Chapter 1
SEA LEVEL RISE AND
WETLAND LOSS: AN OVERVIEW
by
James G. Titus
Office of Policy Analysis
U.S. Environmental Protection Agency
Washington, D.C. 20460
INTRODUCTION
Along the Atlantic and Gulf coasts of the United States, beyond the reach of the ocean
waves, lies a nearly unbroken chain of marshes and swamps. Part land and part water, our
coastal "wetlands" support both terrestrial and aquatic animals, and boast biological
productivities far greater than found on dry land.
Many birds, alligators, and turtles spend their entire lifetimes communing between wetlands
and adjacent bodies of water, while land animals that normally occupy dry land visit the
wetlands to feed. Herons, eagles, sandpipers, ducks, and geese winter in marshes or rest there
while migrating. The larvae of shrimp, crab, and other marine animals find shelter in the marsh
from larger animals. Bluefish, flounder, oysters, and clams spend all or part of their lives
feeding on other species supported by the marsh. Some species of birds and fish may have
evolved with a need to find a coastal marsh or swamp anywhere along the coast (Teal and Teal
1969). Wetlands also act as cleansing mechanisms for ground and surface waters.
The importance of coastal wetlands was not always appreciated. For over three centuries,
people have drained and filled marshes and swamps to create dry land for agriculture and urban
development. Flood control levees and navigation channels have prevented fresh water,
nutrients, and sediment from reaching wetlands, resulting in their conversion to open water.
Marshes have often been used as disposal sites for channel dredging, city dumps, and
hazardous waste sites.
In the 1960s, however, the public began to recognize the importance of environmental
quality in general and these ecosystems. In 1972, the U.S. Congress added Section 404 to the
federal Clean Water Act, which strengthened the requirement that anyone wishing to fill a
coastal wetland obtain a permit from the Army Corps of Engineers, and added the requirement
of approval by the Environmental Protection Agency. Several coastal states enacted legislation
to sharply curtail destruction of coastal wetlands.
These restrictions have substantially reduced conversion of wetlands to dry land in coastal
areas. The rate of coastal wetland loss declined from 1000 to 20 acres per year in Maryland
(Redelfs 1983), from 3100 to 50 acres per year in New Jersey (Tiner 1984), and from 444 to 20
acres per year in Delaware (Hardisky and Klemas 1983). The rate of conversion to dry land in
South Carolina has been reduced to about 15 acres per year (South Carolina Coastal Council
1985).1
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Nevertheless, these restrictions have not curtailed the conversion of wetlands to water. The
majority of coastal wetland loss in the United States is now taking place in Louisiana, which
loses fifty square miles of wetlands per year, mostly to open water. Navigation channels, canals,
and flood control levees have impeded the natural mechanisms that once enabled the wetlands of
the Mississippi Delta to keep pace with subsidence and rising sea level. The majority of coastal
wetland loss in South Carolina results from impoundments that have converted wetlands to open
water during part of the year.2
In the next century, moreover, conversion of wetlands to open water may overshadow con-
version to dry land throughout the coastal zone of the United States. Increasing concentrations
of carbon dioxide and other gases are expected to warm our planet a few degrees Celsius (C) by
a mechanism commonly known as the "greenhouse effect." Such a warming could raise sea level
one meter or so by expanding ocean water, meeting mountain glaciers, and causing polar ice
sheets to melt or slide into the oceans. Because most of America's coastal wetlands are less than
one meter above sea level, a large fraction of our coastal wetlands could be threatened by such a
rise.
Offsetting this potential threat are two compensating factors. A rise in sea level would flood
areas that are now dry land, creating new wetlands. Moreover, wetlands can grow upward by
accumulating sediment and organic material. The potential of these two factors to prevent a
major loss of wetlands in the next century, however, may be limited. People who have
developed the land just inland of today's wetlands may be reluctant to abandon their houses,
which new wetland creation would require. Although wetlands have been able to keep pace
with the rise in sea level of the last few thousand years, no one has demonstrated that they could
generally keep pace with an accelerated rise.
This report examines the vulnerability of U.S. coastal wetlands (excluding Alaska and
Hawaii) to a possible rise in sea level of one or two meters through the year 2100. By coastal
wetlands, we refer to marshes, swamps, and other plant communities that are flooded part, but
not all, of the time, and that are hydraulically connected to the sea. This chapter, written for the
general reader, summarizes the other chapters and their implications, as %ell as the basis for
expecting a global warming and rise in sea level; nature's response to a rising sea; the impacts of
human interference with the mechanisms by which wetlands adjust to sea level rise; and policies
that might limit future loss of coastal wetlands.
Chapters 2 (Kana, Baca, & Williams) and 3 (Kana, Eiser, Baca & Williams) describe field
surveys that were used to estimate the potential impacts of sea level rise on wetlands in the area
of Charleston, South Carolina, and Long Beach Island, New Jersey, respectively. In Chapter 4,
Armentano, Park, & Cloonan use topographic maps to estimate the potential loss for 52 regions
throughout the United States. Finally, in Chapter 5, EPA's Office of Wetland Protection
responds to the challenges presented in the preceding chapters.
This report leaves unanswered many questions that will need to be investigated for society
to rationally respond to the implications of a substantial rise in sea level: What portion of our
wetlands will he able to keep pace with rising sea level? In how many areas would it be
economical for communities to hold back the sea by erecting levees and bulkheads, at the
expense of their wetlands? Should wetland protection policies seek to slow an inevitable loss of
coastal marshes and swamps, or to ensure that a particular fraction of wetlands are maintained
in perpetuity?
We hope that this report will stimulate the additional research and policy analysis necessary
for society to rationally respond to the risk of wetland loss caused by a rise in sea level.
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THE BASIS FOR EXPECTING A RISE IN SEA LEVEL
Post Changes In Climate and Sea Level
Throughout geologic history, sea level has risen and fallen by over three hundred meters (one
thousand feet). Although changes in the size and shape of the oceans' basins have played a role over
very long periods of time (Hays and Pitman 1973), the most important changes in sea level have been
caused by changes in climate. During the last ice age (18,000 years ago), for example, the earth was
about five degrees Celsius colder than today, glaciers covered most of the northern hemisphere, and
sea level was one hundred meters (three hundred feet) lower than it is today (Donn, Farrand, and
Ewing 1962).
Although most of the glaciers have melted since the last ice age, polar glaciers in Greenland
and Antarctica still contain enough water to raise sea level more than seventy meters (over two
hundred feet) (Untersteiner 1975). A complete meeting of these glaciers has not occurred in the last
two million years, and would take tens of thousands of years even if the earth warmed substantially.
However, unlike the other glaciers, which rest on land, the West Antarctic Ice Sheet rests in the ocean
and is thus more vulnerable. Warmer ocean water would be more effective than warmer air at
melting glaciers and could melt the ice shelves that prevent the entire glacier from sliding into the
oceans. Mercer (1970) suggests that the West Antarctic Ice Sheet completely disappeared during the
last interglacial period (which was one or two degrees warmer than today and occurred 100,000 years
ago), at which time sea level was five to seven meters (about twenty feet) above its present level.
Over periods of decades, climate can influence sea level by heating and thereby expanding (or
cooling and contracting) sea water. In the last century, tidal gauges have been available to measure
relative sea level in particular locations. Along the Atlantic Coast, sea level has risen about 30
centimeters (one foot) in the last century (Hicks, Debaugh, and Hickman 1983). Studies combining
tide gauge measurements around the world have concluded that average global sea level has risen ten
to fifteen centimeters (four to six inches) in the last one hundred years (Barnett 1983; Gornitz,
Lebedeff, and Hansen 1982). About five centimeters of this rise can be explained by the thermal
expansion of the upper layers of the oceans resulting from the observed global warming of 0.4C in the
last century (Gornitz, Lebedeff, and Hansen 1982). Meltwater from mountain glaciers has contributed
two to seven centimeters since 1900 (Meier 1984). Figure 1-1 shows that global temperature and sea
level appear to have risen in the last century. Nevertheless, questions remain over the magnitude and
causes of sea level rise in the last century.
The Greenhouse Effect and Future Sea Level Rise
Concern about a possible acceleration in the rate of sea level rise stems from measurements
showing the increasing concentrations of carbon dioxide (CO2), methane, chlorofluorocarbons, and
other gases released by human activities. Because these gases absorb infrared radiation (heat),
scientists generally expect the earth to warm substantially. Although some people have suggested
that unknown or unpredictable factors could offset this warming, the National Academy of Sciences
(NAS) has twice reviewed all the evidence and concluded that the warming will take place. In 1979,
the Academy concluded: "We have tried but have been unable to find any overlooked physical effect
that could reduce the currently estimated global warming to negligible proportions" (Charney 1979).
In 1982, the NAS reaffirmed its 1979 assessment (Smagorinsky 1982).
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 its atmosphere retains heat. When
sunlight strikes the earth, it warms the surface, which then reradiates the heat as infrared radiation.
However, water vapor, CO2, and other gases in the atmosphere absorb sow of the radiation
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FIGURE 1-1
GLOBAL TEMPERATURES AND SEA LEVEL TRENDS IN THE LAST CENTURY
Year
Sources: Temperature curve from: HANSEN, J.E., D. JOHNSON, A. LACIS, S. LEBEDEFF, D.
RIND, AND G. RUSSELL, 1981. Climate Impact of Increasing Atmospheric Carbon Dioxide,
Science 213:957-966. Sea level curve adapted from: GORNTTZ, V., S. LEBEDEFF, and J.
HANSEN, 1982. Global Sea Level Trend in the Past Century. Science 215:1611-1614.
4
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rather than sowing 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 generally known as the "greenhouse effect." Without the green-
house effect of the gases that occur in the atmosphere naturally, the earth would be approximately
33 °C (60 °F) colder than it is currently (Hansen et al. 1984).
In recent decades, the concentrations of "greenhouse gases" have been increasing. Since the
industrial revolution, the combustion of fossil fuels, deforestation, and cement manufacture have
released enough CO2 into the atmosphere to raise the atmospheric concentration of carbon dioxide
by 20 percent. As Figure 1-2 shows, the concentration has increased 8 percent since 1958
(Keeling, Bacastow, and Whorf 1982).3 Recently, the concentrations of methane, nitrous oxide,
chlorofluorocarbons, and a few dozen other trace gases that also absorb infrared radiation have also
been increasing (Lacis et al. 1981). Ramanathan et al. (1985) estimate that in the next fifty years,
these gases will warm the earth as much as the increase in CO2 alone.
Although there is no doubt that the concentration of greenhouse gases is increasing, the future
rate of that increase is uncertain. A recent report by the National Academy of Sciences (NAS)
examined numerous uncertainties regarding future energy use patterns, economic growth, and the
extent to which CO2 emissions remain in the atmosphere (Nordhaus and Yohe 1983). The
Academy estimated a 98 percent probability that CO2 concentrations will he at least 450 parts per
million (1.5 times the year-1900 level) and a 55 percent chance that the concentration will be 550
parts per million by 2050. The Academy estimated that the probability of a doubling of CO2
concentrations by 2100 is 75 percent. Other investigators had estimated that a doubling is likely by
2050 (Wuebbles, MacCracken, and Luther 1984).
If the impact of the trace gases continues to be equal to the impact Of CO2, the NAS analysis
implies that the "effective doubling" of all greenhouse gases has a 98 percent chance of occurring
by 2050.4 An international conference of scientists recently estimated that an effective doubling by
2030 is likely (UNEP, WMO, ICSU 1985). However, uncertainties regarding the emissions of
many trace gases are greater than those for CO2. Although the sources of chlorofluorocarbons
(CFCs) are well known, future emissions involve regulatory uncertainties. Because these gases can
cause deterioration of stratospheric ozone, forty nations have tentatively agreed to cut emissions of
the most important CFCs by 50 percent. However, additional cutbacks may be implemented, and
other nations may sign the treaty; on the other hand, emissions of gases not covered by the treaty
may increase.
Considerable uncertainty also exists regarding the impact of a doubling of greenhouse gases.
Physicists and climatologists generally agree that a doubling would directly raise the earth's
average temperature by about I°C if nothing else changed. However, if the earth warmed, many
other aspects of climate would be likely to change, probably amplifying the direct effect of the
greenhouse gases. These indirect impacts are known as "climatic feedbacks."
Figure 1-3 shows estimates by Hansen et al. (1984) of the most important known feedbacks.
A warmer atmosphere would retain more water vapor, which is also a greenhouse gas, and would
warm the earth more. Snow and floating ice would melt, decreasing the amount of sunlight
reflected to space, causing additional warming. Although the estimates of other researchers differ
slightly from those of Hansen et al., climatologists agree that these two feedbacks would amplify
the global warming from the other greenhouse gases. However, the impact of clouds is far less
certain. Although recent investigations have estimated that changes in cloud height and cloud
cover would add to the warming, the possibility that changes in cloud cover would offset part of
the warming cannot be ruled out. After evaluating the evidence, two panels of the National
Academy of Sciences concluded that the eventual warming from a doubling of greenhouse gases
would be between 1.5° and 4.5°C (3°-8°F) (Charney et al. 1979; Smagorinsky 1982).
5
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FIGURE 1-2
CONCENTRATIONS OF SELECTED GREENHOUSE GASES OVER TIME
1. Keeling, C.D., R.B. Bacastow, and TP. Whorf, 1982. Measurements of the Concentration of
Carbon Dioxide at Mauna Loa, Hawaii. Carbon Dioxide Review 1982, edited by W. Clark. New
ybrk: Oxford University Press, 377-382. Unpublished data from NOAA after 1981.
2. Weiss, R.F., 1981. 'The Temporal and Spatial Distribution of TYopospheric Nitrous Oxide."
Journal of Geophysical Research. 86(C8):7185-95.
3 Cunnold DM, et al., 1983a. The Atmospheric Lifetime Experiment. 3. Lifetime Methodology
and Application to Three Mars of CFCL3 Data. Journal of Geophysical Research.
88(C13):8401-8414.
4. Cunnold, DM., et al., 1983b. The Atmospheric Lifetime Experiment. 4. Results for CF2CL2
Based on Three Years Data, Journal of Geophysical Research. 88(C13):8401-8414.
5. Rasmussen, RA„ andMA.K. Khalil, 1984. Atmospheric Methane in the Recent and Ancient
Atmospheres: Concentrations, Trends, and Interhemispheric Gradient, Journal of Geophysical
Research. 89(D7): 11599-605.
6
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FIGURE 1-3
ESTIMATED GLOBAL WARMING DUE TO A DOUBLING OF GREENHOUSE
GASES: DIRECT EFFECTS AND CLIMATIC FEEDBACKS
C Ci
•3
co a>
.c a.
O E
a>
III
Direct Effect Water Decreased Cloud Cloud
of CC>2 Vapor Reflectivity Height Cover
and Other Feedback Due to
Greenhouse Retreat of
Gases Ice and
Snow
NOTE: Although Hansen et al. estimate a positive feedback from the clouds, a negative feedback
cannot be ruled out.
Sources: Adapted from: HANSEN, J.E., A. LAC1S, D. RIND, and G. RUSSELL, 1984. Climate
Sensitivity to Increasing Greenhouse Gases. In Greenhouse Effect and Sea Level Rise: A
Challenge for This Generation, edited by M.C. Barth and J.G. Titus. New Ybrk: Van Nostrand
Reinhold, p. 62.
A global warming could raise sea level by expanding ocean water, melting mountain
glaciers, and causing ice sheets in Greenland and Antarctica to melt or slide into the oceans.
Four major reports have assessed the possible significance of these factors, as shown in Table
14 and Figure 14. All predict that the global warming will cause the rate of sea level rise to
accelerate.
Revelle (1983) estimated that Greenland and mountain glaciers could each contribute 12
cm to sea level in the next century, and that thermal expansion could contribute 30 cm. Based on
current trends, Revelle concluded that other factors could contribute an additional 16 cm, for a
total rise of 70 cm, plus or minus 25 percent. Hoffman et al. (1983) developed a variety of sea
level rise scenarios based on high and low assumptions for all the major uncertainties. They
estimated that sea level was most likely to rise between 26 and 39 cm by 2025 and 91 to 137 cm
by 2075.
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The National Academy of Sciences Polar Research Board Report Glaciers, Ice Sheets,
and Sea Level (Meier et al. 1985) examined the possible glacial contribution to sea level rise by
the year 2100. The panel endorsed estimates that alpine (Meier 1984) and Greenland
(Bindschadler 1985) glaciers mould each contribute 10 to 30 centimeters. Thomas (1985)
estimated that the antarctic contribution resulting from a four-degree warming would most
likely be 28 cm, but could be as high as 2.2 meters. However, the panel concluded that the
antarctic contribution could be anywhere from a 10-centinxter drop (due to increased snowfall)
to a one-meter rise.
Hoffman et al. (1986) revised their earlier projections in light of the glacial process
models developed in the Polar Board report and new information on future concentrations
provided by Nordhaus and Yohe (1983) and Ramanathan et al. (1985). Although the revised
assumptions had a minor impact on their estimates of thermal expansion, it substantially
lowered their estimates of snow and ice contributions until after 2050. They estimated the rise
by 2025 to be between 10 and 21 cm, and by 2075 to be between 36 and 191 cm.5 Thomas
(1986) estimated the likely rise through 2100 to be 64 to 230 cm.
TABLE 1-1
ESTIMATES OF FUTURE SEA LEVEL RISE (centimeters)
Year 2100 by Cause (2085 in the case of Revelle 1983):
Revelle (1983)
Hoffman et al.
(1983)
Meier et al.
(1985)
Hoffman et al.
(1986)
Thomas (1986)
Thermal
Expansion
30
28-115
28-83
28-70
Total Rise in Specific Years
2000
Revelle (1983)
Hoffman et al. (1983)
low 4.8
mid—range low 8.8
mid-range high 13.2
high 17.1
Hoffman et al. (1986)
low 3 .5
high 5.5
Alpine
Glaciers
12
b
10-30
12-37
14-35
• d
2025
13
26
39
55
10
21
Greenland
12
b
10-30
6-27
9-45
2050
23
53
79
117
20
55
Antarctica
a
b
-10
+100
2075
38
91
137
212
36
191
12-220
13-80
2085
70
Total
70
56-345
50-200°
57-368
64-230
2100
56.0
144.4
216.6
345 . 0
44
258
57
368
a Revelle attributes 16 cm to other factors.
b Hoffman et al. (1983) assumed that the glacial construction would be one to two times the
contribution of thermal expansion.
c This estimate includes extrapolation of thermal expansion from Revelle (1983).
d Only Hoffman et al. made year-to-year projections for the next century.
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FIGURE 1-4
GLOBAL SEA LEVEL RISE SCENARIOS
(inches) (cm)
160 -
140 -
120 .
100 -
80 -
60 -
40 -
20 -
0 -
Key
Current Trend
EPA high, mid-high, mid-low, and low scenarios
(Hoffman et al. 1983)
| Hoffman et al. (1986) high and low estimates
0 NAS estimate (Revelle 1983)
Polar Research Board high and low estimates (Meier et al. 1985)
O Thomas (1986) high and low estimates
Note: The EPA 1983 Mid-Low and Mid-High scenarios am called "low " and "high "for the
remainder of this chapter and throughout Chapters 2, 3, and 4.
In this study, we examine the implications of the mid4ow and mid-high scenarios from
Hoffiman et al. (1983), shown in Table 1-1 and Figure 1-4. (For simplicity, we call these
scenarios "low" and "high.") Although it might be desirable to undertake a worst-case analysis
of a larger rise, the scenarios me used are broadly representative of the studies that have been
undertaken so far. Because much of the U.S. coast is sinking, the relative rise at a particular
location will generally be greater. Table 1-2 lists the expected rise in sea level under the low and
high scenarios for different areas of the United States.
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TABLE 1-2
RELATIVE SEA LEVEL RISE IN THE UNITED STATES
Historic
Historic Relative Low High
Subsidence Sea Level Scenario Scenario
Rate Trend 1980-2100 1980-2100
(mm/yr) (mm/yr) (cm) (cm)
Portland, Maine
1.1
2.3
157.6
229.8
Boston, Massachusetts
1.1
2.3
157.6
229.8
Newport, Rhode Island
1.4
2.6
161.2
233.4
New London, Connecticut
1.0
2.2
156.4
228.6
New York, New York
1.6
2.8
163.6
235 .8
Sandy Hook, New Jersey
3.0
4.2
186.4
252.6
Atlantic City, New Jersey
2.8
4.0
178.0
250.2
Philadelphia, Pennsylvania
1.4
2.6
161.2
233.4
Baltimore, Maryland
2.0
3.2
168.4
240.6
Annapolis, Maryland
2.5
3.7
174.4
246.6
Hampton Roads, Virginia
3.1
4.3
181.6
253.8
Charleston, South Carolina
2.2
3.4
170.8
243.0
Fernandina, Florida
0.5
1.7
150.4
222.6
Miami Beach, Florida
1.1
2.3
157.6
229.8
Cedar Key, Florida
0.8
2.0
154.0
226.2
Pensacola, Florida
1.2
2.4
158.8
231.0
Eugene Island, Louisiana
8.8
10.0
250.0
322.2
Galveston, Texas
5.1
6.3
205.6
277.8
San Diego, California
0.7
1.9
152.8
225.0
Los Angeles, California
-0.6
0.6
137.2
209.4
San Francisco, California
0.0
1.2
144.4
216.2
Astoria, Oregon
-1.7
-0.5
124.0
196.2
Seattle, Washington
0.7
1.9
152.8
225.0
Sitke, Alaska
-3.6
-2.4
101.2
173.4
Worldwide
0
1.2
144.4
216.6
Source: Derivations of historic rates of relative sea level rise due to subsidence are based on an
assumption of a 1.2 mm/yr global rise in sea level. Projections are based on mid-low and mid-
high estimates from Hoffman et al. 1983, with historic subsidence (from Hicks, Debaugh, and
Hickman 1983) added.
NATURAL IMPACTS OF SEA LEVEL RISE
There are three major ways by which sea level rise can disrupt wetlands: inundation,
erosion, and saltwater intrusion. In some cases, wetlands will be converted to bodies of open
water; in other cases, the type of vegetation will change but a particular area will still be
wetlands However, if sea level rises slowly enough, the ability of wetlands to grow upward-by
trapping sediment or building upon the peat the sediment creates-can prevent sea level rise from
disrupting the wetlands.
In explaining potential impacts of sea level rise, we focus on what the impact would be if
wetlands did not grow upward, and leave it to the reader to remember that this potential
"vertical accretion" can offset these impacts. The actual impact will depend on the "net substrate
change," i.e., the difference between sea level rise and wetland accretion. In this report, all
estimates of future wetland loss are based on the assumption that current rates of vertical
accretion continue. An important area for future research will be to determine whether future
climate change and sea level rise will accelerate or slow the rate of wetland accretion. Even if
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wetlands are able to accrete more rapidly in the future, however, existing literature provides little reason to
believe that wetlands will generally be able to keep up with a one- or two-meter rise in sea level.
Tidal Flooding
Because periodic flooding is the essential characteristic of salt marshes, increases in the frequency and
duration of floods can substantially alter these ecosystems. Salt marshes extend seaward to roughly the
elevation that is flooded at mean tide, and landward to roughly the area that is flooded by spring tide (the
highest astronomical tide every 15 days). Salt marsh plants are different from most plants found inland in
that they tolerate salt water to varying degrees (Teal and Teal 1969). Coastal wetlands flooded once or
twice daily support "low marsh" vegetation, while areas flooded less frequently support high marsh species.
Transition wetlands can be found above the high marsh, in areas flooded less frequently than twice a month.
The natural impact of a rising sea is to cause marsh systems to migrate upward and inland. Sea level rise
increases the frequency and/or duration of tidal flooding throughout a salt marsh. If no inorganic sediment
or peat is added to the marsh, the seaward portions become flooded so much that marsh grass drowns and
marsh soil erodes; portions of the high marsh become low marsh; and upland areas immediately above the
former spring tide level are flooded at spring tide, becoming high marsh. If nearby rivers or floods supply
additional sediment, sea level rise slows the rate at which the marsh advances seaward.
The net change in total marsh acreage depends on the slopes of the marsh and upland areas. If the land has
a constant slope throughout the marsh and upland, then the area lost to marsh drowning will be equal to the
area gained by the landward encroachment of spring high tides. In most areas, however, the slope above
the marsh is steeper than the marsh; so a rise in sea level causes a net loss of marsh acreage. Two
extreme examples are noteworthy: marshes immediately below cliffs in New England and along the Pacific
Coast could drown without being replaced inland. In Louisiana, thousands of square miles of wetlands are
within one meter of sea level, with very narrow ridges in between and very little adjacent upland between
one and two meters above sea level. A one-meter rise in sea level could drown most of the wetlands there
without necessarily creating any significant new marsh (Louisiana Wetland Protection Panel, 1987;
Gagliano et al. 1981).
Figure 1-5 illustrates why there is so much more land at marsh elevation than just above the marsh.
Wetlands can grow upward fast enough to keep pace with the slow rise in sea level that most areas have
experienced in the recent past (Kaye and Barghoorn 1964; Coleman and Smith 1964; Redfield 1967).
Thus, areas that might have been covered with two or three meters of water (or more) have wetlands
instead (Figures 1-5A, 1-5B). If sea level rise accelerates only slightly, marshes that are advancing today
may have sufficient sediment to keep pace with sea level. But if sea level rise accelerates to one centimeter
per year (projected for 2025-2050), the sea will be rising much more rapidly than the demonstrated ability
of wetlands to grow upward in most areas (Armentano et al., Chapter 4) and the increase in wetland
acreage of the last few thousand years will be negated (Figure 1-5C). If adjacent upland areas are
developed, all the wetlands could be lost (Figure 1-5D).
An important factor in determining the vulnerability of marshes to sea level rise is the tidal range, the
difference in elevation between the mean high tide and mean low tide. Coastal wetlands are generally less
than one tidal range above mean sea level.6 Thus, if the sea rose by one tidal range overnight, all the
existing wetlands in an area would drown. Tidal ranges vary greatly throughout the United States. Along
the open coast, it is over four meters in Maine, somewhat less than two meters (about five feet) along the
mid-Atlantic, and less than one meter (about two feet) in the Gulf of Mexico (NOAA 1985). The shape of
an embayment can amplify or dampen the tidal range, however. Most notably, the estuaries behind barrier
islands with widely separated inlets can have tidal ranges of thirty centimeters (one foot) or less. The tidal
range of Chesapeake Bay is about fifty centimeters (NOAA 1985).
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FIGURE 1-5
EVOLUTION OF A MARSH AS SEA LEVEL RISES
5000 Years Ago
-J—Sea Level
B
Today
n.a
Current
Sedimentation and
Past
Peat Formation
C Future
Substantial Wetland Loss Where There is Vacant Upland
Future
- Sea Level
" Current
Sea Level
Future
Complete Wetland Loss Where House is Protected
In Response to Rise in Sea Level
Future
. Saa Level
Current
Saa Level
Coastal marshes have kept pace with the slow rate of sea level rise that has characterized the last several thousand
years. Thus, the area of marsh has expanded over time as now lands were inundated, resulting in much more
wetland acreage than dry land just above the wetlands (A and B). If in the future, sea level rises faster than the
ability of the marsh to keep pace, the marsh area will contract (C). Construction of bulkheads to protect economic
development may prevent now marsh from forming and result in a total loss of marsh in some areas (D).
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To investigate some of these issues, Kana et al. (Chapters 2 and 3) estimate the impact of accelerated
sea level rise on wetlands in the areas of Charleston, South Carolina, and Long Beach Island, New Jersey.
Charleston has a tidal range of almost two meters, while the New Jersey area has tidal ranges between sixty
and one hundred centimeters. In each area, they surveyed a dozen marsh profiles to develop a "composite
transect," an average cross section of the marsh. Based on previous studies, they assume that the marshes in
both areas could grow upward at a rate of five millimeters per year.
Figure 1-6 illustrates the composite transect of the Charleston marshes. The low marsh, whose
elevation is between 45 and 90 centimeters 0.5 to 3.0 feet) is 550 meters (1800 feet) wide. The high marsh,
with elevation between 90 and 120 centimeters (3.0 to 4.0 feet), is about 210 meters (700 feet) wide; the
transition wetlands, with elevation between 120 and 195 centimeters (4.0 to 6.5 feet), are generally about
150 mters (500 feet) wide. Thus, the average slopes found in the low, high, and transition marsh areas are
0.08, 0.14, and 0.50 percent, respectively, confirming that the slope of the profile increases as one moves
inland from the marsh. (The slope immediately above the marsh is approximately 0.55 percent.)
Composite wetlands transect for Charleston illustrating the approximate percent occurrence and
modal elevation for key indicator species or habitats based on results of 12 surveyed transects.
Minor species have been omitted. Elevations are with respect to 1929NGVD, which is about 15
cm lower than current sea level. Current tidal ranges are shown at right.
Source: Kana et al. (Chapter 2)
A word on what we mean by elevation is in order. Old maps often have contours representing, for
example, five feet above sea level. However, because sea level has been rising, a contour that was five feet
above sea level fifty years ago may only be four and one-half feet above sea level today. To avoid potential
confusion, most maps today express elevations with respect to the "National Geodetic Vertical Datum"
(NGVD) reference plane, which is a fixed reference that is unaffected by changes in sea level.
NGVD was developed in 1929 by estimating mean sea level at twenty-six sites along the North
American coast for the preceeding couple of decades. For these sites, zero elevation (NGVD) is the same as
mean sea level over that period. For other sites, however, the zero elevation is not necessarily mean sea
level for that period. NGVD was developed by a surveying
FIGURE 1-6
COMPOSITE TRANSECT-CHARLESTON, S.C
Highland 47%
TYPICAL DISTANCE (FT.)
13
-------
technique, known as "leveling," between the twenty-six sites; mean sea level, on the other hand, may be
higher or lower at a particular location depending on such factors as rainfall, winds, currents, and
atmospheric pressure. This distinction is usually unimportant; even USGS topographic maps printed
before 1973 refer to elevations above "mean sea level" when they really mean NGVD. For most practical
purposes, the reader of this report can assume that zero elevation at a particular site refers to the level of
the sea between 1910 and 1929. All elevations in this report are with respect to NGVD unless otherwise
stated.
The other type of elevational reference is the "tidal datum." Depending upon context, terms such
as "mean sea level" can refer to a theoretical concept or a legal definition. The legal definition of mean
sea level (MSL) is the average water level observed at a location over the period 1960-78; mean high
water (MHW) and mean low water (MLW) are the averages of all high and low tides, respectively, over
that period; mean tidal range is the difference between mean high water and mean low water. However,
wetlands respond to actual conditions, the average water level of today. Thus, unless otherwise stated, the
term mean sea level in this report refers to the average water levels of today, not the legal tidal datum.
Figure 1-7 illustrates the impact on the composite marsh profile of the low scenario for the period
1980-2075, which implies an 87-centinieter (2.9-foot) rise in relative sea level for the Charleston area.
Because Kana et al. assume that sedimentation would enable the surface to rise 48 centimeters, the net
rise in sea level is equivalent to an instantaneous rise of 39 centimeters (15 inches). As the figure shows,
the area of low and high marsh would each decline by about 50 percent as they shifted upward and
inland. For the high scenario rise of 159 centimeters (5.2 feet), the loss would be approximately 80
percent.
FIGURE 1-7
SHIFT IN WETLANDS ZONATION ALONG A SHORELINE PROFILE
Conceptual model of the shift in wetlands zonation along a shoreline profile if sea level rise exceeds sedimentation by 40an. In
general, the response will be a landward shift and altered areal distribution of each habitat because of variable slopes at each
elevation interval
Source: Kana et al. (Chapter 2)
Although Kana et al. considered alternative scenarios of sea level rise, they did not investigate
alternative rates of wetland accretion. However, using the data presented in Figure 1-6, one can derive
Figure 1-8, which shows marsh loss for various combinations of vertical accretion and sea level rise. For
example, an 80 percent loss could occur (1) if the marsh grows upward at I centimeter per year and sea
level rises 1.9 meters by 2100 or (2) if sea level rises 80 centimeters
14
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and the marsh stops accreting. The shaded region illustrates the most likely range based on
current literature: global sea level rise of 50-200 centimeters and accretion of 4-6 millimeters
per year. Within this likely range, a negligible loss of wetlands is possible; however, over half
the shaded region shows an 80 percent loss of marsh by 2100.
FIGURE 1-8
PERCENT MARSH LOSS IN THE CHARLESTON AREA BY 2100 FOR
COMBINATIONS OF SEA LEVEL RISE AND MARSH ACCRETION
100%*
o
ec
<
z
LU
o ^
CO (A
111 Q>
<4%
_iw
LU O
> O
LU
_l CM
< 1
Hi un
CO CO
CT)
LU t-
>
I-
<
-I
LU
CC
MARSH ACCRETION (Millimeters/Year)
The shaded area represents the most likely range of sea level rise (50-200 cm, global; 75-225cm,
relative to Charleston) and marsh accretion (4-6 mm/yr).
'Wetland loss in excess of 80 percent occurs only if today's uplands are protected.
15
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To put the significance of these estimates in perspective, one would expect the Charleston
area to lose less than 0.5 percent of its wetlands in the next century if current rates of conversion
for development continue. Although a substantial amount of marsh was filled as the city was built,
conversion of wetlands to dry land came to a virtual halt with the creation of the South Carolina
Coastal Council. Since 1977, the state has lost only 35 of its 500,000 acres to dry land (South
Carolina Coastal Council 1985). Impoundments have transformed another 100 acres.7 Extrapolating
these trends would imply a loss of about 1,500 acres in the next century, about 0.3
percent of the state's coastal wetlands. Thus, sea level rise would be the dominant cause of
wetland loss.8
In the New Jersey study area, the high marsh dominates. Thus, there would not be a major
loss of total marsh acreage for the low scenario through 2075; the high marsh would simply be
converted to low marsh. For the high scenario, however, there would be an 86 percent loss of
marsh, somewhat greater than the loss in the Charleston area. Table 1-3 illustrates the projected
shifts in wetlands for the South Carolina and New Jersey Case studies through the year 2075;
Table 14 shows projected changes in marsh area for net rises in sea level (over accretion)
ranging from 10 to 100 cm.
TABLE 1-3
IMPACT OF SEA LEVEL RISE ON WETLANDS 1980-2075 (acres)
2075 Abandonment Defend Shore
2075 (Vacant Land) (Bulkheads)
Current Low High Low High
1980 Trend Sea Level Sea Level Sea Level Sea Level
Charleston Case Study
Transition
1500
2820
1355
1420
605
0
High Marsh
2300
3320
690
675
690
0
Low Marsh
5400
3910
3235
860
3235
750
Tidal Flat
2600
2600
5020
1425
5020
1425
Total Marsh
7700
7230
3925
1525
3925
750
Percent Loss (Gain)
High Marsh
-
(44)
70
71
70
100
Low Marsh
-
28
40
84
40
86
Marsh
-
6
49
80
49
90
New Jersey Case Study
Transition
1400
6600
1300
1130
-
-
High Marsh
9200
3300
1200
530
-
-
Low Marsh
500
1700
8100
1200
-
-
Tidal Flat
2410
2400
1200
900
-
-
Total Marsh
9700
5000
9300
1730
-
Percent Loss (Gain)
High Marsh
-
64
87
88
-
*
Low Marsh
-
(240)
(1520)
(140)
-
All Marsh
"
48
4
82
Source: Kctna et al. (Chapters 2 and 3).
16
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TABLE 1-4
WETLAND AREA AS A PERCENT OF TODAY'S ACREAGE FOR A 10- to 100-cm
RISE IN SEA LEVEL IN EXCESS OF VERTICAL ACCRETION*
Charleston, SC Tucker-ton, NJ Great Bay, NJ
Level
Rise
High
Marsh
Low
Marsh
Total
Marsh
High
Marsh
Low
Marsh
Total
Marsh
High
Marsh
. j f
Low
Marsh
Total
Marsh
0 cm
29.9
70.1
100.0
93.9
6.1
100.0
95.8
4.2
100.0
10
22.6
64 .6
87.2
60.1
43.2
103.4
76.0
23.9
99.9
20
15.4
59.0
74.4
26.3
80.5
106 .8
56.2
43.5
99.7
30
8.1
52.9
61.0
11.5
98.6
110.2
36.4
63.3
99.7
40
7.8
41.1
48.9
11.5
102.0
113 .6
16.5
70.3
86.8
50
7.8
30.7
38.5
11.5
89.5
101.0
5.2
61.9
67.1
60
7.8
23.4
31.2
11.5
55.8
67. 3
5.2
42.0
47.2
70
7.8
16.2
24.0
11.5
22.0
33.5
5.2
22.2
22.4
80
7.8
11.7
19.5
11.5
21.6
33.2
5.2
3.9
9.1
90
7.8
11.7
19.5
11.5
21.6
33.2
5.2
3.8
9.0
100 cm
7.8
11.7
19.5
11.5
21.6
33.2
5.2
3.8
9.0
*Calculations are based on the assumption that development does not prevent new wetlands
from forming inland. If adjacent lowlands are protected, rises of between 1 and 1.6 m would
destroy the remaining marsh.
Barrier Islands, Deltas, and Saltwater Intrusion
Although most marshes could probably not keep pace with a substantial acceleration in sea level
rise, three possible exceptions are the marshes found in river deltas, tidal inlets, and on the bay sides of
barrier islands. River and tidal deltas receive much more sediment than wetlands elsewhere; hence they
might be able to keep pace with a more rapid rise in sea level. For example, the sediment washing down
the Mississippi river for a long time was more than enough to sustain the delta and enable it to advance
into the Gulf of Mexico, even though relative sea level rise there is approximately one centimeter per
year, due to subsidence (Gagliano, Meyer Arendt, and Wicker 1981). A global sea level rise of one
centimeter per year would double the rate of relative sea level rise there to two centimeters per year; thus,
a given sediment supply could not sustain as great an area of wetlands as before. It could, however,
enable a substantial fraction to keep pace with sea level rise.
In response to sea level rise, barrier islands tend to migrate landward as storms wash sand from
the ocean side beach to the bay side marsh (Leatherman 1982). This "overwash" process may enable
barrier islands to keep pace with an accelerated rise in sea level. However, it is also possible that
accelerated sea level rise could cause these islands, to disintegrate. In coastal Louisiana, where rapid
subsidence has resulted in a relative sea level rise of one centimeter per year, barrier islands have broken
up. The Ship Island of the early twentieth century is now known as "Ship Shoal" (Pendland, Suter, and
Maslow 1986).
Marshes often form in the flood (inland) tidal deltas (shoals) that form in the inlets between
barrier islands. Because these deltas are in equilibrium with sea level, a rise in sea level would tend to
raise them as well, with sediment being supplied primarily from the adjacent islands.
17
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Moreover, if sea level rise causes barrier islands to breach, additional tidal deltas will form in the new
inlets, creating more marsh, at least temporarily. In the long run, however, the breakup of barrier islands
mould result in a loss of marsh. Larger waves would strike the wetlands that form in tidal deltas and in
estuaries behind barrier islands. Wave erosion of marshes could also be exacerbated if sea level rise deepens
the estuaries. This deepening would allow ocean waves to retain more energy and larger waves to form in
bays. Major landowners and the government of Terrebonne Parish, Louisiana, consider this possibility a
serious threat and are taking action to prevent the breakup of Isle Demiere and others around Terrebonne Bay
(Terrebonne Parish 1984).
Sea level rise could also disrupt coastal wetlands by a mechanism known as saltwater intrusion,
particularly in Louisiana and Florida. In many areas the zonation of wetlands depends not so much on
elevation as on proximity to the sea, which determines salinity. The most seaward wetlands are salt marshes or
their tropical equivalent, mangrove swamps. As one moves inland, the fresh water flowing to the sea reduces
salinity, and brackish wetlands are found. Still farther inland, the freshwater flow completely repels all salt
water, and fresh marshes and cypress swamps are found.
Although these marshes may be tens (and in Louisiana, hundreds) of kilometers inland, their elevation
is often the same as that of the saline wetlands. A rise in sea level enables salt water to penetrate upstream and
inland, particularly during droughts. In many areas, the major impact would be to replace freshwater species
with salt-tolerant marsh. However, many of the extensive cypress swamps in Louisiana, Florida, and South
Carolina, as well as sow "floating marshes," lack a suitable base for salt marshes to form. These swamps could
convert to open water if invaded by salt, which is already occurring in Louisiana (Wicker et al. 1980).
HUMAN INTERFERENCE WITH NATURE S RESPONSE TO
SEA LEVEL RISE
Although the natural impact of the projected rise in sea level is likely to reduce wetland acreages, the
ecosystems would not necessarily be completely destroyed. However, human activities such as development
and river flow management could disable many of the natural mechanisms that allow wetlands to adapt to a
rising sea, and thereby substantially increase the loss of wetlands over what would occur naturally. In some
areas the impacts could be so severe that entire ecosystems could be lost.
Development and Bulkheads
Although environmental regulations have often prevented or discouraged people from building on
wetlands, they have not prevented people from building just inland of the marsh. As the final box in Figure 1-5
shows, wetlands could be completely squeezed between an advancing sea and bulkheads erected to protect
developed areas from the sea. A few jurisdictions, such as Massachusetts, currently prohibit additional
construction of bulkheads that prevent inland advance of marshes9 However, these provisions were enacted
before there was a concern about accelerated sea level rise; it is unclear whether they would be enforced if sea
level rise accelerates. Moreover, bulkheads are already found along much of the shore and are generally
exempt from such provisions.
The amount of sea level rise necessary for development to prevent new marsh from forming would
depend on the extent to which development is set back from the wetlands. In Maryland, for example, the
Chesapeake Bay Critical Areas Act forbids most new development within 1,000 feet of the marsh; thus, if the
sea rises 50 centimeters (the highest part of the marsh) in excess of the vertical accretion, there may still be
1,000 feet of marsh. Additional rises in sea level, however, would eventually squeeze out the marsh.
18
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In the Charleston area, development is prohibited in the transition wetlands, which extend 75
centimeters (2.5 feet) above the high marsh. Thus, Kana, Baca, and Williams (Chapter 2) estimate that in the
low scenario, protecting development will not increase the loss of marsh through 2075, although it would
increase the loss of transition wetlands. For the high scenario, however, protecting development would result
in a 100 percent loss of high marsh (compared with a 71 percent loss), and would increase the loss of low
marsh slightly (from 84 to 86 percent) by 2075. As Figure 1-8 shows, a two-meter rise by 2100 could result in
a 100 percent loss of all marsh if development is protected.
Kana et al. do not explore the implications of protecting development in the New Jersey study. About
one half of the marsh in that study falls within Brigantine National Wildlife Refuge, and hence is off-limits to
development. New development in the other part of the study area must be set back 50 to 300 feet from the
marsh.10 Although the buffer zone would offer some protection, eventually the marshes here would also be
squeezed out.
The development of coastal areas may have one positive impact on the ability of marshes to adapt to a
rising sea. The development of barrier islands virtually guarantees that substantial efforts will be undertaken
to ensure that developed islands do not break up or become submerged as the sea rises. Thus, these coastal
barriers will continue to protect wetlands from the larger ocean and gulf waves for at least the next several
decades and, in some cases, much longer.11
This positive contribution may be offset to some extent by human interference with the natural
overwash process of barrier islands. Under natural conditions, storms would supply marshes on the bay sides
of barrier islands with additional sediment, to enable them to keep pace with sea level rise. On developed
barrier islands, however, public officials generally push the overwashed sand back to the Oceanside beach,
which could inhibit the ability of these barrier marshes to keep pace with sea level rise. In many instances,
however, these marshes have already been filled for building lots.
Louisiana and Other River Deltas
Although natural processes would permit a large fraction of most river deltas to keep pace with sea
level, human activities may thwart these processes. Throughout the world, people have dammed, leveed, and
channelized major rivers, curtailing the amount of sediment that reaches the deltas. Even at today's rate of sea
level rise, substantial amounts of land are converting to open water in Egypt and Mexico (Milliman and Meade
1983).
In the United States, Louisiana is losing over 100 square kilometers (about 50 square miles) per year of
wetlands (Boesch 1982). Until about one hundred years ago, the Mississippi Delta gradually expanded into the
Gulf of Mexico. Although the deltaic sediments tend to settle and subside about one centimeter per year, the
annual flooding permitted the river to overflow its banks, providing enough sediment to the wetlands to enable
them to keep pace with relative sea level rise, as well as expand farther into the Gulf of Mexico.
In the middle of the 19th century, however, the Corps of Engineers learned of a new way to reduce
dredging costs at the mouth of the Mississippi River. Two large jetties were built to confine the river flow,
preventing the sediment from settling out and creating shoals and marsh in and around the shipping lanes.
Instead, the sediment is carried out into the deep waters of the Gulf of Mexico. The "self-scouring" capability
of the channels has been gradually increased over the years. The banks of the lower part of the river are
maintained to prevent the formation of minor channels that might carry sediment and water to the marsh,
and thereby slow the current. The system works so well that dredging operations in the lower part of the
river often involve deliberately resuspending the dredged materials in the middle of the river and sowing
it to wash into the Gulf of Mexico, rather than disposing of the dredged spoils nearby. Although the
channelization of the river has enabled cost-effective improvements in navigation, it prevents
sediment, fresh water, and nutrients from reaching the wetlands near the mouth of the river.
19
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Since the 1930s, levees have been built along both sides of the river to prevent the river from
overflowing its banks during spring flooding, and several minor "distributaries" (alternative channels that lead
through the wetlands to the Gulf of Mexico) have been sealed off. Although these actions have reduced the risk
of river flooding in Louisiana, they also prevent sediment and fresh water from reaching the wetlands. As a
result, wetlands are gradually submerged, and salt water is intruding farther inland, killing some cypress
swamps and converting freshwater marsh to brackish and saline marsh. Finally, dams and locks on the upper
Mississippi, Arkansas, Missouri, and Ohio Rivers (and improved soil conservation practices) have cut in half
the amount of sediment flowing down the river, limiting the growth of wetlands in the Atchafalaya delta, the
one area that has not (yet) been completely leveed and channelized.
Canals and poor land use practices have also resulted in wetland loss (Turner, Costanza, and Scaife
1982). However, levees and channels are particularly important because they disable the mechanisms that
could enable the wetlands to repair themselves and keep pace with sea level. With almost no sediment reaching
the wetlands, an accelerated rise in sea level could destroy most of Louisiana's wetlands in the next century.
Figure 1-9 illustrates the disintegration of wetlands at the mouth of the main channel of the Mississippi
River between 1956 and 1978. Because there are no levees this far downstream, this marsh loss is attributable
to navigation projects. Figure 10 illustrates changes in Terrebonne Parish's wetlands from 1955 to 1978. Note
the extensive conversion of fresh marsh to saline and brackish marsh, as well as the conversion of cypress
swamps to open water. Figure 141 shows the generally expected shoreline for Louisiana in the year 2030 if
current management practices and sea level trends continue. Although projects to slow the rate of wetland loss
may improve this picture, accelerated sea level rise could worsen it. Figure 142 shows the loss expected if sea
level rises 55 cm by 2050.
20
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witland lou at we mourn or n» Mississippi mvhi
ACTIVE DELTA 1111 ACTIVE DELTA 1S7I
Q O#pos^
Source; National Coastal Ikotysfmm Tmm, U.S. Fish and Wildlife Strata.
21
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FIGURE 1-10
CHANGES IN TERREBONNE PARISH HABITATS: 1955-1978
22
-------
LOUISIANA SHORELINE IN THE YEAR 2030
SOURCE: COASTAL ENVIRONMENTS, INCORPORATED
0
i—r
so
I i r
miles
COASTAL ZONE WETLANDS
PREDICTED LOUISIANA COASTLINE
IN 50 YEARS AT PRESENT
LAND LOSS RATES
23
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FIGURE 1-12
PROJECTED FUTURE COASTLINE OF LOUISIANA FOR THE YEAR 2033
GIVEN A RISE IN SEA LEVEL OF S4.9 CM
I Projected Land SurUce
Mexico
• C*ol
-------
NATIONWIDE LOSS OF WETLANDS:
A FIRST APPROXIMATION
Methods
The case studies of South Carolina and New Jersey illustrate the hypothesis that a rapid rise in sea
level would drown more wetlands than it would create. Nevertheless, to demonstrate the general applicability
of this hypothesis requires more than two case studies. Although this project did not have the resources
necessary to conduct additional field surveys, we wanted to develop at least a rough estimate of the likely
nationwide loss of coastal wetlands.
Armentano et al. (Chapter 4) use topographical maps, information on tidal ranges, and a computer
model to estimate the impacts of sea level rise on 57 sites comprising 4800 square kilometers (1,200,000
acres) of wetlands, over 17 percent of all U.S. coastal wetlands. For each square kilometer they assigned a
single elevation. If the map has ten-foot contours, and most of a square is between five and fifteen feet above
sea level, they assigned the entire square an elevation of ten feet. If the map shows that a particular area is
marsh, they gave it the marsh designation and an elevation based on a linear interpolation between the
shoreline and the first contour, generally at elevation 10 feet. Their data base also considered whether a
particular area is developed or undeveloped, and whether there is an existing flood-protection wall or
bulkhead.
Although their data base was much more coarse, Armentano et al. use a more sophisticated model for
projecting the impact of sea level rise than Kana et al. The latter simply subtracted estimated vertical accretion
from relative sea level rise for the year 2075, to yield an estimate of net substrate change for the entire period.
Armentano et al. also subtract vertical accretion from relative sea level rise, but in five-year increments. Once
an area is below spring high tide, it is assumed to be marsh; once it is below mean low water, it converts from
marsh to open water. This procedure makes it possible to display results of wetland loss for particular years,
and to consider changes in marsh accretion rates during the forecast period. Armentano et al. also account for
changes in exposure to waves due to destruction of barrier islands and spits.
Because elevations are estimated crudely, one should be suspicious of individual results. Although
marsh is generally found at elevations ranging from mean sea level to spring tide, Amientano et al. assign it all
to a single elevation for a particular cell based on contours that generally describe elevation of adjacent dry
land, not the elevation of the marsh, rounded to the nearest half meter. If the change in water depth (relative
sea level rise minus accretion) is small, the model assumes no loss of marsh; whereas some marsh would
actually be lost. Conversely, for a water depth greater than the estimated elevation above man low water, all
the marsh is assumed lost; whereas the marsh between that elevation and spring high tide would actually
remain marsh. Similarly, the model may tend to underestimate marsh creation for small rises in sea level while
overestimating creation for larger rises.
The estimates by Armentano et al. were based on a number of conservative assumptions that may tend
to understate wetland loss. They assumed that the New England, Florida, and Texas marshes are not subsiding,
whereas tide gauges indicate that these areas are subsiding between one and two millimeters per year (Hicks et
al. 1983). Moreover, they assumed that sea level rise would not convert marsh until mean low water had risen
above the marsh; by contrast marsh is often not found below mean sea level, and in the case of Charleston,
Kana et al. found that it is generally at least 30 centimeters above today's mean sea level (NGVD elevation 45
centimeters). Finally, the linearity assumption tends to understate marsh loss in areas where the profile is
concave, as in Figures 1-5 and 1-6 and most coastal areas.
25
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Regional Results
Armentano et al. emphasize that their estimates should not be considered as statistically valid estimates
of wetland loss in particular U.S. coastal regions. Nevertheless, we believe that the results provide a
useful and indicative first approximation.
Table 1-5 summarizes their estimates for the low and high sea level rise scenarios. The first two
columns of the bottom half show their estimates of the wetland loss that would take place if development
prevented new marsh from forming inland. The other two columns show their estimates of the net change
in wetland acreage assuming that development does not prevent new marsh from forming except where
the shoreline already has bulkheads, levees, or other shore protection structures. These assumptions are
both extreme. Complete protection of all existing dry land would be very unlikely, as would a total
abandonment of all (currently) unprotected areas just inland of the wetlands. The extent to which
development retreats would depend both on economics and on public policies regarding the appropriate
level of wetland protection in the face of rising sea level. An investigation of these issues, however, was
outside the scope of that study.
TABLE 1-S
SAMPLE CHANGES IN COASTAL WETLANDS: 1975-2100
2100
Defend
Shore
Abandonment
REGION
1980
Low
High
Low
High
Wetland Area (square kilometers)
New England
60
58
22
58
22
Mid Atlantic
454
277
0
366
66
South Atlantic
913
652
208
954
420
Florida
598
596
357
770
517
N.E. Gulf Coast
736
672
520
685
544
Mississippi Delta*
1509
298
45
298
45
Chenier Plain, Tex
299
190
0
258
49
Californian Prov.
265
174
0
263
218
Columbian Prov.
12
11
9
127
133
TOTAL
4846
2928
1161
3779
2014
Percent Loss (gain)
New England
-3
-63
-3
-63
Mid Atlantic
-39
-100
-20
-85
South Atlantic
-29
-77
+4
-54
Florida
-.3
-40
+29
-14
N.E. Gulf Coast
-9
-29
-7
-26
Mississippi Delta*
-80
-97
-80
-97
Chenier Plain, Tex
-36
-100
-14
-84
Californian Prov.
-35
-100
-1
-18
Columbian Prov.
-8
-25
+958
+1000
TOTAL
-40
-76
-22
-58
* These estimates do not consider the potential wetland creation that could result from
possible diversion of the Mississippi River.
Source: 1980 data from Appendix 4-A; 2100 data from Table 4-8 of Armentano et aL (Chapter 4).
—a®
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Armentano et al. estimate that the low scenario would have relatively little impact on New England's
marshes, largely due to their ability to keep pace through peat formation. Nevertheless, peat formation
would not be likely to keep pace with the more rapid rate of sea level rise implied by the high scenario,
which could result in two-thirds of these marshes being lost. Similar situations could be expected in
Florida and the Northeast Gulf Coast although a flatter coastal plain in these regions would offer a greater
potential for wetland creation if development did not stand in the way. The assumption by Armentano et
al. that Florida wetlands could accrete one centimeter per year may be unduly optimistic.
The middle and southern Atlantic coastal marshes would be more vulnerable than New England to the
low sea level rise scenario, largely because smaller tidal ranges there imply that existing wetlands are
found at lower elevations than the New England wetlands, while vertical accretion was generally assumed
to he less than in the case of Florida and the Northeast Gulf Coast. These estimates appear to imply less
wetland loss than the case studies by Kana et al. In the high scenario, however, estimates by Armentano et
al. are considerably higher and more closely consistent with Kana et al., as w discuss below.
To understand the implications of Armentano et al., it is useful to compare their procedures and results
-with those of Kana et al., where there is site-specific information. In the case of Charleston, Armentano
et al. estimate that the low scenario (net substrate change, III centimeters) implies a 37 percent loss and a
21 percent gain through 2100, for a net loss of 16 percent. The transacts of Kana et al. imply that the low
scenario would result in a 100 percent loss of existing marsh with an 18 percent gain, for a net loss of 82
percent Had the Armentano et al. approach been applied to the Charleston case study, it would have
attributed an initial elevation of 1.0 meters to the marsh,12 which is not unreasonable given that it ranges
from 0.5 to 1.3 meters-although 80 percent of the marsh is below 1.0 meters. However, their procedure
would require the net substrate change to be one meter plus one-half the tidal range, for a total rise of 1.8
meters, before the marsh would convert to water. Thus, the model of Armentano et al. estimates
Charleston's wetlands to be much less vulnerable than the field surveys by Kana et al. suggest.13
In the case of the New Jersey wetlands, the groups arrived at similar results. Armentano et al. estimate
a 75 percent wetland loss through 2075 in the high scenario and no loss in the low scenario, while Kana et
al. estimate an 86 percent loss in the high scenario and a 6 percent gain in the low The tendency of
Armentano et al. to assign a fairly high elevation to the marsh is more appropriate in areas where high
marsh dominates. Moreover, five-foot contours were available in this case. Table 1-6 summarizes the
Armentano et al. and Kana et al. finding.
TABLE l-»
COMPARISON OF ARMENTANO ET AL. AND KANA ET AL. STUDY RESULTS
SHOWS THAT USE OF TOPOGRAPHIC MAPS CAN UNDERESTIMATE
VULNERABILITY OF WETLANDS TO SEA LEVEL RISE (percent loss of wetlands)
Low Scenario
High Scenario
Defend
Abandonment Shore Abandonment
Defend
Shore
Charleston, South Carolina (2100)
Armentano et al.
Kana et al.1
16
62
37
100
28
84
55
100
Tuckerton, New Jersey (2075)
Armentano et al.
Kana et al.
0
4
75
82
There results are derived from the profiles estimated by Kana et al.
27
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The Mississippi Delta and Texas Chenier Plain wetlands appear to be the most vulnerable. As Table 1-
5 shows, 36 percent of the latter would be lost in the low scenario, and all could be lost in the high scenario.
Abandonment would increase the portion of wetlands surviving the next century by about 15 percent of today's
acreage. Armentano et al. estimate that 80 and 97 percent of Louisiana's wetlands would be lost for the low
and high scenarios, respectively. However, me caution the reader that their model did not consider the
potential positive impacts of a diversion of the Mississippi River, which could enable a fraction of the wetlands
to survive a more rapidly rising sea level.
Although the Pacific Coast wetlands examined appear to be as vulnerable to sea level rise as Atlantic
and Gulf coast wetlands, Armentano et al. found that the former have greater potential for wetland creation
with sea level rise. In the Californian study areas, 35 to 100 percent of the existing wetlands could be lost;
however, the net loss would be I to 18 percent if developed areas were abandoned.
The Pacific Northwest study site could experience a tenfold increase in metland area for either scenario,
if uplands are abandoned. However, me suggest that the reader not attribute undue significance to the
Columbia River results. This study site accounted for less than 5 per- cent of the Pacific Coast marshes
considered. The result is a useful reminder of the fact that some areas could gain substantial amounts of
wetland acreage. We do not recommend, however, that any of the regional results be taken too seriously until
they can be verified by additional study sites and a more detailed examination of wetland and upland transacts,
such as those in Chapters 2 and 3.
Nationwide Estimate
The results of Armentano et al. can be used to derive a rough estimate of the potential nationwide loss
of coastal wetlands. However, the reader should note that Armentano et al. did not use a completely random
method for picking study areas, and that their elevation estimates mere rounded to the nearest quarter meter.
Thus, they warn the reader that estimates based on their projections are not statistically valid.
Armentano et al. sought to include study sites for all major sections of coast. However, they did not
attempt to ensure that the wetland acreage of the sites in a particular region are directly proportional to the total
acreage of wetlands in that region. Therefore, to derive a nationwide estimate of the loss of wetlands one
should meight estimates of "percentage loss by region" by actual wetland acreages in the various regions.
A recent study by the National Ocean Service estimates coastal wetland acreage by state (Alexander,
Broutman, and Field 1986). We modified those estimates to exclude swamp acreage in regions where
Armentano et al. did not investigate swamps. The term "coastal wetland" in this report refers to tidal wetlands
and non-tidal wetlands that are hydraulically connected to the sea, such as cypress swamps in Louisiana. The
NOS study includes all swamps in coastal counties, some of which are well inland and not hydraulically
connected to the sea, particularly in North Carolina and New Jersey.
The first column of Table 1-7 shows the adjusted estimates of wetlands acre4e by region. Because the
Pacific Coast wetlands represent such a small fraction of the total, me have combined the California and
Pacific Northwest regions. The rest of the table shows the implied wetland losses and gains estimated using
the percentages reported by Armentano et al. The greatest losses would appear to be in Louisiana and the
southern and middle Atlantic coast. However, we caution the reader that the region-specific estimates have
less credibility than the nationwide estimate.
Of the estimated 6.9 million acres of coastal wetlands, 3.3 million could be lost under the low scenario.
If human activities do not interfere, however, 1.1 million acres might be created. Under the high scenario, 5.7
million acres (81 percent) would be lost, while 1.9 million acres could potentially be created.
28
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These estimates of the nationwide loss of wetlands are based on dozens of assumptions.
Nevertheless, they seem to support the simple hypothesis that the area of wetlands today is
greater than what would be at the proper elevation for supporting wetlands if sea level rose a
meter or two. Thus, if rates of vertical accretion remain constant, a rise of this magnitude in the
next century would destroy most U.S. coastal wetlands.
TABLE 1-7
PROJECTED U.S. COASTAL WETLAND LOSS AND POTENTIAL GAIN
(thousands of acres)
2100
1985
Low
Hish
Lost
Gained
Lost
Gainei
Northeast
(ME, NH, MASS, RI)
120.9
4.0
0
7.7
0
Mid-Atlantic
(CN,NY,NJ,DE,MD,VA)
733.3
285.9
193.7
733.3
108.2
South Atlantic
(NC.SC.GA)
1376.6
393.5
455.3
1062.9
319.6
Florida
736.3
2.5
214.2
296.7
197.0.
AL,MS
401.4
34.9
70.9
117.8
13.1
Louisiana*
2874.6
2306.9
0
2781.2
0
Texas
609.4
222.1
138.6
642.0
132.4
Pacific Coast
89.1
29.6
65.9
31.5
54.0
TOTAL
6941.6
3279.4
1138.6
5673.1
824.3
Percent
-
47.2
16.4
81.7
11.9
'These estimates do not consider the potential wetland creation that could result from possible
diversions of the Mississippi River planned and authorized by the State of Louisiana.
Source: 1985inventory from Alexander, Broutman, and Field1986. Nationwide losses calculated
by applying percentages from Table 1-5 to 1985 inventory. "Lost" refers to wetlands inundated.
Gained" refers to potential increases in wetland acreage if upland areas are not developed or if
development is removed.
PREVENTING FUTURE WETLAND LOSSES
Future losses of wetlands from sea level rise could be reduced by (1) slowing the rate of
sea level rise, (2) enhancing wetlands' ability to keep pace with sea level rise, (3) decreasing
human interference with the natural processes by which wetlands adapt to sea level rise, or (4)
holding back the sea while maintaining the marshes artificiafly.14
Society could curtail the projected future acceleration of sea level rise by limiting the
projected increases in concentrations of greenhouse gases. Seidel and Keyes (1983) projected
29
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that reducing C02 emissions with bans on coal, shale oil, and synfuels (but not oil and gas) would
delay a projected two degree (C) warming from 2040 to 2065; because of the thermal delay of the
oceans, the resulting thermal expansion of ocean water would be delayed ten to fifteen years.1 Other
trace gases might also be controlled. Hoffman et al. (1986) showed that the acceleration of sea level
rise could be significantly delayed through controls of greenhouse gas emissions.
Although limiting the rise in sea level from the greenhouse effect might be the preferred
solution for most parties involved in the wetland protection process, it would also be largely outside of
their control. The nations of the world would have to agree to replace many industrial activities with
processes that do not release greenhouse gases, perhaps at great cost. A decision to limit the warming
would have to weigh these costs against many other possible impacts of the greenhouse warming
which are understood far less than wetland loss from a rise in sea level, including the economic
impacts of sea level rise; environmental consequences for interior areas, such as an increase in
desertification; and possible disruptions of the world's food supply. Perhaps the most important
challenge related to this option is that it would have to be implemented at least fifty years before the
consequences it attempts to avert would have taken place.
Because me may have passed the time when it would be feasible to completely prevent an
accelerated rise in sea level, wetland protection officials may also want to consider measures that
would enable wetlands to adapt to rising sea level. Enhancing the ability of wetlands to keep pace
with sea level rise has the advantage that such measures, which include marsh building, enhanced
sedimentation, and enhanced peat formation, would not have to be implemented until sea level rise
has accelerated.
Current environmental policies often require marsh building to mitigate destruction of
wetlands. Although this measure will continue to be appropriate in many instances, it can cost tens of
thousands of dollars per acre, which would imply tens of billions of dollars through 2100 if applied
universally. Enhanced sedimentation may be more cost-effective; it is generally cheaper to save an
acre of marsh than to create an acre of new marsh. Technologies that promote vertical growth of
marshes generally spray sediment in a manner that imitates natural flooding (Deal 1984). Although
these technologies look promising, they are barely past the development stage and may also prove too
costly to apply everywhere. Although processes for enhancing peat formation might prove feasible,
reduced peat formation might also result from climate change.
Allowing wetlands to adapt naturally to sea level rise would not prevent a large reduction in
acreage, but might allow the ecosystems themselves to survive. This option would consist primarily
of removing human impediments to sedimentation and the landward migration of met- lands. The
sediment washing down the Mississippi River, for example, would be sufficient to sustain a large part
of Louisiana's wetlands, if human activities do not continue to force sediment into the deep waters of
the Gulf of Mexico. However, the costs of restoring the delta would be immediate, while the benefits
would accrue over many decades. Similarly, measures could be taken to ensure that the wetlands in
tidal deltas adjacent to barrier island inlets are not deprived of sediment by groins and jetties built to
keep sand on the islands and out of the inlet.
For the extensive mainland marshes not part of a tidal delta, a natural adaptation would require
the wetlands to migrate landward and up the coastal plain. Such a policy would also be costly. It
would be necessary to either prevent development of areas just upland of existing wetlands, or to
remove structures at a later date if and when the sea rises. Preventing the development of the upland
areas would require either purchasing all the undeveloped land adjacent to coastal marshes or
instituting regulations that curtailed the right to build on this property. The former option would be
costly to taxpayers, while the latter option would be costly to property owners and would face legal
challenges that might result in requirements for compensation.
Developing upland areas and later removing structures as the sea rises would allow costs to be
deferred until better information about sea level rise could be obtained. This option could be
30
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implemented either through an unplanned retreat or a planned retreat. Howard, Pilkey, and Kaufman (1985)
discuss several measures for implementing a planned retreat along the open coast. Although North Carolina
and other coastal areas have required houses to be moved inland in response to erosion along the open coast-
where shore protection is expensive-it may be more difficult to convince people that the need for wetland
protection also justifies removal of structures.
There is also a class of institutional measures that increases the flexibility of future generations to
implement a retreat if it becomes necessary, without imposing high costs today. For example, permits for new
construction can specify that the property reverts to nature one hundred years hence if sea level rises so many
feet. Such a requirement can ensure the continued survival of coastal wetlands, yet is less likely to be opposed
by developers than policies that prohibit construction. Moreover, with the government's response to sea level
rise decided, real estate markets can incorporate new information on sea level rise into property values. The
State of Maine (1987) has adopted this approach, specifying that houses are presumed to be moveable. In the
case of hotels and condominiums, the owner must demonstrate that the building would not interfere with
natural shorelines in the event of a rise in sea level of up to three feet, or that he or she has a plan for removing
the structure if and when such a rise occurs.
Finally, it might be possible to hold back the sea and maintain wetlands artificially. For small amounts
of sea level rise, tidal gates might be installed that open during low tide but close during high tide, thereby
preventing saltwater intrusion and lowering average water levels. For a larger rise, levees and pumping
systems could be installed to keep wetland water levels below sea level. Although these measures would be
expensive, they would also help to protect developed areas from the sea. Terrebonne Parish, Louisiana, is
actively considering a tidal protection system and a levee and pumping system to prevent the entire jurisdiction
from converting to open water in the next century (Edmonson and Jones 1985). They note, however, that
effective measures to enable shrimp and other seafood species to migrate between the protected marshes and
the sea have not yet been demonstrated.
Measures to ensure the continued survival of wetland ecosystems as sea level rises need to be
thoroughly assessed. We may be overlooking opportunities where the cost of implementing solutions in the
near term would be a small fraction of the costs that would be required later. Only if these measures are
identified and investigated will it be possible to formulate strategies in a timely manner.
CONCLUSIONS
An increasing body of evidence indicates that increasing concentrations of greenhouse gases could
cause sea level to rise one or two meters by the year 2100. If current development and river management
practices continue, such a rise would destroy the majority of U.S. coastal wetlands. Yet these losses could be
substantially reduced by timely anticipatory measures, including land use planning, river diversion, and
research on artificially enhancing coastal wetlands, as well as by a reduction in emissions of greenhouse gases.
Case studies of South Carolina and New Jersey marshes indicate that a two-meter rise would destroy 80
to 90 percent of the coastal marshes, depending on development practices, while a one-meter rise would
destroy 50 percent or less. The large body of research previously conducted in Louisiana suggests that its
marshes and swamps would be far more vulnerable. Yet anticipatory measures, if implemented soon, could
save a large fraction of these wetlands.
For the rest of the nation, no site-specific research has been undertaken. Most of these wetlands are
also within one or two meters of sea level. Preliminary analysis by Armentano et al.
31
-------
suggests that coastal wetlands throughout the nation would be vulnerable to such a rise, with
the possible exception of areas with large tidal ranges or substantial terraces two or three
meters above sea level.
Basic and applied research on the ability of wetlands to adjust to rising sea level would be
valuable. Because sea level rose one meter per century on average from 15,000 B.C. until 5,000
B.C., it may be possible to better assess the response of wetlands to such a rise in the future.
Research on how to artificially promote vertical accretion or control water levels is also
important. Such research could benefit coastal states throughout the nation in the long run,
although the short-run benefits of protecting Louisiana's wetlands—40 percent of the
total—suggests that such research should be initiated soon.
When is the appropriate time to respond to the potential loss of wetlands to a rising sea?
If technical solutions are possible, it might be sufficient to wait until sea level rise accelerates.
Where planning measures are appropriate, a thirty- to fifty-year lead time might be sufficient.
Where policies are implemented that will determine the subsequent vulnerability of wetlands to
sea level rise, it would be appropriate to consider sea level rise when those decisions are made.
If society intends to avert a large rise in sea level, a lead time of fifty to one hundred years may
be necessary.
Wetland protection policies and related institutions such as land ownership are currently
based on the assumption that sea level is stable. Should they be modified to consider sea level
rise today, after the rise is statistically confirmed, or not at all? This question will not only
require technical assessments, but policy decisions regarding the value of protecting wetlands,
our willingness to modify activities that destroy them, and the importance of preparing for a
future that few of us will live to see.
NOTES
1 Several reviewers suggested that these figures may overstate the decline in me6and loss
because they exclude conversion for agriculture and other nonregulated wetland destruction.
2 U.S. Fish and Wildlife Service, Charleston, South Carolina Office, personal communication,
March 1986.
3 This curve shows the concentration for Mauna Loa, Hawaii, which is sufficiently remote to
represent the average northern hemispheric concentration. Measurements at the South Pole
suggest that the concentration for the southern hemisphere lags at most a couple of years,
since most of the sources are in the northern hemisphere.
4 Studies on the greenhouse effect generally discuss the impacts of a carbon dioxide doubling:
By "effective doubling of all greenhouse gases" we refer to any combination of increases in
the concentration of the various gases that causes a warming equal to the warming caused by a
doubling of carbon dioxide alone over 1900 levels. If the other gases contribute as much
warming as carbon dioxide, the effective doubling would occur when carbon dioxide
concentrations have reached 450 ppm, 1.5 times the year-1900 level.
5 These estimates did not consider meltwater from Antarctica or ice discharge from Greenland.
6 Low marsh is found below mean high tide, which is defined as one-half the tidal range above
sea level; high marsh extends up to the spring high tide, generally less than three quarters of a
tidal range above sea level; and transition wetlands are somewhat higher.
7 Personal communication. U.S. Fish and Wildlife Service, Charleston Office. The estimates
exclude -forested wetlands and freshwater marshes, which are cleared_for agriculture and
silviculture.
32
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8 A few reviewers noted that this hypothesis remains to be demonstrated. If insufficient flooding limits vertical
accretion, a more rapid sea level rise would accelerate wetland accretion. However, there is little doubt that
wetlands in Louisiana cannot keep pace with a rise of I cm/year in the absence of substantial sediment nourishment.
9 For Massachusetts, see M.G.L. Ch. 13, S. 40 Reg. 310 C.M.R. 9.10 (2) of Massachusetts General Laws.
10 As specified by the New Jersey Administrative Code, Wetland Buffer Policy, 7:7E-3.26.
11A few reviewers pointed out that coastal protection structures such as snowfences and seawalls can increase the
probability of an eventual breakup. However, the longer-ten-n strategy of raising the beach profile and island with
fill does not share that liability.
12The marsh would range from 0 to 2,500 feet from shore, while the ten-foot contour would be 3,500 feet from shore;
the midpoint of the marsh would be about 1,200 feet from shore. A linear interpolation implies that this point has a
one-meter elevation.
13The Armentano et al. model has additional complexities, but the factors described here are most important in
explaining the discrepancy with the Kana et al. results.
14 This report does not address the issue of whether wetlands should be maintained. It is possible that in some cases
open water areas replacing wetlands would support sea grasses that provide ecological benefits as great as the
benefits of the wetlands they replace.
15 Computer printout of results from Seidel and Keyes 1983
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