal scientists, engi-
neers, and policy analysts suggest that some communities may
plan an orderly retreat from the shoreline rather than retreat-
ing in response to future rises. The National Research Council
report cautions that because methods for dealing with coastal
erosion are evolving rapidly, communities should periodically
reconsider any plans to retreat.
The projected sea level rise should cause neither alarm nor
complacency, researchers stress. The severity of impacts from
sea level rise depends on how society responds and adapts.
People are adept at using and advancing technology when nec-
essary and appropriate.
So far, the projected acceleration in the rate of sea level rise
has not been detected. If the rate does accelerate, the increase
initially would be so slow that we might not detect it until the
coming century. But some responses, especially those requiring
construction of costly and massive structures, can take three to
four decades from planning stages to completion. Some deci-
sions must be made soon if defenses are to be ready in time.
No one can anticipate all the changes that a warmer climate
will cause. Of the changes that are expected, rising sea level
could have the most visible effects on society, literally forcing
people to abandon their homes or to build virtual fortresses
around their cities. Whereas the industrialized countries of the
world that have contributed most heavily to the increase in
greenhouse gases conceivably could afford the technological
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THE FACES OF GLOBAL ENVIRONMENTAL CHANGE
remedies to cope with rising waters (though it will cost them
dearly), many developing nations couIcl not. As Tom Goemans,
a consultant with KPMG in The Hague, notes, accelerated sea
level rise is a "manmade phenomenon that presents a perfect
example of external costs, that is costs not included in the present
price of energy from fossil fuel burning."
Representative terms from entire chapter:
level rise
