United Nations
Environment Programme
United States
Environmental Protection
Agency
October 1986
o.
V
t
EFFECTS OF CHANGES IN STRATOSPHERIC
OZONE AND GLOBAL CLIMATE
Volume 4' Sea Level Rise
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Library of Congress Cataloging - In - Publication Data
Effects of changes in stratospheric ozone and global climate.
Proceedings of a conference convened by the United Nations Environment
Programme and the U.S. Environmental Protection Agency.
Contents: v. 1. Overview — v. 2. Stratospheric ozone — v. 3. Climate
change. — v. 4. Sea level rise
1. Atmospheric ozone—Reduction—Congresses. 2. Stratosphere—Con-
gresses. 3. Global temperature changes—Congresses. 4. Climatic
changes—Congresses. 5. Sea level—Congresses. 6. Greenhouse effect,
Atmospheric—Congresses. 7. Ultraviolet radiation—Congresses. I.
Titus, James G II. United States Environmental Protection Agency. III.
United Nations Environment Programme.
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EFFECTS OF CHANGES IN STRATOSPHERIC
OZONE AND GLOBAL CLIMATE
Volume 4: Sea Level Rise
Edited by
James G. Titus
U.S. Environmental Protection Agency
This report represents the proceedings of the INTERNATIONAL CON-
FERENCE ON HEALTH AND ENVIRONMENTAL EFFECTS OF
OZONE MODIFICATION AND CLIMATE CHANGE sponsored by the
United Nations Environment Programme and the U.S. Environmental
Protection Agency. The purpose of the conference was to make available
the widest possible set of views. Accordingly, the views expressed herein
are solely those of the authors and do not represent official positions of
either agency. Mention of trade names or commercial products does not
constitute endorsement or recommendation for use.
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PREFACE
This document is part of a four volume report that examines the possible
consequences of changes in stratospheric ozone and global climate resulting
from emissions of chlorofluorocarbons, carbon dioxide, methane, and other
gases released by human activities. In June 1986, the United Nations
Environment Programme and the U. S. Environmental Protection Agency sponsored
an International Conference on the Health and Environmental Effects of Ozone
Modification and Climate Change, which was attended by scientists and
officials, representing twenty-one countries from all areas of the world.
This volume examines the effects of the rise in sea level that might
result from a global warming. Volume 1 of the proceedings provides an
overview of the issues as well as the introductory remarks and reactions from
top officials of the United Nations and the United States. Volumes 2 and 3
focus on the effects of ozone depletion and climate change, respectively.
This report does not present the official views of either the U.S.
Environmental Protection Agency or the United Nations Environment Programme.
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TABLE OF CONTENTS
Overview of the Effects of Changing the Atmosphere
James G. Titus and Stephen R. Seidel 1
Future Sea Level Rise and Its Early Detection by Satellite Remote
Sensing
Robert H. Thomas 19
Flooding in Taipei, Taiwan and Coastal Drainage
Chin Y. Kuo 37
The Sea Also Rises:
The Ongoing Dialogue of the Dutch with the Sea
Tom Goemans M7
Planning for Sea Level Rise Under Uncertainty: A Case Study of
Charleston, South Carolina
Micnael J. Gibbs 57
Coastal Geomorphic Impacts of Sea Level Rise on Coasts of South America
Stephen P. Leatherman 73
Potential Effects of Sea Level Rise on the Coasts of Australia,
Africa, and Asia
Eric C. F. Bird 83
Worldwide Impact of Sea Level Rise on Shorelines
Per Bruun 99
Predicting the Effects of Sea Level Rise on Coastal Wetlands
Richard A. Park, Thomas V. Armentano, and
C. Leslie Cloonan 129
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Increased Storms and Estuarine Salinity and other Ecological
Impacts of the "Greenhouse Effect"
Donald P. de Sylva 153
Rising Sea level and Damming of Rivers: Possible Effects
in Egypt and Bangladesh
James M. Broadus, John D. Milliman, Steven F. Edwards
David G. Aubrey, and Frank Gable 155
Sea Level Rise: The Reaction of a Coastal Realtor
Kenneth J. Smith 191
vi
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Overview of the Effects of Changing the Atmosphere
James G. Titus and Stephen R. Seidel
Environmental Protection Agency
Washington, DC USA
INTRODUCTION
Society is conducting a global experiment on the earth's atmosphere.
Human activities are increasing the worldwide atmospheric concentrations of
chlorofluorocarbons, carbon dioxide, methane, and several other gases. A
growing body of scientific evidence suggests that if these trends continue,
stratospheric ozone may decline and global temperature may rise. Because the
ozone layer shields the earth's surface from damaging ultraviolet radiation
(UV) future depletion could increase the incidence of skin cancer and other
diseases, reduce crop yields, damage materials, and place additional stress on
aquatic plants and animals. A global warming from tire "greenhouse effect"
could also threaten human health, crop yields, property,\ fish, and wildlife.
Precipitation and storm patterns could change, and the level of the oceans
could eventually rise.
To improve the world's understanding of these and other potential
implications of global atmospheric changes, the United Nations Environment
Programme (UNEP) and the U.S. Environmental Protection Agency (EPA) sponsored
an International Conference on the Health and Environmental Effects of Ozone
Modification and Climate Change during the week of June 16-20, 1986. The
conference brought together over three hundred researchers and policy makers
from approximately twenty nations. This four-volume report presents the
seventy-three papers that were delivered at the conference by over eighty
speakers, including two U.S. Senators, top officials from UNEP and EPA, some
of the leading scientists investigating the implications of atmospheric
change, and representatives from industry and environmental groups. Volume 1
presents a series of overview papers describing each of the major areas of
research on the effects of atmospheric change, as well as policy assessments
of these issues by well-known leaders in government, industry, and the
environmental community. Volumes 2, 3, and 1 present the more specialized
papers on the impacts of ozone modification, climate change, and sea level
rise, respectively, and provide some of the latest research in these areas.
This paper summarizes the entire four-volume report.
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OZONE MODIFICATION
Atmospheric Processes
The ozone in the upper part of the atmosphere—known as the strato-
sphere— is created by ultraviolet radiation. Oxygen (02) is continuously
converted to ozone (Oo) and back to 62 by numerous photochemical reactions
that take place in the stratosphere, as Stordal and Isaksen (Volume 1)
describe. Chlorofluorocarbons and other gases released by human activities
could alter the current balance of creative and destructive processes.
Because CFCs are very stable compounds, they do not break up in the lower
atmosphere (known as the troposphere). Instead, they slowly migrate to the
stratosphere, where ultraviolet radiation breaks them down, releasing
chlorine.
Chlorine acts as a catalyst to destroy ozone; it promotes reactions that
destroy ozone without being consumed. A chlorine (Cl) atom reacts with ozone
(Oq) to form CIO and 02. The CIO later reacts with another 0^ to form two
molecules of 02, which releases the chlorine atom. Thus, two molecules of
ozone are converted to three molecules of ordinary oxygen, and the chlorine is
once again free to start the process. A single chlorine atom can destroy
thousands of ozone molecules. Eventually, it returns to the troposphere,
where it is rained out as hydrochloric acid.
Stordal and Isaksen point out that CFCs are not the only gas released by
human activities that might alter the ozone balance. Increasing
concentrations of methane in the troposphere increase the water vapor in the
stratosphere, which helps create ozone. Carbon dioxide and other greenhouse
gases (discussed below) warm the earth's surface but cool the upper
atmosphere; cooler stratospheric temperatures slow the process of ozone deple-
tion. Nitrous oxide (^0) reacts with both chlorine and ozone.
Stordal and Isakson present results of possible ozone depletion over
time, using their two-dimensional atmospheric-chemistry model. Unlike one-
dimensional models which provide changes in ozone in the global average, this
model calculates changes for specific latitutdes and seasons. The results
show that if concentrations of the relevant trace gases grow at recent levels,
global average ozone depletion by 2030 would be 6.5 percent. However,
countries in the higher latitudes (60°N) would experience 16 percent depletion
during spring. Even in the case of constant CFC emissions, where global
average depletion would be 2 percent by 2030, average depletion would be 8
percent in the high northern latitudes.
Watson (Volume 1) presents evidence that ozone has been changing recently
more than atmospheric models had predicted. As Plate 1 shows, the ozone over
Antarctica during the month of October appears to have declined over 40
percent in the last six to eight years. Watson also discusses observations
from ozone monitors that suggest a 2 to 3 percent worldwide reduction in ozone
in the upper portion of the stratosphere (thirty to forty kilometers above the
surface), which is consistent with model predictions. Finally, he presents
preliminary data showing a small decrease since 1978 in the total (column)
ozone worldwide. However, he strongly emphasizes that the data have not yet
been fully reviewed and that it is not possible to conclusively attribute
observed ozone depletion to the gases released by human activities. While
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there are several hypotheses to explain why ozone concentrations have
declined, none have been adequately established; nor did any of the
atmospheric models predict the measured loss of ozone over Antarctica.
Ultraviolet Radiation
Many of the chemical reactions investigated by atmospheric scientists
take place only in the stratosphere because they are caused by types of
radiation only found in the upper atmosphere. As Frederick (Volume 1)
explains, the sun emits radiation over a broad range of wavelengths, to which
the human eye responds in the region from approximately 400 to 700 nanometers
(nm). Wavelengths from 320 to 400 nm are known as UV-A; wavelengths from 280
to 320 nm are called UV-B, and wavelengths from 200 to 280 nm are known as
UV-C.
Frederick explains why attention has primarily focused on the UV-B part
of the spectrum. The atmosphere absorbs virtually all UV-C, and is expected
to continue to do so under all foreseeable circumstances. On the other hand,
UV-A is not absorbed by ozone. . By contrast, UV-B is partially absorbed by
ozone, and future depletion would reduce the effectiveness of this shield.
We now examine the potential implications of such changes on human
health, plants, aquatic organisms, materials, and air pollution.
Effects on Human Health
The evidence suggests that solar ultraviolet radiation induces skin
cancer, cataracts, suppression of the human immune response system, and
(indirectly through immunosuppression) the development of some cutaneous
infections, such as herpes. Emmett (Volume 1) discusses the absorption of UV
radiation by human tissue and the mechanisms by which damage and repair may
occur.
Emmett also examines UV radiation as the cause of aging of the skin and
both basal and squamous skin cancers. In reviewing the role of UV radiation
in melanoma (the most frequently fatal skin cancer), he states that some
evidence suggests this link, but that currently there is no acceptable animal
model that can be used to explore or validate this relationship. He concludes
that future studies must focus on three major factors—exposure to solar
radiation, individual susceptibility, and personal behavior. Waxier (Volume
1) presents evidence of a link between UV-B exposure and cataracts.
Volume 2 presents specific research results and provides more detail on
many of the aspects covered in this volume. Scotto presents epidemiological
evidence linking solar radiation with skin cancers, other than melanoma. His
analysis suggests that Caucasians in the United States have a 12 to 30 percent
chance of developing these cancers within their lifetimes, even without ozone
depletion. Armstrong examines the role of UV-B exposure to melanoma in a
study of 511 matched melanoma patients and control subjects in Western
Australia. He shows that "intermittent exposure" to sunlight was closely
associated with this type of cancer.
However, 02 and ^ reflect some UV-A back to space.
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In a paper examining nonmelanoma skin cancer in Kuwait, Kollias and Baqer
(Volume 2) show that despite the presence of protective pigmentation, 75
percent of cancers occur on the 10 percent of the skin exposed to sunlight. A
second paper on skin cancer presents experimental evidence suggesting that the
mechanism by which skin cancer could occur involves disruption of the
cytoskeleton from exposure to UV-A and UV-B light (Zamansky and Chow,
Volume 2).
The pathways by which suppression of the immune response might be
triggered are explored in papers by DeFabo and Noonan, Daynes et al., and
Elmets et al (all Volume 2). Davies and Forbes (Volume 2) show that mice
exposed to UV-B radiation had a decrease in lifetime that was proportional to
the quantity of radiation and not directly related to the incidence of skin
cancer.
Possible implications of immune suppression of diseases and the
mechanisms by which it occurs are still uncertain. However, several papers in
Volume 2 suggest that in addition to skin cancer and contact hypersensitivity,
diseases influenced by UV-B induced immune suppression include leishmaniasis
and herpes infections. Fisher et. al (Volume 2) show that at least one
sunscreen effectively protects mice exposed to UV-B radiation from sunburn;
but it does not stop the immune suppression from interfering with a contact
hypersensitivity (allergic) reaction.
Effects on Plants
The effects of increased exposure to UV-B radiation on plants has been a
primary area of research for nearly a decade. Teramura (Volume 1) reports
that of the two hundred plants tested for their sensitivity to UV-B radiation,
over two-thirds reacted adversely; peas, beans, squash, melons, and cabbage
appear to be the most sensitive. Given the complexities in this area of
research, he warns that these results may be misleading. For example, most
experiments have used growth chambers. Studies of plants in the field have
shown them to be less sensitive to UV-B. Moreover, different cultivars of the
same plant have shown very different degrees of sensitivity to UV-B
radiation. Finally, Teramura suggests that potential effects from multiple
stresses (e.g., UV-B exposure plus water stress or mineral deficiency) could
substantially alter a plant's response to changes in UV-B alone.
In Volume 2, Teramura draws extensively from the results of his five
years of field tests on soybeans. His data show that a 25 percent depletion
in ozone could result in a 20 to 25 percent reduction in soybean yield and
adverse impacts on the quality of that yield. Because soybeans are the fifth
largest crop in the world, a reduction in yields could have serious
consequences for world food supplies. However, some soybean cultivars appear
to be less susceptible to UV-B radiation, which suggests that selective crop
breeding might reduce future losses, if it does not increase vulnerability to
other environmental stresses.
Bjorn (Volume 2) examines the mechanisms by which plant damage occurs.
His research relates specific wavelengths with those aspects of plant growth
that might be susceptible, including the destruction of chloroplast, DNAt or
enzymes necessary for photosynthesis.
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Aquatic Organisms
Aquatic plants would also be adversely affected by increased ultraviolet
radiation. Worrest (Volume 1) points out that most of these plants, which are
drifters (phytoplankton), spend much of their time near the surface of the
water (the euphotic zone) and are therefore exposed to ultraviolet
radiation. A reduction in their productivities would be important because
these plants directly and indirectly provide the food for almost all fish.
Furthermore, the larvae of many fish found in the euphotic zone would be
directly affected, including crabs, shrimp, and anchovies. Worrest points out
that fish account for 18 percent of the animal protein that people around the
world consume, and 40 percent of the protein consumed in Asia.
An important question is the extent to which current UV-B levels are a
constraint on aquatic organisms. Calkins and Blakefield (Volume 2) conclude
that some species are already exposed to as much UV-B as they can tolerate.
Thomson (Volume 2) shows that a 10 percent decrease in ozone could increase
the number of abnormal larvae as much as 18 percent. In a study of anchovies,
a 20 percent increase in UV-B radiation over a 15-day period caused the loss
of all the larvae within a 10-meter mixed layer in April and August.
Many other factors could affect the magnitude of the impacts on specific
species, ecosystems, and the food chain. An important mechanism by which
species could adapt to higher UV-B incidence would be to reduce their exposure
by moving further away from the water's surface during certain times of the
day or year when exposure is greatest. Haeder (Volume 2) suggests, however,
that for certain species such avoidance may be impaired by UV-B radiation.
Even for those organisms that could move to avoid exposure, unwanted
consequences may result. Calkins and Blakefield present model results showing
that movement by phytoplankton away from sunlight to reduce exposure to a 10
percent increase in UV-B would result in a 2.5 to 5 percent decrease in
exposure to the photosynthetically active radiation on which their growth
depends. Increased movement requires additional energy consumption, while
changes in location may affect the availability of food for zooplankton, which
could cause other changes in shifts in the aquatic food chain.
To a certain extent, losses within a particular species of plankton'may
be compensated by gains in other species. Although it is possible that no net
change in productivity will occur, questions arise concerning the ecological
impacts on species diversity and community composition (Kelly, Volume 1).
Reductions in diversity may make populations more susceptible to changes in
water temperatures, nutrient availability, diseases, or pollution. Changes in
community composition could alter the protein content, dry weight, or overall
food value of the initial stages of the aquatic food chain.
Polymer Degradation and Urban Smog
Current sunlight can cause paints to fade, transparent window glazing to
yellow, and polymer automobile roofs to become chalky. These changes are
likely to occur more in places closer to the equator where UV-B radiation is
greater. They are all examples of degradation that could accelerate if
depletion of the ozone layer occurs. Andrady and Horst (Volume 2) present a
case study of the potential magnitude of loss due to increased exposure to
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UV-B radiation on polyvinyl chloride (PVC). This chemical is used in outdoor
applications where exposure to solar radiation occurs over a prolonged
period. It is also used in the construction industry in siding and window
frames and as a roofing membrane.
To analyze the potential economic impact of future ozone depletion on
PVC, the authors assumed that the future service life of polymers would be
maintained by increasing the quantity of light stabilizers (titanuim dioxide)
used in the product. As a result, the costs associated with increased UV-B
radiation would be roughly equal to the costs of increased stablizers.
Preliminary results show that for a 26 percent depletion by 2075, the
undiscounted costs would be $4.7 billion (1984 dollars).
Increased penetration of UV-B radiation to the earth's surface could play
an important role in the formation of ground level oxidants (smog). UV-B
affects smog formation through the photolysis of formaldehyde, from which
radicals are the main source for deriving chain reactions that generate
photochemical smog. Whitten and Gery (Volume 2) analyze the relationship
between UV-B, smog, and warmer temperatures. The results of this preliminary
study of Nashville, Philadelphia, and Los Angeles show that large depletions
in stratospheric ozone and increases in temperature would increase smog by as
much as 50 percent. In addition, because oxidants would form earlier in the
day and closer to population centers (where emissions occur), risks from
exposure could increase by an even higher percentage increase. Whitten and
Gery also report a sensitive relationship between UV-B and hydrogen peroxide,
an oxidant and precursor to acid rain.
CLIMATE CHANGE
The Greenhouse Effect
Concern about a possible global warming focuses largely on the same gases
that may modify the stratospheric ozone: carbon dioxide, methane, CFCs, and
nitrous oxide. The report of a recent conference convened by UNEP, the World
Meteorological Organization, and the International Council of Scientific
Unions concluded that if current trends in the emissions of these gases
continue, the earth could warm a few degrees (C) in the next fifty years
(Villach 1985). In the next century, the planet could warm as much as five
degrees (MAS 1983), which would leave the planet warmer than at any time in
the last two million years.
A planet's temperature is determined primarily by the amount of sunlight
it receives, the amount of sunlight it reflects, and the extent to which the
atmosphere retains heat. When sunlight strikes the earth, it warms the
surface, which then reradiates the heat as infrared radiation. However, water
vapor, COp, and other gases in the atmosphere absorb some of the energy rather
than allowing it to pass undeterred through the atmosphere to space. Because
the atmosphere traps heat and warms the earth in a manner somewhat analogous
to the glass panels of a greenhouse, this phenomenon is commonly known as the
"greenhouse effect." Without the greenhouse effect of the gases that occur
naturally in the atmosphere, the earth would be approximately 33°C colder than
it is currently (Hansen et al. 1984).
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In recent decades, the concentrations of greenhouse gases have been
increasing. Since the beginning of the industrial revolution, the combustion
of fossil fuels, deforestation, and a few other activities have released
enough C02 to raise atmospheric concentrations by 20 percent; concentrations
have risen 8 percent since 1958 (Keeling, Bacastow, and Whorf 1982). More
recently, Ramanathan et al. (1985) examined the greenhouse gases other than
C02 (such as methane, CFCs, and nitrous oxide), and concluded that these other
gases are likely to double the warming caused by C02 alone. Using these
results, the Villach Conference estimated that an "effective doubling" of C02
is likely by 2030.2
Hansen et al. (Volume 1) and Manabe & Wetherald (Volume 1) present the
results that their climate models predict for an effective doubling of C02.
Both models consider a number of "climatic feedbacks" that could alter the
warming that would directly result from C02 and other gases released by human
activities. Warmer temperatures would allow the atmosphere to retain more
water vapor, which is also a greenhouse gas, thereby resulting in additional
warming. Ice and snow cover would retreat, causing sunlight that is now
reflected by these bright surfaces to be absorbed instead, causing additional
warming. Finally, a change in cloud cover might result, which could increase
or decrease the projected warming. Although the two models differ in many
ways, both conclude that an effective doubling of greenhouse gases would warm
the earth's surface between two and four degrees (C).
Hansen et al. project the doubling to occur between 2020 and 2060. They
also provide estimates of the implications of temperature changes for
Washington, D.C., and seven other U.S. cities for the middle of the next
century. For example, Washington would have 12 and 85 days per year above
38°C (100°F) and 32°C (90°F), respectively, compared with 1 and 35 days above
those levels today. While evenings in which the thermometer fails to go below
278C (80°F) occur less than once per year today in that city, they project
that such evenings would occur 19 times per year. (See Plates 2 and 3 for
worldwide maps of historical and projected temperature changes.)
Water Resources
Manabe and Wetherald (Volume 1) focus on the potential changes in
precipitation patterns that might result from the greenhouse warming. They
project substantial increases in summer dryness at the middle latitudes that
currently support most of the world's agriculture. Their model also projects
increased rainfall for late winter.
Beran (Volume 1) reviews the literature on the hydrological and water
resource impacts of climate change. He expresses some surprise that only
twenty-one papers could be found that address future water resource impacts.
One of the problems, he notes, is that there is a better scientific
understanding of how global average temperatures and rainfall might change,
than for the changes that specific regions may experience. Nevertheless, he
Studies on the greenhouse effect generally discuss the impacts of a carbon
dioxide doubling. By "effective doubling" we refer to any combination of
increases in concentrations of the various gases that causes a warming
equal to the warming of a doubling of carbon dioxide alone.
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demonstrates that useful information can be extracted by studying the
implications of particular scenarios.
Nicholson (Volume 3) shows how historical changes in water availability
have caused problems for society in the past. The best lesson of climatic
history, she writes, "is that agricultural and economic systems must be
flexible enough to adapt to changing conditions and, in the face of potential
water scarcity, systems must be designed that require minimum use of
resources." Wilhite (Volume 3) examines drought policies in Australia and the
United States, concluding that the lack of national drought plans could
substantially impair the ability of these two nations to successfully adapt to
hydrologic changes resulting from the greenhouse warming.
Cohen (Volume 3) examines the potential implications of the global
warming for water levels in the Great Lakes that separate Canada from the
United States. Using results from the models of both Hansen et al. and Manabe
& Wetherald, he concludes that lake levels could drop 10 to 30 centimeters.
This drop would significantly reduce the capacity of ocean-going vessels that
enter the Great Lakes. On the other hand, such a drop might be viewed as a
benefit by the owners of critically eroding property whose homes are currently
threatened by historically high lake levels. Street-Perrott et al. (Volume 3)
discuss the historic impacts of changes in climate on the levels of lakes in
North America, South America, Australia, and Africa.
Gleick (Volume 3) uses scenarios from the Hansen et al. and Manabe &
Wetherald models (as well as a third developed by the National Center for
Atmospheric Research) to drive a water-balance model of the Sacramento Basin
in California. He finds that reductions in runoff could occur even in months
where precipitation increases substantially, because of the increased rates of
evaporation that take place at higher temperatures. He also points out that
the models predict that changes in monthly runoff patterns will be far more
dramatic than changes in annual averages. For seven of ten scenarios, soil
moisture would be reduced every month of the year; for the other three cases,
slight increases in moisture are projected for winter months. Mather (Volume
3) conducts a detailed analysis for southern Texas and northern Mexico;
examines in less detail twelve regions around the world; and projects shifts
in global vegetation zones.
Agriculture and Forestry
The greenhouse warming could affect agriculture by altering water
availability, length of growing season, and the number of extremely hot
days. Increased COp concentrations could also have two direct impacts
unrelated to climate change: At least the laboratory, plants grow faster (the
COp fertilization effect) and retain moisture more efficiently. The extent to
which these beneficial effects offset the impacts of climate change will
depend on the extent to which global warming is caused by C02 as opposed to
other greenhouse gases, which do not have these positive impacts.
Parry (Volume 1) provides an overview of the potential impacts of climate
change on agriculture and forestry. He points out that commercial farmers
plan according to the average year, while family and subsistence farmers must
ensure that even in the worst years they can make ends meet. Thus, the
commercial farmer would be concerned about the impact of future climate change
8
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on average conditions and average yields, while farmers at the margin would be
most concerned with changes in the probability of (for example) a severe
drought that causes complete crop failure. Parry notes that the probability
of two or more anomalous years in a row could create disproportionately
greater problems for agriculture. For example, a persistent drought in the
U.S. Great Plains from 1932 to 1937 contributed to about two hundred thousand
farm bankruptcies.
Parry discusses a number of historical changes in climate. The Little
Ice Age in western Europe (1500-1800 A.D.) resulted in the abandonment of
about half the farms in Norway, an end to cultivation of cereals in Iceland,
and some farmland in Scotland being permanently covered with snow. Concerning
the late medieval cooling (1250-1500) he writes: "The failure to adapt to the
changing circumstances is believed to explain much of the Norse decline. The
Norse continued to emphasize stock-raising in the face of reduced capacity of
the already limited pastures. The option of exploiting the rich seas around
them, as the Inuit (Eskimos) successfully did, was not taken up ... This is
an extreme example of how governments can fail to identify and implement
appropriate policies of response." It also suggests that effective responses
can reduce damages from climate change.
The paper reviews a number of studies that project impacts of climate
change on agriculture. "Warming appears to be detrimental to cereals in the
core wheat-growing areas of North America and Europe." If no precipitation
changes take place, a one-degree warming would decrease yields 1 to 9 percent
while a two-degree (C) warming would decrease yields 3 to 17 percent. Parry
also discusses how particular crop zones might shift. A doubling of C02 would
substantially expand the wheat-growing area in Canada due to higher winter
temperatures and increased rainfall. In Mexico, however, temperature stresses
would increase, thereby reducing yields.
A number of studies have been conducted using the models of Hansen et
al., Manabe & Wetherald, and others. Although these projections cannot be
viewed as reliable forecasts, they do provide consistent scenarios that can be
useful for examining vulnerability to climate change. Parry indicates that
investigations of Canada, Finland, and the northern USSR using the model by
Hansen et al. show reduced yields of spring-sown crops such as wheat, barley,
and oats, due to increased moisture stress early in the growing period.
However, switching to winter wheat or winter rye might reduce this stress.
Parry goes on to outline numerous measures by which farmers might adapt to
projected climate change.
Waggoner (Volume 3) points out that the global warming would not affect
plants uniformly. Some are more drought-resistant than others, and some
respond to higher C02 concentrations more vigorously than others. Co plants,
such as wheat, respond to increased COo more than Cjj plants such as maize.
Thus, the COp fertilization effect woulcf not help the farmer growing C^ crops
accompanied by Co weeds. Waggoner also examines the impact of future climate
change on average crop- yields and pests, and the probability of successive
drought years. He concludes that although projections of future changes are
useful, historical evidence suggests that surprises may be in store, and that
"agricultural scientists will be expected to aid rather than watch mankind's
adaptation to an inexorable increase in COj and its greenhouse effect."
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The impact of future climate change on yields for spring wheat in
Saskatchewan, Canada, is the subject of the paper by Stewart (Volume 3).
Using the output from the Hansen et al. model (Volume 1), which projects that
the effective doubling of carbon dioxide would increase average annual
temperatures in that region by 4.7°C, he estimates that the growing season
would start two or three weeks earlier and end three or four weeks later.
Although average precipitation during the growing season would increase, he
also finds that the area would become more prone to drought. The impact of
climatic change would be to reduce yields 16 to 26 percent. Stewart estimates
that the fertilization effect of a CC^ doubling would reduce the losses to 6
to 15 percent. Cooter (Volume 3) examines the economic impact of projected
climate change on the economy of Oklahoma, concluding that the Gross State
Product would decline 75 to 300 million dollars. (The state's gross product
in 1985 was approximately 50 billion dollars.)
Fritts (Volume 3) examines tree rings to assess how past changes in
climate have affected forests, and concludes that tree rings are useful for
estimating past changes in climate. Solomon and West (Volume 3) discuss the
results of their efforts to model the future impacts. Considering the impact
of climate change caused by doubled C02 without the fertilization effect, they
find that "biomass (for boreal forests) declines for 50-75 years as warming
kills off large boreal forest species, before new northern hardwoods can grow
into the plot."
"Warming at the transition site causes an almost immediate response in
declining biomass from dieback of mature trees, and in decline of tree mass as
large trees die and are temporarily replaced by small young trees," they
write. "The deciduous forest site . . . results in permanent loss of dense
forest. One might expect the eventual appearance of subtropical forests
similar to those in Florida today, but the real difficulty is the moisture
balance (which is) more similar to those of treeless Texas today, than to
those of southern Florida." Solomon and West go on to show how the
fertilization effect from increased concentrations of C02 could offset part
but not all of the drop in forest productivity.
Sea Level Rise
One of the most widely recognized consequences of a global warming would
be a rise in sea level. As Titus (Volume 1) notes, global temperatures and
sea level have fluctuated over periods of one hundred thousand years, with
temperatures during ice ages being three to five degrees (C) lower and sea
level over one hundred meters lower than today. By contrast, the last
interglacial period (one hundred thousand years ago) was one or two degrees
warmer than today, and sea level was five to seven meters higher.
The projected global warming could raise sea level by heating and thereby
expanding ocean water, melting mountain glaciers, and by causing polar
glaciers in Greenland and Antarctica to melt and possibly slide into the
oceans. Thomas (Volume 4) presents new calculations of the possible
contribution of Antarctica and combines them with previous estimates for the
other sources, projecting that a worldwide rise in sea level of 90 to 170
centimeters by the year 2100 with 110 centimeters most likely. However, he
also estimates that if the global warming is substantially delayed, the rise
in sea level could be cut in half. Such a delay might result either from
10
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actions to curtail emissions or from the thermal lag induced by the oceans'
ability to absorb heat.
On the other hand, Thomas also estimates that if a warming of four
degrees results from a C02 doubling (which the model of Hahsen et al.
projects) and concentrations continue to grow after 2050, the rise could be as
great as 2.3 meters. He also notes that an irreversible deglaciation of the
West Antarctic Ice Sheet might begin in the next century, which would raise
sea level another six meters in the following centuries.
Titus (Volume 1) notes that these projections imply that sea level could
rise 30 centimeters by 2025, in addition to local subsidence trends that have
been important in Taipei, Taiwan; Venice, Italy; the Nile Delta, Egypt; and
most of the Atlantic and Gulf Coasts of the United States. The projected rise
in sea level would inundate low-lying areas, destroy coastal marshes and
swamps, erode shorelines, exacerbate coastal flooding, and increase the
salinity of rivers, bays, and aquifers.
Bruun (Volume 4) argues that with a combination of coastal engineering
and sound planning, society can meet the challenge of a rising sea. He
discusses a number of engineering options, including dikes (levees) and
seawalls, and adding sand to recreational beaches that are eroding, with a
section on the battle that the Dutch have fought with the sea for over one
thousand years. Goemans (Volume 4) describes the current approach of the
Dutch for defending the shoreline, and estimates that the cost of raising
their dikes for a one meter rise in sea level would be 10 billion guilders,
which is less than 0.05 percent of their Gross National Product for a single
year.
Goemans concludes that there is no need to anticipate such a rise because
they could keep up with it. However, he is more concerned by the two-meter
scenario: "Almost immediately after detection, actions would be required. It
is not at all certain that decision-makers act that fast. . . . The present
flood protection strategy came about only after the tragic disaster of 1953.
When nobody can remember a specific disaster, it is extremely difficult to
obtain consensus on countermeasures." For his own country, Goemans sees one
positive impact: Referring to the unique experience of Dutch engineering
firms in the battle with the sea, he suggests that "a rising sea may provide a
new global market for this expertise." But he predicts that "the question of
compensation payments may come up," for the poorer countries who did not cause
climate change but must face its consequences.
Broadus et al. (Volume 4) examine two such countries in detail: Egypt
and Bangladesh. The inhabited areas of both countries are river deltas, where
low-lying land has been created by the sediment washing down major rivers. In
the case of Egypt," the damming of the Nile has interrupted the sediment, and
as the delta sinks, land is lost to the Mediterranean Sea. Broadus et al.
estimate that a 50-centimeter rise in global sea level, when combined with
subsidence and the loss of sediment, would result in the loss of 0.3 to 0.4
percent of the nation's land area; a 200-centimeter rise would flood 0.7
percent. However, because Egypt's population is concentrated in the low-lying
areas, 16 and 21 percent of the nation's population currently reside in the
areas that would be lost in the two scenarios.
11
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The situation would be even more severe in Bangladesh. As Plate 4 shows,
this nation, which is already overcrowded, would lose 12 to 28 percent of its
total area, which currently houses 9 to 27 percent of its population.
Moreover, floods could penetrate farther inland, which could leave the nation
more vulnerable to the type of tropical storm that killed 300,000 people in
the early 1970s, especially if the frequency of tropical storms doubled due to
warmer water temperatures, which deSylva (Volume 4) projects. Broadus et al.
conclude that the vulnerability of Bangladesh to a rise in sea level will
depend in large measure on whether future water projects disrupt land-creating
sediment washing down the Ganges.
Bird (Volume 4) examines the implications of sea level rise for other
African and Asian nations, as well as Australia. While holding back the sea
may be viable in Australia, he shows areas in New Guinea where people live in
small cottages on the water's edge on a barrier island that almost certainly
would be unable to justify construction of a dike. He also points to the
Philippines, where many people have literally "taken to the water," living in
small boats and maintaining fishing nets in their own plots of bay instead of
land. Current wetlands, he suggests, may convert to these shallow bays, with
people converting to a more water-based economy.
Leatherman (Volume 4) examines the implications of sea level rise for
South America. He notes that such popular resorts as Copacabana Beach,
Brazil; Punta del Este, Uruguay; and Mar del Plata, Argentina, are already
suffering serious erosion. He concludes that because of the economic
importance of resorts, governments will allocate the necessary funds to
maintain their viability. However, he predicts that "coastal wetlands will
receive benign neglect" and be lost.
Park et al. (Volume 4) focus on the expected drowning of coastal wetlands
in the United States. Using a computer model of over 50 sites, they project
that 40-75 percent of existing U.S. coastal wetlands could be lost by 2100.
Although these losses could be reduced to 20-55 percent if new wetlands form
inland as sea level rises, the necessary wetland creation would require
existing developed areas to be vacated as sea level rises, even though
property owners would frequently prefer to construct bulkheads to protect
their property. Because coastal wetlands are important for many commerically
important seafood species, as well as birds and furbearing animals, Park et
al. conclude that even a one-meter rise in sea level would have major impacts
on the coastal environment.
DeSylva (Volume 4) also examines the environmental implications of sea
level rise, noting that in addition to wetlands being flooded, estuarine
salinity would increase. Because 66 to 90 percent of U.S. fisheries depend on
estuaries, he writes that these impacts could be important. He also suggests
that coral reefs could become vulnerable because of sea level rise, increased
temperatures, and the decrease in the pH (increased acidity) of the ocean.
Kuo (Volume 4) examines the implications of sea level rise for flooding
in Taipei, Taiwan, and coastal drainage in general. Although Taipei is
upstream from the sea, Kuo concludes that projected sea level rise would cause
serious problems, especially because Taiwan is also sinking. He recommends
that engineers around the world take "future sea level rise into consideration
... to avoid designing a system that may become prematurely obsolete."
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Gibbs (Volume 4) estimates that sea level rise could result in economic
damages in Charleston, South Carolina, equal to as much as 25 percent of the
annual product of the community. Anticipatory measures, however, could reduce
these impacts by half. Gibbs finds that in some areas actions should be taken
today, in spite of the current uncertainty regarding future rates of sea level
rise, while for other areas it would be more prudent to wait until uncertain-
ties are resolved.
Ken Smith, a realtor from coastal New Jersey, reacts to the other papers
presented in Volume 4. He argues that the issue of sea level rise should be
taken seriously today, but laments the fact that many of his fellow realtors
make comments such as "What do you care? You won't be around to see it!" and
the scientific community is "a bunch of eggheads who don't want us (to build
on the coast) anyway," Smith suggests that part of the resistance to taking
the issue seriously is that there are a number of "naturalists" who oppose
building near the shore, and "most of the discussion seems to come from the
'naturalist1 camp." Nevertheless, Smith argues that "the solutions—if there
are any—should be contemplated now as part of a concerted global effort.
This is a beautiful world, and we are its stewards."
Human Health and Ecological Impacts
Climate and weather have important impacts on human health. A global
warming would increase the stresses due to heat, decrease those due to cold,
and possibly enable some disease that require warm year-round temperatures to
survive at higher latitudes. Kalkstein et al. (Volume 3) present a
preliminary statistical assessment of the relationship of mortality rates to
fluctuations in temperature in New York City. They find that a two to four
degree (C) warming would substantially increase mortality rates in New York
City, if nothing else changed. However, they caution that if New Yorkers are
able to acclimatize to temperatures as well as people who currently live in
U.S. cities to the south, fewer deaths would occur. Kalkstein et al. write
that knowledgeable observers disagree about whether and how rapidly people
adapt to higher temperatures; some people undoubtedly adjust more readily than
others.
Although people may be able to adapt to changes in climate, other species
on the planet would also be affected and may not be as able to control their
habitats. Peters and Darling (Volume 3) examine the possibility that changes
in climate would place multiple stresses on some species which would become
extinct, resulting in a significant decline in biodiversity. (Mass extinc-
tions appear to have accompanied rapid changes in temperatures in the past.)
Throughout the world reserves have been set aside where targeted species
can remain relatively free of human intrusion. Peters and Darling ask: Will
these reserves continue to serve the same function if the climate changes? In
some cases, it will depend on whether the reserve's boundaries encompass areas
to which plants and animals could migrate. Some species may be able to
migrate "up the mountain" to find cooler temperatures; coastal wetlands could
migrate inland. A northerly migration of terrestrial species would be
possible in the undeveloped arctic regions of Alaska, Canada, and the Soviet
Union; but human development would block migration of larger animals in many
areas.
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POLICY RESPONSES
Papers by UNEP Deputy Director Genady Golubev and EPA Administrator Lee
Thomas (both in Volume 1) provide official views on the nature of the effects
from projected changes in the atmosphere and the role of their institutions in
addressing those changes. Golubev notes that while "the global issues are
complex, uncertainty exceeds understanding, and patience is prudence," there
is an other side to the story: "Our legacy to the future is an environment
less benign than that inherited from our forbearers. The risks are sufficient
to generate a collective concern that forebodes too much to wait out the
quantifications of scientific research. Advocating patience is an invitation
to be a spectator to our own destruction."
Golubev also points out that UNEP has worked for the achievement of the
Vienna Convention for the Protection of the Ozone Layer, in which many nations
have agreed to act in concert to address an environmental issue whose impacts
have not yet been detected. Yet he notes that the agreement is for coopera-
tion in research and does not yet bind nations to observe limits in production
and emissions of gases that could deplete stratospheric ozone.
Thomas points out that both the potential depletion of ozone and the
global warming from the greenhouse effect are examples of environmental
problems that involve the "global commons." Because all nations contribute to
the problem and experience the consequences, only an international agreement
is likely to be effective. He urges scientists around the world to discuss
this issue with their colleagues and key officials.
Richard Benedick, Deputy Assistant Secretary in the U.S. Department of
State (Volume 1), describes the emerging international process addressing the
ozone issue. Although the process for addressing climate change has not yet
proceeded as far, he writes, "from my perspective as a career diplomat, it
appears that the greenhouse effect has all the markings of becoming a high
visibility foreign policy issue. . . . How we address this issue internation-
ally depends to a great extent on our success or failure in dealing with the
ozone depletion issue."
J.P. Bruce (Volume 1) of Environment Canada presents the issue of
atmospheric change in the context of air pollution in general. He writes that
ozone modification and climate change are "urgent issues," especially because
important long-term decisions are being made today whose outcomes could be
strongly affected by changes in climate and the ozone layer. Bruce recommends
that emissions of CFCs be reduced, and concludes that "a new approach, a new
ethic towards discharging wastes and chemical materials into the air we all
breathe must soon be adopted on a international scale."
Two U.S. Senators also provide their reactions. John Chafee from Rhode
Island (Volume 1) describes hearings that his Subcommittee on Environmental
Pollution held June 10-11, 1986. "Why are policy makers demanding action
before the scientists have resolved all of the questions and uncertainties?"
he asks. "We are doing so because there is a very real possibility that
society—through ignorance or indifference, or both—is irreversibly altering
the ability of our atmosphere to perform basic life support functions for the
planet." Albert Gore, Jr. from Tennessee, who has chaired three congressional
hearings on the greenhouse effect, explains why he has introduced a bill in
-------
the U.S. Senate to establish an International Year of the Greenhouse Effect.
"The legislations would coordinate and promote domestic and international
research efforts on both the scientific and policy aspects of this problem,
identify strategies to reduce the increase of carbon dioxide and trace gases,
investigate ways to minimize the impact of the greenhouse effect, and
establish long-term research plans." Senator Gore closes by quoting Sherwood
Rowland (discussed below): "What's the use of having developed a science well
enough to make predictions, if in the end all we're willing to do is stand
around and wait for them to come true?" Both Senators call for immediate
action to reduce global use of CFCs.
John S. Hoffman (Volume 1) emphasizes the inertia of the atmosphere and
oceans. Because there are time lags between changes in emission rates,
atmospheric concentrations, and changes in ozone and global warming
temperatures, the types of management strategies must be different from those
that are appropriate for controlling, for example, particulate pollution,
where the problem goes away as soon as emissions are halted. CFC emissions
would have to be cut 80 percent simply to keep concentrations from
increasing. Although constant concentrations would prevent ozone depletion
from worsening, Hoffman points out that even if we hold the concentrations of
greenhouse gases constant once the earth has warmed one degree, the planet
would warm another degree as the oceans come into equilibrium. Thus it might
be impossible to prevent a substantial warming if we wait until a small
warming has taken place.3
The final section of this volume presents the papers from the final day
of the conference. Peter Usher of UNEP recounts the evolution of the ozone
issue. Following Rowland and Molina's hypothesis that chlorofluorocarbons
could cause a depletion of stratospheric ozone in 197M, UNEP held a conference
in 1977 that led to a world plan of action to assess the issue and quantify
risks. Since that time, UNEP has held numerous coordinating meetings leading
up the the Vienna Convention. However, Usher suggests that motivating inter-
national effort on the greenhouse effect will be more difficult: "Prohibition
of nonessential emissions of relatively small amounts (to control ozone
depletion) is one thing, limiting emissions of carbon dioxide from coal- and
oil-burning is quite another."
Dudek and Oppenheimer of the Environmental Defense Fund (U.S.) analyze
some of the costs and benefits of controlling emissions of CFCs. They
estimate that by holding emissions constant, 1.65 million-cases of nonmelanoma
skin cancers could be prevented worldwide, and that the cost of these controls
would be 196 to 455 million dollars, depending on the availability of alterna-
tive chemicals.
Two former high-ranking environmental officials in the United States
argue that we should be doing more to address these problems. John Topping
recommends that CFCs in aerosol spray cans, egg cartons, fast-food containers,
and other nonessential uses be phased out, and that people recognize that
Titus (Volume 1) and Thomas (Volume U) also explore inertia, noting that
even if temperatures remained constant after warming somewhat, sea level
would rise at an accelerated rate as the oceans, mountain glaciers, and ice
sheets came into equilibrium with the new temperature.
15
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along with energy conservation, nuclear power is the most likely alternative
to fossil fuels over the nexc generation or two. He also recommends that
society take steps to minimize the impacts of climate change and sea level
rise, for example, by requiring environmental impact statements to consider
the likely impacts.
Gus Speth, president of the World Resources Institute, recommends
international efforts to stop tropical deforestation; a production cap for
chlorofluorocarbons; increased energy conservation; advanced technologies for
producing electricity from natural gas; and tighter regulations to limit
carbon monoxide from automobiles, which would indirectly limit increases in
atmospheric methane. He agrees with Topping that environmental impact
statements for projects that could contribute to or be affected by climate
change or ozone modification should consider these impacts.
Doniger and Wirth, from the Natural Resources Defense Council (U.S.),
argue that the current uncertainties are no longer a reason to wait for
additional information: "With the stakes so high, uncertainty is an even more
powerful argument for taking early action." These authors conclude that sharp
reductions in CFCs are necessary, pointing out that even with a production
cap, atmospheric concentrations of these gases will continue to grow.
Therefore, Doniger and Wirth propose an 80 percent cut in production over the
next five years for CFCs 11 and 12, the halons, and perhaps some other
compounds, with a complete phaseout in the next decade.
Richard Barnett of the Alliance for a Responsible CFC Policy (which
represents CFC using industries) agrees that we should not delay all action
until the effects of ozone depletion and climate change are felt; but he
"would hardly characterize the activities over the last twelve years as 'wait
and see1 . . . The science, as we currently understand it, however, tells us
that there is additional time in which to solidify international consensus.
This must be done through discussion and negotiation, not through unilateral
regulation."
Barnett adds that industry should "take precautionary measures while
research and negotiations continue at the international level. We will
continue to examine and adopt such prudent precautionary measures as
recapturing, recycling, and recovery techniques to control CFC emissions;
transition to existing alternative CFCs that are considered to be more
environmentally acceptable; practices to replace existing systems at the
expiration of their useful lives to equipment using other CFC formulations;
practices in the field to prevent emissions where possible; encouragement of
CFC users to look for processes or substances that are as efficient, safe, and
productive—or better—than what is presently available."
Barnett concludes that "these environmental concerns are serious, but
their successful resolution will require greater global cooperation in con-
ducting the necessary research and monitoring, and in developing coordinated,
effective, and equitable policy decisions for all nations."
We hope that this paper has provided the reader with a "road map" through
the papers of this four-volume report on the potential effects of changing the
atmosphere. But we have barely scratched the surface of each, Just as the
existing research has barely scratched the surface in discovering and
16
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demonstrating the possible risks of ozone modification and climate change. A
continual evolution of our understanding will be necessary for our knowledge
to stay ahead of the global experiment that society is conducting.
REFERENCES
Hansen, J.E., A. Lacis, D. Rind, and G. Russell. 1984. Climate sensitivity to
increasing greenhouse gases. In Greenhouse effect and sea level rise; A
challenge for this generation, eds. M.C. Barth and J.G. Titus. Mew
York: Van Nostrand Reinhold.
Keeling, C.D., R.B. Bacastow, and T.P. Whorf. 1982. Measurements of the
concentration of carbon dioxide at Mauna Loa, Hawaii. Carbon Dioxide
Review 1982. 377-382, ed. by W. Clark. New York: Oxford University
Press, Unpublished data from NOAA after 1981.
NAS. 1983. Changing Climate. Washington, D.C.: National Academy Press.
Nordhaus, W.D., and G.W. Yohe. 1983. Future carbon dioxide emissions from
fossil fuels. In Changing Climate. Washington, D.C.: National Academy
Press.
Villach. 1985. International assessment of the role of carbon dioxide and of
other greenhouse gases in climate variations and associated impacts.
Conference Statement. Geneva: United Nations Environment Program.
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Future Sea Level Rise and Its Early
Detection by Satellite Remote Sensing
Robert Thomas
Space Department
Royal Aircraft Establishment
Franborough, Hants UK
ABSTRACT
During the past 100 years, sea level appears to have risen by 10-15 cm,
probably due to the combined effects of thermal expansion of ocean surface
waters and net melting of glaciers and ice caps, associated with a small
increase in global temperatures. This trend will almost certainly continue
and accelerate if steadily increasing levels of carbon dioxide and other
"greenhouse" gases in the atmosphere cause warming of the magnitude widely
predicted by climate modellers (see Hansen et al. Volume 1). Rising air
temperatures will cause increased melting from glaciers and ice caps, and
rising seawater temperatures will cause thermal expansion of the oceans.
Moreover, warmer ocean waters could melt and weaken the many floating ice
shelves that surround Antarctica, permitting increased ice discharge from
glaciers. This paper estimates the total sea level rise that could occur
during the next century as a result of these factors.
If, as predicted by many climate models, global temperatures increase by
an average of about 3°C, there is a good probability that 'sea level will rise
by approximately 1.m by the year 2100. Ultimately, such a rise would become
very apparent to coastal populations, but the initial change would be slow.
Consequently, it is important to devise an "early warning system" for prompt
detection of changes that will precede a detectable rise in sea level. These
include changes in:
• Surface temperatures on land, oceans, and ice sheets
• Sea-ice distribution
• Extent of summer melting on the polar ice sheets
• Areal extent and surface elevations of the ice sheets in Greenland and
Antarctica.
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All of these parameters can be measured from space by satellites that are
operating now or are planned for launch during the next few years.
INTRODUCTION
Greenhouse gases trap heat within the atmosphere. Carbon dioxide (C02)
is the most prevalent greenhouse gas, but there are more than a dozen others
present in the atmosphere. Most are becoming increasingly abundant, and if
present trends continue, their combined effect could be equivalent to a
doubling of present-day COp levels by about 2050. Most climate models
indicate that this would ultimately cause global warming of between 1.5° and
4.5°C. Moreover if greenhouse gases continue to increase beyond 2050, global
warming will be even greater. How long it takes for the warming to be
realized will depend largely on the ocean. If only the mixed layer (the upper
100 m or so) is affected, then the global warming would be realized with a lag
of only a few decades. But if, by contrast, the deep ocean responds rapidly
to increasing air temperatures, then global warming could be delayed by a
century or more, much of the additional heat being used to warm the ocean
instead of the atmosphere. Further uncertainty is introduced by the effects
of clouds, which cannot be accurately simulated with present models. However,
although we cannot be absolutely certain that sustained increases in
greenhouse gases will significantly affect the climate, there is a strong
probability that they will. In this paper, I review possible effects of this
warming on sea level.
Heat trapped by greenhouse gases raises the temperature of the atmosphere
and the ocean (Figure 1). The response of sea level to this warming is
strongly determined by the partition of available heat between these two
processes. If most of the heat remains in the atmosphere, air temperatures
rise rapidly and sea level is affected most by increased melting of ice.
Alternatively, rapid transfer of heat into the sea would increase ocean
temperatures, and sea level would rise because of thermal expansion and by
accelerated Antarctic ice discharge associated with increased melting from
beneath the floating ice shelves. Moreover, sea-ice distribution both
influences, and is affected by, thermal interactions between atmosphere and
ocean.
At present we cannot determine the partition of heat between ocean and
atmosphere. Consequently, even for well-defined rates of increasing
greenhouse gases, we cannot accurately predict the response of either the
climate or sea level. Nevertheless, we can estimate probable limiting rates
of sea level rise by examining "reasonable" climate scenarios that would
either favor or inhibit sea level change.
Past Trends in Global Sea Level
Numerous studies of long-term tide-gauge measurements suggest that global
sea level has risen 10-15 cm during the last hundred years (e.g., Barnett
1983). The reasons for this have been variously described to thermal expan-
sion of a warming ocean (Gornitz, Lebedeff, and Hansen 1982); melting of ice
from Greenland and Antarctica (Etkins and Epstein 1982) or from small ice caps
and glaciers in lower latitudes (Meier 1984); or systematic downwarping of
coastlines due to the effects of sedimentation (Pirazolli 1986). There is a
consensus that the observations can be explained by the combined effects of
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GREENHOUSE OASES
GLOBAL WARMNQ
ATMOSPHERE
i
SEA-ICE
DISTRIBUTION
^^^^
MCREA8ED ICE
MELTING
PARTICULARLY
IN GREENLAND
INCREASED
SNOWFALL ON
THE ICE SHEETS
SEA-LEVEL
RISE
SEA-LEVEL
FALL
1.
fS
X
OCEAN
' /
THERMAL
EXPANSION
\
V
INCREASED
MELTWQ
BENEATH
ICE SHELVES
INCREASED
ICE DISCHARGE
FROM ANTARCTICA
INTO THE OCEAN
SEA-LEVEL
RISE
Figure 1. The Major Processes Relating Greenhouse Warming
to Average Worldwide Sea Level
21
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melting from small ice caps and glaciers and of thermal expansion of ocean
waters. But there are no data confirming thac this is so.
We do know that glaciers and small ice caps have retreated during the
past 100 years, but we cannot accurately estimate their contribution to sea
level rise. Information from the ocean presents a yet more confusing
picture. There appears to have been an increase in sea surface temperatures
since 1900, but part of this apparent increase may be due to changes in
measuring techniques. Moreover, the longest time series (26 years) of deep
hydrographic stations (near Bermuda) provide a very noisy signal, with large
decadal temperature oscillations superimposed on a net cooling in the upper
1,000 m, and a more systematic warming trend at greater depths (Roemmich
1985). Further north, repeat measurements along a traverse across the north
Atlantic reveal a cooling and freshening since 1972 at all depths down to
4,000 m (Swift 1984). Considerably more measurements from many parts of the
ocean will be required before these differing trends can be explained.
Estimates of ice-sheet mass balance in Greenland and Antarctica are no
less ambiguous. There is good evidence both for retreat of the ice margin in
west Greenland and for an increase in surface elevation near the ice sheet
summit, and the present consensus is for an approximate balance between
snowfall and losses by melting and iceberg calving. However, errors are so
large that the Greenland ice sheet could be thickening or thinning
sufficiently to cause a sea level fall or rise of up to 0.4 mm/yr (Reeh
1985). Antarctic mass balance is even less well measured. Here, consensus
opinion is that ice volume is slowly increasing, but available data cannot
rule out thickening or thinning equivalent to a sea level change ranging from
a drop of 3 mm/yr to a rise of 1.5 mm/yr.
The possibility that the observed apparent sea level rise is due to a
tendency for coastal downwarping has intuitive attraction, since there is a
continuous transfer of sediments towards the coast. However, the spatial
correlation of observed sea level increase suggests that there has been
eustatic sea level rise during the past 100 years (Barnett 1983).
In summary, there probably has been a sea level rise of 10-15 cm during
the past 100 years. Some of this increase (perhaps 4 or 5 cm) was caused by
net melting of glaciers and small ice caps, and the rest was probably
associated with thermal expansion of a warming ocean. Our greatest
uncertainty regards possible contributions from the ice sheets in Greenland
and Antarctica. Indeed, the comparatively small apparent change in sea level
provides better evidence for near-equilibrium mass balance of the ice sheets
than is available from many decades of glaciological observations.
THE EFFECTS OF A CLIMATIC WARMING
No matter what was responsible for the recent apparent rise in sea level,
it is certain that a significant climate warming will cause an increase in
ocean volume. Rising air temperatures will increase surface melt rates on
glaciers, small ice caps, and the Greenland ice sheet; a warming ocean will
increase in volume by thermal expansion, and it will cause an increase in melt
rates from beneath the vast floating ice shelves around Antarctic. This in
itself will not change sea level, but as the ice shelves thin and weaken, ice
discharge from their tributary glaciers will increase, causing sea level to
rise. (See Figure 2 for a graphic illustration of the contribution that each
22
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2000
2050
2100
a THERMAL
EXPANSION
b GLACIERS AND SMALL
ICE CAPS
2000
2050
YEAR
2100
Figure 2. Estimates of Eustatic Sea Level Rise During the Next
Century. This four-part figure depicts sea level rise 'due to: (a) thermal
expansion; (b) melting of glaciers and small ice caps; (c) melting from the
Greenland ice sheet; and (d) increased ice discharge from Antarctica.
Surface air temperatures are assumed to increase linearly until 2050, to
an average value 3°C higher than at present, and then to remain constant.
Melting from beneath Antarctic ice shelves is assumed to increase in the same
way to either 1 m/yr or 3 m/yr greater than the present rate. The dark
shading indicates the "most probable" response, based on our current
understanding of these processes.
23
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of these factors makes to sea level rise.) In order to assess the contribu-
tion to sea level rise from each of these effects, I shall make some assump-
tions about climate change during the next century.
Present trends in atmospheric concentrations of greenhouse gases suggest
that together, they may have an effect equivalent to a doubling of CCU by the
year 2050, possibly sooner. Most climate studies have indicated that, once
equilibrium has been achieved, earth-surface temperatures will increase by an
average of 1.5°-4.5°C, depending on a variety of feedback mechanisms. Warming
will occur with a lag of a few decades if only the ocean mixed layer is also
warmed, but with a lag of at least 100 years if there is efficient heat
transfer to much greater depths. Climate models take account of this transfer
by allowing heat to leak into the deep ocean at a rate determined by an
effective diffusion coefficient K (Oeschger et al. 1975). Estimates of K show
a large geographical variation, with values near the poles up to 100 times
greater than those near the equator (Hansen et al. 1984). However, in order
to examine the transient climate responses to increases in greenhouse gases,
simple unidimensional models are used with a "global" value of K of 1 to 2
cm2/sec. These studies indicate that the thermal inertia of the ocean delays
climate warming by 100 years or more (Hansen et al. 1984; Siegenthaler and
Oeschger 1984).
In order to provide a "baseline" estimate of sea level rise during the
next century, I assume that earth-surface temperatures increase by an average
of 3°C by the year 2050 and remain constant thereafter. Warming in the polar
regions will be amplified by a factor of two, and ocean conditions will change
sufficiently to increase melt rates beneath Antarctic ice shelves from
present-day values of a few tens of centimeters per year by between 1 and 3
meters per year. Moreover, I assume that snow accumulation on the ice sheets
remains unchanged. These conditions correspond to:
* A growth in greenhouse gases consistent with many current estimates,
but with an arbitrary termination of growth in 2050
• A swift climatic response to these gases.
This probably represents a "reasonable" scenario, with some attempt being
made next century to limit growth rates of greenhouse gases. Continued growth
beyond 2050, a larger climate sensitivity to C02 doubling, or a greater polar
amplification could result in a more rapid sea level rise. Alternatively, a
significantly delayed climate response to increasing COj and probably
increased snow accumulation over the ice sheets would reduce the increase in
sea level. In a later section, I try to indicate probable impacts of these
more extreme cases.
Ocean Response
Heating of both the mixed layer and the deeper ocean will cause sea level
to rise by thermal expansion. The calculations of Hoffman, Wells, and Titus
(1985) using a value of diffusivity (K) = 1.7 cnr/sec yield a sea level rise
of almost 30 cm for a global warming of 3°C by 2055, and I have adapted their
estimates to compile Figure 2a. After 2050, sea level continues to increase,
but at a slower pace, since further heating is mainly by leakage of heat into
the deeper ocean (given the assumption that temperatures remain constant after
2050).
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Such a major climatic change is bound to be accompanied by changes in
ocean circulation, atmospheric pressure, and surface winds. These also will
affect sea level. They will not affect the ocean volume; they will simply
redistribute it. At present, we cannot predict these effects, but regionally,
they may cause a raising or lowering of sea level of up to a few tens of
centimeters superimposed on the eustatic change.
Changes in sea-ice distribution, rainfall, winds, temperatures, and
freshwater inflow will all affect the density structure and mixing rate in the
upper layers of the ocean. These, in turn, will affect how rapidly heat
diffuses into the deeper ocean. So the effective diffusivity, K, may change
with time. In particular, the present-day areas of high diffusivity at high
latitudes may be capped by layers of low-density surface water which would
inhibit cooling of deeper water by convective overturning (Figure 3). The net
result could be a significant increase in the amount of heat penetrating
beneath the Antarctic ice shelves.
Figure 3. Heat Transfer Within the Ocean
There is rapid cooling in specific areas within the polar regions where
convective overturning brings warm water to the surface. A significant
change in climate could alter ocean stratification sufficiently to
inhibit this process and trigger a long-term warming of the deep ocean.
This in turn would cause a major increase in melting from beneath the
Antarctic ice shelves.
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Surface Melting from Glaciers and Ice Sheets
Meier (1984) calculated that, during the past 60 years, melting of
glaciers and small ice caps has contributed to sea level rise at an average
rate of about 0.5 mm/yr. He further estimated that a rise in air temperatures
of 1.5° to 4.5°C could lead to glacier wastage equivalent to 1.7-5.2 mm/yr of
sea level rise and a total rise by the year 2100 of 8-25 cm. In the climate
scenario that I have adopted, temperatures rise more rapidly than assumed by
Meier, which yields a rise in sea level of 12-42 cm by 2100. In Figure 2b,
however, I indicate the preferred value to be about 20 cm because, as Meier
stresses, the area of glacier ice would progressively diminish, and
precipitation on many of the glaciers may increase. The preferred value of 20
cm is, of course, arbitrary, but the actual rise is unlikely to be much
greater, since the total volume of glaciers and small ice caps is estimated as
equivalent to between 30 and 60 cm of sea level.
Meier's estimates did not include possible increases in melting from the
ice sheets in Greenland and Antarctica. At present, there is negligible
wastage from the Antarctic ice sheet by surface melting, and temperatures are
so low that a 3°C rise in global temperatures may not cause significant
wastage by 2100. In Greenland, by contrast, about half the wastage is by
melting, at an average rate exceeding 1 m/yr, from approximately 15$ of the
ice-sheet surface. The effect of a warming climate would be twofold: higher
temperatures would increase the melt rate; and the equilibrium line separating
accumulation from ablation would be elevated, exposing a greater percentage of
the ice-sheet area to melting. There may also be an increase in glacier flow
rates (and hence more ice discharged into the sea) associated with enhanced
lubrication by drainage of greater amounts of surface meltwater to the glacier
bed. We cannot at present assess the importance of this process, but we
should flag it as one of the many wild cards in the deck of greenhouse-warming
impacts.
Bindschadler (1985) developed a simple model to predict the response of
the Greenland ice-sheet to warmer climates. Using available measurements of
the atmospheric lapse rate, he estimated that the equilibrium line would be
elevated between 500 m and 1,000 m for a 6°C rise in air temperatures over
Greenland. Because of the anticipated polar amplification of global warming,
this corresponds to a 3°C rise in average global temperatures. Bindschadler
estimated that the resulting excessive melting would cause a sea level rise of
1.3 mm/yr for a 500-m elevation in equilibrium line, and 3-5 mm/yr for
1,000 m. Bindschadler estimated that the rate of sea level rise would be
approximately 30% greater if the rate of iceberg calving is related to
meltwater discharge, as suggested by Sikonia's (1982) study of calving
glaciers. The total rise by the year 2100 would thus be between 13 cm and 34
cm (Figure 2c).
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Accelerated Ice Discharge from Antarctica
Most of the ice flowing from Antarctica drains via (relatively) rapidly
moving ice streams into floating ice shelves (Figure 4). The ice streams push
the ice shelves seaward past their irregular sides and over and around areas
where the ice shelf has run aground on shoaling sea beds. To do this, the ice
streams build up a "gravitational head" by becoming thicker than they would if
the ice shelves were not there. Consequently, removal or weakening of the ice
shelves would permit the ice streams to thin and discharge their excess
thickness into the ocean. In many cases, an ice stream would contribute a
small amount to sea level rise and then acquire a new thickness profile in
equilibrium with the modified ice-shelf conditions. For other ice streams,
however, particularly many of those in West Antarctica which flow over rock
that is well below sea level, they may thin sufficiently to float free of
bedrock. Then, if the ice shelves continue to weaken, most of the ice in West
Antarctica could progressively drain into the ocean. This possibility is
often raised as the major disastrous consequence of climatic warming. Indeed,
it may ultimately occur, but many studies have shown that it would take
several hundred years (see for instance, Thomas, Sanderson, and Rose 1979;
Bentley 1984).
Figure 4. An Ice Stream Flowing into a Floating Ice Shelf
This ice stream is grounded on bedrock that is well below sea level, as
occurs today in many parts of Antarctica. The larger ice shelves occupy
large embayments which, together with locally grounded ice rises,
restrict seaward motion of the ice. This results in a back pressure
exerted on the ice stream. Weakening of the ice shelf by increased basal
melting would reduce the back pressure and permit more rapid ice
discharge along the ice stream.
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At present, the Antarctic ice shelves are able to survive because there
is virtually no melting from their upper surfaces, and melting from beneath is
limited by the restricted amount of ocean heat that reaches the ice shelves.
This is because the ocean bordering Antarctica is very weakly stratified,
permitting convective overturning of warm deep water, which loses heat to the
sea ice and the atmosphere before penetrating beneath the ice shelves. This
is the principal mechanism by which the deep ocean is cooled (See Figure 3).
One consequence of climatic warming may be a more strongly stratified southern
ocean (Gordon 1983; Jacobs 1985). This could have two consequences: it could
limit cooling of the deep ocean and, hence, accelerate the rate of ocean
warming (and associated thermal expansion) and it could allow significantly
more heat to penetrate beneath the ice shelves and increase basal melting
rates.
In order to estimate associated rates of sea level rise, Thomas (1985)
assumed that climatic warming would increase basal melt rates beneath all ice
shelves, starting in the year 2000, by either 1 m/yr or 3 m/yr by 2050. The
value of 1 m/yr was chosen because it is consistent with Gordon's (1983)
estimate of possible ocean warming close to Antarctica and MacAyeal's (1984)
analysis of the dependence of basal melting on ocean temperatures. The value
of 3 m/yr represents an extreme upper limit, with circumpolar deep water
reaching the ice shelves without major cooling, as appears to be happening
today on the west coast of the Antarctic Peninsula.
In the simplest case, Thomas assumed that ice discharge rates from ice
streams into the ice shelves would increase sufficiently to replace ice lost
by enhanced melting. A more elaborate model attempts to simulate the dynamic
response of the tributary ice streams and to estimate the corresponding ice
discharge rates. The two models are in close agreement, with an estimated
total sea level rise by the year 2100 of about 20 cm for ice shelf melting of
1 m/yr, and about 80 cm for shelf melting of 3 m/yr (Figure 2d). Thomas also
attempted to asssess the consequences of a massive increase in iceberg calving
from the ice shelves. If this occurred in 2050, then total sea level rise by
2100 for either melting rate could be close to 2 m. However, such calving is
unlikely unless air temperatures rise considerably higher than is predicted
during the next century. Of course, if temperatures continue to rise beyond
the year 2100, the ice shelves probably will ultimately break up, thus causing
a much larger rise in sea level.
Our present understanding of ocean/ice interactions indicates that
melting beneath the very large ice shelves is unlikely to increase by more
than 1 m/yr, but I stress that our understanding is still rudimentary.
Nevertheless, in Figure 2d, I indicate the lower curve as preferred.
Sea Level Rise Through 2100
Summing the plots for each of the four processes in Figure 2 gives the
estimate of total sea level rise shown in Figure 5. The total sea level rise
by 2100 for the assumed scenario of climate change is estimated to be 0.9-
1.7 m, with a preferred value close to 110 cm.
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2000
2050
2100
2000
2050
YEAR
2100
Figure 5. Total Sea Level Rise During the Next Century
The dark shading indicates the most probable response to the climate
scenario described in the caption to Figure 2. The broken line depicts
the response to a warming trend delayed 100 years by thermal inertia of
the ocean. A global warming of 6°C by 2100, which represents an extreme
upper limit, would result in a sea level rise of about 2.3 m, but errors
on this estimate are very large.
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FACTORS THAT MAY INFLUENCE THE RATE AT WHICH THE SEA RISES
No extreme assumptions were made in deriving the above estimate.
Therefore, I suggest that we should take very seriously the possibility that
sea level will rise by approximately 1 m during the next century. Clearly
many factors could either delay or accelerate this rise: I now try to
establish some probable limits by quantifying these factors.
Delayed Warming
The assumed climate sensitivity of a 3°C average warming for an effective
doubling of C02 represents a fair consensus of the many attempts that have
been made to model the climate response. Moreover, there is a good
probability that equivalent doubling of C02 will occur next century. However,
there will almost certainly be a lag of many decades between the COp doubling
and climatic warming {Hansen et al. 1984; Siegenthaler and Oeschger 1984) due
to diffusion of heat into the deep ocean. This will have two important
effects: Air temperatures prior to 2100 will be lower than I have assumed,
reducing the amount by which the sea will rise because of surface melting from
glaciers, small ice caps, and the Greenland ice sheet. In addition, the ocean
mixed layer will warm more slowly than assumed earlier, and a greater
proportion of available heat will escape into the deep ocean.
In order to assess how sensitive sea level is to these effects, I
recalculate the curves in Figure 2 and obtain a 3°C increase in average global
temperature by 2150, one hundred years later than in the earlier scenario.
For Greenland, I have attempted to correct the estimates for drainage of
meltwater into the underlying, porous firn by requiring meltwater to saturate
the firn before any runoff into the ocean can occur. In this case, the
Greenland ice sheet contributes 9 ± 4 cm to sea level rise by 2100, and
glaciers and ice caps contribute an additional 14 ± 8 cm. This last estimate
is close to the preferred value in Figure 2b, which was constrained to be low
by the limited volume of this source of meltwater. The thermal expansion
values calculated by Hoffman, Wells, and Titus (1985) for their low scenario
provide an estimate appropriate to global warming of 2.3°C by 2100, which
corresponds closely to a 3 C increase by 2150. Their estimate of total sea
level rise by 2100 due to thermal expansion is approximately 28 cm. For the
Antarctic contribution, I assumed that basal melt rates will increase to 1
m/yr by 2150, and I have not considered the possibility of higher melting
rates beneath the large ice shelves, since these would require major changes
to ocean characteristics around Antarctica. Sea level rise by 2100 from
Antarctic ice discharge then becomes approximately 13 cm. Moreover, the
effects of enhanced snow accumulation over Antarctica associated with climate
warming could reduce this to less than 10 cm (National Research Council 1985,
p. 64). The sum of these modified estimates, which probably represent a lower
bound, yields a total approximately equivalent to a 60-cm rise by 2100.
Increased Warming
Three factors could result in a sea level rise significantly greater than
the "baseline" value of 110 cm by 2100:
• Continued growth in greenhouse gases beyond an equivalent COo doubling
by 2050
30
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• Polar amplification greater than doubling of the global warming
• Greater climate sensitivity to doubled CC^*
To estimate the effect of these factors, I assume a 4.5°C climate sen-
sitivity to doubled CCU with a threefold polar amplification, and an
equivalent 150$ increase of C02 by 2100. This represents an extreme scenario,
and 1 shall try to retain realism by imposing a 30-year delay on climate
response, and a requirement that Greenland meltwater saturate porous firn
before draining into the ocean. The large increase in high-latitude
temperatures implies that there probably would be appreciable surface melting
in Antarctica. However, I do not attempt to estimate how much; instead, I
assume it will balance effects of increased snow accumulation.
Extrapolation of the impact on glaciers and ice sheets of such an extreme
warming is at best approximate, and I stress that errors on the resulting
estimate are large. Many small glaciers and ice caps would disappear with an
associated sea level rise of about 35 cm. All of the Greenland ice sheet
would become an area of net ablation by 2100, contributing approximately 9
mm/yr to sea level rise (Bindschadler 1985), with a total associated increase
of perhaps U5 cm. For the Antarctic, I adopt the upper limit of an 80-cm rise
by 2100 shown in Figure 2d, corresponding to an increase in ice-shelf basal
melting of 3 m/yr. This may be a low estimate primarily because such warm
climatic conditions could result in accelerated break-up of parts of the ice
shelves to permit far greater ice discharge into the sea. Under the Hoffman,
Wells, and Titus (1985) "warm scenario," thermal expansion is estimated to
increase sea level by a 6°C rise in global temperatures over 100 years. The
total increase in sea level by 2100 then becomes 2.3 m. I regard this as an
extreme upper limit corresponding to a climatic change so severe that the
effects of impacts on global weather and agriculture could well dwarf those of
even such a dramatic rise in sea level. Nevertheless, it highlights the need
for us to develop a better understanding of just what increased concentrations
of "greenhouse" gases will do to the climate.
INDICATORS OF A GLOBAL WARMING TREND CAN BE DETECTED
No matter what the total rise in sea level turns out to be by the year
2100, it is certain that initial acceleration will be very slow. It will
probably not be detectable before 2020 at the earliest. Thereafter, the rate
at which sea level will rise will progressively increase to about 1 cm/yr by
2100, after which the rate will probably continue to increase. In order to
plan for such a comparatively rapid rise in sea level, it would be valuable to
establish an early warning program to detect the first signs of the greenhouse
warming. 1 describe a set of satellite measurements that would contribute
significantly towards such a program.
A sustained climatic warming will inevitably cause changes in surface
temperatures on the oceans, ice cover, and land, with associated changes in
surface winds. These changes, in turn, will affect the extent, distribution
and compactness of sea ice in the Arctic and around Antarctica. Moreover, if
air temperatures increase in the polar regions, there will an increase in the
area of summer melt zones on the ice sheets in Greenland and Antarctica.
Finally, after a considerable delay, the extent and surface elevation of these
ice sheets will change, probably in a complex way. All of these parameters
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can be measured from space with instruments that are operating now or are
planned for launch during the next few years (Thomas 1986).
Changes in Surface Temperatures
Infrared radiometers have been flown aboard a series of polar-orbiting
weather satellites since 1972. By scanning a broad swath along the orbit
track, they obtain global coverage every day. These data provide estimates of
surface temperatures on the oceans, land, and ice. Unfortunately, calibration
problems make it difficult to intercompare data from different sensors.
Moreover, atmospheric effects limit accuracy of the estimated temperatures to
about ± 1°C. However, calibration of future instruments will be improved, and
a new instrument, which will reduce the uncertainty attributable to the
atmosphere, will fly aboard the European Space Agencies Remote Sensing
Satellite (ERS-1) in the early 1990s. Infrared measurements are limited by
cloud cover, and to overcome this a Low Frequency Microwave Radiometer (LFMR)
with an all-weather capability for measuring sea-surface temperatures will be
flown aboard a U.S. Navy spacecraft, also in the early 1990s.
Rigorous intercomparison of these various techniques for measuring
surface temperature will improve the accuracy of the long-term measurements
from weather satellites (Njoku 1985). Moreover, the large polar-orbiting
satellites planned as part of NASA's Space Station and ESA's Columbus program
will provide opportunities for long-term continued operation of the new
infrared and microwave radiometers.
Changes in Sea Ice Cover
Estimates of the extent, distribution, and compactness of polar sea ice
have been made almost continuously since 1972 using data from microwave
radiometers aboard NASA's Nimbus satellites (Zwally et al, 1983a; Cavalieri,
Gloersen, and Campbell 1984; Swift, Fedor, and Ramseier 1985;). These data
have already revealed major interannual changes in sea ice extent, but no
clear long-term trend. Similar data will be acquired by future weather
satellites of the U.S. Navy and Air Force, promising a time series of sea ice
observations extending over several decades.
Summer Melting in Greenland and Antarctica
Because surface slopes are very small over the vast ice sheets, a small
rise in the surface elevation of the 0°C summer isotherm causes a large
increase in the area subject to surface melting. Wet snow has a microwave
emission that is quite distinct from that of dry snow, and the extent of
summer melting can readily be mapped using data from radiometers aboard the
Nimbus and U.S. Navy/Air Force spacecraft (Zwally and Gloersen 1977).
Topography and Extent of Ice Sheet
Significant change in the size of the continental ice sheets will be a
comparatively slow response to climatic warming. Nevertheless, it is
important, periodically (perhaps every 5-10 years) to map accurately their
areal extent and surface topography. Here, it is useful to note that a 1-cm
rise in sea level is equivalent to a 25-cm change in thickness over the entire
Antarctic ice sheet. Moreover, actual ice-sheet changes are more likely to
32
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occur within individual drainage basins than uniformly over the entire ice
sheet. This would further increase sensitivity so that, for a sea-level
change of less than 1 cm, ice thickness within the affected drainage basin
would probably change by several meters, which would be readily detectable by
a satellite radar altimeter (Zwally et al. 1983b).
The areal extent of the ice sheets can be accurately mapped using images
obtained at visible wavelengths (from the U.S. Landsat/Eosat and the French
SPOT missions), and at microwave frequencies by synthetic aperture radar
(SAR). Although Landsat-type data have been obtained since 1972 there is but
poor and intermittent coverage of Greenland and Antarctica. A SAR was carried
aboard NASA's Seasat in 1978, but very few images were obtained over
Greenland, and none from Antarctica. However, there will be several
opportunities during the next decade to map the ice sheets using high-
resolution radar imagery.
Changes in the Speed and Direction of Ocean Winds
Estimates of ocean surface winds can be derived from measurements of
either microwave emissions or radar backscatter. The necessary passive-
microwave data (providing estimates of wind speed but not direction) have been
acquired since 1978 by Nimbus-7 and will be extended into the 1990s by U.S.
Navy/Air Force weather satellites. A radar scatterometer aboard NASA's Seasat
provided estimates of sea-surface winds over almost the entire globe every two
days. Scatterometers will be included aboard three satellites planned for
flight during the early 1990s.
Changes in Ocean Surface Topography
Many radar altimeters are planned for flight during the next decade and
similar instruments will almost certainly be included aboard Space Station
and/or Columbus polar platforms. They are designed primarily to measure very
accurately the surface topography of the ocean, with the effects of waves
smoothed. Range measurements from satellite to sea surface are accurate to
between 2 and 10 cm. However, radar altimeter data are unlikely to reveal
sea-level trends because satellite orbit errors limit the accuracy of absolute
sea surface elevation to a few tenths of a meter. Nevertheless, significant
changes over time in ocean currents will be detectable, since they manifest
themselves as changes in the regional tilt of the ocean surface.
SUMMARY
The conclusion that sea level will probably rise by about 1 m by 2100 if
climate becomes significantly warmer due to increasing atmospheric con-
centrations of manmade gases is not new (see for instance, Revelle 1983;
Hoffman, Keyes, and Titus 1983). Earlier reports, however, focused on sea
level rise due to thermal expansion with almost ad hoc estimates of the
effects of melting ice. Here, I have tried to address explicity all potential
contributions to a eustatic rise in sea level, incorporating results of
analyses presented in a report published by the National Research Council
(1985). Three conclusions can be drawn from this summary:
• Sea level will probably rise by 0.6-2.3 m by 2100, with a "most
probable" rise of about 1 m.
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• The increase in sea level will be very slow for the first few decades,
but thereafter rates will increase progressively. This increase will
probably not be unambiguously detectable until the year 2020 or later.
• There will probably be little net contribution to sea level rise
during the next century from the ice in Antarctica unless conditions
in the southern ocean change dramatically. If climate warming is
sustained, however, ice discharge from Antarctica will ultimately
become the dominant cause of sea level rise, with the possibility of a
several-metre increase during the following centuries.
Because sea level rise will initially be very slow, it is important to
develop an early warning system for detection of proxy indicators that will
herald the predicted climatic warming. This could be achieved principally by
monitoring from space these parameters:
• Surface temperatures (on the oceans, ice, and land).
• Extent and compactness of the sea-ice cover in both polar regions.
• Areal extent of summer melt zones on the Greenland and Antarctica ice
sheets.
• Areal extent and surface topography of these ice sheets.
The instruments needed to measure all these parameters are either aboard
existing satellites or are planned for launch during the next few years. It
will be important to coordinate the observation and analysis programs for
these various instruments in order to ensure compilation of a long time series
of compatible measurements.
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Flooding in Taipei, Taiwan and Coastal Drainage
Chin Kuo
Virginia Polytechnic Institute and State University
Blacksburg, Virginia USA
ABSTRACT
Coastal drainage problems associated with the existing inadequate drain-
age system and land subsidence in the city of Taipei are discussed. Impacts
on the system due to potential sea level rise are illustrated. General
implications for all coastal communities of designing today a drainage system
that will protect against rise in sea level or retrofitting the system in the
future are examined.
INTRODUCTION
The uniqueness of flooding in the coastal areas can be characterized by
two key parameters: flat terrains and tidal effects (Kuo 1984). The hydrau-
lic head available for gravity drainage, either by storm sewer pipes or by
channels, is generally small. A side effect of the poor and slow drainage is
a decrease in the transport rate of sediment and debris, which further reduces
the pipe and channel flow capacities. During extraordinary storms, such as a
hurricane or typhoon, the tide elevation associated with the storm surge is
high. As a result, flood protection measures, such as flood walls, levees,
pumping stations, and flap gates are necessary. If sea level rises and/or
land subsides, the head against which pumps work will further increase,
creating an undesirable planning and design situation for coastal drainage
(Kuo 1986). This paper discusses these coastal stormwater management problems
using the city of Taipei as an example to illustrate how the existing
inadequate drainage problems are intensified subject to ongoing land
subsidence and future sea level rise.
BASIN CHARACTERISTICS AND BASIC HYDROLOGY DATA
The city of Taipei is situated in the Taipei Basin about 17 km from the
ocean and is surrounded by the Hsintien River on the south, the Tamsui River
on the west, the Keelung River on the north, and hills on the east and
37
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southeast. The elevation in the basin ranges from 1.3 m to 7 m in the city
with an average slope of 0.1% running from southeast to northwest. Three
rivers (the Tamsui River is the largest) are all tidal rivers subject to
moderate tidal influence. The Tamsui River has a watershed of 2726 m , annual
mean flow of 6835 x 10" nP and a 200-year flood discharge of 25,000 cubic
meters per second. Mean annual rainfall in the basin is 295 cm. Based on the
recorded rainfall data, 15% of storms have their peak discharge within the
first 20 minutes of the storm, due to short river reach, steep river bed, and
high rainfall intensity.
The normal high tide of the Tamsui River at the Taipei Bridge is 1.32 m
above sea level, which is higher than part of the city. The river stage for
5- and 200-year storms are 4.58 m and 8.43 m, respectively (Public Works
Department, City of Taipei 1982).
INADEQUACIES OF THE PRESENT DRAINAGE SYSTEM
There are six major drainage subsystems in the city (Figure 1), which
drain about 6,700 ha. The total length of the storm sewer trunk line is
424 m, and the total length of levee along the three rivers is 53,408 m.
There are 30 pumping stations. The total volume of storm runoff pumped is
374.1 cubic meters per second (Public Works Department, City of Taipei
1982). The drainage system is by gravity for normal tidal stage in the rivers
and under pumping for high river stages during the typhoon events. More
pumping stations and drainage mains are under construction.
The current design criteria for the storm sewer system is based on a
typical 5-year summer thunderstorm event. Pumping stations are designed for a
5-year typhoon for normal operation, and their capacity is for a 200-year
storm.
Traditionally, the metropolitan area has frequently experienced major
floodings along the rivers due to inadequate channel capacity and heavy
rainfalls. In addition, there are a lot of problems associated with the
planning, design, construction, and maintenance of the city storm drainage
systems. Major problems include:
• Surface runoff within the city and flood flow in the surrounding
rivers have increased due to rapid urbanization in the past 25
years. The drainage system at the downstream end of the basin does
not have enough capacity to handle the runoff generated from headwater
areas where most of the urbanization takes place.
• The existing drainage system, initially designed more than 50 years
ago for a 2-year storm, has proven to be inadequate to accommodate the
urban growth.
• There exists a serious problem of pipe clogging due to deposition of
debris, sediment, residuals of construction, and other factors. At
the foot of the hill, massive sediment deposition often blocks the
entrances of the storm sewer pipes.
38
-------
Figure 1. Sub-drainage Basins in the City of Taipei
Source: Public Works Department, City of Taipei 1982
39
-------
• During the typhoon period, the river stages are high. Some areas do
not have levees. Leakage of flap gates occurs frequently.
• The number of pumping stations is not sufficient and capacities of
pumps are not adequate.
• Subsidence of the entire basin due to excessive groundwater pumping
creates many local impoundments, lowers the river levees, reduces the
hydraulic head available for gravity drainage, and increases the head
against which water must be pumped. Land subsidence has aggravated
the existing inadequate drainage system. Figure 2 shows that sub-
sidence of 2 m has been observed in the field during the period of
1955-1977 (Public Works Department, City of Taipei 1980). The sub-
sidence problem has been under control of the groundwater pumping
law. However, it takes at least 20 years for the subsidence to
stabilize. A computer simulation of drainage systems in Taipei has
been carried out to assess the current flooding and to predict future
flooding due to land subsidence (Yen et al. 1979). Using 1979 as the
reference time, the authors predict that in subdrainage area III the
flood area less than 3 m above sea level (which was 0.005 ha in 1979)
will be 65 ha in 1989 and 346 ha in 1999. The flooding will be mainly
due to land subsidence, the incapability of gravity drainage by storm
sewer pipe networks, and forced drainage by pump and levee systems.
Current plans to upgrade the existing system recommend that the city
increase the number of pumping stations, expand the drainage pipe networks,
and improve the levee system at a cost of $41 million (U.S.) for the next
eight years. The improvements would compensate for urbanization and land
subsidence problems expected through the year 2000.
THE IMPACT OF SEA LEVEL RISE ON THE DRAINAGE SYSTEM IN TAIPEI
With the anticipated global sea level rise and possible increase in the
amount of rainfall due to the greenhouse effect, the current design criteria
will be inadequate and the city will be subjected to more intense flooding.
Neither the existing system nor the upgraded design take sea level rise into
consideration. The drainage problem that would result from land subsidence
only (with no sea level rise) approximates the impact of sea level rise with
no land subsidence. The problems will be compounded if both sea level rise
and land subsidence take place simultaneously. Certainly, long-range planning
is needed to incorporate into the master drainage plan a rise in sea level in
the next 50 years.
There are two major types of coastal flooding due to sea level rise:
storm surge, which takes place in the vicinity of the coast, and the backwater
effect which occurs when floodwater from upstream backs up along the river
because of a rise in water level at the basin outlet. The latter effect is
observed less and less as one moves farther away from the coast. To illus-
trate the backwater effect due to a rise in sea level during an extraordinary
typhoon event at the Taipei Bridge (17 km from the coast), results of the
computer modeling of the Tamsui River (Yen and Lin 1982) are presented.
-------
Teipei
Tamsui River
iwan
N
1 2 3
Km
Units: m
Figure 2. Land Subsidence in Taipei Basin, 1955-77
Source: Yen et al. 1979
-------
Typhoon Gloria hit the area September 10-12, 1963. During the -flood,
most parts of the Taipei City and adjacent areas were inundated with 1-3 m of
water, and thirty-one lives were lost. This typhoon produced the record
rainfall and flooding, which is equivalent to a 200-year storm event. The
observed high tide at the river mouth was about 3 m. Based on this tide
elevation and the 200-year river discharge (250,000 cubic meters per second),
time variation of river levels at the Taipei Bridge is shown in Figure 3, as
well as river levels based on sea level rises of 1 m and 2 m. Sea level rise
at the river mouth of the Tamsui River does produce a backwater effect, which
would worsen the stormwater drainage problem in the city of Taipei.
As shown in Figure 3, a 30-cm rise at the river mouth would not simply
translate to a 30-cm rise in river stage at the Taipei Bridge. There are a
few hydraulic problems associated with the backwater effect: Floodwater in
the river may overtop the levee, and the levee height would have to be raised;
an increase in river stage would necessitate an increase in head for pumping;
more and/or larger pumps would have to be installed; and pumping capacity
would be reduced. The backwater effect lasts for many hours. The time period
in which gravity drainage is sufficient is shortened and pumpings must be used
more. Although the flooding caused by the sea level rise itself, as
illustrated in this example, may not be great, its ultimate impact (that is,
effects such as those caused by the typhoon Gloria) would certainly increase
the degree of flood damage.
Cities located closer to the coast will experience a more severe impact
than the city of Taipei in terms of greater backwater effect and possible
storm surge threat. As described before, the city of Taipei would experience
severe flood-related damage when sea level rise effects are combined with the
problems of inadequate existing drainage system and land subsidence. Houston,
Texas, has a similar land subsidence problem. This coastal city also is
vulnerable to sea level rise (although to a lesser extent) and, as a
consequence, will experience similar flood-control and stormwater management
problems (Amandes 1980).
DESIGN RECOMMENDATIONS OF TWO CASE STUDIES
Precise predictions of future sea level rise are currently not avail-
able. Scientific communities need to improve the prediction capability. In
addition, international bodies, such as the United Nations Environment Pro-
gramme, need to investigate the costs and benefits of controlling emissions of
greenhouse gases. While they do so, local officials and engineers in coastal
cities need to evaluate the implications of sea level rise, so that they do
not inadvertently design a system that is prematurely obsolete. Case studies
conducted in the cities of Charleston, South Carolina, and Fort Walton Beach,
Florida, have examined the question of whether to design a system now based on
a reasonable estimation of projected sea level rise or to retrofit the system
later after sea level rise has occurred.
The residentially and commercially developed Grove Street watershed in
Charleston has a drainage area of 68.4 ha. Average surface slope is less than
Q.5%, Ground elevation ranges from 1.52 to 3.96 m above mean sea level
(MSL), The existing drainage facilities, storm sewer pipes and culverts, are
undersized for the design criteria of a 5-year storm. To upgrade the system,
the master drainage plan was altered to accommodate a 10-year storm event
42
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iver Stage
(M)
12
2m above recorded tide
lm above recorded tide
recorded tide for 200-year
typhoon Gloria, 1963
12
18
24
30
36
Time (Hr.j
Figure 3. River Stages at the Taipei Bridge for Various Tide Elevations
Source: Yen and Lin 1982
-------
(19.89 cm rainfall in 24 hours) and a spring-tide elevation of 1.33 m above
MSL. Design scenarios, shown in Table 1, are based on a hypothetical 10%
increase in precipitation, and projection of sea level rise by Hoffman, Keyes,
and Titus (1983). Various alternatives were explored for each scenario to
determine the most economical system to provide protection for the design
storm. The retrofit alternatives that were considered were pumping stations,
retention/detention basins, and pipe networks. The study concluded that best
strategy was to design now for a 0.32 m rise in sea level (LaRoche and Webb,
Titus et al., in press).
The Gap Creek watershed in Fort Walton Beach has a mixed commercial,
industrial, and residential development; an average surface slope of 0.2$; and
a ground elevation ranging from 0.61 m to 11.28 m above MSL. The groundwater
table is high because of a large swamp situation to the north of the
watershed. Scattered storm sewer pipes are connected to the creek, which is
the major drainage way for the watershed. The current design criteria are a
25-year storm with 24-hour rainfall of 28.58 cm and a tide elevation of
0.15 m. Design scenarios are shown in Table 2. Upgrade alternatives included
increasing the capacity of present retention/detention basins, adding other
large detention facilities, dredging channels, and floodproofing homes. It
was concluded that retrofit to the Fort Walton Beach drainage system should
not be implemented until a rise in sea level takes place (Waddell and
Blaylock, in press; Titus et al., in press 1985).
It is evident from the two case studies that whether or not to design a
coastal drainage system against future sea level rise primarily depends upon
the individual watershed of interest. As a result, planners and engineers
responsible for coastal drainage need to examine the sea level rise issue
during the preliminary design and planning phase in terms of its expected
effect on each watershed in order to avoid excessive cost for future retrofit
of the system.
CONCLUSION
Potential sea level rise is an issue that scientists, engineers, planners
and decision makers worldwide need to address in a joint effort. To avoid
excessive damage from coastal flooding, plans upgrading the existing system
and altering the master drainage plan to account for future sea level rise
must be considered. A major challenge for scientists is improving the
accuracy of predictions of sea level rise. Information concerning design
strategies that protect drainage systems against future sea level rise and the
costs of such strategies will assist researchers and decision makers through-
out the world who are trying to determine the benefits of decreasing emissions
of greenhouse gases in order to reduce global warming and thus the rise in sea
level.
44
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Table 1. Design Scenarios for the Grove Street Watershed
A.
B.
C.
D.
Scenario
Current Sea Level
and Climate Conditions
Low Sea level Rise
and Current Climate
Conditions
Low Sea Level Rise
and Ten Percent
Increase in Design
Storm Precipitation
High Sea Level Rise
and Current Climate
Conditions
Year
1980
2025
2075
1980
2025
2075
1980
2025
2075
1980
2025
2075
Sea
Level
Rise
(m)
0
0
0
0
0,32
1,04
0
0,32
1.04
0
0.45
1.49
Table 2. Design Scenarios for the
A.
B.
C.
D,
Scenario
Current Sea Level
and Climate Conditions
Low Sea level Rise
and Current Climate
Conditions
Low Sea Level Rise
and Ten Percent
Increase in Design
Storm Precipitation
High Sea Level Rise
and Current Climate
Conditions
Year
1984
2025
2075
1984
2025
2075
1984
2025
2075
1984
2025
2075
Sea
Level
Rise
(m)
0
0
0
0
0.32
1.04
0
0.32
1.04
0
0.45
1.49
1984
Tide
Elevation
(m)
1.33
1.33
1.33
1.33
1.33
1.33
1.33
1.33
1.33
1.33
1.33
U33
Gap Creek
Base
Tide
Elevation
(m)
0.15
0.15
0.15
0.15
0.15
0.15
0.15
0.15
0.15
0.15
0.15
0.15
Design
Elevation
(m)
1.33
1.33
1.33
1.33
1.65
2.37
1.33
1.65
2.37
1.33
1.78
2.82
Watershed
Design
Elevation
(m)
0.15
0.15
0.15
0.15
0.47
1.19
0.15
0.47
1.19
0.15
0.60
1.64
Design
Storm
Depth
(cm)
19.89
19.89
19.89
19.89
19.89
19.89
21.89
21.89
21.89
19.89
19.89
19.89
Design
Storm
Precipi-
tation(cm)
28.58
28.58
28.58
28.58
28.58
28.58
31.44
31.44
31.44
28.58
28.58
28.58
45
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REFERENCES
Amandes, C. 1980. Effects of subsidence on stream hydraulics. In Urban stormwatej*
management in coastal areas, ed. C. Y. Kuo. New York: American Society of
Civil Engineers.
Hoffman, J.S., D. Keyes, and J.G. Titus. 1983. Projecting future sea level rise,
GPO #055-000-0236-3. Washington, D.C.: Government Printing Office.
Kuo, C.Y. 1984. Some hydraulic problems related to stormwater drainage design in
coastal areas. In Proceedings of the southeastern conference on theoretical
and applied mechanics. Auburn University, Alabama.
Kuo, C.Y. 1986. Sea level rise and coastal stormwater drainage. In Proceedings oj
the Water Forum '86. New York: American Society of Civil Engineers.
LaRoche, T.B., and M.K. Webb. Impact of sea level rise on stormwater drainage
systems in the Charleston, South Carolina, Area. In Sea level rise and coastal
drainage systems. Washington, D.C.: U.S. Environmental Protection Agency, in
press.
Public Works Department, City of Taipei 1980. Assessment of the storm sewer pipe
system, City of Taipei. Bureau of New Projects, (in Chinese).
Public Works Department, City of Taipei 1982. A briefing for urban stormwater
management workshop. Bureau of Maintenance, (in Chinese).
Titus J.G., C.Y. Kuo, M.J. Gibbs, T.B. LaRoche, M.K. Webb, and J.O. Waddell.
Greenhouse effect, sea level rise and coastal drainage systems. Journal of
Water Resources, (American Society of Civil Engineers), in press.
Waddell, J.O., and R.A. Blaylock. Impact of sea level rise on Gap Creek watershed
in the Fort Walton Beach, Florida, Area. In Sea level rise and coastal
drainage systems. Washington, D.C.: U.S. Environmental Protection Agency, in
press.
Yen, C.L., et al. 1979. A study on the influence of land subsidence on the capacity
of drainage system in Taipei, Research Report No. HY 6805. Department of Civil
Engineering, National Taiwan University, Taipei, (in Chinese).
Yen, C.L., and J.L. Lin. 1982. Tidal hydraulics of the Tamsui River, Research
Report No. HY 7104. Department of Civil Engineering, National Taiwan
University, Taipei, (in Chinese).
46
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The Sea Also Rises:
The Ongoing Dialogue of the Dutch with the Sea
Tom Goemans
SIBAS Joint Institute for Policy Analysis
Delft, Netherlands
INTRODUCTION
The Netherlands belongs to the alluvial coastal region of the North Sea,
which means that the western and northwestern part of the country was formed
by alluvial deposits. The rivers Rhine, Meuse, and Schedule flow through the
country and out into the North Sea (Figure 1). Together they built and shaped
the land with regular flooding.
The first efforts to protect this country can be traced back to the 3rd
century B.C., when people built mounds to live on. During the Roman period
these mounds were linked by elevated roads; dikes were also constructed along
some rivers. By the 13th century, there was a more organized way of dike
building, not only in a defensive way but also in a more offensive way to
reclaim land from the sea (Figure 2). Any excess water could flow out of the
Polders during low tide period through sluices, which were closed during high
tide. Both land subsidence and rising sea level made artificial drainage
necessary, at first by means of windmills, later by steam-driven pumps, and
today by electric pumps. The height of the dikes was based on the highest
Previous flood level. It is hardly surprising that most of the previously
"Declaimed land was frequently lost again; but every time the people fought
back. It is said that the Dutch have had a constant dialogue with the sea.
(See Bruun, this volume, for additional discussion of the history of shore
Protection.)
The history of The Netherlands is marked with storm surge disasters. The
first known disaster occurred in the year 1287 and hit the whole country,
drowning fifty-thousand people. In recent history, major floods occurred in
the years 1877, 1881, 1883, 1889, 1894, 1906, 1916, and finally the most
recent one in 1953. During the night of February 1, 1953, storm surges of
unprecedented height hit the southwestern coastal region (called the Delta
region because of the major rivers). Dikes were seriously damaged over a total
47
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Figure 1. The Netherlands in MW-Europe
Areas Reclaimed from
the Sea since 1200
Figure 2.
48
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length of 190 km. Through 89 breaches in the dikes, 150,000 hectares of
polderland were inundated (Figure 3) causing the death of 1835 people. Total
losses were estimated at some 2 billion guilders (one billion U.S. dollars)
measured in 1953 currency.
Once areas below sea level are flooded they remain inundated after the
flood has retreated. As the tide flows in and out through breaches in the
dikes, it widens them further. Fertile farmlands are then covered with marine
sands, and salt penetrates the soil. The only way to return to normal is to
repair the dikes and pump the inundated areas dry, after which it still takes
several years for the land to recover.
PROTECTION STRATEGY
In reaction to the 1953 disaster, the Dutch government embarked on a
massive building program, the Delta plan. A special committee was given the
task of establishing a design philosophy for the sea defenses along the coast.
In the work of this committee two central issues were addressed: (1) shorten-
ing the coastline by closing off the estuaries in the Delta region and (2)
choosing the design water level for the country's system of the sea
defenses. In this paper only the second issue is addressed.
With respect to the design water level the committee used a philosophy
introduced in 1939 by the Dutch hydraulic engineer Wemelsfelder. He analyzed
the probability of exceedance of storm surge levels along the coast of the
Netherlands and presented probability exceedance lines, using a linear scale
for the water level and a logarithmic scale for frequencies of occurrence. Due
to the tidal system of the North Sea, the tidal amplitude varies from 1 to 4
meters along the coast, and for each location a separate exceedance line can
be constructed (Figure 4). By setting an acceptable probability of exceedance
it is possible to establish a design water level.
The 1953 storm surge had an annual probability of 1:250, corresponding to
a surge level of about +4 m in the disaster area. The committee concluded
that the entire coast of the country was inadequately protected and recom-
mended strengthening of the main sea defense structures. Considering the high
economic value of the central part of the country (major industrial and
agricultural areas, large population in metropolitan areas), the probability
of exceedance was set at 1:10,000. The corresponding water level was derived
from an extrapolation of the exceedance lines; this resulted in a design water
level of +5m. For coastal areas north and south of central Holland, the
committee used lower design water levels (corresponding 'to a probability of
1:4,000) because the area protected has a system of secondary dikes which
limits any flood disaster there to smaller sections.
A dike that has to resist a water level of +5 m must be much higher than
5m, mainly because waves can run up and overtop the dike. Other phenomena
taken into account are seiches and gust bumps, settlement of the dike
material, and sea level rise for the next century. Adding all these
contributions together results in design criteria for all sea defenses; since
49
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Dark iriis indi-
cate flooding
NETHERLANDS
GERMANY
BELGIUM
Figure 3. Flooded Area During the 1953 Disaster
I
103 102 10
1
EXTRAPOLATED
MEASURED
r
10~1 10~2 10~3 1CT4 10~* 10"* n 10~T
probability of exceedance par year
Figure 4. Storm Surge Water Levels for Hoek van HoJland
50
-------
the publication of the committee's final report in 1962 this is called the
Delta level. For a typical dike the following figures are representative:
Storm surge level 5.00 m above MSL
Wave runup
Seiches and gust bumps
Sea level rise
Settlement
Total Height 15.75 m
The core of the typical dike is made of sand, covered with a clay layer
1 m thick. The sea side of the dike is normally also covered with asphalt and
large stone blocks. The outer slope is 1:5, the inner slope, 1:3 (Figure 5).
The Netherlands has about 400 km of sea dikes in addition to the 200 km of
dunes, all of which require constant and careful maintenance [totals mainten-
ance cost is presently about 70 million guilders (U.S. $35 million)]. If the
country was not protected against the sea, more than half of it would be
uninhabitable (Figure 6). About 8 million people live in the protected area.
IMPACT OF SEA LEVEL RISE
The Dutch coast is part of a longer coastline stretching from Belgium to
Germany and Denmark. Some of the problems experienced in The Netherlands also
apply to small parts of these other countries. This paper, however, only
covers the Dutch situation.
Tides
The tidal system for the North Sea is rather complex with several
amphidromic points. Simulation runs with a two-dimensional water movement
model for the North Sea and the continental shelf show that these amphidromic
points might move as a result of mean sea level rise. For most of the Dutch
coast this would mean an increase of the tidal amplitude and of the relative
high-water level. This phenomenon may be supported by past measurements along
the Dutch coast. Whereas for the period 1933-1984 the mean sea level
increased approximately 0.18 m per century, the tidal amplitude increased 0.25
m and the high-water level 0.30 m per century. Similar data were found for
the coast of northwestern Germany.
Changes in Storm Surge Level
Water surges into the North Sea during storms. Calculations have shown
that even for a mean sea level rise of 5 m the water setup does not increase
significantly for a given wind field. From this one might conclude—in first
approximation—that the exceedance line as shown in Figure 4 moves to the left
when mean sea level rises. The protection level of the present sea dikes
decreases when the sea level rises; Figure 7 shows this relation for central
Holland. It- is, however, by no means certain that storm tracks and wind
fields will stay the same when the climate changes. The shape of the
exceedance lines might, therefore, change as they move to the left. Wave
characteristics may also change.
51
-------
1970
1930
MSL
T '
Figure 5. Cross Section of a Typical Dike in Various Years
. . Areas Presently Protected by Dikes
[ ! Against Floods and High Tides
Figure 6.
52
-------
o
o
a
•Q
O
a
= 0
3 S
a 2
A a.
O •_
aS
0.5
2.0
mean sea level rise, m
Figure 7.
Changes in Saltwater Intrusion
The tide penetrates into the country through rivers flowing into the
North Sea, which means that during high water, salinity will increase for a
certain point along the river. As a result of mean sea level rise, saltwater
intrusion will be felt further upstream, but only when the river bed does not
change. Due to sedimentation the flow cross section will tend to come back to
the original one. We believe that the increased saltwater intrusion will not
be a major problem for a sea level rise of up to 2 m. Occasionally, however,
the availability of fresh water for agricultural purposes may be reduced.
Changes in Saltwater Seepage
The difference between the seawater level and the groundwater and
surface-water level in the country determines the amount of seepage. At this
moment the western and northern part of the Netherlands suffers from saltwater
seepage. In order to keep surface-water salinity in the polders acceptably
low, a certain amount of fresh water is needed to flush the canal system. As
a first approximation, a sea level rise of 1 m would require ]Q% more water
for flushing; a sea level rise of 2 m would require 2Q%. In times of drought
53
-------
it is not certain that this amount of water will be available; economic losses
in the agricultural sector could be the result. In any case saltwater seepage
will be felt further east in the country, which makes water management—
already a complex problem in the Netherlands—more difficult.
The freshwater bubble under the dune area will not be directly threatened
by an increasing seawater level, unless the dunes are narrow. The dunes
provide a substantial part of the drinking water needed in the country.
Morphological Changes
Part of the Dutch coast consists of dunes, varying in height between 10 m
and 40 m above mean sea level. At some locations the coast line is advancing
(about 40 m in the past 100 years), but at most locations it is retreating (up
to 100 m). Any sea level rise will result in erosion of the dune area; the
retreat may be on the order of 100-200 m for a mean sea level rise of 1-2 m.
At some places the dune area is narrow and low, which means that dangerous
situations could develop. At other places the dunes are more that 2,000 m
wide, providing more than ample protection.
In the north of the country the Wadden Islands are continuously moving:
retreating at the North Sea side and advancing at the Wadden Sea (landward)
side. Hence they are slowly migrating to the mainland. Any sea level rise
will accelerate this trend. In the Wadden Sea sedimentation will diminish and
intertidal areas will decrease; this may have serious consequences for the
population of wetland birds. The estuaries in the southwestern parts of the
country will experience a large tidal volume with rising sea level, resulting
in erosion of the gullies. Here as well, there will be a reduction of
intertidal areas, which will adversely affect the bird population. If sea
level stabilizes at a higher level for a few centuries, the wetlands may
eventually return. However Thomas (this volume) and others indicate that sea
level is likely to continue rising.
Changes in River Water Level
The water level for the rivers flowing into the North Sea will increase
with the sea level, and because the Netherlands is so flat this will be felt
far upstream. The following are some of the impacts we can expect:
• River dikes, protecting against inundation at high discharge rates,
will be too low for the required protection level (presently set at
1/1,250) especially in the western part of the country.
• Clearance for fixed bridges will decrease and hamper ship traffic on
the rivers.
• Drainage of the low-lying polders will be more difficult, requiring
more energy and modification of pumping systems.
• A decision will have to be made about the water level for Lake Yssel
(a 1,200 sq. km freshwater lake created in 1983 by building a barrier
dam to shut the lake off from the North Sea).
-------
• Harbor facilities along the rivers and especially at the coast—the
port of Rotterdam is the largest in the world—will need adjustments
to the higher water level.
• Shipping locks and discharge sluices will have to be rebuilt,
especially for a water-level rise of more than 1 m.
REACTION TO SEA LEVEL RISE
From the preceding paragraphs it will be clear that the protection
against the sea is an anchorpoint in Dutch politics. Throughout the
centuries, a relatively large amount of money has been spent on defense
structures and on water management. On the basis of the value of the area
protected, a safety level has been chosen; construction is still going on, but
around 1990 the required safety level for the whole country will be reached.
An expected sea level rise of 0.25 m for the next 100 years is taken into
account.
Any sea level rise in excess of that value will result in a reduced
actual safety level. There is no reason to assume that the government will
not stick to the present protection policy; hence the dikes will have to be
heightened. In first approximation this will cost:
Mean Sea Level Rise Expenditures
05m 5 billion guilders
(U.S. $2.2 billion)
10m 10 billion guilders
(U.S. $4.4 billion)
2.0 m 20 billion guilders
(U.S. $8.8 billion)
About 60$ of these expenditures are needed for the dikes and dunes, 30%
for the water management, and 10$ for rivers and ports. Dikes will most
probably be heightened in steps of 0.5 m, each step requiring a minimum of 20
years of construction time for the country as a whole. In the extreme case of
a 2 m rise in 100 years, this means a yearly expenditure of approximately 250
million guilders (U.S. $100 million), which is somewhat less than 0.1 percent
of the Dutch GNP. For the less extreme cases, more time is available for one
step and the yearly costs will correspondingly be lower.
From the above data it may be concluded that there is no need for
anticipation. That is, we can keep up with sea level even it rises more
rapidly than the present rate. The scenario of a 2-m rise in 100 years,
however, results in a critical situation; almost immediately after detection,
actions would be required. It is not at all certain that decision makers act
that fast; in this respect the energy crises of 1973 and 1978 and the acid
rain issue are good lessons. Even more relevant is the fact that the present
flood-protection strategy of the Netherlands came about only after the tragic
disaster of 1953 occurred. When nobody can remember a specific disaster, it
is extremely difficult to obtain consensus on countermeasures. The conclusion
must be that monitoring the sea level change is extremely important in order
to detect "the signal" as early as possible. In addition, it would be wise to
start thinking about the decision-making process that will follow detection of
an accelerated rate of sea level rise.
55
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Considering the position of the Netherlands in northwestern Europe,
multilateral cooperation with the border countries Belgium and Germany will be
necessary. This brings us to the fact that the impacts of sea level rise will
be unequally distributed among countries in the world. No country with a
coastline can escape from the phenomenon once it unfolds. Yet it is not a
natural but a manmade phenomenon; it presents a perfect example of external
costs, that is costs not included in the present price of energy from fossil
fuel burning. Hence the question of compensation payments may come up—
especially in the poorer countries—not only because of sea level rise but
also in the broader context of climate change. Much will depend on how
international relations develop in the next century.
Whereas the impacts of sea level rise are mostly negative, there may be
one positive point, at least for the Dutch. The Delta Plan included not only
heightening of dikes, but also closing off estuaries with the objective to
shorten the coastline. This provided a unique opportunity for Dutch
contractors, consultants, and research institutes to gain experience in
innovative hydraulic engineering. Skills and knowledge are applied all over
the world. A rising sea level may provide a new global market for this
expertise.
ACKNOWLEDGMENT
The author is grateful for the use of data and internal memoranda
provided by the Ministry of Transport and Public Works and by the State
Geological Service in the Netherlands. However, the opinions set forth in
this paper are solely those of the author.
56
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Planning for Sea Level Rise Under Uncertainty:
A Case Study of Charleston, South Carolina
Michael Gibbs
ICF Incorporated
Washington, DC USA
ABSTRACT
The Charleston, South Carolina, area could be significantly affected by
accelerated sea level rise caused by global warming. Preparing for this rise
by building protective structures or adapting to the changing environment by
altering investment patterns can eliminate most of the adverse impacts. This
case study of the Charleston area demonstrates that in some areas actions
should be taken today, despite the current uncertainty surrounding the future
rate of sea level rise, while for other areas it is better to wait for the
resolution of uncertainty before taking actions.
SEA LEVEL RISE: A NEW FACTOR TO CONSIDER IN COASTAL PLANNING
People throughout the world exploit coastal resources to provide many
diverse services. In the United States we are balancing the competing demands
on this sensitive environment through coastal zone management programs. A new
factor to consider in managing coastal zones is a rapid rise in the global sea
level. Although rises and falls in sea level are not new, the impending rapid
rate of rise is a significant departure from recent trends. Adapting to it
will be an important new challenge for coastal planners.
Over the next 50 to 100 years, global sea level may rise at a much faster
rate than historical global and local rates. Waiting to observe the effects
(e.g., erosion and inundation) before responding to them may not be a
satisfactory management strategy because damages may occur too rapidly to
allow for effective responses. However, planning ahead for sea level rise is
difficult because of the considerable uncertainty regarding how rapidly the
global sea level may rise. Only recently have scientists begun to quantify
the potential for sea level rise and precise estimates are not yet available.
57
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Should planners wait until better estimates of sea level rise are avail-
able, or should they identify options and act now? This paper addresses these
questions using a case studv of the potential economic impacts on portions of
Charleston, South Carolina. It examines planning options available to the
city and quantifies the implications of alternative planning strategies.
Four sea level rise scenarios (shown in Table 1) were drawn from a recent
EPA report for this analysis (Hoffman, Keyes, and Titus 1983). The baseline
Table 1. Potential Future Relative Sea Level Rise in Charleston,
South Carolina [in Centimeters (Feet)j
Sea Level Rise Scenario
Baseline
Low
Medium
High
1980
0
0
0
0
YEAR
2025
11 (0.4)
28 (0.9)
46 (1.5)
64 (2.1)
2075
24 (0.8)
88 (2.9)
159 (5.2)
232 (7.6)
scenario, 24 cm by 2075, assumes a continuation of the recent historical rates
of relative sea level rise in the Charleston area (this scenario is unlikely
and is viewed as a lower bound); the low and medium scenarios, 88 cm and 159
cm by 2075, represent the likely range of future sea level rise; the high
scenario of 232 cm is used as an upper bound. Although the analysis only
covers the period up to 2075, the global sea level is expected to continue to
rise beyond that time.
THE CASE STUDY AREA
The Charleston area consists of the land around Charleston Harbor, which
is formed by the confluence of the Cooper, Ashley, and Wando Rivers (see
Figure 1). Included in this area are the peninsula portion of the city of
Charleston, portions of the city of Charleston in West Ashley and James
Island, portions of the city of North Charleston, the towns of Mount Pleasant
and Sullivans Island, and unincorporated portions of Charleston County. This
analysis focuses on two portions of the study area: the peninsula and
Sullivans Island.
The peninsula has a maximum elevation of up to 10 meters (33 feet) above
mean sea level and includes several low-lying areas that have been reclaimed
from the harbor. The only major flood protection structure, the Battery, is
' The Charleston case study analysis presented in this paper is an extension!
of the analysis described in Gibbs (1984).
58
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DANIEL
tsland
..--•:;-'•' MT. PLEASANT
L-CHARLESTO
eninsula
WES
ASHLEYx:
JAMES
Island
Figure 1. Charleston Study Area
59
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located at the southern end of the peninsula. This seawall has a maximum
elevation of six feet above mean sea level.
Property values on the peninsula exceed $875 million (1980 dollars). The
southern end of the peninsula has a densely populated historic district and
other residential, commercial, and port areas. The central peninsula consists
of industrial parks and marshland, and the upper peninsula (North Charleston)
has a combination of residential areas and industry, including a large naval
reservation. Because most of the peninsula is already highly developed, its
potential for growth is limited. Some shifts in land use are expected,
primarily to more high-density development.
Sullivans Island is a narrow barrier island with an average elevation of
less than 3 meters above sea level. A residential and resort community, it
has been developed with single-family homes. Its zoning regulations currently
prohibit high-rise and condominium construction. Current property values
total approximately $65 million (1980 dollars).
Vulnerability to Sea Level Rise
The peninsula is the portion of the Charleston area least vulnerable to
sea level rise. It is currently protected by the Battery at the southern end,
and its port facilities provide stabilization. While the peninsula has
various low-lying areas threatened by inundation and erosion, its high
development density will likely make protection of the areas economically
justified.
Sullivans Island, on the other hand, is a low-lying barrier island of
highly erodable materials. Exposed to open-ocean wave attack, it is extremely
vulnerable to sea level rise. The low- to medium-density development on the
island may be insufficient to make protection with a sea wall economical.
Additionally, the substantial recreational value of its beaches would be lost
if a sea wall were built.
Table 2 shows the potential economic impact of sea level rise on these
two areas. For each area, estimates are shown for all four sea level rise
scenarios. First, the estimate of the economic value for each scenario is
shown. This value indicates the level of economic activity that will go on in
the study area between 1980 and 2075. For example, in the peninsula area, the
economic value is estimated at $2.21 to $2.27 billion in the low sea level
rise scenario.
The second column in Table 2 shows the change in economic value from the
baseline scenario. This change in economic value is the estimate of the
impact of the sea level rise scenario. The last column displays this impact
as a percent of the baseline economic value estimate.
Economic impacts include the value of shorelines movement due to erosion and
inundation, increased storm damage, and changes in investment patterns. The
method used to estimate economic impacts is described in Gibbs (1984).
60
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Table 2. Potential Economic Impacts of Sea Level Rise on the Charleston
Peninsula and Sullivans Island: 1980 to 2075a
Area
Sea Level
Rise
Scenario
Estimate of
Economic Value
(millions of
1980 dollars)
Estimate of
Economic Impact
(millions of
1980 dollars)
Estimate of
Economic Impact
(percent
of baseline
economic value)
Charleston
Peninsula
Sullivans
Island
Baseline
Low
Medium
High
Baseline
Low
Medium
High
2,520
2,210 - 2,270
1,770 - 1,870
1,435 - 1,550
85
40
150
• 115
• 90
27 - 75
250 - 310
650 - 750
970 -1,085
35 - 65
60 - 110
75 - 125
10
26
38
23
60
40
50
12
30
43
43
110
73
83
a Estimates represent impacts of sea level rise in the absence of actions
taken to respond to sea level rise. All estimates are present values eval-
uated at a three percent real rate of discount.
The range of impacts shown in Table 2 (e.g., $250 to $310 million for the
low scenario on the peninsula) reflects a range of assumptions regarding how
investment patterns change in response to sea level rise. The upper bound of
the economic impact estimate assumes that people incorporate the potential for
sea level rise very slowly into their decision making. The lower bound
assumes that people immediately alter their investment patterns in
anticipation of future sea level rise. The actual investment behavior will
likely fall within this range.
The estimates in Table 2 do not incorporate community protection systems
such as levees, sea walls, zoning ordinances, or beach stabilization. The
magnitude of the impacts indicate that such community responses may be war-
ranted. The impacts are expected to be large, both in absolute terms
(hundreds of millions of dollars) and in relative terms (a large fraction of
the economic activity in the study area will be affected). The question is:
what actions should be taken now, given current uncertainties about the future
rate of sea level rise? To answer this question, the benefits of a variety of
actions in each of the two areas are evaluated below.
61
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EVALUATION OF ALTERNATIVE PROTECTION STRATEGIES
To evaluate whether it is preferred to act now (and invest funds to
protect areas from potential future sea level rise) or to wait (and act later
when the rate of sea level rise can be projected with greater precision), a
series of protection strategies for the peninsula and for Sullivans Island
were examined. Each of the strategies was evaluated across the range of
potential rates of sea level rise.
On the one hand, acting now runs the risks of
• Under-Protecting — sea level rise turns out to be more extensive than
expected, and too little protection was provided
• Over-Protection — sea level rise turns out to be less extensive than
expected, and too much protection was provided, at too great an
expense.
Waiting for more precise estimates of sea level rise runs the risks of
• Being unable to respond rapidly enough if the sea level rises rapidly
in the future
• Making investments in structures today that are subsequently lost to
the sea.
The tradeoffs among these risks, and the ability to make decisions today
(in the face of uncertainty), depend on the sensitivity of the decisions to
the rate of sea level rise. If the same decision (i.e., protection strategy)
is preferred regardless of the future rate of sea level rise, then the
decision can be made today. Alternatively, if the preferred decision is very
sensitive to the expected rate, then improved estimates of sea level rise may
be required before making a decision.
Protection Strategies for Charleston Peninsula
While a variety of protective measures can reduce the impacts of sea
level rise on the peninsula, this analysis focuses on the most likely
response, resource construction of a sea wall or levee around the peninsula.
The sea wall would stop shoreline retreat as well as protect against storm
surge. It would cost about $150 million (1980 dollars), based on a unit cost
of approximately $6,500 per meter ($2,000 per foot) over the 22.5 km (11
miles) of coastline. Operating and maintaining the seawall structure may cost
about 1 percent of the original costs, or $1.5 million per year. The
questions here are whether to build it, when to build it, and how high to
build it.
Three options for protection of the peninsula were examined:
• Option 1; Crash Construction Program. A protective structure is
built as rapidly as possible. For this evaluation, the completion
date is set at 1990. The height of the structure is assumed to be
sufficient to protect from a 90-cm (3-ft) sea level rise.
62
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• Option 2: Routine Construction Program. A protective structure is
built over the next 30 years, with a completion date set at 2020. The
structure is assumed to be sufficient to protect from a 90-cm (3-ft)
sea level rise.
• Option 3; Delayed Construction. Planning for the construction of the
structure is delayed until precise estimates of the expected rate of
sea level rise are obtained. A. completion date is set for 2050, and
the height of the structure is assumed to be appropriate for the rate
of sea level rise that is identified in the future.
The benefits of each of the options relative to doing nothing are
presented in Table 3 for each of the four sea level rise scenarios. For
example, if the low scenario comes true, the economic value of the development
over time is estimated at $2,210 to $2,270 million assuming no structure is
built. If Option 1, Crash Program, is implemented, it is estimated that the
economic value would increase by $830 to $770 million. This increase is due
to reduced shoreline retreat and reduced storm damage. From this increase is
subtracted the present value of the cost of the program, $400 million.3 The
net increased value over no structure is consequently $430 to $370 million.
Finally, the economic value of the development in the area assuming the low
scenario comes true and Option 1 is performed is estimated at approximately
$2.640 million ($2,210 + $430 and $2,270 + $370). The values for the other
scenario/protection option combinations are interpreted similarly.
The information in Table 3 provides considerable insight into strategies
for preparing for sea level rise. It shows that doing nothing is inferior to
the other options across all the scenarios. Therefore, the sea wall should be
built. Similarly, delaying construction to 2050 is inferior, meaning that it
should be built sooner, rather than later. Table 3 also shows that Options 1
and 2 have considerable value and can offset most of the adverse economic
impacts of a rise in sea level.
The costs of the options are computed as follows:
• Option 1: — Construction cost - $150 million
— Present value of operating and maintenance costs of
$1.5 million per year at a 3 percent discount rate
equals $50 million
— Shadow price of the investment equals 2.0
— Present value of the cost at a 3 percent discount
rate equals: (2.0) x (150 + 50) = $400 million.
• Option 2: The cost of Option 1, $400 million, delayed by 30 years
at a 3 percent discount rate equals:
400 * (1.03)30 = $165 million.
• Option 3: The cost of Option 1, $400 million, delayed by 60 years
at a 3 percent discount rate equals: 400 * (1.03)*** =
$70 million.
63
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Table 3. Estimates of the Economic Value of the Development in the
Charleston Peninsula Area for Three Protective Options
(millions of 1980 dollars)
1.
2.
3.
Protection Options
No Structure
— Economic Va lue
Crash Construction Program
— Increased Economic Value over
No Structure
— Increased Value Net of $400 million
cost
— Economic va lue
Routine .Construction Program
— Increased Economic Value over
No Structure
— Increased Value Net of $165 million
cost
— Economic Value
Delay Construction
— Increased Economic Value over
No Structure
— Increased Value Net of $70 million
cost
— Economic Va lue
Base 1 i ne
2,520
570
170
2,690
380
215
2,735
190
120
2,640
Sea Leve
Low
-
2,210
-
830
U30
2,640
600
485
2,645
320
250
2,640
- 2,270
- 770
- 370
- 2,640
- 550
- 385
- 2,655
- 290
- 220
- 2,490
Rise Scenarios
Medi
1,770 -
1, 100 -
700 -
2,470 -
800 -
635 -
2,405 -
380 -
310 -
2,080 -
urn
1,870
1 ,030
630
2,500
750
585
2,455
350
280
2, 150
Hi
1,435
1,090
690
2, 125
870
705
2, 140
460
390
1,825
qh
- 1,550
- 1,040
- 640
- 2, 190
- 810
- 645
- 2, 195
- 420
- 350
- 1,900
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Table 4. Coranunity Protection Options Examined
for Sullivans Island
Option
Description
Present Value of Cost a./
(millions of I960 dollars)
1. Beach Nourishment - 1990
2. Beach Nourishment - 2020
3. Beach Nourishment - 2050
Initiate in 1990 a beach nourishment pro-
gram to stabilize the beach face and build
dunes. Included in the program is the
construction of a levee to stabilize the
backside of the island bordering the
intracoastal waterway.
Initiate in 2020 a beach nourishment pro-
gram to stabilize the beach face at its
2020 position and build dunes. Included
in the program is the construction of a
levee to stabilize the backside of the
island bordering the intracoastal
waterway.
Initiate in 2050 a beach nourishment pro-
gram to stabilize the beach face at its
2050 position and build dunes. Included
in the program is the construction of a
levee to stabilize the backside of the
island bordering the intracoastal
waterway. (Not applicable for the high
SLR scenario.)
Beach Nourishment: $40 to
$200 million, depending on SLR
scenario.
Backside Stabilization: $31
mi 11 ion.
Beach Nourishment: $31 to
$160 million, depending on SLR
scenario.
Backside Stabilization: $13
million.
Beach Nourishment: $20 to $72
million, depending on SLR
scenario.
Backside Stabilization: $5
mill ion.
. Stop Investment - 1990
5. Stop Investment - 2020-
Prohibit future investment on Sullivans
Island following 1990. Repair of storm
damage and construction of protective
structures would be prohibited.
Prohibit future investment on Sullivans
Island following 2020. Repair of storm
damage and construction of protective
structures would be prohibited.
No direct costs.
No direct costs.
a/ Present values estimated at real discount rate of three percent.
-------
The results for Options 1 and 2 are very close. While Option 2 is
preferred to Option 1 for the baseline, low, and high scenarios, the opposite
is true for the medium scenario. This pattern of preferences is reasonable
because: (1) in the baseline and low scenarios, losses are very small in the
early years, so waiting does not cause large losses; (2) the assumed 90-cm (3-
ft) rise provides insufficient protection for the high scenario, so rushing to
completion has little value if the sea wall is built to protect from a 90-cm
rise and the high scenario comes true; and (3) Option 2 provides protection
for the medium scenario for a substantial period of time. Therefore, it is
probably better to proceed with a routine construction schedule and a target
completion date of 2020 than to rush the sea wall to completion by the
1990s. This conclusion is reinforced when the added costs of a crash program
are considered.
Protection Strategies for Sullivans Island
As a low-lying barrier island, Sullivans Island is very vulnerable to a
rise in sea level. The medium and high rates of sea level rise would cause
most of the island to be lost by 2075, assuming no protective measures are
taken. The low scenario would cause substantial, although possibly more
manageable, land loss (Kana et al. 1984).
This analysis focuses on two major planning options for Sullivans
Island: a beach stabilization program in the form of beach nourishment, and a
prohibition of future investment on the island, with the goal of returning the
island to its natural (i.e., undeveloped) state. Table 4 lists variations on
these protection options and their costs.
Options 1 through 3 involve beach nourishment programs, starting at
different times. The costs of the beach nourishment options were estimated
using the following assumptions:4 the length of the beach is approximately
7,600 m (25,000 ft); the beach face is approximately 900 m (3,000 ft) wide;
the amount of sand required to stabilize the beach increases with sea level
rise (i.e., a 30-cm rise in sea level requires that 30 cm of sand be spread
across the entire beach length [7,600 m] and width [900 m]); 10 percent of the
placed sand is lost every year from erosion and must be replaced; the unit
cost of sand is $13 per nH C$10 per yd^) for the initial application of sand
and $6.5 per nH ($5 per yd^) for replacement sand (the higher cost of the
initial application of sand includes the cost of program design and the
identification of a suitable offshore source); the backside of the island is
approximately 7,600 m (25,000 ft) and is stabilized at a cost of $2,000/m
($6lO/ft), including operation and maintenance costs; the shadow price (i.e.,
opportunity cost) of the program funds is 2.0 when evaluated at a 3 percent
discount rate. Because the amount of sand needed to stabilize the beach will
Unit costs are derived from Sorensen, Weisman, and Lennon (1984). The
estimate of a 10 percent loss of sand every year is based on reports of
previous beach nourishment projects and is conservative (i.e., probably low)
for a beach facing open-ocean wave attack and possible hurricanes.
66
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vary with the amount of sea level rise, the cost of the program differs for
each sea level rise scenario.->
Options 4 and 5 are nonstructural alternatives. Under these options no
investment is permitted on the island following 1990 and 2020, respectively.
The prohibition of investments includes prohibiting not only new investment
but also investments to upgrade existing properties and to repair storm
damage. The goal of these two options is to move toward an undeveloped
island, possibly designating the area as a park.
To implement either Option 4 or Option 5, the city would have to exert
control over the private properties on the island, either through zoning or by
purchasing the properties. The property is currently valued at over $65
million. The loss of the value of the use of the properties on the island (by
prohibiting future development) is incorporated into the estimated economic
values." The payment of $65 million by the government to current property
owners would be a transfer, and consequently does not influence the estimate
of net economic impacts. Issues regarding equity and the implementability of
the program are raised by the potential $65 million government payment. These
issues are not addressed here.
An additional option for protecting the island would be to encircle it
with a sea wall/levee system. This option would protect the structures on the
island, but unlike the other options, the recreational beaches would be lost.
Because the beaches are important to this area, we did not investigate this
option.
To develop a strategy for preparing for sea level rise given current
uncertainties, the five options were evaluated for each sea level rise
scenario relative to doing nothing. The results of this evaluation are shown
in Table 5. For example, the estimates for Option 1 are interpreted as
follows for the low scenario:
• Implementing the option results in an increase in economic value of
$185 to $158 million.
• Implementing the program costs $121 million.
• The net increased economic value is $185 - 121 = $64 million to $158 -
121 = $37 million.
In the medium and high scenarios, the sea rises so much by 2075 that
nearly all of the island would have to be raised in order to prevent
flooding. Because these costs of raising existing structures and
regrading the island are not included in the costs of Options 1 through
3, the cost of these options in the medium and high scenarios are
probably underestimated. The addition of these costs would reinforce the
conclusions drawn in this analysis.
In fact, the current method overestimates the losses caused by this
strategy because the value of creating a recreational area, such as a
park with beaches, is not estimated separately.
67
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Table 5. Estimates of the Economic Value of the Development
in the Sullivans Island Area for Five Protection Options
(millions of 1980 dollars)3
00
1.
2.
3.
1.
5.
Protection Option
Np_ Action
-- Economic Valun
Beach Nourishment - 1990
— Increased Economic Value over
No Action
— Cost of Beach Nourishment Program
— Net Increased Economic Value
— Economic Value
Beach Nourishment - 2020
*
— Increased Economic Value over
No Action
-- Cost of Beach Nourishment Program
— Net Increased Economic Value
— Economic Value
Beach Nourishment - 2050
— Increased Economic Value over
No Action
— Cost of Beach Nourishment Program
— Net Increased Economic Value
— Economic Value
§. t pj>._l_n vest nw?I!.t_r_i9 9_Q
-- Increased Economic Value over
No Act ion
— Economic Va lue
Stop Investment - 2020
— Increased Economic Value over
No Action
-- Economic Value
Basel ine
153
131
71
60
213
81
11
10
193
38
26
12
165
-35
1 18
-18
135
81
185
61
118
130
16
130
52
2
86
31
1 15
31
115
Sea Leve
Low
- 115
- 158
121
- 37
- 152
- 101
8'l
- 20
- 135
- 38
50
- -12
- 103
1
- 1 16
9
- 121
Rise Scenario
Mod i urn
10 - 8/
165 - 123
178
-13 - -55
27 - 32
101 - 68
128
-27 - -60
13 - 27
12-8
78
-66 - -70
-26 - 17
72 - 25
1 12 - 112
52 - 21
92 - III
...
High
28 - 75
1 15 - 90
235
-120 - -115
-82 - -70
38 - 31
162
-121 - -131
-96 - -56
b/
NA
80 - 31
108 - 109
13 - 33
71 - 108
a/ All estimates evaluated at a three percent real discount rate.
b/ Option 3 was not examined for the high scenario because by 2050 the high scenario would cause
sufficient damage and land loss so as to make beach stabilization infeasible.
-------
• The total economic value of this option is the economic value for No
Action, plus the incremental value for the low scenario for Option 1,
or: $84 + 64 = $148 million to $115 + 37 = $152 million.
The results for the other sea level rise scenario/protection option
combinations are similarly interpreted. Of note is that there are no direct
expenditures for implementing Options 4 and 5, and that Option 3 is not
feasible for the high scenario.
The information in Table 5 provides considerable insight into the
difficulty in preparing for sea level rise given current uncertainties. For
the baseline and low scenarios, Options 1 and 2 are preferred. If initiated
right away, a beach nourishment program could have an incremental value as
high as $60 million. In the low scenario, the program can eliminate nearly all
the adverse impacts of sea level rise. Reducing investment (Options 4 and 5)
in the baseline case would be counterproductive, producing a net loss.
Although reducing investment in the low scenario may produce a gain over doing
nothing, its gain is not as large as the potential gain from beach
nourishment. Thus, Options 4 and 5 are inferior for the baseline and low
scenarios.
The opposite is true for the medium and high scenarios. For these
scenarios, beach nourishment is so expensive it makes Options 1 through 3
uneconomical. Options 4 and 5 are preferred if the sea level rises at these
rates.
If beach nourishment is initiated, and the medium or high scenario is
found to be true in, say, 2020, officials may wish to stop beach nourishment-
and reduce future investments at that time. However, with this strategy,
funds would be spent unnecessarily to nourish the beach between now and 2020,
and investments made on the island would subsequently be lost. There is also
the risk of not being able to stop the nourishment program once it has begun,
because people's expectations will have been established regarding
protection. Similarly, if investment is reduced today, and the baseline or
low scenario is true, then officials may wish to initiate beach nourishment
in, say, 2020. Again, however, if the city has purchased property, and park
development has proceeded, there may be no turning back. Finally, the city
may elect to do nothing, in the hope that by 2020 sufficient information will
be available to make the correct choice at that time.. Table 6 quantifies
these various strategic possibilities.
Table 6 shows the returns from four strategies, and their expected
values:
* Strategy A: Begin a beach nourishment program. This strategy is best
when the future rate of sea level rise is expected to be low.
• Strategy B: Begin a disinvestment program. This strategy is best
when the future rate of sea level rise is expected to be high.
* Strategy C: Wait until 2020 to act. This strategy assumes that a
correct decision cannot be made today. Consequently, nothing is done
(in the way of a coordinated community response) until 2020, when it
is assumed that sufficient information is available about sea level
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rise to correctly choose between beach nourishment (if the sea level
rises by a small amount) and disinvestment (if the sea level rises by
a large amount).
• Strategy D: Act correctly today. This strategy is not currently
possible because it requires knowing whether the future rate of sea
level rise will be below the low scenario or above the medium
scenario.
The estimates of the economic value of each strategy under each sea level
rise scenario are drawn from Table 5. The expected value is computed using
the following subjective weights for the scenarios: baseline = 0.1; low =
0.4; medium = 0.4; and high = 0.1 (different weights would produce different
expected values). Based on the expected values, which are driven by the
subjective weights, Strategy C (Wait Until 2020) is preferred to Strategies A
and B. Strategy B would maximize the lowest possible return (i.e., the worst
one would do with B is $109 million), and Strategy A maximizes the highest
return (i.e., the best one would do with A is $213 million). For Strategy A
to be preferred to Strategy C (using the expected value criterion), the
baseline and low scenarios must be much more likely to occur than the medium
and high scenarios. Similarly, for Strategy B to be preferred, the medium and
high scenarios must be most likely.
Table 6. Estimates of the Value of Alternative Protection Strategies
for Sullivans Island (millions of 1980 dollars)a
Protection Option Chosen Today
A. Beach Nourishment - 1990
B. Stop Investment - 1990
C. Wait until 2020 to Act
D. Able to Act Correctly Today
Beach Nourishment - 1990
Stop Investment - 1990
Sea Level Rise Scenario
Baseline
213
118
193
213
Low
150
115
130
150
Medium
30
112
-
112
High
-76
109
-
109
Expected Value
86.
113.
120.
137.
a All estimates evaluated at a 3 percent real discount rate.
Strategy D in Table 6 is not feasible because there is currently
uncertainty about the future rate of sea level rise. However, if one were
sufficiently knowledgeable to choose correctly by 1990, the expected value
would increase by $17 million over Strategy C ($137 - 120 million).
Therefore, although waiting is preferred to acting today and potentially
making a mistake, the sooner one can act correctly, the better. In fact,
having better information today could be worth $17 million for Sullivans
Island.
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THE NEED FOR REDUCING UNCERTAINTY
These findings indicate that in the case of Sullivans Island, there is
considerable value in reducing uncertainty about future rates of sea level
rise. Reliable, narrow ranges of estimates of sea level rise will enable
preferred protection options to be unambiguously identified for such areas as
Sullivans Island. Although the decision of whether to build a sea wall to
protect the peninsula is not sensitive to the future rate of sea level rise,
improved estimates may be required to establish design criteria and schedules
for projects such as sea walls. Perhaps most important, improved estimates
may be a precondition for initiating actions at the local level. Individuals
and governments may not invest the time and resources necessary to plan for
sea level rise unless and until the threats are clear and well known.
A recent book on the greenhouse effect and sea level rise contains the
following recommendations (Earth and Titus 1984):
• Federal research on the physical, environmental, and economic impacts
of sea level rise should be substantially expanded.
• Federal support for scientific research on the rate of future global
warming and sea level rise should be greatly expanded.
To make progress, the issue of sea level rise must be elevated to its
appropriate level of priority to command attention among decisionmakers and
leaders. In the past, natural disasters (hurricanes and tsunamis) have
provided an impetus for inclusion of such issues on local planning agendas.
Although disasters may call attention to the need to begin planning for sea
level rise, I hope that analyses such as this one may begin to form a basis
for taking action before disasters occur.
REFERENCES
Earth, M.C., and J.G. Titus (eds). 1984.
rise: A challenge for this generation.
Greenhouse effect and sea level
New York: Van Mostrand Reinhold.
Gibbs, M.J. 1984. Economic analysis of sea level rise: Methods and results.
In Greenhouse effect and sea level rise, eds. M.C. Earth and J.G. Titus.
Hoffman, I.S., D. Keyes, and J.G. Titus. 1983. Projecting future sea level
rise: Methodology, estimates to the year 2100. and research needs.
Washington, D.C.: Government Printing Office.
Kana, T.W., J. Michel, M.O. Hayes, and J.R. Jensen. 1984. The physical impact
of sea level rise in the area of Charleston, South Carolina, In
Greenhouse effect and sea level rise, eds. 105-150, M.C. Barth and J.G.
Titus. New York: Van Nostrand Reinhold.
Sorensen, R.M., R.N. Weisman, and G.P. Lennon. 1984. Control of erosion,
inundation, and salinity intrusion caused by sea level rise. In
Greenhouse effect and sea level rise, eds. M.C. Barth and J.G. Titus.
New York: Van Nostrand Reinhold.
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Titus, J.G. 1984. Planning for sea level rise before and after a coastal
disaster. In Greenhouse effect and sea level rise, M.C. Barth and J.G.
Titus, eds. New York: Van Nostrand Reinhold
Titus, J.G., and M.C. Barth. 1984. An overview of the cause and effects of sea
level rise, 43-44. In Greenhouse effect and sea level rise. New York:
Van Nostrand Reinhold.
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Impacts of Sea Level
Rise on the Coasts of South America
Stephen P. Leatherman
University of Maryland
College Park, Maryland USA
ABSTRACT
South America exhibits a wide diversity of coasts from icy, deep fjords
along the Chilean Pacific Coast to the muddy, tropical mangrove coast of
Venezuela. Fortunately, this continent is not subject to hurricanes or
typhoons; but winter coastal storms can be quite damaging, especially along
the Atlantic sandy shores and cliffs of unconsolidated sediments. Accelerated
sea level rise due to the greenhouse effect will increase the rate of beach
erosion and cliff retreat, and result in the loss of significant coastal
Wetlands, principally mangroves and salt marshes.
Popular coastal resorts along sandy beaches in South America, such as
Copacabana Beach, Rio de Janeiro, Brazil; Punta del Este, Uruguay; and Mar del
Plata, Argentina, are already adversely affected by eroding shorelines.
Copacabana Beach, perhaps the most famous beach in the world, was replenished
in the early 1970s by pumping sand onshore and will require another
nourishment project within a decade if current trends continue. The Argentine
government has recognized the problem of critically narrow beaches at Mar del
Plata, the nation's chief coastal resort, which boasts the largest casino in
the world; beach fill is to be undertaken as soon as suitable supplies of sand
can be located on- or offshore. These examples illustrate the nature of the
problems along sandy coasts, which are similar in type and magnitude to those
experienced along the U.S. coastlines.
Coastal wetlands also vary geographically in type and extent. Along
little-inhabited, muddy coasts of Guyana and Venezuela, accelerated sea level
fise may merely shift the coastal flora inland. Elsewhere, however, little
lowland (coastal plain) exists behind the coastal fringe so that the wetlands
will be squeezed out with sea level rise. The problem is particularly acute
in areas like Guanabara Bay, Rio de Janeiro, Brazil, where the large, poor-
class people are cutting down the mangrove forests for fuel. At the same time
the Brazilian government seems to lack the environmental regulations and/or
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enforcement to stop the onslaught of mangrove obliteration by urbanization
(e.g., construction of factories on filled wetlands and encroachment of
squatters building shanty towns).
Accelerated sea level rise poses very serious problems to a continent of
peoples that are struggling to even maintain their present standard of
living. The greenhouse-induced coastal changes will result in two radically
different strategies: the popular coastal resorts will receive prompt
attention (e.g., sand nourishment as required to maintain recreational
beaches) at the expense of other critical public needs through the
reallocation of scarce resources; while coastal wetlands will receive benign
neglect for the most part since governments at all levels seem unable to cope
with the present, severe problems in urbanized areas.
INTRODUCTION
South America is characterized by an extremely diverse coastline, ranging
from the hard-rock, steeped cliffs of Peru and Chile to the low, flat coastal
plain of the Amazon region. To assess the impacts of accelerated sea level
rise on South America, this continent must be divided into regions that will
exhibit similar responses. This partitioning of the coast is based on the
geologic framework of the continent and is further stratified by considering
the specific landforms and dominant processes operative within each coastal
zone.
South America is almost a mirror image of North America, with some quali-
fications (McGill 1958). The Pacific Coast along Ecuador, Peru, and Chile is
dominated by the Andes Mountains with no true coastal plain. Much of the
coast is hard (crystalline) rock, resulting in steep cliffs and rocky
promontories. Sandy to cobbly beaches occur as scattered pocket beaches
interspersed along the cliffed coast. The U.S. Pacific Coast is quite similar
to its South American counterpart as cliffs predominate; small spits at river
mouths and pocket beaches at shoreline reentrants are the only low, sandy
landforms along the coastal margin. Some of the cliffs, such as those in
Delmar and Malibu, California, are composed of unconsolidated, erodable
sediments, and urbanization of these areas involves considerable risk as the
sea level rises and the cliffs retreat. Similar areas are found along the
Pacific Coast of South America, but few have population centers in equivalent,
hazardous locations.
In contrast to the technically active coast of the Pacific, the Atlantic
Coast of the Americas is largely characterized by a low coastal plain, with
some exceptions. Parts of Mew England, particularly Maine, in the United
States have rocky coasts (nonerodable crystalline granitic rock), which form
steep vertical cliffs in some areas. The mirror image comparison fits nicely
when one considers that the southern portion of Argentina is also a hard-rock
coast with similar characteristics.
The Atlantic Coast of South America is generally low, but the specific
landforms are climatically controlled. The northeast Atlantic Coast of
Venezuela, French Guiana, Surinam, and the Amazon region of Brazil are
dominated by mangroves. This low, muddy coast is akin to the Mississippi
River delta region of the U.S. Gulf Coast. While Louisiana is already
experiencing major problems with shoreline retreat and coastal wetlands loss
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with relative sea level rise, equivalent changes in this South American region
are of little notice or importance as the coastal fringe is largely
uninhabited. The most extensive beaches and concentration of people extend
from Salvador, Brazil, along the Pio de Janeiro coast, including Punta del
Este, Uruguay, and terminate south of Mar del Plata, Argentina.
The effects of accelerated sea level rise depend upon the specific
landform type that it affects, which is determined by the regional geology and
dominant processes. The regional geology can be explained by continental
drift (Inman and Nordstrom 1971). The Atlantic Coast is a "trailing edge"
coast and hence characterized by wide coastal plains and extensive sandy
beaches and barrier rslands. Coastal plains, such as the pampas of Argentina,
are of low relief, gently slope, and are composed of unconsolidated (erodable)
sediments. By contrast, the Pacific Coast is tectonically active and subject
to both volcanic eruptions and violent earthquakes as have been recently
recorded in Colombia and historically documented in California (e.g., 1906
disaster in San Francisco). This "collision coast" in plate tectonic
terminology has little to no coastal plain; instead, the coastal range
stretches along the U.S. Pacific Coast and the corresponding Andes Mountains
in the southern hemisphere. Cliffs predominate, making the coast rather
ragged in outline and steep in relief. Because sea level rise effects will be
very localized and the South American Pacific Coast is not highly populated,
the major concerns are wetlands loss and beach erosion along the Atlantic
shore.
COASTAL WETLANDS
The Atlantic Coast of South America has two distinct types of coastal
wetlands: mangroves and salt marshes. Mangroves are the principal vegetation
along the deltaic coast of the tropical northeast Atlantic. While the Amazon
River continues to deliver billions of tons of fine sediments to the coast
yearly (which is moved In a northwest direction along this coastal compart-
ment), evidence from field studies and air photo analysis shows shore erosion
(Wells et al. 1980). As sea level rises, the entire coastal zone shifts
landward up the gradual, seaward-sloping coastal plain. The mangroves rooted
in the silty substrate along the open ocean are being eroded and undermined
along the outer edge, which indicates active shore retreat. Therefore, the
overall amount of coastal wetlands has probably changed little, with their
spatial distribution changing temporally. Also, the affected areas have
little population, and no major urbanized areas along the coastal margin are
threatened by probable future sea level changes.
This situation changes drastically to the south, particularly in the
state of Rio de Janeiro, Brazil. Here the mountain range meets the coast at
places, creating the fascinating skyline of bald mountain stacks (sugarloafs)
rising out of the sea. Wetlands, particularly mangroves in this tropical
environment, have formed on the interface between land and sea within
protected bays. (The wave climate is too high and the sediment too
restrictive to allow mangrove development along the open coast in this
geomorphic province.)
Mangroves in Guanabara Bay, which separates the cities of Rio and
Niteroi, are already rapidly declining in areal extent due to population
pressures. The large poor class uses the mangrove trees as fuel, and resi-
75
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dents of shanty towns have clear-cut the forest in adjacent Bay areas.
Furthermore, industrial expansion has resulted in the loss of wetlands as the
upper margins of Guanabara Bay are filled to provide inexpensive construction
sites on the new land. Brazilian federal and Rio state regulations to the
contrary, the onslaught continues largely unabated to the point that thousands
of square kilometers of mangroves have been reduced to a fraction of the total
(estimated at less than 100 knr by some Brazilian coastal ecologists; Vallejo,
personal communication 1985). Accelerated sea level rise will exacerbate this
serious problem, because the mangroves on the tidal flats are backed by the
cliffs of the coastal mountain range. Therefore, these wetlands cannot shift
landward (migrate upslope as in the case of deltaic and chenier plains of the
northeast coast). Instead, the mangroves will be gradually squeezed out of
existence in this region due to natural conditions and artificial constraints
(human development in the coastal fringe).
BEACH AND CLIFF EROSION
South Americans are fortunate that no hurricanes have been reported in
this hemisphere. However, the Atlantic Coast from Salvador, Brazil, to Mar
del Plata (the heavily populated coastal area with sandy beaches) is subject
to winter storms. This moderate to high wave climate in a microtidal (less
than 2 m tidal range) setting (Davies 1973) indicates the potential for storm-
induced damage and the long-term problems of sea level rise.
The Argentine coast from south of Mar del Plata along the Atlantic Coast
up to the Rio de La Plata estuary is presently eroding. Cliff erosion of the
unconsolidated to loosely compacted sediments is a major concern in the Mar
del Plata region (Figure 1). The pocket and mainland beaches in this region
are eroding at variable rates, approaching 2-4 m per year in the most severely
affected areas (Schnack 1985). In fact, the beaches of Mar del Plata are
already critically narrow, and a groin field (of little utility) has been
installed. The federal government has recognized the problem and is planning
a major beach nourishment project as soon as suitable fill material can be
located. This coastal resort, which is Argentina's most popular and hosts the
largest single casino in the world, will require continued beach restoration
with ever-increasing amounts of fill as sea level rise accelerates.
Just to the north of Mar del Plata, the cliffs subside to meet the gently
sloping pampas of northern Argentina. These broad, flat features are the
Argentine equivalents of the U.S. Atlantic coastal plain (Figure 2).
Therefore, estimates of shore retreat with sea level rise should correspond
closely with their North American counterparts and depend upon such factors as
slope, 3-D geomorphology, and wave climate (Leatherman 1983, 1984). Holocene-
age (5,000-year-old) beach ridges can be found several meters above sea level
in the Mar Chiquita area. If this area has been geologically stable, as now
believed (Schnack, Fasano, and Isla 1982), then these inland beach ridges on
the pampas record a previous sea level high stand during the Holocene.
Similar age-dated and stratigraphically equivalent beach ridges have been
described in Brazil (Suguio and Ostrom 1982; Suguio and Martin 1976), but no
such features have ever been documented along the U.S. East and Gulf coastal
plains. While this occurrence provides ample food for thought for coastal
geologists and neotectonists, the present and future evolution of these
coastal plains in both hemispheres should follow similar pathways with
accelerated sea level rise.
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Figure 1. Near Mar del Plata, the Argentine coast is characterized by
low bluffs (5-10 m high), which are generally comprised of highly erodable,
unconsolidated materials. The coastal highway (main thruway) from Mar del
Plata to Mar Chiquita is threatened by cliff recession, indicating an already
severe beach erosion problem which will only be exacerbated with accelerated
sea level rise.
Figure 2. At Mar Chiquita, salt marsh and lagoonal deposits are
Presently being eroded on the active beach face. This situation indicates a
long-term and continuing trend of barrier recession, which is similar to that
found on U.S. East and Gulf Coast barrier islands.
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The state of Rio de Janeiro, as already mentioned, is largely charac-
terized by the coastal mountain range intersecting the coast. However, small
coastal barriers to either side of Rio proper will be greatly influenced by
sea level rise (Muehe 1979; Figure 3). As these burgeoning coastal
communities derive their potable water from the groundwater table at shallow
depths, sea level rise will diminish the size of this freshwater lens through
time. (Even without such a rise, it seems likely that the ensuing
overpopulation of these fragile coastal environments will virtually ruin the
potable water supply by waste discharge directly into barrier substrate.)
The sandy beaches of Rio and Niteroi along Guanabara Bay are all artifi-
cial, having been created by beach fill (Figure 4). Although the wave climate
of this sheltered embayment is low, sea level rise will cause shore erosion as
has been documented in similar environmental settings along the U.S. coast
(e.g., Calveston Bay, Texas; Leatherman 1984).
The outer, open ocean beaches of Copacabana and Ipanema are more subject
to rapid retreat. The world famous Copacabana Beach was nourished in the
early 1970s by Dutch engineers who pumped offshore sand onto the beaches.
Unfortunately, the grain size of the introduced material was slightly larger
than the native sand so that the beaches have become noticeably steeper since
nourishment (Julio Gongalez, personal communication 1985). This beach profile
alteration actually reduced the usable intertidal foreshore beach (apart from
the widening of the dry backshore beach region). In addition, larger waves
now break directly on the beach face, which makes even the shallow water zone
dangerous for small children. Copacabana Beach is already quite narrow and
heavily utilized (Figure 5) so that another beach fill project will be
required soon. Beaches are clearly a national priority in Brazil so that as
sea level rises and the beaches recede, money will be allocated to maintain
this international tourist area if necessary, at the expense of other public
needs.
New high-rise residential developments are in the suburbs to the west of
Rio along the Atlantic Ocean barriers. Here high-rise construction of
condominiums is proceeding at a rapid pace (Figure 6) despite the apparent
shore retreat. This type of building along an already eroding, low shoreline
is reminiscent of the Miami Beach, Florida, development in the 1950s and more
recently the high-rise construction at Ocean City, Maryland, in the 1970s
(Figure 7; Leatherman 1981, 1986). Parallels can easily be drawn among the
construction practices, lack of governmental control and planning, present
hazardous situation for the development, and the impending problems with
accelerated sea level rise. At Miami Beach and Ocean City, large-scale beach
nourishment projects costing $65 million and $30 million, respectively, have
already been conducted or will be shortly undertaken. The high-rise buildings
along the barrier shorelines of the swanky Rio suburbs will also soon be in
the same predicament, and one can assume that political muscle and power will
be applied so that public funds are used to protect private investment.
CONCLUSIONS
South America has a diverse coastline, but similar conditions and
problems exist in both American hemispheres especially along the Atlantic
Coast. While large portions of the coastal zone have not been mapped in
detail and frequently only general descriptions exist, the present erosional
78
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Figure 3. Rapid urbanization is occurring along this coastal barrier,
just to the east of Rio. The populace draws their water from the near-surface
(unconfined) ground water table. As sea level rises, this low barrier will be
subject to increased flooding and reduced supplies of potable water due to
contraction of the fresh water lens.
Figure 4. The Rio de Janeiro area lacks any true coastal plain.
Historical erosion and cliff instability are evident across Guanabara Bay in
Niteroi. The beaches in this area are all artificial in that sand has been
Manually emplaced from dredging operations offshore. Although the wave
climate is low in the sheltered bay, increased sea level rise will cause beach
erosion and eventually reactivate cliff retreat in the absence of
litigating measures.
79
any
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Figure 5. Copacabana Beach in Rio is perhaps the most famous beach in
the world. This beach was nourished in the early 1970s with offshore sands.
The beaches are already overcrowded by U.S. standards, and another beach fill
project will be necessary in the near future.
Figure 6. To the west of Rio is a major suburban development occupied by
the relatively wealthly Brazilians. Unfortunately, there is little to • no
planning, and high-rise condominiums are being constructed along the waters
edge. Shore recession rates are not available, but the presence of a wave-cut
scarp along a high energy coast indicates ongoing beach erosion and the poten-
tial for major problems in the future for this urbanizing coastal barrier.
80
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Figure 7. Ocean City, Maryland, is overbuilt with high-rise condominiums
located along the eroding beach strand.
;-rend along populated, sandy coastal areas is obvious, and we can expect
increasing, critical problems with accelerated sea level rise. In summary,
the major problems will be experienced at the coastal resort and population
°enters (e.g., Rio de Janeiro, Brazil, and Mar del Plata, Argentina) due to
beach erosion and cliff retreat. Coastal wetlands, particularly mangroves,
**ill be virtually eliminated in these coastal areas due to overpopulation and
Orowding as well as to loss of suitable intertidal areas with continued sea
ievel rise.
ACKNOWLEDGMENTS
This paper was made possible by the Partners of America program,
^shington, D.C., in that Dr. Leatherman was able to exchange ideas with the
^esident South American coastal scientists and travel along a large portion of
the Brazilian and Argentine coasts. This exchange of ideas and papers has
Resulted in closer international relationships and helped to provide the
Scientific framework for future studies along the sandy beaches in both
hemispheres.
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REFERENCES
Davies, J.L. 1973. Geographical variation in coastal development. New York:
Hatner Publishing Co.
Inman, D.L., and C.E. Nordstrom. 1971. On the tectonic and morphological
classification of coasts. Journal of Geology. 79:1-21.
Leatherman, S.P. 1981. Barrier beach development: A perspective on the
problem. Shore and Beach. 49:2-9.
Leatherman, S.P. 1983. Coastal hazards mapping on barrier islands. Proc. of
National Symposium on Preventing Coastal Flood Piasters, Natural Hazards.
Res. & Appl. Spec. Publ. #1, Boulder, Colorado, p. 165-75.
Leatherman, S.P. 1984, Geomorphic response of coastal landforms to projected
sea level rise. In Greenhouse effect and sea level rise, eds. M. Earth
and J. Titus, 151-178. New York: Van Nostrand Reinhold.
Leatherman, S.P. 1986. Shoreline response to sea-level rise: Ocean City,
Maryland. Proc. of Icelandic Conference on Coasts and Rivers, Reykjavik^.
Iceland, in press.
McGill, J.T. 1958. Map of coastal landforms of the world. Geographical Review.
48:402-5.
Muehe, D. 1979. Sedimentology and topography of a high energy coastal
environment between Rio de Janeiro and Cabo Frio, Brazil. An. Acad...
Brazil. Ciena. 51:473-81.
Schnack, E.J. 1985. Argentina. In The world coastline, eds. E.G. Bird and M.L.
Schwartz, 69-76. New York: Van Nostrand Reinhold Co.
Schnack, E.J., J.L. Fasano, and F.I. Isla. 1982. The evolution of Mar Chiquita
lagoon coast, Buenos Aires province, Argentina. In Holocene sea level
fluctuations, magnitude and causes. 143-55. IGCP #61 Project, INQUA.
Suguio, K., and L. Martin. 1976. Brazilian coastline quaternary formations—
the states of Sao Paulo and Bahia littoral zone evolution schemes. Proc.
of Intern. Symp. Continental Margin of Atlantic Type. Acad. Bras. Cien.
48:325-34.
Suguio, K., and L.M. Ostrom. 1982. Progress in research on quaternary sea
level changes and coastal evolution in Brazil. In Holocene sea level
fluctuations, ed. D.J. Colquhaun, p. 166-81. IGCP #61 Program.
Wells, J., D. Prior, and J. Coleman. 1980. Flowslides in muds and extremely
low angle tidal flats, northeastern South America. Geology 8:272-75.
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Potential Effects of Sea Level Rise
on the Coasts of Australia, Africa, and Asia
Eric C. F. Bird
University of Melbourne
Melbourne, Australia
INTRODUCTION
Recent studies (e.g., Barth and Titus 1984) have indicated the possi-
bility that human -induced global climatic changes will result in a rise of sea
•Level: it is thought that atmospheric warming will lead to a reduction in
glaciers and ice sheets and a consequent addition of water volume to the
°ceans, as well as to thermal expansion of ocean water. Hoffman (1984) esti-
toated a rise of 0.24-1.17 m (0.78-3.8 3 ft) by 2050, and 0.56-3.45 m (1.83-
3-83 ft) by 2100. According to these predictions, sea level will stand 1 m
[3-3 ft) higher than it is now between 2045 (high scenario) and 2140
(conservative scenario).
Factors that complicate the prediction of actual changes on particular
Actors of the world's coastline include tectonic uplift or subsidence of the
land margin, the effects of the additional water load, which may hydro-
isostatically depress the submerged land, and the spatial variability of ocean
surface levels, which make it unlikely that the rise of sea level will be
equivalent around the world's coastlines, even on sectors where the land
remains stable.
This paper is concerned with the potential geomorphological, ecological,
socio-economic consequences of a rise of mean sea level, relative to the
°°astal land margin, of the order of one meter within the next century, and
Defers particularly to the coastlines of Australia, Africa, and Asia (Bird and
Schwartz 1985).
A mean sea level rise of 1 m (3.3 ft) implies that the mean high spring
limit will rise a similar amount, subject to any modification of tidal
tude due to the deepening of nearshore waters in relation to coastal
configuration. As a first approximation, the forecast coastline may be
etermined by surveying a contour 1 m (3.3 ft) above high spring tide line,
allowance must then be made for the effects of erosion or deposition as
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submergence proceeds. Submergence will result in coastline recession, except
where the coast consists of a vertical cliff, or where deposition of sediment
continues at a sufficient rate to maintain or prograde the coast.
Progradation may continue in the vicinity of the mouths of major sediment-
yielding rivers which have built large deltas, such as the Ganges, the
Irrawaddy, and the Mekong in Asia, but erosion is already prevalent on some
deltas, notably the Nile and Volta deltas in Africa.
EFFECTS OF SUBMERGENCE
The general effects of coastal submergence may be listed as follows:
• On cliffed coasts submergence is likely to accelerate coastline
recession, except on outcrops of hard rock formations, where the high
and low tide lines will simply move up the cliff face. Existing shore
platforms and abrasion ramps will disappear beneath the sea.
• The shores of deltas and coastal plains will retreat, except where
they are maintained by coastal sedimentation.
• Beaches will be narrowed, and beach erosion will become much more
extensive and severe than it is now.
• Inlets, embayments, and estuaries will be enlarged and deepened, and
increasing salinity penetration will cause a regression of coastal
ecosystems: where possible, mangrove and salt marsh communities will
move back into terrain presently occupied by freshwater vegetation.
• Coastal lagoons will also become larger and deeper, but the enclosing
barriers may transgress landward on to them. If the barriers are
submerged, or destroyed by erosion, the lagoons will become coastal
inlets or embayments.
• Low-lying areas on coastal plains, such as sebkhas (saline depressions
now subject to occasional marine flooding) on arid coasts, will be
flooded to form permanent lagoons.
• Upward growth of coral and associated organisms will be stimulated on
fringing biogenic reefs, keeping pace with the marine transgression or
lagging somewhat behind it (Neumann and Macintyre 1985).
• Erosion, structural damage, and marine flooding caused by storm surges
or tsunamis will intensify because of the greater heights of waves
arriving through deepening coastal waters.
• Water tables will rise in coastal regions, and soil and water salinity
will be augmented.
If the sea level rise continues, the sea will eventuallyreoccupy levels
it held in the Late Pleistocene (e.g., the 3-m and 7-m emerged coastlines in
southeastern Australia), reviving raised beaches and rejuvenating Pleistocene
bluffs as active cliffs.
-------
It should be noted that many existing coastal features have developed
during a period of relatively stable sea level. Over the past 6000 years, the
sea has remained at or close to its present level on much of the Australian,
African, and Asian coastline, in contrast with the slow marine transgression
recorded on parts of the Atlantic coast of North America, and the secular
emergence resulting from postglacial isostatic rebound in Scandinavia and
northern Canada (Bloom 1977). The Holocene sea level "stillstand" has been
"larked by the development of broad shore platforms on many cliffed coasts, of
beach-ridge plains, prograded barriers, deltas, and depositional terraces
formed under mangroves and salt marshes. Such features will not immediately
redevelop at higher levels as the sea rises: they will evolve only gradually,
after a new sea level stillstand has become established. Such a stillstand
requires the attainment of a new balance between global atmospheric tempera-
tures and ice volumes, which could occur at any level up to 60 m, the
estimated height of the ocean surface if all the world's glaciers and. ice
sheets were to melt (Donn, Farrand, and Ewing 1962).
On the coasts of Australia, Africa, and Asia the most dramatic changes in
a century of rising sea level are likely to be on beaches, around coastal
lagoons, and in mangrove ecosystems. These will now be considered further.
SEA LEVEL RISE AND BEACH EROSION
Studies by the International Geographical Union's Commission on the
Coastal Environment have demonstrated that there is already a prevalence of
erosion on sandy beaches: more than 70% of the world's sandy coastline has
shown net erosion over the past few decades, and less than 10$ net prograda-
fcion; the remaining 20% -30% have remained stable, or shown no measurable
change within this period (Bird 1985). Beach erosion is partly due to a sea
level rise of the order of 1.2 mm/yr (0.05 in/yr) (i.e., about one-eighth the
rate of sea level rise forecast for the next century) recorded on tide gauges
at>ound the world's coastline since the beginning of the present century
(Gornitz et al. 1982; Pirazzoli 1984). However, this is only one of fourteen
^actors identified by the Commission as initiating or accelerating beach
eposion (Table 1). A sea level rise of 1 m (3.3 ft) during the forthcoming
century will certainly become the major factor causing recession of sandy
beaches, but most of the other factors will continue to operate. An exception
"fcy be found locally where beaches that were nourished with sediment derived
y^om nearby cliffs actually receive an increased sediment supply because sea
level rise has accelerated cliff erosion. On the other hand the supply of
"luvial sediment to beaches will diminish, as greater proportions are retained
submerging river mouth areas.
Because there are so many variables, it is difficult to forecast the
e*tent of erosion or deposition that will accompany submergence by a rising
f|ea (Leatherman 1984). Bruun (1962) devised the rule that beaches in equili-
b^iuni with the processes at work on them will respond to a sea level rise by
°elng cut back and lowered until a new equilibrium profile is established. It
3 possible to estimate the extent of recession necessary -to accomplish this
Under the conditions specified by Bruun (Figure 1), but in practice many beach
^sterns are complicated by gains and losses of sediment from offshore, along-
shore and hinterland sources (Figure 2), and prediction of the response to sea
level rise then becomes much more difficult. A beach could be maintained
period of sea level rise if it were supplied with sufficient
85
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Table 1. Factors That Favor Initiation or Acceleration of Beach Erosion
1. Diminution of fluvial sand supply to the coast, as a result of reduced
runoff and sediment yield from a river catchment (e.g., because of
reduced rainfall, or dam construction leading to sand entrapment in
reservoirs, or successful soil conservation works).
2. Reduction in sand supply from eroding cliffs and shore outcrops (e.g.,
because of diminished runoff, a decline in the strength and frequency of
wave attack, or the building of sea walls to halt cliff recession).
3. Reduction of sand supply to the shore where dunes that had been moving
from inland are stabilized, either by natural vegetation colonization or
by conservation works, or where the sand supply from this source has run
out.
U. Diminution of sand supply washed in by waves and currents from the adja-
cent sea floor, either because the sand supply has run out, or because
the transverse profile has attained a form that no longer permits such
shoreward drifting.
5. Reduction in sand supply from alongshore sources as the result of inter-
ception (e.g., by a constructed breakwater).
6. Increased losses of sand from the beach to the backshore and hinterland
areas by landward drifting of dunes, notably where backshore dunes have
lost their retaining vegetation cover and drifted inland, lowering the
terrain immediately behind the beach and thus reducing the volume of sand
to be removed to achieve coastline recession.
7. Removal of sand from the beach by quarrying, and losses of sand from
intensively used recreational beaches.
8. Increased wave attack resulting from the deepening of nearshore water
(e.g., where a shoal has drifted away, where seagrass vegetation has
disappeared, or where dredging has taken place).
9. Submergence and increased wave attack as the result of a rise of sea
level relative to the land.
10. Increased wave attack due to a climatic change yielding a higher
frequency, duration, or severity of storms in coastal waters,
11. Diminution in the volume of beach material as the result of weathering,
solution, or attrition of beach sand grains leading to a. lowering of the
beach face and a consequent increase of wave attack on the backshore.
12. Increased beach erosion resulting from a rise in the water table, due to
increased rainfall or local drainage modification, rendering the beach
sand wet and more readily eroded.
13. Increased losses of sand alongshore as a result of a change in the angle
of incidence of waves (e.g., as the result of shoal or reef growth, or of
breakwater construction).
14. Intensification of wave attack as the result of lowering of the beach
face on an adjacent sector (e.g., as the result of reflection scour
induced by sea wall construction).
SOURCE: Bird (1985)
86
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Figure 1. The Bruun Rule states that a beach that has attained
ibrium with coastal processes (Profile 1) will respond to a rise in sea
level by losing sand from the upper part of the profile and gaining it in the
nearshore area until a new equilibrium (Profile 2) is established. The
c°astline will thus retreat from A to B as the direct result of the sea level
rise, and from B to C as a result of the transference of sand seaward. It is
Possible to predict the extent of coastline recession where the conditions
Proposed by Bruun apply, but it should be noted that other factors (see Table
1) also influence the changes that will occur on a beach as a result of a sea
level rise.
SUPPLY OF SAND TO A BEACH
|beacn quarrying!
sand volume reduced by
weathering and
attrition
LOSSES OF SAND FROM A BEACH
Figure 2.
rr>om a beach.
The various ways in which sand is supplied to, and removed
87
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quantities of sediment from nearby rivers, melting glaciers or spilling
dunes. Erosion of a beach undergoing marine submergence would also be reduced
if at the same time the coastal environment became calmer and drier, or if
swash-dominant wave regimes replaced drift-dominant wave regimes (i.e., if
losses by longshore drifting are reduced) as a result of changes in coastal
configuration that accompany a sea level rise.
A great deal of additional site-specific research is therefore needed to
determine the extent of beach erosion that will result from a sea level rise
of the order of 1 m (3-3 ft) over the next century.
EFFECTS OF SEA LEVEL RISE ON COASTAL LAGOONS
Coastal lagoons, formed where inlets, estuaries, or embayments have been
partly or wholly sealed off from the sea by the formation of depositional
spits, barrier islands, and barriers, are numerous on the coasts of Australia,
Africa, and parts of Asia. Typically they show salinity gradients from fresh
river inflow through brackish to sea water, and are important to coastal
fisheries, especially as breeding and feeding areas. In Asia, especially in
India and Sri Lanka, some have been extensively converted into brackish-water
fish ponds (e.g., the lagoons south of Madras), while others have been modi-
fied by increased freshwater outflow resulting from development of irrigated
rice fields in their immediate hinterlands, e.g., Kalametiya Lagoon, Sri Lanka
(see Mahinda Silva 1986).
A sea level rise of 1 m (3-3 ft) over the next century is likely to
increase marine influences in such lagoons by deepening and widening their
tidal inlets. At the same time the depositional spits and barriers that
enclose them will be trimmed back on their seaward shores and possibly
breached locally: as sand is washed and blown across them, they will trans-
gress landward (Bird 1983). In turn, the lagoons will -become deeper and
(unless deposition offsets this) larger in area, inundating marginal lands,
and initiating or intensifying erosion on their shores. Where enclosing
barriers are submerged or removed by erosion, lagoons will open out as marine
inlets or embayments.
Some of the likely consequences of sea level rise can be deduced from
studies of changes in coastal lagoons where sea water inflow has increased as
the result of the opening or enlargement of a tidal entrance, as in the
Gippsland Lakes in southeastern Australia, where the cutting of an artificial
entrance in 1889 resulted in previously freshwater lakes becoming brackish
estuarine lagoons, with die-back of freshwater vegetation fringes and their
replacement by salt marsh, the displacement of freshwater biota by estuarine
and marine species, and the onset of extensive shoreline erosion (Bird
1978). Similar changes have followed the opening or widening of tidal
entrances to other coastal lagoons, notably the Ebrie Lagoon on the Ivory
Coast and Lake St. Lucia in South Africa. A sea level rise would have similar
effects, and would accentuate changes in already-estuarine lagoons such as the
Gippsland Lakes. By contrast, the Murray-mouth1 lagoons in South Australia
were brackish until 19^0, when natural tidal entrances were sealed by bar-
rages, and they have since developed into freshwater lakes, with associated
ecological changes that are the reverse of those seen in the Gippsland
Lakes. Where coastal lagoons have thus been artificially separated from the
marine environment, they will only be maintained in their present condition by
raising the enclosed barrages to prevent the rising sea from reinvading.
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SEA LEVEL RISE AND MANGROVE ECOSYSTEMS
Mangrove-fringed coastlines have become much less extensive in Australia,
Africa, and Asia in recent decades because of the impacts of land reclamation,
fishpond construction, mining, and waste disposal, but where they persist they
typically stand in front of zones of salt marsh and freshwater vegetation
(fen, scrub, or swamp forest). In many cases there has been coastal prograda-
tion accompanied by the advance of the mangroves (often arranged in species
zones), with salt marsh and freshwater communities following in succession as
sedimentation builds up the substrate to appropriate levels, forming an inter-
tidal depositional terrace.
A sea level rise will reverse this, unless sedimentation continues at a
Efficient rate to maintain or prograde the coastline. Submergence will kill
th® seaward mangroves and initiate erosion of the previously prograded
terrace. As the coastline retreats the mangrove zone will migrate landward to
d*splace the salt marsh, which in turn will invade the freshwater hinter-
land. Mangroves regenerate quickly in areas that have been cleared then
bandoned, and it is likely that they will spread back on the suitable hinter-
habitats as the sea rises, but where the hinterland is steep or rugged
mangrove zone may be extinguished by submergence.
On the Asian coast the land immediately behind the mangroves is commonly
intensively for fish ponds or rice fields, and it is these that would be
G°lonized by the regressing mangroves, if local people permit. In drier
a^as, as in northern Australia, mangroves are backed by bare, hypersaline
Plains, and in these areas a sea level rise will result in the colonization of
Presently unvegetated tracts by mangroves as the coastline retreats.
RESPONSES TO COASTAL SUBMERGENCE AND EROSION
Secular coastal submergence rates of as much as a meter per century have
j been documented in historical times. Goudie (1977) quoted Vasilev's
\1969) report of submergence of parts of the Black Sea coast at rates of 30-
?2-5 cm (11.8-20.5 in) per century; Tjia (1970) recorded up to 30 cm (11.8
Tn)/century from parts of Indonesia, and Veenstra (1970), 10-20 cm (4-8
ln)/century from the southern shores of the North Sea. Sudden submergence has
Recurred locally as the result of earthquakes, as on Homer Spit, Alaska, parts
f which sank nearly 2 m (6.6 ft) in the 1964 earthquake, around Tumaco,
Colombia, which subsided up to 1.6 m (5.2 ft) in the 1979 earthquake, and in
he Rann of Kutch, Pakistan, which has been subject to recurrent tectonic
subsidence (Bird 1985), but such events have been highly localized, and their
ffects are not necessarily indicative of the changes that would accompany a
gradual but sustained global sea level rise.
A more rapid rise occurred in prehistoric times. About 18,000 years ago,
the end of the Last Glacial cold period, sea level stood about 140 m (460
lower than it does now. Global warming then produced a rapid sea level
isei the Flandrian Transgression, which brought the sea up close to its
Resent level about 6000 years ago, at an average rate of 1.16 m (3.8
^/century. It was probably an oscillatory rise, phases of more rapid
^mergence alternating with phases of slower submergence or episodes of
(Fairbridge 1961; Bloom 1977). Primitive societies that occupied
shelf areas were forced to retreat as this transgression proceeded.
89
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Thus, the Australian Aborigines became separated from their Tasmanian
counterparts with the submergence of Bass Strait between 13,500 and 12,000
years ago and were cut off from Papua Mew Guinea by the submergence of Torres
Strait 8,000 to 6,500 years ago (Bird 1986). Unfortunately, we have no record
of their perceptions and reactions to a sea level rise proceeding at a rate
similar to that expected over the coming century.
The ways in which modern coastal societies will respond to a relatively
rapid rise in sea level will vary considerably with political and economic
factors. Within the regions here considered, the response in developed
countries such as Australia, South Africa, Japan, and Singapore will differ
from that of less developed countries in tropical Africa and Asia. The former
will have the organization, technology, and resources to counter the effects
of a sea level rise, whereas the latter will in general have to choose between
evacuation and adaptation to their changing coastal environments (Figure 3).
On coastlines close to urban and industrial centers it is likely that a
sea level rise of about 1 m over the next century will stimulate massive
expenditure on structures designed to prevent submergence and erosion. It is
very likely that where land has recently been reclaimed from the sea for use
by large coastal populations (as in Singapore, Hong Kong, and Tokyo Bay)
strenuous efforts will be made to retain it, using techniques familiar from
the history of the Netherlands coast. Where eroding beaches are valued for
recreation or tourism, as in Australia close to the state capitals and other
seaside towns and at coastal resorts in Africa and Asia, it is likely that
they will be maintained by expensive beach renourishment programs of the kind
already used in some of these areas.
Away from such highly developed centers, especially on the coastlines of
less wealthy countries, there may have to be evacuation and abandonment of
coastal fringes as submergence and erosion proceed, and modification of
developed land and water uses as the water table rises and soil and water
salinity increase. Efforts will certainly be made to prevent the sea from
inundating farmland. The Yilan Plain in northeast Taiwan is an already-
subsiding coastal area where dykes have been built to protect an intensively
used lowland from marine invasion (Hsu 1985). But as submergence proceeds
there is a strong possibility, already in evidence in parts of the
Philippines, of increasing the use of nearshore waters for mariculture and the
occupation of shallow areas by people who live on boats and who may eventually
occupy floating villages and towns.
In Asia, brackish-water fish ponds have been constructed extensively on
the fringes of deltas and coastal plains, sometimes Just behind the mangrove
fringe, sometimes replacing it (Figure 4). On the deltaic north coast of
Java, fishponds are constructed on mud deposited at river mouths by
floodwaters directly after the floods disperse, and before mangroves are able
to colonize (Bird and Ongkosongo 1980). On prograding deltas and coastal
plains, where such fish ponds become gradually farther from the coast, and
less easy to supply with sea water, they are progressively converted into rice
fields irrigated with fresh water from rivers. The effects of sea level rise
in such situations can be Judged with reference to sectors where the coastline
90
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Figure 3. Village on the narrow sand barrier which borders the subsiding
lrthern shore of the Sepik Delta, Mew Guinea. As submergence proceeds the
3rrio,- is migrating intermittently landward as the result of occasional storm
The village has been repeatedly rebuilt on the retreating
Photo: Eric Bird.
91
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Figure 4. Intensive use of the
ponds) and shallow nearshore areas
Phillipines. Photo: Eric Bird
coastal fringe (brackish-water fish
(shellfish farms) in Manila Bay,
is eroding (usually because the river mouth has been diverted, naturally or
artificially, to some other outlet) and fish ponds are being destroyed (Figure
5). In such areas, the landward spread of salt water has prompted the conver-
sion of ricefields back to brackish-water fish ponds. Such changes raise
socio-economic and political problems, for people who live on eroding sectors
are losing land and resources, whereas those on accreting sectors are gaining
them, and it is difficult to organize equitable redistributions.
A sea level rise of 1 m (3-3 ft) over the next century will thus cause
major problems on the intensively utilized and densely populated Asian coastal
plains—producing coastline recession of up to several kilometers, displacing
coastal villages, and depriving many people of their land and resources. On
the low-lying coast of the Bight of Bangkok (Figure 6), for example, the
mangrove fringe has been largely cleared, the seaward parts being converted
into brackish-water fish and shrimp ponds and salt pans, while landward,
canals have been built to bring fresh water to irrigate the rice fields
(Figure 7). Tide range there is up to 2.3 m (7.6 ft) and a sea level rise of
a meter on this gently sloping coastal plain could submerge the whole of the
previous mangrove area and an additional zone up to 5 km (3.2 mi) inland
(Figure 8). The existing fishponds, shrimp ponds, and salt pans are likely to
be destroyed by marine erosion, but the local people will
92
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Figure 5. Brackish-water fish ponds on the coastline of the Citarum
, northern Java, being destroyed by marine erosion, which has developed
as a sequel to the diversion of the mouth of the river that formerly supplied
Se
-------
Figure 6. Coastal plain behind the Bight of Bangkok, Thailand, showing
fish ponds and some uncleared mangrove remnants behind a narrow mangrove
fringe. Photo: Eric Bird
Figure 7. Probable extent of submergence on the low-lying coast at the
head of the Gulf of Thailand if sea level rises 1 m (3.3 ft).
-------
:---.4 M-GVy Ji
Jl | Rice fields
|pl Zone of fish ponds
J and salt pans
?J^ Existing mangroves
&
vi'J
024
THAILAND
8 10km
Figure 8. Part of the low-lying coastline south of Bangkok, Thailand,
showing the natural coastline in 19^5 (inner limit of mangroves), the present
features (including areas reclaimed for fish ponds and rice cultivation), and
tne predicted coastline after a sea level rise of one meter, assuming no
c°untermeasures are taken.
CONCLUSION
The effects of a sea level rise can be deduced theoretically, or inferred
studies of coastlines where submergence is already in progress, as in
s°utheast England (e.g., the Thames estuary), the Netherlands (e.g., the Rhine
Delta), northeast Italy (e.g., Venice and Ravenna), parts of the Gulf coast of
the United States (e.g., Galveston), and Taipei, Taiwan. Alternatively, some
°f the effects may be deduced from studies of retreating coastlines where the
recession is due partly or wholly to factors other than active submergence, as
°n parts of the deltaic coastline of northern Java. There is a need for site-
sPecific predictive studies of the physical and ecological consequences of a
sea level rise on various types of coastline, especially those that are low-
ying, densely populated, and intensively utilized.
95
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The human response to sea level rise may be countermeasures (where
resources and technology are capable) evacuation, and adaptation to the
changing coastal environment. With the prospect of a continuing sea level
rise, some countries may decide to enclose and reclaim embayments, such as
Port Phillip Bay (Figure 9) and Botany Bay in Australia, and Tokyo Bay and
parts of the Inland Sea in Japan, on the grounds that the cost of protecting a
shortened, artificial coastline can be justified by increased economic yields
from the land gained. But on the coasts of less-developed countries, the most
likely outcome is an intensification of use and occupance of nearshore sea
areas.
I
MELBOU
PORT PHILLIP BAY
Sectors likely to be
submerged or eroded
if sea level rises one
metre
MORNINGTON
POINT
LONSDALE
PORTSEA
SORRENTO
Figure 9. Port Phillip Bay, southeastern Australia, showing sectors
likely to be submerged or eroded if sea level rises 1 m (3.3 ft). If the sea
continues to rise, a possible response would be to build a dam across the
outlet at Point Lonsdale, with canals to the ports of Melbourne and Greelong>
and to reclaim the rest of the bay in the manner of the Dutch polders.
96
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REFERENCES
Earth, M.C., and J.G. Titus. 1984. Greenhouse effect and sea level rise.
New York: Van Nostrand Reinhold.
Bird, E.C.F. 1978. A geomorphological study of the Gippsland Lakes. Ministry
for Conservation, Victoria, Australia, Publication 186.
Bird, E.C.F. 1983. Changes on spits and barriers enclosing coastal lagoons,
45-53. Oceanologica Acta. Symposium International sur les Lagunes
Cotieres.
Bird, E.C.F. 1985. Coastline changes. Chichester: Wiley Interscience.
Bird, E.C.F. 1986. Man's response to changes in the Australian coastal
zone. Man and coastline change, ed. K. Ruddle, in press.
Bird, E.C.F., and O.S.R. Ongkosongo. 1980. Environmental changes on the
coasts of Indonesia. Tokyo: United Nations University.
Bird, E.C.F., and M.L. Schwartz, eds. 1985. The world's coastline. New
York: Van Nostrand Reinhold.
Bloom, A.L. 1977. Atlas of sea-level curves. Ithaca, New York: Cornell
University.
Bruun, P. 1962. Sea level rise as a cause of shore erosion. J_. Waterways and
Harbors Div.. Proc. Amer. Soc. Civil Engnrs. 88:117-130.
°onn, W.L., W.R. Farrand, and M. Ewing. 1962. Pleistocene ice volumes and
sea level lowering. J. Geol. 70:206-214.
^airbridge, R.W. 1961. Eustatic changes in sea level. Physics and chemistry of
the earth. 4:99-185.
Gornitz, V., S. Lebedeff, and J. Hansen. 1982. Global sea level trend in the
past century. Science. 215:1612-1614.
Goudie, A. 1977. Environmental change. Oxford: Clarendon Press.
Hoffman, J.S. 1984. Estimates of future sea level rise, 79-103. In
Greenhouse effect and sea level rise, ed. M.C. Barth and J.G. Titus. New
York: Van Nostrand Reinhold.
Hsu, T.L. 1985. In The world's coastline, ed. E.C.F. Bird and M.L. Schwartz,
829-831. New York: Van Nostrand Reinhold.
Weatherman, S.P. 1984. Coastal geomorphic responses to sea level rise:
Galveston Bay, Texas, 151-178. In Greenhouse effect and sea level rise,
eds. M.C. Barth and J.G. Titus. New York: Van Nostrand Reinhold.
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Mahinda Silva, A.T. 1986. Environmental changes, ecological conditions, and
sociological aspects of two lagoon ecosystems in southern Sri Lanka. In
Man in the mangroves, eds. E.C.F. Bird, P. Kunstadter, and Sabhasri.
Tokyo: United Nations University.
Neumann, A.C., and I. Macintyre. 1985. Reef response to a sea level rise:
Keep-up, catch-up, or give-up. Proc. 5th Internat. Coral Reef Congress,
3:105-110.
Pirazzoli, P.A. 1984. Secular trends of relative sea level changes indicated
by tide-gauge records, 82-85. Proc. Internat. Symp. Late Quaternary Sea-
level Changes. Mar del Plata.
Ruddle, K., and R.E. Johannes. 1985. Traditional knowledge and management
systems in Asia and the Pacific. Jakarta: UNESCO Regional Office.
Tija, H.D. 1970. Rates of diastrophic movement during the quaternary in
Indonesia. Geol. en Mi.lnbouw. 49:335-338.
Veenstra, H.J. 1970. Quaternary North Sea coasts. Quaternaria 12:169-179.
98
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Worldwide Impact of Sea Level
Rise on Shorelines
Per Bruun
Chairman (Ret) Technical University of Norway
Hilton Head, South Carolina USA
INTRODUCTION
Sea level is rising around the world. As Hansen (Volume 1) and others
Point out, anthropogenic emissions may result in a climatic warming through
the next few centuries, which could overcompensate the natural cooling that
^ight otherwise be expected. As Thomas (this volume) notes, the warming might
be a threat to the continued existence of the Greenland and the West Antarctic
ice sheets.
This paper discusses evidence from tidal gauges and archeology that sea
level has been rising in the past centuries. Next, it describes methods for
assessing erosion caused by sea level rise. It then describes how people have
held back the sea in the past, focusing on the low countries of Europe.
finally, it describes the options available to society for responding to
shoreline changes resulting from projected sea level rise.
HISTORIC EVIDENCES OF SEA LEVEL RISE
Tide level recordings tell us of sea level movements over the last 200
years. For longer-term developments, we must seek the.evidence of such rises
in, for example, increased flooding of low coastal areas, drowning of valleys
and shore features, including some ancient manmade features, and density
currents penetrating further upstream. The findings are not always easy to
interpret. Tectonic movements, glacial rebounds, consolidation of softer
Materials in the ground and re-balancing of land masses due to offshore sedi-
have interfered with the relative movement of land and sea. The
authors who have written about above-mentioned phenomena come from a
Variety of disciplines, including geology, geography, engineering, and
archeology, to name a few. Other evidence of sea level rise comes from
historic records of past construction activities.
99
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Recordings of the events were solely descriptive rather than by
instruments, although surveys have been made to state the actual situation.
An impressive geophysical review was given by Fairbridge (1961) who referred
to works by Marmar (1948), Stommel (1960), and Guilcher (1958). Others
include Milliman and Emery (1968) and Emery (1980).
Important reviews of tide level fluctuation are given by U.S. Dept. of
Commerce (1983) for the United States' shores. The recent contribution by
Earth and Titus (1984) was the first to examine future conditions.
The following text presents a few examples of these reviews, some of them
tracked down by the author and not known before (see Bruun 1985).
Early Harbor Works
The Port of Alexandria in Egypt was founded by Alexander the Great, who
destroyed the ports at Tyre and Sidon. Alexander built the port by connecting
through reclamation the island of Pharos (the scene, centuries earlier, of the
great harbor works, which had by then mostly disappeared) with the mainland.
This formed two harbors protected by the island, in which basins were formed
by the construction of walls in the form (on plan) of semicircles, the
northeastern basin having an outer mole protecting the single central entrance
channel. This was a form of layout that was later adopted by both Greeks and
Romans.
The first mention of a lighthouse for the benefit of shipping is that
built upon a rock off the northeast of the island of Pharos. This lighthouse
appears to have been of considerable height and was circular in plan, tapering
to the top.
Of the Greek ports, perhaps the most interesting are.those of Piraeus,
Zea and Munychia, Phodes, and Cnidus. All these were natural bays protected
on the seaward sides by moles or breakwaters. In the Mediterranean, where
there is practically no rise or fall in water level—and the earlier Greeks
had apparently not evolved any means of drying out the sites by cofferdams—
these structures were all founded upon beds of tipped impervious material,
which were built up until they reached the surface or thereabouts, when they
were leveled to receive the masonry that formed the superstructures.
The period of the Roman Empire provides considerably more information in
regard to the construction of many harbors that were built on the coasts of
Italy and Sicily, both from the accounts written by historians and from
remains of the actual works. These works were much more substantial than
anything previously existing, both in design and in the means and methods of
construction employed. For example, in Italy, as distinct from Greece,
natural bays that could readily be formed into harbors were scarce, and the
character of the works was therefore somewhat different. Moreover, Roman
cement was in general use, which contributed not a little to stability and
lasting qualities; again, methods of constructing underwater works were
evolved, many of which were founded on piles driven into the sea-bed, for all
Roman moles and breakwaters were constructed of masonry founded at sea-bed
level. Many moles were of arched formation, probably to avoid the siltation
that in some places would be induced by solid walls; and to reduce foundation
work and masonry in exposed situations, double arcades were formed some
100
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distance apart, with the arches staggered to break the force of waves but
still allow the passage of water. Cellae-covered recesses for ships of war,
which protected the vessels from the sun and inclement weather, were a feature
°f many Roman ports, but vessels used for purely commercial purposes were
berthed at marginal quays or moles.
For all these ancient ports, the piers and wharves today are covered by
2-3 meters of water. However, because the structures have deteriorated, this
does not provide quantitatively firm figures for the amount of sea level rise
occurring in the last 1,500-3,000 years.
Ports
We know little about port development during the centuries following the
downfall of the Roman Empire. Sea voyages and trade undoubtedly continued.
ports were river based; quays or piers hardly existed or were uncommon. The
big seafarers for several centuries were the Vikings, whose large fleets
Cabled them to invade England, France, Ireland, Spain, and the Baltic
countries all the way down to Constantinople (Istanbul). Next they crossed
Atlantic Ocean to Iceland, Greenland, and Vineland (North America). We
about their powers from historic accounts of what they accomplished. But
s require naval bases for operation. We have found them during recent
decades of excavations.
From archeological discoveries in Denmark and in the southern part of
Sweden we know that the Vikings built large forts at rivers and in fiords.
Their vessels were probably moored, or they may have been beached, but we have
also found large pile walls, for example, at Hedeby, South Jutland,
demonstrating that their vessels, whether built for commerce or for war, used
Piers for loading or unloading. Sometimes they also built large submerged
walls or dams from sunken ships, or from piles across a navigation channel to
stop enemies from penetrating into their harbors. A magnificent example of
latter is in Roskilde Fiord on the Island of Zealand in Denmark.
The remains of these 1,000- to 1,500-year-old Viking structures do not
Provide direct data on elevations, but some of them were built in a way that
indicates that the water table must have been lower than it is today. The
conclusion of the Danish National Museum regarding water tables is that sea
level about 1,000 years ago must have been 1 to 1.5 ra below present sea level.
^gdimentalogical Evidence
Coastal geomorphologists observe the elevation of beach ridges and
ines. In emerged condition they provide evidence of a higher sea level
., as may be seen in Denmark where strandlines are found in up to about 60
70 meters elevation) generated by the postglacial arctic waters. After
of the ice cap, the ground experienced a rebound effect, which was
faster than the sea level rise. That this situation still continues was
revealediby Svante Arhenius' famous drilling of holes in vertical rock walls
°n the Swedish west coast outside Gothenburg. The holes were drilled in 1890
^nd are now found about 0.6 m above present mean sea level (MSL). (Svante
ftr
-------
On the Skagen Spit, the northernmost tip of Jutland, the elevation of old
beach ridges elevated by the glacial rebound decreases to the north as
deposits get younger. One of these changes in elevation occurs rather
abruptly, revealing a slowdown in the relative movement of land and sea about
2,500 years ago, demonstrating a rise in sea level elsewhere.
Two examples of sedimentation provide indirect confirmation of the
mechanics of the sea/land interaction process. At Penang, West Malaysia,
rivers discharge known quantities of sediments on the estuary bottom. If the
area of the estuary bottom (+500 km ) is multiplied by a sea level rise of
0.002 m/yr one arrives at 1 million m^, which corresponds to the annual river
discharge of sediments. This is consistent with the observation that water
depths have not changed in the last 200 years.
The situation on the 140-km pocket shore on the South Coast of Iceland
between Porlakshofn and Dyrholaey is described by Bruun (1986). The discharge
of river sediments of sand size is 4.5 million nwyear. This quantity is
balanced by deposits of 0.0035 m/yr (which corresponds to the recorded sea
level rise in the Port of Reykjavik) on the 140-km long, 9,000-m wide
platform, of which 2,000 m is beach area, extending to a 50-m to 60-m depth
where there is a steepening of the offshore bottom leaving the impression of a
deposit boundary slope. Grain size distributions confirm the existence of an
equilibrium condition. A forth horizontal rock platform is found at a 100-m
depth, possibly an abrasion platform from a lower sea level during an extended
glacial period.
Tidal Gauges
As described by Hicks (1985):
Tidal datums are the fundamental base from which most coastal and
marine boundaries are determined. They are also the fundamental
references for sounding and depiction of the shoreline on all
nautical charts. As such, they are intrinsic to the activities of
coastal engineering, surveying, hydrography, and photogrammetry.
The most important tidal datums are defined, amplified, and their
specific uses listed in abbreviated outline form. The definitions
are in terms of the procedures used in their computation.
As "tide authority" (in the international and legal sense) of
the United States of America, the National Ocean Service (NOS) of
the National Oceanic and Atmospheric Administration (NOAA), U.S.
Department of Commerce, has statutory responsibility for tidal
measurements, analyses, predictions, and datum determinations.
Responsibility for tidal datums included the development and any
subsequent modifications of the methods of computation.
The method used for the computation of a tidal datum is
specified in the official definition of that datum. The purposes
of these "method definitions" are to:
• Ensure uniformity of computation among oceanographers
102
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e Ensure uniformity of computation among the various tidal
regimes of the U.S.
• Provide for comparisons with foreign datums
• Enable outside scientific verification of results through
duplication
• provide precise terminology for judicial and legislative
acceptance in cases involving coastal and marine
boundaries.
Hicks gives twelve references of recordings of sea levels. Tide level
Recordings, however, have been undertaken on a larger scale for some decades
Only. Exceptions are the Dutch and British recordings, which date back a few
centuries; however, the accuracy of those tidal observations is
Questionable. In Holland, the influence of sea level rise is reinforced by
subsidence due to the soft underground caused by the large Rhine, Maas, and
Scheldt rivers.
In the United States the trend of relative sea level rise is fairly well
documented by surveys on all shores (Hicks 1980; Lisle 1982; U.S. Dept. of
Commerce 1983; Nummedal 1983, for the Louisiana Coast, are among the most
important contributions). Sea level rise during the latest decades ranges
from 1 to 4 mm/yr depending upon the location. The subsidence in Louisiana is
about 10 mm/yr. On the East Coast the rise is 2-3.5 mm/yr, on the Gulf, 2-3
Nro/yr, and on the Pacific, about 2 mm/yr. Rises for east and west coasts of
Canada are similar. Germany has kept good records of the rises on the North
s«a coast (Siefert 1984), as may be seen from Table 1. The conclusion of the
Siefert paper reads as follows:
The knowledge of the probable development of the water level
on the German North Sea coast—in general on all flat coasts
--is of greatest importance for coastal engineering. In
Germany an increase of MThw (mean high water) of 25 cm per
century has been taken as a basis for dimensioning of coastal
protection constructions (e.g. dikes or storm surge
barrages). If instead one had to reckon with a rise of MThw
of 2 m or more in the next 100 years and with at least the
same increase of the storm surge levels, all coastal
protection works would be ineffective. It is doubtful that
an elevation of the dikes for such a water -level rise would
be possible everywhere. As one can conclude from the
development of MThw at all locations on the German North Sea
coast for the last 400 years in connection with the
development of temperature in Central England, an increase of
2 m or more would be thinkable in the case of an increase of
the temperature of about 4°C. The increase of temperature is
assumed to be 1°C since the culminating-point of the "Little
Ice Age". In the same period MThw rose about 1 m.
Therefore it is necessary to intensify investigations in
order to predict the probable development of the global sea
level as well as that of certain coastal sections for the
103
-------
next 100 years as exactly as possible.
research are deemed necessary:
Three routes of
1) One has to analyze all the factors responsible for the
development of water level in the ocean and on the coasts
as exactly as possible. This demands an extensive and
close co-operation of the different scientific disci-
plines of climatology, meteorology, oceanology, glaci-
ology, geodesy and coastal engineering. This co-opera-
tion has to be international. Coastal engineers should
contact such projects as the International Climate
Research Program and the Project No. 200 of the Inter-
national Geological Correlation Program (Sea-Level
Correlation and Applications). The results of these
projects are of greatest practical importance for coastal
engineering.
Table 1. Trends in Sea Level Along the German North Sea Coast
Mean High Water Mean Low Water
Emden
Wilhelmshaven
Cuxhaven
Busum
Husum
Dagebull
(1857)
(1873)
(1843)
(1871)
(1870)
(1873)
till 1955
1
0.25
0.23
0.25
0.22
0.30
0.28
1873-1955
2
0.23
0.23
0.24
0.21
0.28
0.28
1956-1983
2
0.54
0.37
0.51
0.62
0.63
0.65
1956-1983
4
-0.33
-0.15
-0.21
-0.25
-0.09
-0.32
Slope "b" of the regression lines y = a + bx of mean high
water in three different periods and of mean low water in
the last period.
From these figures it appears that high tides apparently have risen from
2 to 6 mm/yr, while low tides have dropped 1-3 mm/yr; therefore, tidal ranges
have increased. The large differences occur during the 1956-1983 period when
the means of high and low tide levels have increased 2-5 mm/yr.
In Denmark the rebound from the glacial period was believed to dominate
until recently. The latest decades, however, show that relative sea level is
rising in the entire country, being highest (1-2 mm/yr) in the southwest part,
but close to 0 at the northernmost part of Jutland. At the tip of Jutland,
the Skagen Spit, the apparent 3-mm rise may be related to the consolidation of
thick layers of silt upon which the spit grew out by littoral drift deposits
in water depths of about 200 m in "the Norwegian trench."
In Norway it appears that the Oslo fiord area of granite and old lavas
may still be rising in comparison with sea level. The rest of the Norwegian
coast up to North Cape seems to be in a neutral situation, but at some places
sea level is slightly rising compared to land despite the fact that glaciers
were roaming here only about 10,000 years ago. Movements range from a 1-2
104
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sea level rise in the northern part to a 3-4 mm/yr land level rise
(compared to sea level) in the Oslo fiord areas.
In Sweden, glacial rebound is still very predominant, as shown in Figure
1- The southern shore of Sweden, however, now experiences a sea level rise.
The situation in Iceland is described by the already-mentioned recordings in
^eykjavik, which show a rise of 3.5 mm/yr.
In Holland and Belgium, sea level is rising 2-3 mm/yr (Table 2). The
general situation in northwest Europe is seen in Figure 1. The glacial re-
bound in Fenno-Scandinavia is still active, but not along most of the shores.
Table 2. Average Rate of Rise of Relative Mean Sea Level (MSL), and Mean
High Water Level (HW), Mean Low Water Level (LW) and Mean Tidal
Difference (TD) in cm per Century in Holland and Belgium
Station
^ostende
flushing
Terneuzen
Hansweert
Bath
Zierikzee
H°ok of Holland
JJmuiden
Jen Helder
Harlingen
°elfzijl
MSL
22
17
19
21
16
16
16
HW
17
33
40
no
44
26
22
24
15
27
21
LW
5
19
18
10
16
5
16
18
7
10
7
TD
12
13
22
30
28
21
6
6
8
17
14
period
used
1925-1980
1900-1980
1900-1980
1900-1980
1900-1980
1900-1980
1900-1980
1900-1980
1933-1980
1933-1980
1900-1900
Figure 1. Changes in Relation to Sea Level (cm per year)
105
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It has not been possible to secure data from the USSR. In Japan, sea
level has been observed (e.g., in the Osaka Bay) and is of the order of
magnitude of 2 mm/yr. On the Northern Island, settling takes place to a
considerable extent due to the extraction of gas and water. In China the rise
in sea level has been noticed mainly because of the increasing problems of
saltwater intrusion in the rivers.
In summary, sea level is rising worldwide at a rate of from 2 to 4 mm
annually in the areas we have discussed. However, it appears to be stable in
a number of places, such as in the vicinity of coral reefs in the Pacific and
Australia. As a result, most authors estimate the eustatic rate of rise at 1
to 1.5 mm/yr. There is some indication of an increase in the rise, but the
evidence is not too clear at this point.
THE IMPACT OF SEA LEVEL RISE ON SHORELINES
Erosion
It has long been known that high tides, particularly storm tides, can be
very destructive. The influence of the eustatic rise of sea level on shore
stability was "rediscovered" in the early 1960s.
The Bruun Rule of erosion (Bruun 1962), so named by American coastal
geomorphologists (Schwartz 1967, 1968, 1980), concerns a long-term budget of
onshore/offshore movement of material. The rule is based on the assumption of
a closed material balance system between (1) the beach and nearshore and (2)
the offshore bottom profile. This topic is dealt with extensively in theory
(Hallermeier 1972; Allison 1980; Bruun 1980) and through observations in the
field (Bruun 1954a,b, 1962, 1980; Dubois 1976; Fisher 1980; Hands 1980). The
"rule" has sometimes been used rather indiscriminately without realizing its
limitations. One should always remember that it is basically two-dimensional*
but it is almost always applied three-dimensionally.
The theory has now been tested extensively in the field (e.g., by
Schwartz 1967; DuBois 1976; Fisher 1980; Hands 1980; and others). Its overall
validity seems unquestionable when boundary conditions are well defined, but
this is not always the case. Its mathematics has been proven for two-
dimensional conditions (Allison 1980, 1981). The difficulty lies in clear
definitions of boundaries in relation to the composition of the materials of
which the shores are built up, usually sand particles decreasing in size as
distance offshore increases, until silt- and perhaps clay-sized particles
finally prevail. Such fine materials may originate from erosion of the shore,
but may also originate from rivers or from deserts and volcanoes as windblown
dust (Bruun 1980; Hands 1980).
As explained above, the theory of the influence of sea level rise on
erosion is proposed as being two-dimensional, but nature is three-
dimensional. This in turn means that in practice one must consider a certain
uninterrupted length offshore, when the material transport is contained in a
"box", xyz, where x is the distance offshore from a defined point on the
shoreline, y, the length of the box along the shore, and z, the depth from a
defined water table. The numerical material balance in and out of two x-z
sections y meters apart is initially assumed to be zero. If there is no
balance between the two quantities, this must be considered in the total
material balance equations. There are two boundary y-z sections, one located
106
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°n the beach or in the dunes, the other at a certain water depth, which
separates the "nearshore drift" from the "offshore drift" in the x-
direction. On the beach, the effect of wind-drift may then have to be
added. It is usually negligible, but it may in some cases present a non-
negligible quantity. The outer y-z boundary is more difficult to define,
Because there is no clear distinction between the limit of exchange between
beach and offshore drifts of material. In practice these limits are "fading-
out ."
Often the term "wave base" is used for material agitation on the
bottom. Recent research by Hands (1980) and Hallermeier (1972) attempts to
Present rational methods for calculation of the limits of active agitation of
the sand bottom and the long-term erosion rates based on long-term rises of
sea level. The short-term "seasonal" limit may be close to depth 2Hb, where
Hb is the height of the highest breaking waves. The "wave base" is thereby
related to the capacity or limit of the wave action in agitating the bottom
material "actively," which in a statistical sense depends upon the time
interval considered (Hallermeier 1972). A "five-year wave base" obviously may
n°t be the same as a "fifty-year base."
In addition to this, there is the problem of diffusion of material along
the bottom and other boundary layer phenomena (Bruun 1980). The original
theory (Bruun 1962), however, is based on a quantitative two-dimensional
balance between the beach and the nearshore and offshore bottom and there will
be a certain transition area between "nearshore" and "offshore" adjustments to
a change in sea level. If the physical forces do not change in the nearshore
apea, the equilibrium-balance condition apparently will be maintained. But
the situation may be different in the offshore area, where forces and supplies
of> materials are very different from those governing the nearshore area. No
equiiibrium balance situation needs to exist in the offshore area, where
QUrrents are offshore-originated. Bottom sediments in movement are clay and
silt, occasionally fine sand where currents are strong enough to carry the
Current-generated ripple marks have been found at very great depths
g., at 5,000 ft (1,500 m) on the Blake Plateau), which include indications
scour due to current movement over shells.
Depending upon the grain size, the "base" may extend to shallower or
r water. This refers to open sea coasts, where sediments of silt and
size may be carried long distances in suspension before being
^Posited. In defining the area of exchange between nearshore and offshore
^ifts, one therefore has to consider the grain sizes and materials of certain
°haracteristics available on the shore. The finest-parts of this material,
^hich may stay in suspension for long periods, therefore have to be included
in the materials balance equations. This refers to a certain depth beyond
which fines may still be transported to much deeper water for deposition
(Bruun 1980; Hands 1980).
The ultimate limit for movement characterized by threshold velocities is
ssed by Bruun (1985, 1986) based on research by Losada and Desire
(1985). The results from Iceland mentioned by Bruun (1986) point in the
of ultimate depth
d * 3.5 Hs
1
107
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where Hs then refers to wave heights occurring once, or over a certain number
of hours, every 1, 10, 20, 50, 100, and 500 years.
Bruun (1962, 1983) points out that his two-dimensional theory depends
upon a "closing depth" for exchange of material between the offshore bottom
and the shore. It must also be adjusted for grain sizes that will eventually
wind up far offshore due to their small size and consequent slow settling
velocities (i.e., silts and clays). For three-dimensional applications, major
adjustments may become necessary, particularly when features like tidal
inlets, canyons, and other "sinks" of materials enter the balance picture.
The report by Titus et al. (1985) on the erosion at Ocean City, Maryland, is
remarkable because it emphasizes the great importance of sea level rise on a.
long unabstructural shore.
A quantification of erosion caused by the rise in sea level is attempted
by Bruun and Schartz (1985). Sea level rise may contribute from 10$ to 100$ to
the total erosion or from 1 to 6 nH/m yr. If erosion is not caused by natural
or artificial inlets, breakwaters, groins, or sea walls, the sea level rise is
the only explanation for the occurring erosion. Figures 2-5 are examples of
erosion where sea level rise has been the major contributor. Examples were
taken from Queensland in Australia, Dajeng Bay in northeast China, Kerala in
southwest India, and Kamchatka in the USSR. Shoreline recessions are of the
order of 0.3 rn/yr. If sea level rises 1.5 m by the year 2100, shoreline re-
cession will be about 150 meters. For the lower bay lagoon, estuary, and
other tidelands such development may approach the catastrophic. Huge areas in
Bangladesh, Brazil, China, Egypt, Iraq, Iran, India, Lagos, Nigeria, and areas
on the U.S. East and Gulf coasts will be flooded.
Erosion of shores is a worldwide phenomenon. Shores always attempt to
adjust their condition to the impact of the acting forces, thereby
establishing an "equilibrium condition." But why do they then continue_to
erode? There is no other explanation than that sea level is rising.
The author has observed erosion in many countries all over the world-
Table 3 gives a record of his experiences, listing six different reasons for
erosion, three "natural" and three "man-induced." Sea level rise is listed as
a reason in all cases, either because no other reason could be given for the
erosion of uninterrupted shorelines of extended length or that sea level was
known to be rising and consequently erosion must result.
Another "natural" reason is subsidence caused by the consolidation of
soft materials, such as that which occurs in Holland; the Po-estuary in Italy*
part of India; Louisiana, USA; and elsewhere. Subsidence could, however, als°
be caused by the extraction of oil and/or gas as it occurs in Japan (Hokaido)
and California (Long Beach). Some areas may experience large- or small-scale
tectonic movements related to fault lines.
108
-------
Figure 2. Erosion in Queensland, Gold Coast, Australia (1985)
109
-------
Figure 3. Erosion at Dajeng Bay, China
Courtesy: Tinjang University (1984)
110
-------
^holograph showing beach erosion at Punnapra, Kerala (b) Photograph showing how the coconut trees were being up-
aur'ng the Monsoon of 1967. rooted at Punnapra, Kerala due to beach erosion (1967).
"otograph shewing erosion problem at a beach at Trivan- (d) Photograph showing the uprooted ccconut trees at Vypcen
um, Kprala Hnrino the- Mrmcr»rvn nf TQ76 Kerala due to the terminal effect rf a seawall.
Kerala during the Monsoon of 1976.
Figure 4. Erosion on the Southeast Coast of India, Kerala State
Source: NIO, GOA, 1983
111
-------
Figure 5. Erosion in the USSR, Kamsjatka (1962)
112
-------
Table 3. Overview of Causes of Erosion Worldwide
Country
Algeria
Argentina
Australia
Belgia
Brazil
Canada
China
Columbia
Denmark
Ecuador
Egypt
France
Holland
Iceland
Ireland
India
Iran
Israel
Italy
Japan
Lebanon
Malaysia
Mexico
Nigeria
New Zealand
Nicaragua
Norway
Pakistan
Portugal
Saudi Arabia
Spain
Sweden
Sri Lanka
Tripoli
Turkey
UK
USA
USSR
Venezuela
West Germany
I 40
NATURAL
Sea Level
Rise
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
40
Sub-
sidence
X
X
X
X
X
X
anti
(x)
X
7
Tidal
Inlets
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
28
MAN- INDUCED
Navig .
Channels
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
37
Man-made
Structures
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
33
Mining
X
X
2
113
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The following are other causes of erosion:
• Tidal entrances interrupt the normal littoral longshore
drift causing erosion because they discharge shore
material in the bay and/or in the ocean. Tidal entrances
accounted for 28 out of 40 cases in the 40 countries where
erosion was observed by the author; some of these cases
were very severe (Bruun 1978).
• Navigation channels, like the tidal entrances upon which
they were often based, constitute littoral barriers to the
normal drift by accumulating materials that otherwise
would drift to downdrift shores thereby contributing to
their stability.
• Mantnade structures such as breakwaters, jetties, groins,
and other shore-perpendicular structures are also barriers
to the longshore drift and have also caused severe damages
to downdrift shores. It can safely be said that the
coastal protective groins have caused more erosion than
accretion. According to the author's experience, manmade
structures (listed in Table 3) are second only to sea
level rise and head of natural tidal inlets as causes of
erosion.
• Mining for heavy minerals has caused damages in several
countries until the adverse effects were observed. Mining
also includes extruction of sand, gravel, and stones.
Such operations are now largely prohibited, but they still
occur in countries where the problems they cause have not
yet been realized.
Short-term erosion events of a severe or extreme nature usually catch
eye of the public. To evaluate occurrences of rapid dune erosion due to stor"1
tides, like hurricanes and typhoons, the large-scale experiments by Veiling
(1983), Vellinga and Bruun (1984), Vellinga et al. (1985) and van de Graa*
(1986) are important for assessing events of an extreme nature for whicl1
preparedness is warranted.
In summary, erosion due to sea level rise is obvious. Compared to
reasons, particularly the manmade, it is less severe, but as it occurs almost
everywhere, it undoubtedly counts for the largest part of erosion worldwide*
It is therefore very unfortunate that we may expect (during the next sev
decades) an accelerated rise in sea level.
Shoreline Recession
Shoreline recession can be expected on the ocean shore as a result of
level rise. The actual shoreline recession depends upon the exposure,
offshore profile-steepness, and the character of the material that builds UP
the shore and the offshore bottom.
One may get a general impression of the rate of shoreline recession
examining the following data which are representative of recession rates
exposed shores like those of the Atlantic, the Pacific, and the Indian Oceans*
114
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Rise per year Shoreline Recession per year
1 cm 1-1.5 m (4-5 ft)
2 cm 2-3.0 m (7-10 ft)
3 cm 3-4.5 m (10-15 ft)
10 cm 10-15.0 m (33-50 ft)
These numbers represent the volume of material lost to the sea or the
needed to maintain a status quo with respect to erosion. The figures
sPeak for themselves, giving an impression of the severity of the problem we
ar
-------
midst of a waste of water. It was impossible to say whether
the country belonged to the land or to the sea. "They try to
warm their frozen bowels by burning mud, dug with their hands
of the earth and dried to some extent in the wind more than in
the sun, which one hardly ever sees.
No doubt the mud Pliny refers to was the peat which was found
in the "wolds," or swamps, some distance south of the clay
marshes, where the artificial mounds had been made. In all
they built 1,260 of these mounds in the northeastern part of
the Netherlands, an area of a mere 60 x 12 miles. Further East
there are more of them in East Friesland. The areas of the
mounds themselves vary from 5 to 40 acres; they rise sometimes
to a height of 30 feet above normal sea level. The contents of
a single mound may be up to a million yards.
They built their mounds on the shores of the creeks in which
the tide ebbed and flowed. In their scows they went (in their
language in which the roots of so many English words can be
found): "uth mitha ebbe, up mitha flood," out with the ebb, up
with the flood. The tide bore them towards the peat regions,
or perhaps to the woods still farther inland and then brought
them back. Or they went out with the ebb in the morning
towards the sea, where they gathered their food, and returned
in the evening with the incoming tide.
The Coastal Dutch have now lived 24 centuries in their marshes
and of these the first 20 or 21 were spent in peril. It was
not until 1600 or 1700 that some reasonable security from
flooding was achieved. During these long treacherous centuries
the artificial mounds made their survival possible'.
It was a work which might be compared with the building of the
pyramids. The pyramid of Cheops has a content of 3,500,000
cubic yards, that of Chephren 3,000,000 and that of Mycenium
400,000 cubic yards. The amount of clay carried into the
mounds of the northeastern part of the Netherlands can be
estimated at 100,000,000 cubic yards.
In Egypt it was a great and very powerful nation which built
the pyramids throughout a series of dynasties. The aim was to
glorify the Pharaohs. With us it was a struggling people, very
small in number and often decimated, patiently lifting their
race above the dangers of the sea, creating large monuments,
not in stone, but in native clay.
In this Lex Frisionum of 802 there is not yet any mention of
seawalls, but the first attempts at dike building must have
been made shortly afterwards. Frisian manuscripts still
extant, dating from the early Middle Ages, deal chiefly with
the following three points: First, the right of the people to
freedom, all of them, "the bern and the unbern." Secondly, the
"wild Norsemen" whose invasions took place roughly from 800 to
1000, and thirdly: the Zeeburgh or Seawall.
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This novel means of defence against the sea by means of a
continuous clay wall was called a Burgh, or stronghold. The
people were apparently very proud of this seaburgh, because
they described it in poetical language as "the Golden Hop," the
Golden Hoop.
...This is also the Right of the Land to make and maintain a
Golden Hoop that lies all around our country where the salt sea
swells both by day and by night.
The spade, the hand barrow, and the fork were the instruments used for
Diking, the fork presumably for the grass turfs which were used to heighten
dikes and make them stronger. Despite the tremendous efforts the sea was
strongest.
This was due partly to our insufficient technical skill and
partly to lack of co-operation. For a single night, Dec. 14th
1287, the officials and priests estimated that 50,000 people
had been drowned in the coastal district between Stavoren and
the Ems. This is a large number considering that this was the
area where so many dwelling mounds could be used as places of
refuge.
The advances and successes have been tied to a few names. Says van Veen:
We often wondered who was the master engineer who created
marvellous Great Holland Polder, south of Dordrecht, the work
which had included the damming off of the tidal mouth of the
river Maas, and the leading of that river into the Rhine. This
proved to be William I. He had already finished that gigantic
undertaking by 1213. The polder was destroyed in 1421 by the
St. Elisabeth's flood, described in a former chapter. William
was a man of great conceptions. He surrounded the entire area
of Holland-Proper with strong dikes and made several canals
intended to drain the vast moors. They also served as a
splendid network of shipping canals. It is likely that he made
the dikes around the Zeeland islands Walcheren and Schouwen
too, and that he established the still-existing administrations
for the upkeep of these islands. The other part of his clever
and amazing reclamation and construction programme cannot be
described here, but it is very clear that he knew the geography
of his county by heart. No maps as yet existed!
The earliest reference to the art of accelerating the natural rate of
tion is the manuscript "Tractaet van Dijckagie" (Treatise on
ebuilding), written by the Dutch dikemaster Andries Vierlingh between 1576
1579. Vierlingh discusses the construction of "cross-dams" on mudflats
are not yet dry at low water. In this connection he also advises that
ships should be sunk and earth dumped on the top of them so as to make
islands or flats which should hold back the silt and sand suspended
^ the water. These islands should subsequently be connected with low dams.
^though this method has not been used commonly, it is known that shipwrecks
*V been used at numerous places to close dike breaches. These wrecks formed
basis for the fill material which was secured with mats or brushwood.
117
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Vierlingh, however, was much against closing of dike breaches with shipwrecks
due to the non-homogenity they created in the dike structure. Nevertheless,
this method was widely used over a long period of time, not only in Holland
but in the (at that time Danish) Schleswig-Holstein.
Vierlingh was found to be a real master of the dikes and waters, a man of
great ability and spirit - one of the greatest of his kind. Luckily the
greater part of his manuscript has survived. Its ancient picturesque style is
a joy to every hydraulic engineer. This remarkable book already shows the
special vocabulary of the Dutch diking people in all its present-day
richness. In some ways it is even richer.
His advice is simple and sound. The leading thought is: Water
be compelled by any 'fortse' (force), or it will return that fortse onto
This is the principle of streamlines. Sudden changes in curves or cross-
sections must be avoided. It is the law of action and reaction. And truly »
this fundamental law of hydraulics must be thoroughly absorbed by any one who
wants to be a master of tidal rivers.
Andries Vierlingh was a genius. If we had followed his advice on
streamlining, we would have been better off today. His protection was the
streamlined dike. It offered storm tide protection but not necessarily
erosion protection unless the dike was provided with a hard surface. And even
so, erosion might continue below or in front of the hard surface due to the
combined action of waves and currents. To push the currents away, current
breakers or groins were then introduced. The hard surface on the dike was *
revetment with a gentle slope reflecting little wave energy, and so it stil-
ls in Holland, where vertical walls have long been banned. The groin worked
well in certain areas, where currents and waves carried considerable material
as they still do on some parts of the Dutch coast and in Denmark (Bruufl
1984). If not, they were of little importance or only of value as a kind of
toe protection for the dike. Experiences elsewhere are, of course, similar.
PLANNING FOR SEA LEVEL RISE ON THE OCEAN SHORES
We must accept that sea level in all probability is going to accelerate
its rise. Consequently, the beach and bottom profiles must adjust to a neV
situation. As a result erosion will occur. Titus (1984) characterizes the
issue as follows:
Ocean beach resorts in the United States have always faced erosion
and storm damage. At first, these risks were accepted as
inevitable. Development was generally sparse, and people often
built relatively inexpensive cottages along the ocean that they
could afford to lose. When the occasional severe storm destroyed
these houses and eroded the beach, replacement structures were
frequently built farther inland to maintain the original distance
from the shore.
118
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After World War II, beaches became more popular and were developed
more densely than before. The resulting increases in real estate
values enabled greater numbers of communities to justify expensive
engineering solutions to maintain their shorelines. Frequently
subsidized by the federal government, the practice of stabilizing
shorelines replaced the previous custom of accepting erosion as
inevitable.
The projected rise in sea level poses a fundamental question: how
long should these communities hold back the sea? In the decades
ahead, the costs of shoreline protection will rise dramatically
and the relative efficiencies of various measures will change.
But without such efforts, a 1 ft rise would erode most shorelines
over 100 ft, threatening recreational use of both beaches and
adjacent houses. Even under the low scenario, this could happen
by 2025.
Although sea level is not expected to rise rapidly until after
2000, resort communities may have to consider its consequences
much sooner. After the next major storm, in particular,
homeowners whose properties are destroyed will decide whether and
how to rebuild; and local governments will decide whether or not
to let all of them rebuild, and which options are appropriate to
address the storm-induced erosion. How well a community
ultimately adapts to sea level rise will depend largely on the
direction it takes when it reaches this crossroads.
, Quantification of erosion due to sea level rise is of course difficult as
ne rise-induced erosion is mixed with other agents of erosion. Bruun and
(1985) made an attempt and show that beach erosion due to sea level
may amount to about *\5%-20% of the total erosion occurring on heavily
g shores. The above-mentioned report by Titus et al. (1985) (for Ocean
, MD), however, demonstrates that sea level rise could become the major
of the erosion on extended shores far from tidal inlets or other
sinks." See also Everts (1985) and Bruun (1986c).
Nourishment
Bruun (1973) after having stated that future coastal protection must
beach restoration and maintenance as well as storm tide protection by
or revetted dunes stated the following:
In future coastal protection one must think large. It will
therefore develop as a function of the combined political,
administrative and technical structure. There will be little or
no use for "one-man shows." Large groups and large areas will
have to be accomodated - by large scale measures. Needs will be
concentrated on protective and recreational projects and all
combinations thereof. Pressure will increase by the need for
recreational beaches. Protection will be achieved simultane-
ously. The question of which protective measure will be most
practical under such circumstances may be answered by just looking
at Tables 5-10 which clearly demonstrate that artificial
nourishment with suitable material offers the best large-scale
119
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protection. This, however, does not mean that it always
suffices. It may need support from dikes and/or sea walls because
of the possibility of storm surges or it may need groins to break
scouring currents running close to shore. One main technical
advantage associated with artificial nourishment is that it is
"smooth" and "streamlined" and therefore not only has no adverse
leeside effects, but, on the contrary, benefits adjoining shores
by a gradual release of material. Other measures, particularly
groins and offshore breakwaters, have definite adverse effects on
neighbouring shores. The importance of streamlining is obvious.
What shall we then do? We shall improve our artificial nourishment
technology. For that we need (a) suitable sand, as coarse as possible of
coarser than the beach sand (but sand must not be so coarse that it generates
a partly reflecting beach which could become dangerous to bathers); (b) the
sand must be placed by equipment which is as efficient and economic ^
possible, (c) the sand shall be placed with the right profile (which means:
so that we lose as little of it as possible). Although items (a) and (b) have
long been recognized as appropriate, that is not the case with item (c).
Bruun (1986a) discusses in theory, as well as in protective equilibria111'
profiles including geometries and grain sizes. Theoretical and practical
results favor placement of material, not on the beach, but along the entire
cross section, all at one time, with grains of various sizes placed "exactly
where they belong in the profile. It is unquestionable that this will cause
higher stability and less material loss alongshore as well as offshore.
Table M (Bruun 1986a) describes an example of profile nourishment, W
which three different grain sizes (distributions) are placed in three
different depth ranges. This, of course, requires meticulous planning
supervision of the actual work in the field under construction.-
Table 4. Profile Nourishment
Tide
High Spring
Mean
Neap
Mean Spring
Mean
Neap
Low Spring
Mean
Neap
Area
A: + 1, -1
C
C
C
c
-
-
-
-
—
B: -1, -3
C
M
M
M
M
M
M
-
—
C: -3, -5
—
F
F
F .
F
F
F
F
F
120
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Three different grain sizes C (0.25mm), M (0.21mm), and F (0.18mm), are dumped
*t three tidal elevations (High, Mean, and Low) on three different bottom
areas for the portion of the beach profile extending from 1-m above sea level
to 5-m below sea level (A, B, and C) (Bruun 1986a).
How shall we accomplish profile nourishment? — using equipment that is
al)le to dump on the beach as well as to whatever depth the material shall be
Placed. This includes pipelines and hopper-dredges as well as split-hull
b^rges (Bruun et al. 1985) Figure 6 shows the principle of "profile
n°urishment;" Figure 7 is a Danish example. As explained in Bruun (1986a)
Profiie nourishment is highly economical compared to conventional beach
JJ°Urishment due to the lesser costs in offshore dumping bid prices in
Queensland .
For storm tide protection we need dikes of adequate elevation. At some
where the beach has narrowed excessively, dikes must have a hardened
on the ocean side to resist wave attack and overflows by uprushes.
to keep the dike intact a beach stabilization is needed in front of the
. if we are not able to stabilize the beach, we will have to move the
back sooner or later and keep moving it back if shore recession
°°nUnues. We have no other option.
In the large scale we apparently have to consider three different
p°ssibiiities as shown in Figure 8:
* Stabilization of the shoreline: No further shoreline recession due to
the existing land developments.
' Let nature take its course: Accept the erosion; establish a beach
park area instead.
* Compromise: Establish set-back lines valid for a certain time period
before reviewing them for further set-backs.
. Stabilization. This is a common case, where stabilization is a must.
in Holland, parts of Denmark, the United Kingdom, and Germany" will
to be closely monitored in case stabilization becomes necessary. Shore
opments like townships and barriers protecting bays or lagoons do not
w further shoreline recession. Consequently, it is necessary to nourish
beach and to stabilize the dunes. This will be done for many shores in
fornia and Florida as well as for other shores on the east coast of the
United States.
Let nature take its course. This may be done where plenty of undeveloped
is fOLmd behind the shoreline, such as in the Outer Banks of North
in the United States. The most practical way of handling such a
is by constructing withdrawn dikes at considerable distances behind
® shoreline and prohibiting any substantial development outside the
ithdrawn dike. This is already taking place on the Danish North Sea Coast,
e Dutch Friesan Island, and on the Outer Banks. The wisdom of this strategy
°bvious — such national and state parks have become very popular and more of
are probably going to be established.
121
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PROFILE NOURISHMENT
Schematic Example
Beach
0.24 mm = A
Profile: (Depth) 3/2 = PX
p = f(W, H, T, D-)
MSL
Nourishment Material
A: 0.24 mm
B: 0.2 mm
C: 0.18 mm
Dumped on Beach
— Offshore
- Offshore Until Limiting Depth
for Movement Based, e.g., on the One-Year Storm
Figure 6. Profile Nourishment
Figure 7. Profile Nourishment on the Danish Morth Sea Coast
Source: Coastal Directorate, Lemvig 1985
122
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1. STAND-BY
No Recession
Maintained Dune or Dyke
Shoreline Kept
in Position
Wide Beach
Narrow
Beach
Hardened
Dune
2. GIVE WAY
Dunes
3. NATURE'S COURSE
Recreational Area
Park
Park
Park
A, B, C To Be Evaluated By
Economic and Environmental Parameters
Figure 8. Long-Term Shore Management
Organized Retreat
Time-Limited
Development (50, 1 00 Yrs)
Withdrawn Hykoa ^
Ac
v<
^s
^
idE
I
Withdrawn Dyke
'or Dune
Shoreline Moves
ack ^ck
.^
Defence Lines. At Intervals
Set- Backs
Gradua
**
lly
N^~"
2
2
U
o
\ /
Shoreline Moves
Back Gr
actually \
Z
2
8
123
-------
Compromise. Establish a set-back line valid for a certain time-period
(e.g., 50 or 100 years). Allow a controlled development. Houses can be built
in such a way that they can easily be moved back.
While items (1) and (2) are straightforward, item (3) is more complicated
because it must be anticipated that the lifetimes of parts of the development
are time-limited. That would imply, for example, that buildings erected on
the property after some time would be moved back to "safer locations." One
may imagine the administrative difficulties involved in such an arrangement.
One solution, of course would be to lease the coastal properties for limited
periods of time (e.g., 20, 30, 50, or 100 years). The buildings erected on
such property should then be designed in such a way that they could be moved
landward to another location after a certain time period. Technically, this
relocation would not be difficult; the problem would be to make new areas
available for the reconstruction of such buildings, which are expendable after
so many years.
Financing
Financing possibilities vary from country to country. In the United
States the state governments are taking over more and more of the
responsibility which earlier rested with the Federal Government. The Florida
task force "Save our Beaches" suggests a 15% financial contribution by the
state and a 25% local contribution. In the low countries in Europe, costs
will be borne by the national government either entirely or with some matching
funds from the local governments. The contributions by public funds rang6
from 50$ to 100$. In some latin countries, like Spain, the national and state
governments will pay all costs of shoreline protection.
Developing countries in general have no provisions or possibilities f°f
public funding. In some of these countries (e.g., large coastal areas in
Bangladesh) the situation has become critical. Financing by- UN-World Bank ^
a possibility being explored for Bangladesh.
CONCLUSION
* Sea level is rising. An accelerated rise would cause accelerating
erosion of our coastal shorelines.
• Our ability to quantify the magnitude of the rise is steadily
improving.
• We are (largely) able to predict the influence of sea level rise o11
shore stability.
• Proper coastal protective measures are available to mitigate
effects of erosion, wherever this is desirable.
• Future coastal protection will involve beach nourishment and s
tide protection by dikes and revetments, where needed, as well as
establishment of practical set-back lines.
• Progress is being made, scientifically and technically, to impr°ve
available nourishment procedures and equipment.
124
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128
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Predicting the Effects of Sea Level Rise
on Coastal Wetlands
Richard A. Park, Thomas V. Armentano, and
C. Leslie Cloonan
Holcomb Research Institute
Butler University
Indianapolis, Indiana USA
INTRODUCTION
Accelerated sea level rise brought about by global warming may have a
Profound effect on coastal wetland distribution (Titus, this volume). Wet-
lands, including salt-, brackish-, and freshwater marshes and, in subtropical
*nd tropical regions, mangrove swamps, typically occupy low-energy, protected
coastlines in estuaries, bays, and lagoons. Consequently, in the United
States only 10$ to 20% of the Pacific coast is suitable for wetland
development because of the high-energy, rocky shoreline, whereas about 70% of
the shoreline of the Atlantic and Gulf coasts is suitable because of the
extensive estuaries and gently sloping coasts with lagoons ' behind barrier
islands (Emery and Uchupi 1972).
Wetlands are important to the ecology and economy of coastal areas for
several reasons. Their biological productivity is equal to or exceeds that of
any other natural or agricultural system. For example, a Georgia salt marsh
has, on average, an annual yield of 8 tons/acre (1700 g/m2) of organic matter
(Teal 1962) and a Florida mangrove swamp, 4 tons/acre (880 g/m2) (Mann
!972). In comparison, the best wheat field yields 7 tons/acre (1570 g/m2) and
offshore coastal waters yield 1 to U tons/acre (220 to 3^0 g/m2) of organic
About half of marsh productivity is available to marsh animals and
°oastal fisheries (Teal and Teal 1969). Furthermore, salt marshes serve as
fiursery grounds for over half the species of commercially significant fishes
*n the southeastern United States (Thurman 1983), as rookeries for- many water
f°wl, and as refuges for a variety of mammals (Teal and Teal 1969).
Wetlands also remove pollutants, including nitrogen, phosphorus, heavy
Petals, and radioisotopes, from sewage effluent and ground- and surface water
(Pope and Gosselink 1973). Equally important, they store floodwater and
Provide protection from coastal storms and high tides (Lugo and Brinson
129
-------
1978). Based on these functions, it has been estimated that marshes provide
an annual return equivalent to $5,500/acre (Thurman 1983) and mangrove swamps,
a return of $990/acre (Lugo and Brinson 1978).
These valuable systems could be diminished and lost with accelerated sea
level rise, whereas slow sea level rise favors development and expansion of
wetlands. In fact, during the past 4,000 years when the sea level has risen
an average of 1 mm/yr, 0.04 in/yr (Emery and Uchupi 1972), wetlands have kept
pace, and in many areas, actually increased significantly due to sediment
entrapment and peat formation (Davis 1985). However, vertical accretion of
wetlands has not been observed to occur at a rate comparable to the acceler-
ated sea level rise projected for the next century by Hoffman (1984).
Therefore, future inundation and erosion of wetlands has been projected (Earth
and Titus 1984).
Based on available data, wetlands along undeveloped coastlines lacking
protective engineering structures are expected to migrate readily onto
adjacent lowlands, especially in areas less than 3.5 m (10 ft) in elevation.
However, because most coastal lowlands have steeper slopes than present-day
wetlands, the areas of wetlands lost may exceed areas gained, causing a net
reduction in wetlands. Furthermore, many coastlines, especially those which
are or will be developed commercially, probably will be protected by dikes and
levees. These structures will preclude migration of the wetlands, and large-
scale loss of wetlands has been projected (Titus, in press).
In order to assess the potential loss and migration of wetlands, detailed
case studies were conducted in South Carolina and in New Jersey (Kana, Baca,
and Williams 1986; Kana et al., in press), and a more general map and
simulation analysis was conducted on fifty-seven sites along the contiguous
coasts of the United States (Park, Armentano, and Cloonan 1986; Armentano,
Park, and Cloonan, in press). This paper reviews these studies and suggests
possible effects on marshes and mangrove swamps representative of a variety of
coastal situations under a scenario of high sea level rise and protection of
developed areas.
DETAILED CASE STUDIES
Charleston, South Carolina, Area
Twelve wetland transects provided the database for considering the
effects of sea level rise on the harbor of Charleston, South Carolina (Kana,
Baca, and Williams 1986). Figure 1 shows a composite of these transects.
Shoreline changes were assumed to depend on slope, with gentle slopes
experiencing considerable horizontal shoreline displacement and steep slopes,
relatively little. Artificial or engineered shorelines were recognized as
impediments to shifts in shoreline position until a threshold sea level was
exceeded. Detailed maps were prepared based on topographic quadrangles that
were digitized and displayed in computer-generated form. A good database on
historical shoreline trends was available to help project future responses to
sea level rise.
130
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COMPOSITE TRANSECT-
CHARLESTON. S.C.
Hir.Hl.AND 47%
u>
SPBIHO HIGH WATER
MEAN MICH WATER
NEAP HtOH WATER
-e
7OOO 3OOO
TYPICAL DISTANCE.(FT)
4OOO
sooo
Figure 1. Composite Transect of Charleston, South Carolina, Marsh
Source: Kana, Baca, and Williams 1986
-------
A previous study showed that over the past 40 years the Charleston shore-
line has accreted relative to sea level, which is similar to shoreline
responses throughout most of the Atlantic coast. For purposes of the present
study, a vertical marsh accretion rate of 5 mm/yr was employed. Landward
migration of tidal flats, low marsh, high marsh, and transitional wetland-
upland vegetation, was considered as a response to sea level rise. Dikes,
bulkheads, and other protective structures, however, would act as barriers to
marsh migration onto uplands. Under such a scenario, wetland distribution
through 2075 will depend on the potential for wetlands to maintain vertical
sediment elevation relative to specified sea level rise rates, assuming a
constant sedimentation rate throughout all marsh environments, and on the
capacity for horizontal migration as rising seas encroach on the critical
elevational range for each marsh type.
Results of the scenario analyses indicate that by the year 2075 signifi-
cant marsh loss would occur under all scenarios (Figure 2). Even without
additional protective structures, 70$ of high marsh and 40$-84# of low marsh
would be lost under moderate and high scenarios. Total wetland loss (exclud-
ing tidal flats but including transition marsh) would range from 43^-73^
without additional protection, under moderate and high scenarios. Under
specified construction of protective devices for certain sites, losses would
rise to 5*\%-92% of 1980 marsh area. Nearly all of Sullivans Island, a seaward
barrier island, would experience significant erosion and shoreline movement.
In the high scenarios (a rise of 1.6 m, or 5.2 ft, by 2025), few additional
changes would occur except for shifts up to 225 m (740 ft) inland of the
shoreline parts of the mainland. Despite the extensive losses projected for
the Charleston area, the relatively wide tidal range (1.6 m, or 5.2 ft)
suggests less wetland vulnerability to rising seas.
Tuckerton, New Jersey, Area
This site, consisting of two marsh areas with a 0.6-1.0 m (2.0-3.3
tidal range, was evaluated as representative of a microtidal back-barrier
wetland area that is more typical of the Atlantic coast (Kana et al., in
press). Six subenvironments were distinguished at Tuckerton, ranging from
open sea to highland (Figure 3). Field surveys determined exact boundaries of
subenvironments.
The Kana et al. model projects the replacement of high marsh by low marsh
under the low scenario (Figure 4). However, the aggregate change in salt
marsh area is small; if transition marsh is included, wetland changes counter-
balance, with essentially no net change.
With a high scenario sea level rise, significant land and wetland losses
result (Figure 5). Salt marsh is reduced by &5%-90%, favoring open sea.
Transition marsh would be significantly reduced at Tuckerton. The South
Carolina and New Jersey sites differ in projected response to the moderate sea
level rise scenario as a result of the predominance of low marsh at one and
high marsh at the other. The low marsh is lost to tidal flats or open water,
whereas the extensive high marsh in New Jersey would be converted to low
marsh.
132
-------
WATER
2075
33*
•6-
IIJ
o
in
O-
-2-
TBAMSITIOM HIGH MARSH
198O I 198O
3% I 5%
ZO75 MSL
LOW SCENARIO
19BO MSL
EMSTWG
HIGHLAND •
: 1980 *:
47% ,
Figure 2. Conceptual Model of Low-Scenario Sea Level Rise in Charleston Marsh
Source: Kana, Baca, and Williams 1986
-------
Tidal Flat
1%
Waler
33%
MainHighWater
Loc»l MSL
M«»o Low Wjt»c
20QQ 30OO
TYPICAL DISTANCE (FT)
5000
6000
Figure 3. Composite Transect of Tuckerton, New Jersey, Marsh
Source Kana et al., in press
-------
*8
£f +6
LU
-2-
-4
Highland
2075
28%
Transition
2075
3%
High Marsh
2075
5%
Low Marsh
2075
29%
Highland
1980
30%
Transition
1980
7%
Migh Marsh
1980
27%
Low Marsh
1980
2%
Tidal Flat Water
2075
1%
2075
34%
2075 («5.0 fl.)
2075 (»3.3 ft.)
2075 (* 2.5 ft.)
• 2075(» 1.0 ft.)
»075 MSL
?-2075 MSL
Low Scenario
¥-1980 MSL
Tidal Flal Water
--1980 1980
1% 33%
Figure 4. Conceptual Model of Low-Scenario Sea Level Rise in Tuckerton Marsh
Source: Kana et al., in press
-------
z
0
£
>
UJ
111
Highla
207J
1%
*10 -
*6 -
*4 -
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-2 -
-4 -I
nd
5
:'•••:•.
High Marsh
2075
Transition 2%
2075 /Low Marsh
2%/ / 2075
T^JL L ' ' L^Tidalflat Water 2075 (* 5 Oft)
^^^J" , ~\ 2075 2075 , , , '(
KXXXX>L 1 _l „ . ... ,._._. .,.,..._ 2075 (« 3 3 It .)
?*<Kr-\ 2075 (* 2.5 ft.)
:::':&^*/:^&£x:5fc><(£s5r*£-- — ^ 2075 ( » 1.0) \-7 _„_
;-V»«"
:•::•'
'•;.;::"
:/:::
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Highland
1980
30%
^-^Z^/////Sediin«ntation SrnnVyr'V/y VSS/S S / / / 7^ i linh '
-
- '"
1
•
- -
Transition
1980
7%
f T V o U M olr~~
-
High Marsh Low Marsh
1980 1980
27% 2%
y> . ••!)•• -
s/»-
\
+*.
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^-tfc^V/^-^.^^ _ 1 JO
- ^^^^^22777^7
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Tidal Flat Water
-^-- 1980 1980
| 1% 33%
Figure 5. Conceptual Model of High-Scenario Sea Level Rise in Tuckerton Marsh
Source: Kana et al., in press
-------
REGIONAL MODELING
To develop a regional-national analysis of U.S. coastal wetland responses
to sea level rise, a general simulation model was developed, and model-
supportive stratified sampling of the contiguous U.S. coastline was undertaken
for nine regions (Armentano, Park, and Cloonan in press).
Data
A total of 183 7.5-minute USGS topographic quadrangles were used to
characterize the fifty-seven sites depicted in Figure 6. The data were
collected using the Universal Transverse Mercator grid so that each cell is
1 km^ (100 hectares, or 247 acres). Of the sixteen categories of coastal
types, each is based on the dominant category within the square-kilometer
cell. They are summarized in Table 1. The mean elevation is based on the
dominant category in the cell. Although this introduces an element of
imprecision, if a large enough area is considered, the estimate is not
biased. Tidal range for both open sea and sheltered areas is taken from the
topographic maps, or, if necessary, from tide tables.
The presence of naturally sheltered areas (e.g., bays) is coded, as are
"fcjor protective structures such as levees. Finally, the existence of
significant residential and commercial development is noted. The extent of
freshwater and brackish wetlands cannot be determined at the regional level
from topographic maps.
Models
A large number of models have been constructed for fresh- and saltwater
Wetlands (cf. Day et al. 1973; Wiegert et al. 1975; Costanza et al. 1983;
^itsch et al. 1982; Costanza and Sklar 1985). However, few of these models
incorporate the spatial resolution desired in the present study. Two notable
exceptions are recent papers by Browder, Hartley, and Davis (1985) and Sklar,
costanza, and Day (1985) on disintegration and habitat changes in the
Louisiana coastal wetlands. No previous models provided both the spatial
Resolution and the generality required for studying the effects of sea level
on a regional scale for the United States.
e^ SLAMM Model
5escrip_tion. SLAMM (Sea Level Affecting Marshes Model) simulates the long-
term change in coastal areas due to rising sea level. The smallest area
simulated is a 7.5-minute quadrangle (1:24,000); the largest practical area is
*bout the size of four 7.5-minute quadrangles. The assumptions and methodol-
°gy are appropriate for application to entire coastlines.
SLAMM is intended to be used primarily as a diagnostic tool, providing
scientists, resource planners, and decision makers with a basis for consider-
ing- the impacts of future sea level rise on coastal systems, especially wet-
lands and barrier islands. As such, the model employs a reasonably straight-
forward but complex set of decision rules to predict the transfer of map cells
from one category to another (Figure 7). These rules embody assumptions of
137
-------
1 New England
2 Mid-Atlantic
3 South Atlantic
4 Southern Florida
5 East Gulf Coast
6 Mississippi Delta
7 Chenier Plain-Texas Barrier Islands
8 Californian
9 Columbian
8
Figure 6. Location of Sample Sites and Coastal Regions Used in
Modeling Wetlands
Source: Armentano et al., in press
138
-------
Table 1. Coastal Land Categories
Category
Undeveloped Upland
Developed Upland
Undeveloped Lowland
Developed Lowland
Protected Lowland
High Dunes
Exposed Beach
Sheltered Beach
Developed Exposed Beach
Developed Sheltered Beach
Freshwater Marsh
Salt Marsh
Mangrove Swamp
Tidal Flat
Sheltered Water
Open Water
Definition
Undeveloped upland land that is above
3.5 m elevation
Upland that has significant line deleted
development
Land that is below 3.5 m elevation and
above mean high water spring tide (MHW
Spring)
Lowland that has significant residential
or commercial development
Lowland that is protected from
inundation by a dike or levee
Extensive, large sand dunes
Beach exposed to the open ocean
Beach sheltered from the open ocean
Exposed beach with significant resi-
dential or commercial development
Sheltered beach with significant
residential or commercial development
Wetland composed of species intolerant
of salt water
Wetland composed of herbaceous species
that are tolerant of salt water
Wetland composed of mangrove trees
Muddy or rocky intertidal zone
Water that is protected from the open
ocean
Water that is not protected from the
open ocean
139
-------
TIDAL FLAT
(& ROCKY
INTERTIDAL)
UNDEVELOPED
LOWLAND
Figure 7. SLAMM Flow Chart Showing Transfers Among Categories
Source: Park et al.f 1986.
140
-------
linear, average responses and may not apply in detail for any particular area;
however, they are suitable for policy development on a regional basis,
Providing an estimate of the magnitude of the problems and suggesting the
nature of the regional policies needed to mitigate those problems. Because
the model is intended to be applied to many different areas, the algorithms
have been developed to require minimal input from the user.
Each cell category is represented by a pattern and a color, so that the
Primary output from the model is colored maps for user-specified yearly
intervals for a given area and rate of sea level rise. Summary statistics for
all categories are provided for 25-year intervals and for wetlands, for 5-year
intervals, so that the progressive impact on coastal wetlands can be assess-
ed. The model is implemented in Turbo Pascal for the IBM PC.
^gsumptions and Simplifications. Because the model is intended to be used for
Regional analysis of long-term trends, several simplifying assumptions have
been made that may not be appropriate for detailed analysis of local and
short-term conditions:
• Each square-kilometer cell is represented by only one (dominant) cate-
gory and by average elevation; this results in underrepresentation of
pocket beaches, marshes, and narrow barrier beaches; furthermore,
gradual changes seem to occur instantaneously when the threshold
average elevation of the cell is reached.
• Continued residential and commercial development of coastal zones is
ignored; only those areas developed when the maps were published are
subject to protection; given current trends and policies, this may not
be a reasonable assumption.
• Freshwater discharge is ignored in distinguishing freshwater wetlands
from saltwater wetlands; this is most noticeable in the Florida
Everglades, which are modeled as mangrove swamp due to their elevation
near sea level.
• Sedimentation and accretion rates are related to the extent of exist-
ing wetlands; in most areas this results in a decrease in sedimenta-
tion as marshes disappear, coinciding with the decrease brought about
by sediments "hanging up" further inland in the deepening estuaries;
however, in areas where extensive lowlands are inundated and converted
into wetlands, this algorithm will predict increased sedimentation—
perhaps more than is reasonable.
• No distinction is made among East Coast, West Coast, and Gulf Coast
marshes; the same algorithms are used for accretion, erosion, and
position within the tidal range for all three regions; SLAMM also does
not distinguish between mature and new marshes.
• No provision is made for changing vegetation due to global warming
trends; in particular, mangroves will not be simulated in more north-
erly areas where they do not already occur.
* Cliff retreat is not modeled, nor is the increased supply of sediment
to the coastal regime due to cliff erosion; this could affect areas
such as Cape Cod, Massachusetts, and Oxnard, California.
141
-------
• Actual bathymetry is not considered nor is the effect of changing
bathymetry on wave energy; beach migration is permitted in sheltered
water but not in open sea; this seems to be a reasonable simplifica-
tion for essentially all areas.
• The change in erosion by tidal currents with changing morphometry and
bathymetry is not modeled.
• Changes in storm tracks and in the erosive energy of storms con-
comitant with climatic change are not modeled.
Validation. SLAMM has been partially validated by comparison of results with
those of the detailed study of the Tuckerton, New Jersey, area conducted by
Kana et al. (1985). However, it must be recognized that true validation
cannot be obtained because of the radically different approaches and scales
being compared. In the detailed study the predicted major response at the New
Jersey wetland site to a low-scenario sea level rise, as of the year 2075, is
the replacement of high salt marsh with low salt marsh (Kana et al., i°
press). Also projected is the loss of over half the transition marsh in the
Tuckerton area, but an increase of transition marsh in the Great Bay area.
However, at both locations no change in overall wetland area is projected
under the low scenario. The conversion of high salt marsh to low salt marsh
would not be detected in our model; furthermore, because the distinction
between salt marsh and freshwater marsh cannot be made in the input data but
is based on imprecise elevation determinations, we prefer to consider total
wetland changes. Adjustments to transitional marsh in the Kana et al. studies
would occur within the framework of our general freshwater marsh category-
The SLAMM model projects a 9% decline in total wetland area by 2075, growing
rapidly to a 75% decline by the year 2100. For the common year of 2075, the
New Jersey study projects no net change in the total marsh area, whereas ours
projects a 9% decline. However, as late as 2045, SLAMM projects a 1.°*
decline in wetland area, a figure not significantly different from one of no
change, given the limits of both studies. Our simulation through the year
2100, however, suggests that the trend toward migration onto adjacent lowland
would soon come to an end and that many of the gains would be lost.
Agreement is more pronounced under the high scenario. Kana et • al-
project an 84$ decline in salt marshes of the Tuckerton area by 2075, compared
with a loss of 75% by 2075 and a loss of 99$ by 2080 in our study.
Consequently, our conclusion with respect to salt marshes in the Tuckerton
area is that the two methods, despite being dissimilar in many respects and
covering different areas, represent reasonably well an unstable coastal
situation which leads to either salt marsh gains or salt marsh losses»
depending on rates of sea level rise.
Results
A major finding of the modeling study is that regional patterns of
wetland distribution and the potential for loss or gain of wetlands with sea
level rise during the next century depend on two principal factors: (1) the
tidal range within which saltwater wetlands can occur; and (2) the extent of
the lowest Pleistocene terrace (often found at approximately 1.5 m, or 5 ft»
in elevation above present sea level along tectonically stable coasts).
142
-------
Thus in New England, where there is virtually no low terrace, marshes
occur in association with pocket beaches in small coves and behind small sand
spits. Although the tidal range is high and thus favors maintenance of
marshes, there is little lowland to be inundated and colonized by marshes.
Consequently, after 2075, when sea level rise exceeds the present spring high
tide level, present salt marshes will be lost with no compensating gain in new
area (Figure 8).
In contrast, from Long Island to southern Florida, coastal slopes are
gentle, barrier beaches are common, and the low terrace is widespread. Tidal
ranges are also moderately high. Therefore, wetlands are an important compe-
tent of the coastal system. With the high scenario, most marsh areas will be
inundated, such as those behind the Atlantic City barrier beach (Figure 9).
However in a few areas, unless development of resort communities precludes
inundation of the low terrace, some marshes will expand throughout much of the
twenty-first century, decreasing only after the protective beach ridges are
breached (Figure 10).
The Florida Keys and Everglades owe their existence to carbonate deposits
accumulated in shallow water during higher stands of sea level in the
Pleistocene. As the Keys are inundated (in the absence of protective mea-
sures), a slight increase in mangrove swamps can be anticipated; but after
2075 the region will rapidly become open water (Figure 11). The southern
Everglades will also disappear, but wetlands may actually increase where
areas are inundated.
The Gulf Coast is also a region of low slopes and barrier coastlines;
but, unlike the Atlantic, it has higher terraces along the coast and very low
tidal ranges. Therefore, the marshes are more vulnerable to inundation and
cannot migrate inland as readily as the marshes of the Atlantic Coast. With
few exceptions, the Gulf Coast marshes will gradually disappear until the
barrier islands are breached, at which time those marshes not protected by
dikes will decline precipitously (Figure 12). A notable exception to this
Pattern is in the region of the Mississippi delta, where rapid subsidence is
Already overwhelming high sedimentation and accretion rates. In general,
large-scale loss of marshes (far exceeding the current rate) can be expected
ln this region early in the next century (Figure 13).
Most of the West Coast is similar to New England: steep, rocky slopes
Predominate. Wetlands are of minor extent but occupy a wide tidal range, so
fchat they can be expected to persist throughout most of the next century. The
"tore extensive marshes in the tectonic lowlands of San -Francisco Bay and the
Washington coast will probably expand onto adjacent lowlands unless restricted
by protective structures (Figure 14).
Aggregating the individual case studies provides, with one reservation, a
c°nvenient way to detect commonalities in wetland response trends throughout
the diverse U.S. regions. Although the study sites were chosen to achieve a
Representative sample of wetland types without a priori bias as to expected
responses, the case study sites were not randomly chosen nor was adequacy of
sampie size ensured. Therefore, the apparent patterns in any area cannot be
interpreted as statistically valid estimates of region-wide responses to sea
level rise. Instead, the aggregated data are best viewed as indicative of the
class of responses likely to occur in coastal areas similar to the case study
143
-------
21
LEGEND
Undev. Upland
UndevLowland
Prot. Lowland
Exp. Beach
DevExp. Beach
Fresh Marsh
Mangrove
Shelt. Water
Dike or levee
o
u
Dev. Upland
Dev. Lowland
High Dunes
Shelt. Beach
Dev. Shelt. B.
Salt Marsh
Tidal Flat
Open Sea
Blank
a
ft
•
Vt
!
--
I!
Figure 8. Small Marsh Is Lost at Jonesport, Maine, with
High-Scenario Sea Level Rise.
144
-------
LEGEND
Undev. Upland R
UndevLowland Ml
Prot. Lowland o
Exp. Beach '/
DevExp. Beach <>
Fresh Marsh III!
Mangrove u
Shelt. Water
Dike or levee 1
Dev. Upland
Dev. Lowland
High Dunes
Shelt. Beach
Dev. Shelt. B.
Salt Marsh
Tidal Flat
Open Sea
Blank
£
0
•
%
ft
LJ
=
S
Figure 9. Extensive Back-Barrier Marsh Is Lost at Atlantic City,
New Jersey, with High-Scenario Sea Level Rise
145
-------
LEGEND
Undev. Upland
UndevLowland
Prot. Lowland
Exp. Beach
DevExp. Beach
Fresh Marsh
Mangrove
Shelt. Water
Dike or levee
LI
Dev. Upland
Dev. Lowland
High Dunes
Shelt. Beach
Dev. Shelt. B.
Salt Marsh
Tidal Flat
Open Sea
Blank
ii
o
V.
U
Figure 10. Marsh Persists by Migrating onto Adjacent Lowlands at
Sapelo Island, Georgia, with High-Scenario Sea Level Rise
146
-------
21W
fM>
LEGEND
Undev. Upland
UndevLowland
Prot. Lowland
Exp. Beach
DevExp. Beach
Fresh Marsh
Mangrove
Shalt. Water
Dike or levee
•
ft
0
',
fr
Illl
li
1
Dev. Upland
Dev. Lowland
High Dunes
Shalt. Beach
Dev. Shelt. B.
Salt Marsh
Tidal Flat
Open Sea
Blank
it
u-
•
%
%
11
=
n
Figure 11. Everglades and Most of the Florida Keys Are Lost
with High-Scenario Sea Level Rise
147
-------
21
LEGEND
Undev. Upland
UndevLowland
Prot. Lowland
Exp. Beach
DevExp. Beach
Fresh Marsh
Mangrove
Shelt. Water
Dike or levee
R
u
Dev. Upland
Dev. Lowland
High Dunes
Shelt. Beach
Dev. Shelt. B.
Salt Marsh
Tidal Flat
Open Sea
Blank
I!
a
Figure 12. Marsh and Rice Fields Not Protected by Dikes West of Sabine
Pass, Texas, Are Lost with High-Scenario Sea Level Rise
(saltwater intrusion is not modeled)
148
-------
2190
n
LEGEND
Undev. Upland
UndevLowland
Prot. Lowland
Exp. Beach
DevExp. Beach
Fresh Marsh
Mangrove
Shelt. Water
Dike or levee
! 1
Dev. Upland
Dev. Lowland
High Dunes
Shelt. Beach
Dev. Shelt. B.
Salt Marsh
Tidal Flat
Open Sea
Blank
V«
I!
B
Figure 13. All the Lower Atchafalaya Delta in Louisiana Is
Inundated with High-Scenario Sea Level Rise
149
-------
1975
21W
LEGEND
Undev. Upland
UndevLowland
Prot. Lowland
Exp. Beach
DevExp. Beach
Freah Marsh
Mangrove
Shelt. Water
Dike or levee
f>
Illl
•
Dev. Upland
Dev. Lowland
High Dunes
Shett. Beach
Dev. Shelt. B.
Salt Marsh
Tidal Flat
Open Sea
Blank
ft
0
H
II
Figure 14.
The Salt Marshes of Southern San Francisco Bay, California,
Expand into Unprotected, Undeveloped Lowlands with
High-Scenario Sea Level Rise
150
-------
Nationally, the fifty-seven sites selected for study include 485,000 ha
(1,198,000 acres) of coastal wetlands. Under the high scenario, about 7355
(354,000 ha, or 874,000 acres) of all wetlands considered were lost to rising
seas by 2100. However, formation of new wetlands reduced the loss to 56? of
the 1975 wetland area. Under the low scenario, about 40$ (192,000 ha, or
^74,000 acres) of the 1975 wetlands were inundated, but new wetlands extended
°ver 85,100 ha (210,200 acres), leaving a net reduction by 2100 of 107,000 ha
(264,300 acres) or 22% of the 1975 wetlands. The apparent national pattern is
dominated by the Gulf Coast, especially the Mississippi Delta, and by the
South Atlantic regions where the largest wetland areas are found.
In summary, some areas may exhibit an increase in wetlands if lowlands
are permitted to be inundated by sea level rise; and in some areas existing
wetlands may persist well into the the next century. Over extensive areas of
the United States, however, catastrophic wetland losses will have occurred by
2100 if present practices of inadequately protecting existing wetlands are
followed and if adjacent lowlands are not reserved for wetland migration.
REFERENCES
Armentano, T. V., R. A. Park, and C. L. Cloonan. Impacts on Wetlands Through-
out the United States. In Impact of Sea Level Rise on Coastal Wetlands
in the United States, 157-271. Washington, DC: U.S. Environmental
Protection Agency. In press.
Barth, M. C., and J. G. Titus. 1984. Greenhouse Effect and Sea Level Rise.
New York: Van Nostrand Reinhold.
^owder, J. A., H. A. Bartley, and K. S. Davis. 1985. A probabilistic model
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152
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Increased Storms and Estuarine Salinity and Other
Ecological Impacts of the Greenhouse Effect
Donald de Sylva
Rosenstiel School of Marine and Atmospheric Science
University of Miami
!, Florida USA
ABSTRACT
Increased concentrations of CC^ and other trace gases, as well as the
Codification of the ozone layer by chlorofluorocarbons, should contribute to
the atmospheric greenhouse effect and warming of the earth's surface. First,
the principal effects of air temperature rise should result in an increase in
Relative humidity and changes in wind pressure systems which would reduce
G°oling sea breezes along coastal areas. Changes in global rainfall patterns
"^Y result in decreased rainfall in tropical and subtropical regions for
jinking water, industry, and agriculture, and a reduced supply of freshwater
Rowing into brackish estuaries, which are nursery grounds for most recrea-
tional and commercial fisheries. The resulting increased salinity would
Jftduce salt-loving nuisance organisms—some of which are noxious or dangerous
t° human health—to move upstream. In the tropics, loss of coastal
Ve8etation, principally mangroves, would reduce the protective advantage it
beaches and waterfront property against normal wave action and
Pollutant-disposal effectiveness in estuaries would be reduced.
, the principal effects of sea surface temperature (SST) rise will be to
crease the frequency of tropical cyclones which may restore some expected
?°ss in rainfall onto the land. The kinds of marine organisms and fisheries
the tropics will decrease in diversity and shift poleward, while tropical
such as corals will become stressed and may perish, together with
r value as recreational attractions and as protection against shoreline
et>osion. Pollutant effects in coastal ecosystems should be synergistically
*°Pe severe at higher sea surface temperatures. Survival of exotic freshwater
ishes, diseases, and aquatic parasites may increase. Finally the effects of
5ea-level rise may be limited biologically, including coral reef die-off, but
*j°astal beach erosion would be severely exacerbated and would result in
^struction of waterfront property, marinas, roads, and causeways as well as
Damage to coastal energy facilities, freshwater aquifer supplies, and
°°astal toxic waste facilities through saltwater corrosion of metal
c°ntainers. A suggestion is made for aquatic organisms and their ecosystems
153
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to be used as possible indicators of rising sea level and increasing water
temperatures.
INTRODUCTION
The causes and effects of the "greenhouse effect" and associated sea
level rise are well covered by Schneider and Chen (1980), Earth and Titus
(1984), and the various papers of Volumes 1 and 4. This paper considers some
marine ecological effects that apparently have not been considered, together
with suggestions on how to use subtle changes in marine ecosystems as possible
indicators of the greenhouse effect.
INCREASED STORMS
Rising air temperature resulting from an increase in atmospheric carbon
dioxide (COp) and other greenhouse gases will increase sea surface tempera-
tures (SST) (Wendland 1977; Manabe and Stouffer 1980). Warm water of at least
26.8°C (77°F) is needed to supply energy for generation of storms (Wendland
1977; Serra and Buendia 1976). These storms are known as typhoons in the
western Pacific Ocean and as hurricanes in the Atlantic and eastern Pacifi°
oceans. The El Nino/Southern Oscillation of 1982-83 in the eastern Pacifi°
(Reed 1983; Rasmusson and Carpenter 1984; Mechoso, Kitoh, and Arakawa 1985*
Gray 1984) resulted in a doubling of the average frequency of hurricanes fro"1
ten to twenty. Thus, low-lying coastal regions of the world, which are
subjected to 6-7 billion U.S. dollars in damage each year, as well as 20,000
deaths worldwide (Anthes 1982), would be subjected to increased frequency and
intensity of tropical cyclones, an increase in the duration of the cyclone
season, and an increase in the surface water area of warmer sea surface
temperature for cyclone generation. Storm surges caused by rising water
resulting from landward movement of these huge storms would also increase in
frequency, severity, and duration (Bruun 1962).
Two anticipated feedbacks from the greenhouse effect, which will als°
increase damage caused by cyclones, are associated with an expected rise in
water temperature. Two natural buffers against storm destruction of coastal-
areas are coral reefs and mangrove forests. Increased coastal erosion fr°"J
cyclones will result in increased water siltation, which reduces growth °*
corals or kills them (Adey 1978; Tomascik and Sander 1985). A Holocene
barrier reef along the southeastern Florida shelf is believed to have died
around 7,000 years ago as a result of erosion of the shelf soil cover and
consequent smothering of coral (Lighty, Maclntyre, and Stuckenrath 1978)-
Furthermore, an increase in surface temperatures of only slightly above an
ambient temperature of 30°C (86°F) will cause coral mortality (Jaap 19#5f
Glynn 1985; Lasker, Peters, and Coffroth 1984). A doubling of C02 could ais°
lead to a decrease in ocean pH from 8.1 to about 7.8 (Bach 1980; Holm-Hansen
1982; Ausubel 1983), which would cause a dissolution of the calcium carbonate
skeletons of which corals are made.
Mangroves and other vegetation will migrate landward with rising sea
level, but such mitigation will be inhibited by manmade developments such
bulkheads, levees, and other structures (Titus, Henderson, and Teal
Thus, the protective buffering effects of coral reefs and mangroves
cyclones will be reduced as a result of sea level rise and increas
temperatures.
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A third feedback of the greenhouse warming is an increase in evaporation
and precipitation (Manabe and Stouffer 1980), which would lead to increases in
relative humidity in the lower troposphere (Manabe and Wetherald 1975). This
should result in greater electrical demand for air conditioning, and hence a
greater release of C02 into the atmosphere in places where fossil fuels are
used for electricity generation.
SEA LEVEL RISE AND EROSION, INUNDATION, AND SALINITY INTRUSION
Sea level rise should cause increased coastal erosion and shoreline
inundation because of higher normal tide levels as well as increased temporary
surge levels during storms (Sorensen, Weisman, and Lennon 1984). An accelera-
tion of beach erosion in regions already eroding will occur and possibly cause
a start in areas previously unaffected by erosion. This will happen because
erosion from waves and currents can encroach further up the beach profile
(Sorensen, Weisman, and Lennon 1984). Storm surges will be especially
damaging from sea level rise in funnel-shaped bays and estuaries.
Saltwater intrusion will be experienced primarily in estuaries and
aquifers (Parker 1955), especially along karst limestone coastlines (Bowen
1982), which are found in many of the low-lying countries of the world. As
fresh water continues to be withdrawn for consumptive water uses and as human
Population centers continue to migrate toward the coast, the amount of fresh
Water used will increase accordingly. Rainfall will decrease, at least in the
summer, and reduce the amount of fresh water entering estuaries (Manabe and
Stouffer 1980). Saltwater intrusion in estuaries will cause more saline water
to move further upstream, especially during droughts. Problems arising from
saltwater intrusion into ground water include public health risks from higher
concentrations of sodium chloride (a principal component of sea water), which
is believed to exacerbate high blood pressure in humans (Braun and Florin
1963). Also important are increased costs of water treatment to remove salt
from water for human consumption (Hull, Thatcher, and Tortoriello 1986),
agricultural irrigation, manufacturing, and electricity generation (Bowen
1982; Titus and Barth 1984). Plumbing and machinery will also be damaged
(Hull, Thatcher, and Tortoriello 1986). Hazardous waste sites in coastal
areas, which contain metal drums, would be subject to damage because sea water
is highly corrosive to metal (Flynn et al. 1984). All problems of saltwater
intrusion caused by sea level rise will be heightened by projected increases
in freshwater withdrawals for human use (Wilson 1982). It is important to
note, for example, that the State of Florida apparently has no official plan
to cope with sea level rise and saltwater intrusion (Tschinkel and Berquist
1986), although this is being given serious thought in at least the Delaware
River Estuary (Hull and Titus 1986).
SEA LEVEL RISE AND ESTUARINE ECOLOGY
Estuaries are among the most highly productive ecosystems (Ketchum 1967;
°dum 1971; Coutant 1981). They are characterized as areas where freshwater
Divers flow into the sea and where tidal action brings about a mixing of salt
water and fresh water (Odum 1971). Estuaries, salt marshes, mangrove swamps,
and freshwater marshes are all considered important in providing food and
Protection for a variety of fishes, invertebrates, and birds.
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The dominant effects which have been observed following damming, with-
drawal, or diversion of freshwater flows are summarized as follows (Snedaker
et al. 1977), recognizing, however, that such consequences will be greater or
lesser depending upon the absolute reduction in the normal discharge:
• Nearshore waters become more saline.
• Mixing due to salinity differences is diminished.
• A salt-wedge may develop farther upstream in the discharge channel.
• Saltwater intrusion appears in coastal groundwater and surface water.
• The estuary is starved of essential nutrients of terrestrial origin.
• Benthic substrate tends to become anaerobic and heavy metals
sequestered in the substrate are liberated; sulfur cycles become
dominant.
• Particulate and soluble organic matter inputs are reduced and/or are
flocculated and deposited nearshore rather than being dispersed.
• Certain fisheries are lost in their entirety for a variety of reasons >
such as elevated salinity, reduction in food supply, and loss of a
large area of low salinity.
• Euryhaline species lose dominance to stenohaline species and, i°
general, selection is for species adapted to the new conditions.
• Salt-tolerant mosquito and dipteran populations increase.
• Schistosomiasis becomes rampant.
• All negative effects are aggravated during low-flow periods.
• Saltmarshes and/or mangroves and seagrass beds deteriorate under*
constantly elevated salinity.
* Renourishment of sand/silt ceases; downstream shoals erode.
• Littoral drift patterns are altered as well as nearshore circulation.
As sea level rises, salt water will move farther into the estuary
prevent this freshwater outflow (Stone et al. 1978). These effects are
complex and intertwined, as summarized by Snedaker, deSylva, and Cottrell
(1977), and which are reviewed as follows:
Fresh water in estuaries is important as:
• Diluter of salt water. Lighter fresh water runs off land and
rivers, then toward the sea, while heavier salt water moves into
estuary. This forms a salt-wedge, which extends significant distances
up the river channel (Pritchard 1967). A rise in sea level woul"
extend the upstream movement of the salt wedge.
156
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• Protector of fish, shrimp, oysters, and other biota. The temperature,
salinity, food, substrate, and protection found in estuaries is
balanced for a host of organisms to carry out their life cycles, and
this optimum combination cannot be found in environments that are more
saline or less saline (Odum and Heald 1975). Between 66% and 90$ of
the U.S. fisheries depend upon estuaries for their life cycle (Douglas
and Stroud 1971). The dependence of fisheries upon estuaries has been
documented for other parts of the world (e.g., MacNae 1968).
Estuaries provide sites for reproduction and protection of young,
moderate water temperatures, osmoregulation of the salt balance in
marine organisms (Fontaine 1975), and an environment that is optimum
for larval and juvenile fishes and invertebrates (Heald and Odum
1972).
One of the most important functions of estuaries, including the fresh-
water-dependent salt marshes and mangroves, is their contribution in providing
organic and inorganic nutrients from the breakdown of their leaves, stems, and
roots (Heald and Odum 1972). Adverse effects on estuarine productivity have
been caused by diversion of fresh water away from marshlands (Carter et al.
1973). A decrease in the catch of fish and invertebrates in South Vietnam
followed the nearly complete herbicidal defoliation of mangrove forests
(deSylva and Michel 1974; deSylva 1975). Decreases in freshwater runoff
caused by rising sea level would decrease the food resources or nutrients that
form the basis of the food chain (Heald 1970; Reeve 1975).
Estuaries also offer protection to organisms by reducing predation and
competition because the combined conditions of temperature, salinity, and
turbidity, to name a few, cannot be tolerated by organisms normally found in
offshore regions, but which are stenotopic and cannot adjust to the eury topic
conditions of the estuary (Green 1968).
The larvae of marine organisms that utilize the estuary for protection
are able to carry out relatively long-distance horizontal movements toward
toore suitable habitats by selectively responding to either ebb or flood tide
water characteristics (Hughes 1969). Changes in freshwater balance from
estuaries due to increased salinity could thus affect the survival of pelagic
larvae .
Estuaries also act as cutting and filling mechanisms. Rivers carry an
6stimated 30 billion metric tons per year of suspended material to the oceans
(Turekian 1968). The best examples of this material, which is derived from
eposion of the continental landscape, are found in areas where deltas are
forming. These are regions where a river carries more sediment into a
Banding body of water than can be carried away by waves and currents. Here,
the river velocity drops and the suspended sediments are deposited. Much of
this deposition occurs at the river mouth as a sandbar deposit or as an
Alluvial plain, which eventually becomes a salt marsh or a mangrove swamp,
WUch acts as a barrier to storms and surges to protect the land behind them
(Warnke 1967). A rise in sea level would reduce the amount of sediment-laden
^Unoff entering the coastal zone, thus reducing the protective nature of the
sandbars. Salt marshes faced with rising sea level and a reduction in river-
sediments could be converted to open-water habitats, due to drowning
'Orsen, Panageotou, and Leatherman 1985).
157
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The reduction of freshwater inflow into estuarine systems and consequent
increase in salinity has resulted in considerable changes in fisheries,
currents, plankton, and benthos (Neiman 197*0; has caused extensive changes in
salt-marsh ecosystems (Hoese 1967); and has changed spawning migrations of
economically important fishes (Massmann 1971). The latter phenomenon is
believed to occur because many estuarine-dependent fishes, and probably
invertebrates, respond to very slight gradients on brackish water, turbidity,
or dissolved organic compounds to give them olfactory cues that trigger their
migrations (Frolander 1964; Hoese 1967). A high correlation has also been
shown between white shrimp catches and freshwater input into estuaries (Gunter
and Edwards 1967). Salinity increases in estuaries due to sea level rise
would diminish the freshwater runoff of estuaries and concomitantly reduce the
various physio-chemical signals that organisms use to guide them into the
estuaries.
An increase in estuarine salinity would favor the establishment of
nuisance organisms, which normally are excluded from the estuary because of
the low salinity. This effect would be exacerbated if sea surface tempera-
tures were also to increase, because many of these nuisance organisms are
tropical, and can become part of new coastal ecosystem shifts, which already
has occurred as the result of warm-water discharges from electrical power
stations (Krenkel and Parker 1969; deSylva 1969). Among these high-salinity
organisms are the destructive mollusk Teredo (or shipworm), the crustacean
Limnoria, and barnacles, which destroy wooden structures such as docks,
pilings, and boats, causing extensive economic damage. Barnacles, which are
scarcer in estuaries than in areas of higher salinity (Moore and Frue 1959)»
cause fouling of submerged objects, especially boat hulls, which must be
periodically hauled and scraped (Ray 1959). (Barnacle-oyster communities may
prove to be very good indicators of both sea level rise and increasing
salinity in estuaries [Wanless 1982].)
Stingrays are bottom-feeding coastal fishes that can crush mollusks with
their powerful jaws. In coastal areas, they cause extensive damage to clam
beds. Their movement into estuaries as salinity increases would be associated
with extensive damage to oyster beds. Similarly, oyster drills and oyster
parasites, normally absent from estuaries because of their intolerance for low
salinity, should increase their infestation of oyster beds. Sharks, which
normally are not present in estuaries, which have low salinity levels, would
be more likely to enter higher salinity estuaries, thus posing potential
dangers to humans engaging in water sports.
Estuaries play an important function in dispersing pollutants. Normally'
well-mixed estuaries with rapid flushing rates can dilute pollutants rather
quickly (Pritchard 1967). However, reduced circulation from increased
salinity due to sea level rise should diminish the ability of estuaries to
perform this function.
It is clear, based upon the results of the National Symposium on Fresh-
water Flow to Estuaries (Cross and Williams 1981), that freshwater flow play3
an intricate and complex role in the functioning of estuarine ecosystems-
Rising sea level will surely decrease freshwater input into estuaries and
reduce estuarine ecological and economic importance to mankind.
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RESEARCH RECOMMENDATIONS
Meteorologists, physical oceanographers, atmospheric chemists, and
geophysicists have tried to document the "greenhouse effect" and resulting
climate change by attempting to detect statistically significant increases in
air and water temperatures, glacial melting and retreat, and sea level rise.
One problem seems to be that the variance fluctuates greatly about trends in
the mean values, the so-called "signal-to-noise ratio." However, there
appears to be little effort by biologists to use aquatic species or
communities as biological indicators of rises in sea level and sea surface
temperatures, or changes in dissolved gases or ionic concentrations. Sea
level changes have long been known to be reflected in zonation of intertidal
communities (Scholl and Stuiver 1967; Cubit 1985), while more recent evidence
can be seen in a corresponding rise in the location of an oyster-barnacle
community in a Coral Gables, Florida, coastal waterway between 1949 and 1981
of 15 cm {Wanless 1982).
Slight increases in water temperatures result in subtle changes in marine
and freshwater organisms, which show physiological, behavioral, and ecological
responses (Kinne 1984). Species and community shifts have thus been observed
during long-term climate fluctuations on the U.S. Pacific coast (Hubbs 1948),
over rather short periods due to El Nino (Gerard 1984; Glynn 1984), and over
longer periods in the North Sea (Gushing 1978; Blacker 1957) and in North
America (Dow 1969; Gushing and Dickson 1976). Short-term changes are seen as
a result of thermal pollution, in which entire ecosystem shifts along a coast-
line can be documented as a result of warmer sea surface temperatures
resulting from warm-water discharges from electrical power plants (Krenkel and
Parker 1969; deSylva 1969; Kinne 1984).
It would seem that a golden opportunity is thus presented to marine and
freshwater ecologists to benefit from the extensive literature on the rela-
tionships between aquatic organisms and increased temperatures to monitor the
Slobal experiment known as the "greenhouse effect."
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164
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Rising Sea Level and Damming of Rivers:
Possible Effects in Egypt and Bangladesh
James Broadus, John Milliman, Steven Edwards,
David Aubrey, and Frank Gable
Woods Hole Oceanographic Institution
Woods Hole, Massachusetts USA
INTRODUCTION
The projected worldwide rise of sea level during the next 100 years will
be particularly hard-felt in deltaic areas where substantial areas are barely
above sea level. Regional subsidence will increase the relative rate of sea
level rise; and damming of large rivers could prevent sediment influx from
compensating for regional subsidence, increasing coastal erosion. These
e?fects will be felt most in developing countries, where the rivers are large,
deltas extensive and inhabited, and proposed damming of large rivers may
dramatically increase coastal erosion.
To help understand the potential consequences of sea level rise in the
deltaic regions of the world, we have concentrated our attention on two areas
that seem especially vulnerable: the Nile River delta in Egypt, which has
already been dammed, and the delta of the complex Ganges-Brahmaputra-Meghna
River system in Bangladesh, in which river damming has begun and is expected
to increase during the next 15 years.
STUDY APPROACH
Our general approach for characterizing possible consequences of sea
level rise in these two regions has been as follows: First, geological infor-
^tion has been employed to describe likely scenarios of shoreline change for
Scenarios of future dams and relative sea level for the next 100 years. These
scenarios have then been used to describe the geographic areas within Egypt
Bangladesh that will be affected by landward transgression.
Next, demographic and economic information has been examined to portray
he scale of economic activities within each nation that currently originate
the potentially affected areas.
165
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Several limitations of this approach are immediately obvious. Geologi-
cally, we know little about the rates of subsidence in either the Nile or
Bengal deltas, nor do we know the actual sediment flux from the Ganges-
Brahmaputra River which flows into Bangladesh. The economic analysis reports
current, not projected levels of economic activity within the two countries.
Although techniques have been developed and employed elsewhere to measure the
present monetary value of potential economic damages associated with physical
scenarios for sea level rise (Earth and Titus 1984), we employ a more simpli-
fied way of describing the economic significance of future sea level rise. We
have not yet attempted to describe the complex interactions of social
processes that will come into play in response to rising sea level, such as
the subtle social relationships identified by Warrick and Riebsame (1983).
Our approach employs a highly aggregated and very coarse scale of data to
arrive at a rough characterization of the current scale of economic activities
that are at stake as relative sea level rise encroaches on the areas of
interest. It is extremely mechanical, intended only to portray the current
scale of economic activities and population in potentially affected areas. ^
does not take into account future mitigation measures, nor adaptive responses,
nor future changes in the distributions of economic activities and popula-
tion. We suspect that our methods result in conservative approximations °*
the economic stakes, since we do not add future economic and population growth
to our data on current conditions.
Deltaic regions may be particularly susceptible to disruptions
rising sea level because of the delicate balance achieved by deltas at the
river/ocean interface through newly delivered sediment via river transport*
local subsidence as deltas tend to sink, and the erosional forces of oceanic
energy, waves, and storms. In this analysis, we focus on the net effect of
subsidence and rising sea level, which consists of two primary elements: (a'
global changes in sea level; and (b) local effects through subsidence °r
uplift. A third major influence that comes into play in river delta areas lS
the delivery of new sediments to nourish the delta itself. Subsidence can b®
thought of as that associated with tectonic movements and the more superficial
subsidence resulting from compaction and dewatering of the soils that pile UP
to make the delta. Removal of sediments, as through upstream dammit
projects, reduces the ability of the delta to build itself seaward or to
stabilize itself against the forces of subsidence and erosion.
Two other major influences are important for an analysis of potentia1
effects of sea level rise on delta regions. The first is saltwater intru-
sion. As sea level rises the wedge of salt water that underlies the delta's
fresh ground water is forced farther and farther inland, contaminating ground
water used for drinking and agricultural purposes. A second major consider-
ation, particularly in Bangladesh, is increased exposure to violent inundati011
by storm surges.
n
Two scenarios are examined here. The more optimistic scenario -assumes
l-m (3-ft) rise in relative sea level over the next 100 years (Hoffman, Keyes»
and Titus 1983). This case can be thought1 of as a 50-cm (1.5-ft) rise l0
eustatic sea level combined with a 50-cm increase in local subsidence.
more pessimistic high case assumes a 3-m rise in relative sea level:
result of a 200 to 250-cm rise in eustatic sea level and a 50 to 100-cm lo°a
rise due to subsidence and reduced sediment delivery. Such reductions *
166
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sediment delivery have already been experienced in Egypt, resulting in severe
erosion in the delta there; and such effects could also be expected in
Bangladesh in association with future water management projects such as dams
and barrages upstream.
We now consider the nature of human activities taking place in two
deltas.
EGYPT: THE NILE RIVER DELTA
The vital role of the Mile River in the nation's economy has been
recorded for millenia. Herodotus wrote, "Egypt is the Nile and the Nile is
Egypt." The population is densely clustered about the banks of the river and
throughout the river's delta (Figure 1). Of the country's million square
kilometer area, only about 35,000 square kilometers, or 3.5%, is cultivated
and settled (Quarterly Economic Review of Egypt - 1985). This results in an
estimated population density in 1985 of about 1,400 people for every square
kilometer of arable land in the country. The deltaic area examined here
extends from just west of the port city of Alexandria and eastward to Port
Said at the northern entrance to the Suez Canal. Alexandria, exposed since
antiquity to the forces of the sea and historically reached by a causeway
extending to its walled enclosure, today contains about 3 million inhabi-
tants. Port Said, at the eastern extremity of our study area, is the home of
nearly one half million residents. The densely inhabitated delta between is
devoted to intensive, multi-crop agriculture, and to urban and industrial uses
(Figure 2).
Within this century, severe erosion has been documented on the delta
coastline (Milliman 1986). Large reductions in the delivery of nourishing
sediments to the delta began with the construction of the delta barrages in
1881. Additional diversion of Nile sediments resulted from the construction
of the dam at Aswan in 1902 and its enlargement in 1934. Even so, approxi-
mately 80-100 million tons of sediment were delivered annually by the Nile to
the delta before 1964. Closure of the high dam at Aswan in 1964, however,
entirely eliminated sediment delivery to the delta. This loss of sediment,
combined with tectonic subsidence on the order of 5-6 m every few thousand
years, has resulted in spectacular erosion of the Nile delta. As can be seen
in Figure 3, the delta's numerous former distributaries have been closed down
by water management, irrigation, and land reclamation projects, so that the
Nile's flow through the delta is channeled into only two major distributaries:
the Rosetta on the west and the Damietta on the east. Many areas along the
delta's coast have experienced annual erosion rates in excess of 1 m, between
1966 and 1974 (Figure 3), and locally, erosion has exceeded 100 m per year
(Milliman 1985; Bird 1985; Inman, Aubrey, and Pawka 1976).
167
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10,000 - 20,000
5,000 10.000
1,000 • 5,000
400 1,000
| I Ftwtr ihon 400
Figure 1. Population Density in Egypt 1982
Source: Central Agency for Public
Mobilization and Statistics 1982
168
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Figure 2.
Nile Delta
Source: NASA
169
-------
MEDITERRANEAN
Erosion >l m/yr (1965-74)
Figure 3. Coastal Erosion on the Nile Delta
Source: Bird 1985
170
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Figure 4 illustrates the additional landward transgression that might
result from a 1-m or a 3-m rise in relative sea level. The major existing
natural defenses against such a retreat on the shore are the series of sand
dunes along the delta's coast and the increasingly brackish lakes—Maryut,
Idku, Burullos, and Manzalah—that lie behind them. These lakes are a major
source of the nation's approximately 100,000 ton annual fish catch, 80% of
which is freshwater fish. The area that might be inundated in the low
scenario represents approximately 12$-15$ of the nation's arable land and
contains approximately 16$ of the nation's estimated 49 million population, or
almost 8 million people. The area that could be lost to the sea in the high
scenario represents about 20$ of the nation's arable land and is inhabited by
over 10 million people, some 21$ of Egypt's current population.
Assuming that agricultural output in the delta is distributed as arable
area, and assuming that all other sectors of economic activity, as specified
in Table 1, are distributed as population, we estimate that approximately 15$
of Egypt's current gross domestic product (GDP) originates in the area likely
to be affected by the 1-m increase in relative sea level. Similarly, approxi-
mately 20$ of the nation's current GDP originates within the area likely to be
affected by a 3-m increase in relative sea level. The magnitudes of economic
activities comprising GDP that thus originate in the likely affected areas are
disaggregated and reported in Table 1. It is worth noting here that large
areas apparently targeted for priority development in land reclamation
projects, both south of Lake Manzalah in the region of Port Said and around
Lake Maryut in the region of Alexandria, lie within the area likely to be
flooded by even a 1-m increase in relative sea level.
BANGLADESH: THE GANGES-BRAHMAPUTRA-MEGHNA DELTA
We turn now to the even more serious exposure that exists in Bangladesh
(Figure 5). This densely populated nation of an estimated 93 million people
covers an area of 143,000 square kilometers. Eighty percent of this area is
made up of the complex Bengal delta system created by the Ganges, Brahmaputra,
and Meghna Rivers. Together, the Ganges and the Brahmaputra currently deliver
approximately 1.6 billion tons of sediment annually to the face of the delta
(Milliman and Meade 1983). The seaward face of this delta extends some 650 km
from the western boundary with India to the Chittagong Hill Tracts on the
east.
The country's population is widely distributed, with heavy concentrations
in the major city of Dhaka (population about 4 million), in the southwestern
city of Khulna (population 800,000), and in the eastern port city of
Chittagong (population about 1.5 million). Most of the remaining population
is rurally dispersed and dependent on subsistence agriculture. Much of the
Population lives at the very edge of subsistence: it is estimated that 85$
receive less than the 2,122 calories per day considered necessary for minimal
subsistence (Jansen 1983). Per capita GNP is approximately $140, compared
with that of Egypt, which is $670, and that of the United States, which is
over $13,000. Under- and unemployment rates in Bangladesh are estimated in
the 35-40$ range, and population increase is in the range of 2.5$-3.0$ per
annum.
171
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PERSONS PER SQUARE KILOMETER
I I 10,000-20,000
5,000-10,000
E.V.Y.V,
.VAVi
•^•••B
.Y.Y.Y.j .
.v.v.-.vl
•••••
1,000
400
3 M
1 M
- 5,000
-1 000
Figure 4. One-meter and three-meter transgression scenarios for relative
sea level rise in the Nile Delta (detail from Figure 1).
(Note: Dashed segment in three-meter scenario projected in
lieu of exact topographic detail.)
172
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Table 1. Economic Activity and Population Originating in Affected
Region: Egypt (1984-85)
Activity* One-Meter Scenario
Agriculture
Industry and Mining
Oil and Oil Products
Construction
Power, Water and Sanitation
Transportation, Storage, and
Communications
Trade Services and Finance
Housing
Miscellaneous Services
GDP in Affected Region
Total GDP
Percent of GDP
631
674
785
237
41
258
916
95
804
4,441
29,872
15
Three-Meter Scenario
1,052
885
1,030
311
54
338
1,202
125
1,056
6,053
29,872
20
"Values in millions of Egyptian pounds in 1984/85. ($1 U.S. = 1.23/pounds in
1983)
Total Estimated Population 48.6 million 48.6 million
Estimated Population in
Affected Region 8 million 10 million
Percent Population in
Affected Region 16 21
173
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0 6 10 IS 3O 26
HIG
LOW
Figure 5. Infrared Aerial Photograph of Bangladesh
Source: World Bank and Plate 4 of Volume 1
of this report
174
-------
When superimposed on shoreline retreat from the low and high sea level
rise scenarios (Figure 6), the spatial distribution of population shows that
approximately 9% of the nation's present population would be directly affected
by a 1-m rise, and 27% • of the total population would be affected by a 3-m
rise. The densely populated area around the southwestern city of Khulna
where population density exceeds 2,900 per square kilometer, stands between
the 1-m and 3-m transgression lines (Figure 7). Similarly, the heavily popu-
lated environs of the Meghna River eastward of Dhaka would clearly be greatly
affected in the case of a 3-m relative sea level rise.
Exposure of the population to storm surge is an extremely grave consider-
ation in Bangladesh. On average, 1.5 severe cyclonic storms attack the
country each year with storm surge reaching as far as 160 km inland in recent
times (Bird and Schwartz 1985). The May 1985 storm is estimated to have
killed over 5,000 people, and the tragic November 1970 storm surge is believed
to have taken the lives of over a quarter of a million people. Assuming a
similar 160-km reach in the case of a 3-m relative sea level rise, the area
likely to be exposed to storm surges 100 years hence can be seen in Figure 7
as the area extending 160 km beyond the 3-m line. The plight of Dhaka is
obvious. With increasing population densities in the future, through the
combined effects of natural population increase and loss of territory to
coastal transgression, the exposure to severe storm surges will only increase.
In the context of storm surge, a noteworthy feature of today's delta is
the 6,000-tar mangrove and nepa palm Sundarban Forest Preserve in Khulna
District on the southwestern coast. A maze of heavily forested waterways,
this preserve, with no permanent settlement, stands out clearly in Figure 7
Immediately behind, however, are heavily settled areas, including the densely
Populated environs of Khulna. It may be presumed that the Sundarban forest
Provides vital protection for these settled areas by acting as a buffer
against the force of storm surges. Loss of this buffer could'greatly increase
the threat of storm surges, and the forest appears to be vulnerable to even a
1-m rise in relative sea level. Indeed, near-term monitoring for the killing
effects of heightened salinity may be warranted at the Preserve's coastal
margins.
Agricultural production represents approximately 55% of the nation's
gross domestic product, and it is estimated that over 857. of the nation's
Population depends on agriculture for its livelihood. Major crops alone
account for about 40$ of gross domestic product. Net cropped/agricultural
areas (represented by horizontal hatching in Figure 8) are obviously the major
Use of land in Bangladesh. We estimate that about 8.5% of the nation's agri-
cultural output originates in the area seaward of the 1-m relative sea level
rise scenario, this represents about 11* of the nation's crops. Estimated
agricultural output originating seaward of the 3-m scenario line totals 21* of
the nation's agricultural output, or about 21% of the nation's crops.
A major consideration for agricultural productivity is the intrusion of
salt water into the nation's fresh ground water resources. Current estimates
suggest that saltwater intrusion now extends as far as 150 miles or about 240
«n inland (Zaman 1983). Assuming, however, that the high scenario unfolds,
the wedge of saltwater could be driven another 240 km northward. If the
Bengal rivers are effectively dammed, it is conceivable that nearly the entire
nation could be affected by the intrusion of salt water.
175
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BANGLADESH
DIVISION, DISTRICT SUBDIVISION
AND THANA BOUNDARIES
Division Boundary
— — — — District Boundary
Subdivision Boundary
j
---- Thana Boundary
DACCA Capito
KHULNA Division Headquarters
RANGPUR Dl»trlct Headquarters
&R j
O BHALUKA Subdivision or Thana Headquarters
, --- International Boundary
t \ """ ' ""•""
' J°~\"*~ 'O -A. 'j
«« r^-a-r11^ Bj£5
-'™>~
Figure 6. One-meter and Three-meter Transgression Scenarios for Relative
Sea Level Rise in the Ganges-Brahmaputra-Meghna River Delta
176
-------
88-E
89*
90'
91'
95 E
BANGLADESH
DISTRIBUTION OF POPULATION
1971*
70 «0 tO SO 00
18"
Eoch dot represents 2,000 p*rjon$
liSiflliSMSfesv
21'N
88'E
2TN
B9
Figure 1. Distribution of Population in Bangladesh
Source: Ahmad 1976
1971 Distribution used for graphical presentation only. Projected 1985
population by district used in scenario estimates reported in text.
177
-------
92'E
BANGLADESH
LAND UTILIZATION
(BY DISTRCTS)
NET CHOPPED AREA
CULTIVABLE WASTE
CURRENT FALLOW
BAY OF
BENGAL
Figure 8. Land Utilization in Bangladesh
Source: Ahmad 1976
178
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It will be difficult to replace the croplands lost to sea level rise
because the countryside is already so extensively cultivated. Of the 24.5
million acres in the nation estimated to be cultivable, approximately 22.5
million acres, or more than 90$, are already in cultivation (Zaman 1983).
Currently fallow areas (Figure 8) constitute a negligible percentage of land
utilization. Further prospects for increasing agricultural usage in northern
districts (the only area possibly unaffected by salt water intrusion) are also
a relatively small proportion of the total (Figure 8). Increasing the inten-
sity of agricultural land utilization through multiple cropping strategies may
show some promise as a response to loss of agricultural lands. Of the
nation's net cropped area, over half is currently used for only one crop per
year, while 39$ is used for two crops annually and about 1% is fully utilized
in the production of three crops (Quarterly Economic Review of Bangladesh
1985). However, much of the land in the delta least likely to be affected by
coastal transgression is in relative topographic highs produced by old delta
sediments of distinctly poor fertility and is less productive for agriculture
than the extensive new delta most likely to be flooded.
These differences in agricultural productivity can be discerned in Figure
9, which shows that areas with relatively poor rice production, northwest of
Dhaka and along the northwestern boundaries with India, are the old deltaic
soils of low agricultural value. Rice accounts for 90$ of the nation's total
grain output, 73$ of the nation's cropped area, and 28$ of the nation's gross
domestic product. It is the vital food staple for the nation's population.
As seen in Figure 9, a significant portion of the nation's rice producing
areas would be lost from a low rise, an even more would be lost with a 3-m
rise.
The other vital crop in the nation's agricultural output is jute (Figure
10). Jute and jute products represent some 55$ of the nation's export
revenues and approximately 50$ of agricultural earnings. Approximately 2-2.5
million acres are devoted to jute production, representing 11$ of the nation's
total agricultural area. Although only about 0.8$ of the nation's jute
production originates in the area seaward of the shoreline for a 1-m rise, a
3-m rise would affect nearly 16$ of the production, which would be close to 9$
of the nation's current export revenues.
The loss of land would also affect fishing. Although fish production
only accounts for about 5$ of GDP, it represents an estimated 80$ of the
nation's total production of animal protein. About 1.5 million people depend
on fishing activities for their livelihood. The nation annually produces
about 700,000 tons of fish, 80$ of which are freshwater species. Fish
products are the nation's fourth leading export commodity after jute products,
jute, and leather, exceeding even exports of tea. We estimate that 7.5$ of
the nation's major fishing centers are in the area affected by the 1-m
relative sea level rise. Forty-one percent of the nation's fishing centers
are within the area affected by the 3-m scenario. It seems likely, however,
that fishing centers are subject to considerable relocation and the fishing
industry, in part due to its largely artisan nature, may be in a position to
respond with great flexibility over the coming century to changing configur-
ations and conditions in the distribution of the nation's aquatic resources.
More drastic would be the combined effect of decreased river flow and sea
level rise. In the Nile Delta, the offshore sardine industry ceased
179
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BANGLADESH
AMAN RICE
Each dot rvpr*Mnt» 2,000 oort*
BANGLADESH
RICE
Each dot r«pr»s»nt» 2,000 aero
Figure 9. Rice and Araan Rice Production in Bangladesh
Source: Ahmad 1976
180
-------
92'E
X IT
JUTE
- ••*•*..!' „'*
26'N '."*'•• •*• *t • *l •*•'."'•''• 7*C/
22'
«2*E
Figure 10. Jute Production in Bangladesh
Source: Er-Rashid 1977
181
-------
within a few years after the fertile Nile waters stopped entering the
Mediterranean. Similarly, since the construction of the Kotri barrage on the
Indus River, fish catch per fishing boat in coastal waters has fallen by
nearly 75% (Quraishee 1985).
Large industries in Bangladesh are portrayed in Figure 11. Many are
associated with jute products, and as with the case of jute production, we see
that only a relatively minor proportion is found within the area likely to be
affected by a 1-m relative sea level increase. However, a significant portion
of the nation's industrial activity appears to be situated within the area
affected by the 3-m rise.
The current economic activities originating in potentially affected areas
within Bangladesh are summarized in disaggregated form in Table 2. For
purposes of this disaggregation, we assume conservatively and for the sake of
simplicity that, except for the Sundarban and Chittagong forests, agriculture
is uniformly distributed over the nation's entire land area. About 11% of
agricultural GDP is thus contained within the area affected by the low
scenario and about 27.5$ within the area affected in the 3-m scenario.
Economic data indicate that approximately 1.5$ of the nation's industrial
activity originates within the area likely to be affected by a 1-m rise, with
5.5% of industrial production originating within the area affected by a 3-ro
case. We assume that transport, storage, and communications; trade and
services; banking and insurance; and professional and miscellaneous services
are distributed in the same way as industry. In a similar manner, we assume
that construction activities; power, water, and sanitation; housing; and
public administration and defense are distributed as population, whose distri-
bution we know from other sources (Central Agency for Public Mobilization and
Statistics 1982). Thus, between 9$ and 10$ of those activities currently
originate within the area that would be affected if the sea rises 1 m, while
nearly 28$ originate within the area that would be threatened by a 3-m rise in
relative sea level. Altogether, some 8$ of the nation's current GDP is esti-
mated to originate within the area that could be lost in the 1-m scenario,
while about 20$ originates in the area to be threatened by 3-m rise.
As shown in Figure 12, there may be some reason to fear that a 3-m rise
in relative sea level would threaten more areas than portrayed in our
scenario. Notice that a large area surrounding the upper reaches of the 3-ro
contour along the Meghna River already is subject to severe flooding. If the
high scenario unfolds, flooding might intensify in these regions. Similar
additional effects might be expected in the moderately flooded regions
surrounding the Padma River and the confluence of the Ganges and Brahmaputra
Rivers north of the central portion of our 3-m contour and extending
northwesterly and southwesterly into currently highly flooded areas.
We emphasize again the massive delivery of sediments to the Ganges-
Brahmaputra-Meghna Delta by these rivers. The sediment currently delivered to
the mouth of the delta appears to be maintaining a near equilibrium, a
balanced state with the forces of tectonic and deltaic subsidence, to maintain
a nearly static situation with little noticeable deltaic progradation and
little noticeable net erosion. Any significant reduction in the delivery of
these sediments to the delta, however, could disrupt this balance and expose
the delta to the same kinds of erosion that have been witnessed in the Nile.
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61- 92'E
BANGLADESH
DISTRIBUTION
OF
LARGE INDUSTRIES
1974
10 «p to «p 90
V T«« Girdcnt* Factor!** A Ch*mic*l»
Jut* Mill
<* Pr«»
Cotton Tcitll*
Silk Fictory
Sugar Mill
Flour Mill
OI.IMUry
Tobacco Fictory
F«rt!lli»r Factory
Rubb*r Ficlory
Pip.fMill
Match Factory
4 Hydroilcctrlc Plant
S(«el Mill
Q*n*nl Engineering
Q| Dockyard
C«ment Ficlory
Glali Fictory
Oil R«lln»ry
L*lth«r Fictory
• Induttrlal Centra*
21'N
Figure 11. Distribution of Large Industries in Bangladesh
Source: Ahmad 1976
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Table 2. Economic Activity and Population Originating in Affected Region:
Bangladesh (1984-85)
Activity* One-Meter Scenario
Agriculture
Industry
Construction
Power, Water, and Sanitation
Transportation, Storage, and
Communications
Trade Services
Housing 2,735
Public Administration and Defense
Banking and Insurance
Professional and
Miscellaneous Services
GDP in Affected Region
Total GDP
Percent of GDP
20,685
590
1,750
235
400
510
7,895
1,680
95
445
29,125
390,015
8
Three-Meter Scenario
51,450
21,615
5,065
670
1,470
1,870
4,860
355
1,635
775,435
390,015
20
"Values in millions of Takas (TK) in 1984/85. ($1 U.S. = TK 25.35 in 1984)
Total Estimated Population (1985) 92.8 million 92.8 million
Estimated Population in
Affected Region 8.5 million 24.8 million
Percent Population in
Affected Region 9 27
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BANGLADESH
FLOODING AND DRAINAGE
MLF>
Mostly above flood or fair
Land under conilint tidal
action
iii Gravity drainage possible
Highly flooded
Moderately flooded
Pump drainage possible
Large scale tubewells (In use)
Improvement possible
by low embankment!
Coastal embankment*: Existing*
under construction
# C H 6 A L
Figure 12. Flooding and Drainage in Bangladesh
Source: Ahmad 1976
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Careful attention to this possibility seems warranted in the planning and
design of upstream water management projects such as dams and barrages.
Assuring such careful attention, however, is greatly complicated by the
sensitive international nature of the problem. Bangladesh shares the Ganges
Basin with two of its neighboring states, India and Nepal, and the Brahmaputra
Basin with India, China, and Bhutan. Of the total drainage area of the
Ganges-Brahmaputra-Meghna River system, only 7.5% lies within Bangladesh
(Zaman 1983). Already, a serious dispute between Bangladesh and India has
arisen over India's construction and use of the barrage on the Ganges at
Farakka. This dispute led in 1972 to the creation of the Indo-Bangladesh
Joint Rivers Commission and in 1977 to the Ganges Waters Agreement.
Nonetheless, tensions continue over this issue, and it seems very likely that
future difficulties can be expected in allocation and management of the
aquatic and sediment resources of these complex international river systems.
SUMMARY
In Egypt, about 12$-15$ of the nation's arable area falls within the
region threatened by a one meter rise and some 20% falls within the area that
could erode with a three meter rise. (These represent approximately 0.5$ and
0.8$, respectively, of Egypt's total land area.) We estimate that 15% of
current GDP originates within the area affected in the 1-m scenario, with 20%
of current GDP originating in the area affected in the 3-m scenario.
Approximately 11.5$ and 27$ of the land in Bangladesh would be threatened
by rises in sea level of one and three meters, respectively. We estimate that
8.5 and 25 million people (9$ and 27$ of the total population) reside within
these areas. Given our assumptions, 8$ to 20$ of the nation's gross domestic
product currently originates in these areas.
Approximately 12$ to 15$ of Egypt's arable land (0.5$ of the total area)
could be lost if the sea rises one meter, while a three meter rise would
threaten 20$ of the arable land (0.8$ of the total area). These areas are
currently home to 7.7 and over 10 million people (16$ and 21$). We estimate
that 15$ and 20$ of the nations's GDP originates in these areas.
In view of the substantial human and economic stakes involved in these
two sea level rise scenarios for Bangladesh and Egypt, we believe it is clear
that private and public agencies should begin to include consideration of the
possible effects of sea level rise over the next century in their long-range
planning and project development. The types of activities that seem
threatened soonest by the sea level rise are subsistence agriculture, storm
protection, and urban activities. There seems little doubt that major public
works have already increased the vulnerability of areas in Egypt to sea level
rise, and major public works such as upstream dams and barrages in Bangladesh,
or in its co-basin states, have the potential for greatly increasing that
nation's vulnerability to sea level rise. Indeed, as can be seen from Table
3, substantial differences exists in Bangladesh in the economic activities
originating within the areas affected by the 1-m and 3-m relative sea level
rise scenarios.
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Table 3. Summary of Activities Originating in Affected Areas
Account (1985 Value)One-Meter ScenarioThree-Meter Scenario
Percent of Land Area
Bangladesh 11.5
Egypt (3.5/5 settled) 12-15
Percent of Population
Bangladesh 9
Egypt 16
27.5
29
27
21
Polpulation (Millions)
Bangladesh (92.8) 8.5
Egypt (48.6) 7.7
24.8
10.1
Percent of Gross Domestic Product
Bangladesh 8
Egypt 15
20
20
Gross Domestic Product (Millions)
Bangladesh (390,015 Taka) 21,125
Egypt (29,872 Pounds) 4,441
77,435
6,053
There is an obvious need for refined estimates based on better, more
disaggregated data about land-use patterns, population distribution, and
distribution of economic production. Official projections of these variables
would also allowt projection of economic exposure to account for economic and
population growth, assuming no sea level rise, for comparison with trans-
gression scenarios. Equally important, particularly in view of the fact that
in both deltaic areas the rate of subsidence will probably affect local sea
level rise more than global rise over the next 60 years, is documenting the
fluvial and coastal environment. In Egypt it is too late, the Nile being
completely dammed. But in the case of Bangladesh, we need to document how the
river flows, how the sediment is carried in the lower reaches of the delta,
and where the sediment ultimately accumulates. If rivers can be dammed or
diverted in ways that minimize adverse impacts on sedimentation, then coastal
inundation and erosion might be minimized.
Similar scenarios can be constructed for other developing nations. Of
the three factors controlling these scenarios, nothing can be done to control
local subsidence. Nations, however, can have some effect in helping regulate
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both locally and internationally the emissions to the atmosphere that lead to
the greenhouse effect. More importantly, each nation has substantial control
over the managing and damming of its rivers, although extended and difficult
international negotiations may be necessary. With proper measures, coastal
erosion may be minimized to reduce the human acceleration of shoreline
retreat. Further, recognizing the threats of socioeconomic dislocations posed
by sea level rise, nations likely to be affected can begin to plan for such a
rise by taking appropriate actions.
In spite of the simplifications necessitated by a first-order assessment,
it seems indisputable that when added to the expected subsidence of fifty
centimeters through 2100, even a 50-cm rise in global sea level implied by our
low scenario would have serious implications for Bangladesh and Egypt.
However, it seems reasonable to conclude that if the sediment washing
continues to reach the delta, the impacts on Bangladesh can be reduced
substantially, although it would be far more difficult to do so in the case of
Egypt. Neither maintaining natural deltaic processes nor slowing the rise in
sea level due to the greenhouse effect would be easy, but for Bangladesh and
Egypt, even the most modest success in that direction would be well worth the
effort in terms of both economics and human lives.
ACKNOWLEDGMENTS
This research was supported by a Cooperative Research Agreement with the
U.S. Environmental Protection Agency, No. CR-812941-01-0, and with funds from
The Pew Trusts to the Marine Policy Center and the A.W. Mellon Foundation to
the Coastal Research Center of the Woods Hole Oceanographic Institution.
REFERENCES
Ahmad, N. 1976. A new economic geography of Bangladesh. Bombay: Vikas
Publishing House PVT Ltd.
Barth, M.C. and J.G. Titus, eds. 1984. Greenhouse effect and sea level
rise. New York: Van Nostrand Reinhold.
Bird, E.C.F. 1985. Coastline changes: a global review. Chicester: John
Wiley and Sons.
Bird, E.C.F., and M.L., Schwartz, eds. 1985. The world's coastline. Strouds-
burg: Van Nostrand Reinhold.
Central Agency for Public Mobilization and Statistics, Egypt. 1982 October
13, 1982.
Er-Rashid, H. 1977. Geography of Bangladesh. Dhaka; University Presg.
Boulder: Westview Press.
Hoffman, J.S., D. Keyes, and J.G., Titus. 1983. Projecting future sea level
rise. Methodology, estimates to the year 2100 and research needs.
Washington, D.C.: U.S. Government Printing Office.
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Inman, D.L., D.G. Aubrey, and S.S., Pawka. 1976. Application of nearshore
processes to the Nile Delta. In UNDP/UNESCO Proceedings of seminar on
Nile Delta sedimentology. 205-55.
Jansen, E.G. 1983. Rural Bangladesh - competition for scarce resourcesT
DERAP Publication No. 162. Bergen, Norway:Chr. Michelsen Institute.
Khafagy, A. and M. Manohar. 1979. Coastal protection of the Nile Delta.
Nature and Resources 15:7-13.
Milliman, J.D. 1985. Changing sediment influxes from rivers to the ocean; A
real and future problem for Indian Ocean nations. IUC/UNESCO Press, in
press.
Milliman, J.D. 1986. Tropical river discharge and rising sea level; present
and future impacts from man's activities. IOC/UNESCO Special
Publication, in press.
Milliman, J.D., and R.H. Meade. 1983. World-wide delivery of river sediment
to the oceans. Journal of Geology 91(1):1-21.
Quarterly economic review of Bangladesh. (Annual Supplement for 1985).
London: The Economist Publications Ltd.
Quarterly economic review of Egypt (Annual Supplement for 1985). London: The
Economist Publications Ltd.
Quraishee, G.S. 1985. Influence of the Indus River on the marine environ-
ment. In Proceedings of the International Conference on Management of
Environment 111-22. Pakistan: Pakistan Academy of Science. Islamabad.
United Nations Economic and Social Commission for Asia and the Pacific.
1981. Country Monograph Series. No. 8, Population of Bangladesh.
Bangkok, Thailand: UNESCO
Warrick, R.A. and W.E. Riebsame. 1983. Social response to C02-induced
climate change: opportunities for research. In Social science research
and climate change; an interdisciplinary approach, eds. R.S. Chen, E.
Boulder, and S.M. Schneider. Boston: D. Reidel.
World Bank. 1972. Bangladesh land and water resources. Washington, D.C.:
World Bank.
Zaman, M., ed. 1983. River basin development. In Proceedings of the
National Symposium on River Basin Development. Dublin: Tycooly Inter-
national.
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Sea Level Rise: The Reaction of a Coastal Realtor
Kenneth J. Smith
New Jersey Shore and Beach
Manahawken, New Jersey USA
I appreciate the opportunity to present my reaction to the issue of sea
level rise. I am a coastal realtor from New Jersey, and I grew up on a
barrier island just north of Atlantic City called Long Beach Island. I
purchased my current home a couple of years ago, and after reviewing the
preceding papers I am proud of the foresight I must have had to have chosen a
site about 10 kilometers (6 miles) inland and 20 meters (80 feet) above sea
level.
Although this report has an international focus I mention the New Jersey
coast very briefly, because it is a microcosm of developed coasts worldwide.
Many of the challenges and problems of developed shorelines are evident
here. We have everything from bustling deepwater ports to low-lying, fully
habited barrier islands, and an extensive tidal marsh system.
About 25 percent of the U.S. population lives within a 480-kilometer
(300-mile) radius of Atlantic City, and one gets the feeling in the summer
that they are all at the Jersey shore. In the summer we are crowded beyond
belief, and our access roads are backed up for miles. Our 200 kilometers (125
miles) of beach are almost 100 percent developed,.with real estate worth many
billions of dollars.-^To give you an idea of our values, an oceanfront lot, if
you can find one, averages about half a million dollars, and oceanfront houses
rent for about two thousand dollars per week.
What this tells me is that people love the shore and have no qualms about
paying to be near it. The location of New Jersey's beaches, Just a short
drive from the New York and Philadelphia urban areas, has predetermined our
role as a provider to tourism. If millions of people converge on our beaches,
they need the development that facilitates their enjoyment of those beaches.
A place to sleep, take a shower, restaurants, amusement parks, marinas—all of
these combine to enhance a summer vacation. In my view the development is as
valuable to recreation as the beaches and bays.
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It is a paradox that, given the economic and recreational benefits of a
developed coast, we always seem to have insufficient funds for shore protec-
tion—not just in Mew Jersey, but nationwide, and I suspect in other countries
as well. This lack of funds is based partially on budget constraints at every
level of government; yet, even when we can show tremendous benefit-cost
ratios, the funds to protect our coasts are difficult to find.
What does this have to do with sea level rise? Quite a bit. There are
two main philosophies of coastal management which I will call the "stabi-
lizationists" and the "naturalists."
The "stabilizationists" include the "can do" people: the Corps of Engi-
neers, coastal engineering firms, property owners, local officials, and most
tourists. I include myself in this group. We believe that the technology
exists to maintain most beaches in their present positions, not forever, but
for many years to come, and that the value of the existing development, both
economic and recreational, justifies spending the money to do it.
The "naturalists" or "back to nature" proponents insist, with varying
degrees of intensity, that because there is nothing we can ultimately do to
stop the ocean, we should begin the depopulation of our barrier islands and
the abandonment of our coast. They exhibit, at best, a cavalier attitude and,
at worst, an arrogant disdain toward coastal property owners, and have managed
to obstruct meaningful shore protection projects in every locality.
Unfortunately, many of the "naturalists" are government officials who are
responsible for shore protection. So when the issue of sea level rise began
to emerge, it was "right up their alley." Although the predicted sea level
rise has a large range of uncertainty, you can bet that in the future this
issue will become a cornerstone of their argument against funding shore-
protection projects.
The reaction of our local residents, officials, and realtors to projected
sea level rise ranges from a healthy skepticism to a rather macho detachment
from the subject. I have heard comments such as, "What do you care? You
won't be around to see it!" The scientific community is often dismissed as "a
bunch of eggheads who don't want us here anyway." I wince when I hear comments
like that, but I suspect sea level rise will be taken more seriously as more
evidence comes in.
The disdain of the ordinary coastal citizen is understandable, however,
since most of the discussion of sea level rise seems to come from the
"naturalist" camp. Many theories have been used to justify the condemnation
or confiscation of coastal property, so sea level rise is seen by many people
as just another empty theory.
I do not feel that way; I believe the problem is serious. What concerns
me is the rapidity of the increase of carbon dioxide and the other gases. It
appears that we are racing into a climate that is unknown to man. The solu-
tions, if there are any, should begin to be contemplated now, as part of a
concerted, global effort.
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Even a mid-range sea level rise scenario will have a profound effect on
our coastline throughout the world. The resulting retreat of our beaches
would severely threaten and possibly wipe out our barrier island develop-
ment. The flooding of our ports, the salt contamination of our aquifers, the
drowning of our marshes—all of these possibilities need to be taken very
seriously.
People of the "can do" persuasion have relied on the technologies of
beach nourishment and hard structures to protect our coastal development. But
the coastal zone is dynamic, and it is difficult to include long-term
certainty in our planning. Plans to abandon our property or retreat from our
coast at this point would surely produce such enormous litigation as to be
counterproductive. For the near future, I can only hope that the rise will be
perceptible enough to be seriously considered, and slow enough that we can
prepare for it.
Is there a point where a global constituency will say "enough" and begin
to reduce the emissions that produce this climatic change? I don't know.
This earth sometimes seems like a cancer patient who can't stop smoking. But
it is a beautiful world, and we are its stewards. Certainly, the future is
what we make it, and I hope that we will see increased funding for climatic
research. I congratulate UNEP, EPA, and all of the other authors for their
focus on this important issue.
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