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Overview and
Recommendations
EXECUTIVE SUMMARY
i:
This study addresses current scientific understanding of sea-level change-particularly
the processes of sea-level change, their rates, and the record of past change. An important
part of such an assessment is an evaluation of the adequacy of the geophysical knowledge
base and the opportunities to improve upon it. Discussion of engineering and societal
responses to sea-level change is not included in this study as these issues are fully
discussed in a report, Responding to Changes in Sea Level: Engineering Implications
(NRC, 1987).
Average sea level over the oceans has never been constant throughout earth history, and
it is changing slightly today. The entirety of civilization has occurred within a single high
stand of the sea, and yet global sea level was more than 100 m lower than it is at present
only 18,000 yr ago. And during the geologic past there have been repeated variations of
more than 100 m from present sea level, both during times of intense glaciation and during
times of an ice-free Earth.
Relative sea level (RSL; i.e., sea level relative to a fixed point on land) at any particular
place in the ocean varies over a wide range of time and space scales. The direct causes of
these variations are vertical motions of the land to which the tide gauge or other measuring
instrument is attached, and changes in the volume of sea water in which the tide gauge is
immersed. But changes in climate, plate tectonics, ice and snow, and ocean circulation are
all indirect causes of changing sea level. The relative importance of the forcing functions
varies with the time scales of interest.
On the basis of estimates of global warming of the atmosphere and ocean resulting from
increasing concentrations of carbon dioxide and other greenhouse gases, it is possible to
make an approximate forecast of global rise in sea level during the next 100 yr. Two
processes will be principally involved: thermal expansion of ocean waters as they become
warmer and changes in the mass of land ice in both continental ice sheets and mountain
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4
OVERVIEW AND RECOMMENDATIONS
glaciers. One hundred years from now it is likely that sea level will be 0.5 to 1 m higher
than it is at present.
There are two principal uncertainties at present about global sea level. (1) What if any
is the value of the long-term trend over the next few centuries? (2) If a secular trend exists,
what proportion of this trend results from changes in the specific volume of sea water
(called steric changes) and what from changes in the total mass of water in the oceans?
The apparent trend of sea level at any particular place as measured by a tide gauge is the
sum of the trend in motion of the gauge itself as the land on which it is mounted moves
vertically, the trend of change in steric sea level, and the trend of change in water mass
under the tide gauge. To understand what is happening, one needs to be able to make
measurements that will separate these three components of the observed sea level. In
principle, a combination of inverted echo sounders (which in effect measure thermal
expansion or contraction of the water column) or systematic observations of ocean tem-
perature as a function of depth with conductivity-temperature-depth recorders to determine
the steric component, plus one or more of three methods (very-long-baseline interferom-
etry, global positioning system, and absolute gravimetry) for measuring the vertical mo-
tions of the tide gauge, plus tide-gauge measurements at the sea surface should allow us to
separate the three major components of changes in RSL. Within the next few years, it
should be possible to measure accurately the combined effects on global sea level of steric
changes and changes in the mass of sea water from laser or radar altimeters mounted on
Earth-orbiting satellites.
Recommendation 1. Long-term sea-level measurements of sufficient accuracy over the
world's oceans could provide one of the most significant data sets for understanding global
change, particularly climatic change resulting from greenhouse warming. It is for this
reason that the planning committees for the World Climate Research Program and the
Intergovernmental Oceanographic Commission of UNESCO have given a very high prior-
ity to extending the global sea-level network in the Indian, South Atlantic, and South
Pacific oceans. We strongly recommend that national oceanographic and meteoro-
logical communities-lend moral and intellectual support to this sea-level program.
Recommendation 2. Possible changes in the mass balance of the Antarctic and Greenland
ice sheets are fundamental gaps in our understanding and are crucial to the quantification
and refinement of sea-level forecasts (the probable contribution from ice wastage makes up
more than half of various forecasts). A polar-orbiting satellite altimeter would be
invaluable in monitoring the mass balance of these ice sheets.
Recommendation 3. To refine estimates of sea-level change related to greenhouse
warming, it is necessary to develop and improve coupled atmosphere-ocean-cyrosphere
global circulation models in which greenhouse gas concentrations in the atmosphere
are gradually increased.
Recommendation 4. The Cretaceous period offers special opportunities to understand
global processes and their variations, in particular, large, long-term changes in sea level.
One of the major projects of the Global Sedimentary Geology Program, under the auspices
of the International Union of Geological Sciences, is entitled "Cretaceous Resources,
Events, and Rhythms." We urge national and international support of this and similar
programs that will improve our understanding of past sea-level changes and the
processes that produced them.
Recommendation 5. To separate epeirogeny from eustasy and steric components, it is
important to measure repeatedly the absolute heights of tide gauges. We recommend that
global measurements of absolute heights of these gauges be undertaken using abso-
lute gravimetry and space-based techniques.
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OVERVIEW AND RECOMMENDATIONS
s
INTRODUCTION
It is easy to think of sea level as a stable baseline against which changes on land can be
measured, and it is so used by politicians, engineers, land planners, and households. But
as we have become more aware of the Earth and its metabolism, we have recognized that
sea level is highly unstable both in time and in space. We can regain our former confidence
in its usefulness only by learning how and why it varies and compensating for these vari-
ations.
, 1
-
Average sea level over the globe has never been constant throughout earth history: and
it is changing slightly today. The entirety of civilization has occurred within a single high
stand of the sea, and yet global sea level was more than 100 m lower than it is at present
during the maximum of the last glacial period only 18,000 yr ago (yrBP).
Relative sea level (RSL; i.e., sea level relative to a fixed point on land) at any particular
place in the ocean varies over a wide range of time and space scales. Among the causes
of these variations are vertical motions of the land to which the tide gauge or other
measuring instrument is attached and changes in the volume of sea water in which the tide
gauge is immersed. Wind-driven waves produce the shortest-period variations in the
height of the sea surface. Variations with periods of 12 hours or more are caused by lunar
and solar tides. Variations in atmospheric pressure cause inverse variations in sea level-
an atmospheric pressure differential of 1 mbar is equivalent to a sea-level differential of 10
mm. A series of depressions in atmospheric pressure can cause a rise in sea level in a
shallow ocean basin of 0.3 m or more (Hekstra, 1988~. Variations in the runoff of large
rivers can result in local sea-level changes of as much as 1 m. In relatively shallow water,
large variations in sea level are also caused by offshore and onshore winds that pile up
water against the shore or drive it away from the shore. (This process is called wind setup.)
In exceptional circumstances, in the North Sea, along the Chinese coast, and in the Bay of
Bengal, sea level may rise by 5 m or more in a "storm surge" under the action of strong
winds (Hekstra, 19881. Both irregular and seasonal variations in temperature or salinity of
the upper ocean layers cause expansion or contraction of the water volume in different
regions. These relatively short-term steric changes in sea level may persist for a few days,
several months, or even several years, and the magnitude may be as much as 50 to 150 mm.
Changes in sea level have many practical consequences, often disruptive but sometimes
beneficial. The disruptive consequences can sometimes be avoided and the benefits
enhanced if the ways and means by which sea level varies are understood. Although
disruptive consequences, especially from sea-level rise, are far more common, occasional
examples of benefits are being obtained as a result of changes in policy. For example, in
the Wadden Sea on the northwest coast of the Netherlands, where the land has subsided as
much as 260 mm during the past 20 yr, the Dutch government has decided not to follow
age-old tradition by attempting to reclaim more land for agriculture. The area has been
designated as a "Declared UNESCO Biosphere Reserve" in which only those economic ac-
tivities are allowed that do not conflict with natural conditions or processes. In view of
agricultural surpluses in the Netherlands, and indeed throughout the European community,
the Wadden Sea can be used much more effectively as a nursery for young fish and shrimp
and other valuable invertebrates (Hekstra, 1988) than as reclaimed land for agriculture.
Chances of sea level on the order of 300 mm can have significant implications for
J '
coastal communities and coastal engineering practices. engineering responses lo sea-level
change are largely a function of the rate of change. Many of these issues and engineering
responses are described in the report Responding to Changes in Sea Level: Engineering
Implications prepared by the Marine Board of the National Research Council (NRC, 19871.
A 1-m change in average sea level can translate into major shifts in shoreline positions,
positions that have both economic and legal significance.
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6
O VER VIEW AND RECOMMENDA TI ONS
Sea-level change, seemingly so simple and straightforward, is in fact the product of
many interrelated processes. Insight into these processes can be gained by intensive study
of sea-level change in the context of related environmental phenomena, remembering that
changes in sea level are an integrated measure of environmental change, in terms of both
causes and consequences. Changes occur on all space and time scales, from local to global,
and from a few seconds to geologic ages.
This volume is primarily concerned with future sea-level changes over the next few
centuries and past changes over a wide range of times from which greater understanding
can be gained and used as an aid in the prediction of future changes and in the search for
fossil fuels and other natural resources. Also covered are the mechanisms and processes
involved in past changes, in order to gain greater knowledge of the Earth as a dynamic
system, i.e., how the Earth works.
_ ~ ~^ ~_ V _. ~ ~ ~ ~ ~ ~ · ~ ~
Climate, plate tectonics, the cryosphere, and ocean circulation all contribute to changing
sea level. The relative importance of the forcing functions varies with the time scales of
interest. The effects of changing sea level are also broad on the one hand with direct
feedbacks to the causative forcing functions, e.g., albedo change, and on the other hand
with effects on other processes such as sedimentation or coastal ecology.
PROCESSES AND FEEDBACKS
Many processes can cause a change in RSL at any particular location. They include the
following:
1. local or regional uplift or subsidence of the land;
2. changes in atmospheric pressure, winds, or ocean currents;
3. changes in the mass of ocean water brought about by wastage or accumulation of ice
sheets and mountain glaciers (glacio-eustatic) or by increased or decreased retention of
liquid water within or upon the continents, and possibly also by a slow release through
geologic time of juvenile water from the Earth's interior;
4. steric changes in the volume of ocean water without changes in water mass (Patullo
et al., 1955) in response to temperature or salinity changes (also called thermohaline
changes in Table 11; and
5. changes in the volume of the ocean basins owing to changes in the rate of plate
divergence (seafloor spreading), plate convergence (subduction, overthrusting), epeiro-
genic changes in the elevation of the seafloor (largely from mid-ocean volcanism), marine
sedimentation, or isostatic adjustment of the Earth's crust under the sea resulting from
glaciation or deglaciation on land.
The latter three processes can affect global mean sea level or eustatic sea level. But all
processes need to be considered, even though, depending on the time scale of interest or the
magnitude of the sea-level change, some of them may be insignificant. In a following
section on forecasting sea-level change due to greenhouse-induced climate warming, a
projected global sea-level rise of 0.5 + 1 m by the year A.D. 2100 is ascribed to a
combination of thermal expansion of ocean water and melting of glaciers and ice sheets.
Time Scales of Sea-Level Change
Sea-level change encompasses a broad range of time scales, with different mechanisms
associated with change over different times. The oceanographer concerned with storm
tides will not have much interest in the factors explaining Cretaceous sea levels; likewise,
the geologist's glacio-eustatic theories have little application to seasonal events. The
problem of sorting out time scales and processes afflicts studies of climate change in
general. Complexities multiply in attempts to link different processes together.
Table 1 summarizes mechanisms of sea-level change by time scale and magnitude.
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OVERVIEW AND RECOMMENDATIONS
7
TABLE 1 Some Mechanisms of Sea-Level Change
Time Scale
(years)
Order of
Magnitude of
Change (mm)
Ocean Steric (thermohaline) Volume Changes
Shallow (0 to S00 m) 10-~ to 102 10° to 103
Deep (500 to 4000 m) 10' to 104 10° to 104
Glacial Accretion and Wastage
Mountain Glaciers 10t to 102 10' to 103
Greenland Ice Sheet 102 to 105 10~ to 104
East Antarctic Ice Sheet 103 to l 05 104 to l 05
West Antarctic Ice Sheet 102 to 104 103 to 104
Liquid Water on Land
Groundwater Aquifers 102 to 105 1'02 to 104
Lakes and Reservoirs 102 to 105 10° to 102
Crustal Deformation
Lithosphere Formation and Subduction 105 to 108 103 to 105
Glacial Isostatic Rebound 102 to 104 102 to 104
Continental Collision 105 to 108 104 to 105
Sea Floor and Continental Epeirogeny 105 to 108 104 to 105
Sedimentation 104 to 108 103 to 105
Heat exchange is relatively rapid within the uppermost few hundred meters of the oceans.
A one-dimensional treatment implies that thermal expansion of these waters can occur on
time scales of months to decades. Sea level will rise about 100 mm for every degree of
temperature increase throughout the uppermost 500 m. Heat exchange with ocean deep
waters is slower (Chapter 131. If the deep ocean were to warm everywhere by 10°C, as was
perhaps the case during the early Tertiary and Cretaceous, sea level could rise by about 10
m.
The time scales and magnitudes of melting ice can be estimated from both historical
data and mass balance considerations. The present Greenland and Antarctic ice caps are
remnants of the late Pleistocene ice sheets that increased sea levels about 100 m by
disintegrating over a period of several thousand years encompassing the end of the Pleis-
tocene (Chapters 4 and 5~. Mass balance estimates suggest modern ice residence times on
the order of 102 to 1os yr. The Antarctic Ice Sheet contains enough water to raise sea level
by about 60 m, and the Greenland Ice Sheet contains water equivalent to a 6-m sea-level
rise.
Sea Level and the Geoid
The sea surface departs significantly from the geoid, owing to waves, the tidal attraction
of the Sun and Moon, ocean circulation that tilts the ocean surface, atmospheric distur-
bances, and steric regional variations in water temperature and salinity. If these effects can
be taken into account, the resulting mean sea level accurately follows the geoid, which
however is far from being an ideal spheroidal shape. Deep mantle phenomena and gravity
anomalies associated with subduction can cause deviations from the spheroid of many tens
of meters. Local crustal geological features, such as seamounts, fracture zones, and other
abrupt topographic forms that are not in isostatic equilibrium, can cause deviations of
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8
OVERVIEW AND RECOMMENDATIONS
several meters. In fact, it is possible to derive a partial picture of ocean floor topography
from a knowledge of the mean shape of the sea surface.
If the effects of vertical tectonic movements, in addition to the local or regional
variations of sea-surface topography, are removed from tide-~au~e records nresumahlv
=_ _ =- ~ 7 or-- -a
any remaining trend is global (eustatic sea-level change). Eustatic changes can result from
processes that change the mass or volume of water in the ocean basin or those that change
the volume of the ocean basin itself. These processes are discussed in later sections.
Land Elevation Changes
Tide gauges are anchored to the land, which itself can be moving vertically at rates
comparable to sea-level change. These vertical tectonic movements of the land will result
in an RSL change as measured by a tide gauge. If the land where the tide-gauge station is
located is subsiding, RSL will show a rise; likewise, uplift will result in an RSL fall. The
Viking city now called Old Uppsala was a major port for ships sailing Lake Malaren;
however, the port and the city of Uppsala were forced to move downstream around the
eleventh century because of a 10-mm/yr uplift in the Fennoscandian region while the lake
remained connected with the worldwide mean sea level.
Vertical tectonic motions can be very localized, such as subsidence along a coastline
owing to sediment load or from the withdrawal of groundwater or hydrocarbons. Subsi-
dence is occurring along the Louisiana gulf coast because of the deposition and dewatering
of sediment from the Mississippi River system. On the other hand, vertical motions can be
systematically related to one another through isostasy.
Land areas that were ice covered during the last glacial episode (~18,000 yrBP) were
depressed; this depression created a forebulge in adjacent areas (see Pettier, Chapter 4~.
The North American or Laurentide ice sheets began disintegrating about 15,000 yrBP and
by about 7000 yrBP had all but vanished. Along the east coast of North America, those
areas that were ice covered are still being uplifted due to isostatic rebound at rates of up to
about 10 mm/yr. In the peripheral area where the forebulge is collapsing, the land is
subsiding at more than 1 mm/yr. The east coast of the United States is perhaps the best
location illustrating this "drowning" effect, which is responsible for many of the unique
features of its nearshore environment, including the extensive occurrence of salt marshes.
In these areas and with current models of isostasy and the Earth, the relative vertical
motions are predictable; thus, their relative contribution to sea-level changes as measured
by tide gauges can be taken into account in arriving at eustatic sea-level changes.
Effects of Atmospheric Pressure, Winds, and Ocean Currents
Local RSL variability can result from several forcing functions including air pressure,
wind stress, ocean circulation, and thermohaline changes. These processes contribute to
the high-frequency noise in tide-gauge data and need to be compensated for in extracting
eustatic changes.
The difference between the average air pressure over the world ocean and the local
barometric pressure can result in a change of sea level at annual and shorter periods. One
millibar of pressure differential is equivalent to 10 mm of sea-surface change.
Wind stress can have an important effect on the sea level. The coastal sea-level
response to a steady longshore wind stress can be similar in magnitude to the air-pressure
effect. The time to achieve a steady state is of the order of a few days. Sea-surface changes
caused by wind stress are localized and are not a major contributor to low-frequency sea-
level change.
Ocean circulation can also result in sea-level variability from long-period waves, as
Sturges (Chapter 3) shows. Sea-level signals of about 50 to 150 mm with periods of 5 to
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OVERVIEW AND RECOMMENDATIONS
9
10 yr and longer are coherent between the U.S. Pacific coast and Hawaii, and on both sides
of the Atlantic. These signals are out of phase, with delays of several years, and are
consistent in part with baroclinic wave propagation across the ocean.
Also coherent are sea-surface variabilities resulting from El Nino and related effects
(Figure 1~. From these, sea level can vary on the time scale of a few years by 50 to 150
mm resulting from warming of the ocean subsurface waters by a few degrees Centigrade
(TIC).
Changes in the Mass of Ocean Water
For all intents and purposes, the mass of water at or near the Earth's surface is constant
over time scales of less than 104 yr. It is how the water is partitioned between the major
hydrologic reservoirs that is of importance to sea-level change. The four major reservoirs,
in order of abundance, are the oceans (1370 x 106 km3), ice (30 x 106 km3), ground and
surface waters (8 x 106 to 19 x 106 km3), and atmospheric moisture (0.01 x 106 km33. The
principal exchange of water over the past several million years involved ice. Sea level was
over 100 m lower during the peak of the most recent glacial 18,000 yrBP. The melting of
the northern continental ice sheets between 15,000 and 7000 yrBP probably accounted for
most of the rise of the sea to present levels. Indeed, sea-level change within the next few
thousand years probably will also be dominated by water within the global ice budget and
by ice sheet dynamics. If the polar ice sheets were to disappear, sea level would be some
60 to 70 m higher than at present.
Mountain glaciers make up about 1 percent of the volume of land ice. Their potential
contribution to sea level is about 1 m if they were to melt totally and all the meltwater
reached the sea. Data cited by Meter (Chapter 10) suggests that wastage of the world's
Boo
s
Too ~
30°4
ool
S
Ano.
120° 150° E 180° W 150° 120° 90°
1 1
~DEC 19 75
_ to
_
,~1 ,.,6 1 1 ~I I I 1 1 1 1 1 1 _
-so977
.~~ -
,~
l
- 1 1 1 1
120° 150° E 180° W 1
FIGURE 1 Maps of sea-level anomaly for December 1975 and
December 1977. Contours show sea-level anomalies in millime-
ters after removal of seasonal cycle. The two cases were selected
for their large contrast. From Wyrtki and Nakahoro (1984).
OCR for page 10
10
OVERVIEW AND RECOMMENDATIONS
mountain glaciers and small ice caps contributed about 0.46 + 0.26 mm/yr to higher sea
level between 1900 and 1961 corresponding to a total sea-level rise of 28 + 16 mm, which
is about a third of the estimated sea-level rise during that period.
The Antarctic and GreenlandAice sheets gain material mainly through the accumulation
of snow and lose material through several processes. These processes include surface melt
and runoff of meltwater, calving (discharge) of icebergs, and melting of the underside of
floating ice shelves. Surface melt/runoff is a minor process for the Antarctic Ice Sheet, but
is important to the balance of the Greenland Ice Sheet. Iceberg calving is an important loss
process for both ice sheets, and is predominant in Antarctica. Melting of the underside of
ice shelves has no effect on sea level, but does control the speed of discharge of land-based
ice from Antarctica and is important for predicting the behavior of that ice sheet in the next
century.
The increase in air temperature caused by a rise in concentration of CO2 and other
greenhouse gases may cause increased snow and ice melting, and some of this meltwater
may run off to the oceans, causing a rise in sea level. Increased air temperature and/or
meltwater production may also cause some outlet glaciers and ice streams to flow faster,
transferring land-based ice to the ocean and causing a further rise in sea level. On the other
hand, a rise in CO2 concentration may, in some regions, lead to increased snow precipita-
tion on glaciers and ice sheets, which will have the opposite effect on sea level. Predicting
the effects of climate change on ice growth and wastage is a complex problem because
several different, interacting processes must be considered.
Five important factors in future changes of glaciers and ice sheets are (1) the variation
of energy and mass balance components with altitude, (2) the warming of cold firn to allow
meltwater runoff, (3) the dynamic response of ice masses to changes in thickness, (4)
increased flow and iceberg calving of tidal glaciers due to increased meltwater, and (5) the
stability of ice-sheet/ice-stream/ice-shelf systems. The time frame is restricted to the next
100 yr, approximately the time of doubling of the present level of CO2. The first and fifth
points are briefly discussed below; see Chapter 10 for additional details on all five proc-
esses.
observable functions of altitude.
radiation and precipitation of snow.
For glaciers, many of the mass and energy fluxes related to melting are known or
The two most sensitive of these are absorbed solar
The first is critically dependent on the albedo
(reflectivity) of the surface, which in turn depends on how long the surface is covered with
high-albedo snow during the course of the melt season. The persistence of the snow cover
depends, of course, on the amount of snowfall and the intensity of melt processes, both of
which also depend on altitude. The attitudinal dependence of the mass and energy fluxes
leads to a potential instability, which is one of the reasons for the sensitivity of glaciers to
slight climate changes. An increase in melting causes a lowering of the ice surface, which
in turn may cause a further increase in melting or decrease in snow accumulation leading
, _ is,
to further changes accentuating the melting.
Some have suggested that a climatic change due to increased CO2 in the atmosphere
could lead to disintegration of the West Antarctic Ice Sheet, most of which is grounded
below sea level, causing a 6-m rise in global sea level. The discharge of ice from this ice
sheet is mainly through rapidly moving ice streams, which flow into floating ice shelves.
An ice sheet that rests on a flat bed situated below sea level can be inherently unstable.
Floating ice shelves at its seaward edge, which are "pinned" in position by shallow bottom
areas, act as buttresses that prevent the ice sheet from quickly flowing out into the ocean.
The rise of 100 m in sea level during the past 15,000 yr caused a substantial collapse of a
large part of the West Antarctic Ice Sheet until it anorox'imatelv stahili7.er1 a~ its nr~.ce^.n~
~ r r ., A A ~ ,, _ A ~ JO ~ ~ ~ ~ ~ ~ A_~ ~ ~ ~ ~ ~ ~ ~ ~ ~ _ ~ ~ A ~ ~
· ~ . ~ ~ ~ ~ ~ _ _ . _ _ ,, _
size. A relatively small further sea-level rise could act to "unpin" the buttressing ice
shelves that allow the remnant ice sheet to exist (Thomas and Bentley, 1978).
If the climate in the future becomes warmer with the result that warmer ocean water
intrudes under the ice shelves causing increased melting under the shelves, then the back
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OVERVIEW AND RECOMMENDATIONS
11
pressure exerted on the ice streams by the shelves will be reduced and the ice streams will
accelerate, draining the ice sheet itself.
The critical questions then are: (1) How rapidly will the temperature of Antarctic
subsurface waters rise in response to increased atmospheric concentrations of greenhouse
gases? (2) How much sub-ice melting will be caused by the circulation of this warmer
water under the ice shelves? (3) How will the changed conditions affect calving rates and
thus the dimensions of ice shelves? (4) How rapidly will the ice streams react to changes
in the back pressure? For the next one or two centuries we need to consider only the first
question because warming of the ocean south of the Antarctic convergence is likely to be
markedly delayed. Bryan et al. (1988) used a combined oceanic and atmospheric general
circulation model to show that convective mixing from the surface down to 4000 m will
slow the rate of ocean warming because the entire water column must be warmed by the
same amount. If we consider the added heat energy of around 4 watts/m2 transferred from
the atmosphere to the surface ocean layers, a time of several hundred years would be
required to accomplish this warming. Bentley (1985) has summarized a number of other
reasons why disintegration of the West Antarctic Ice Sheet should not occur within the next
one or two centuries. Accepting Bentley's arguments, sea level will not rise catastrophi-
cally in the near future resulting from the demise of this ice sheet.
Scientific concern about possible disappearance of the West Antarctic Ice Sheet has
largely been based on Mercer's (1978) hypothesis that this body of ice disappeared during
the last interglacial, 125,000 yrBP, with the result that a terrace about 5 m above present
sea level was created around many shorelines around the world. An alternative hypothesis
is that the surface of the East Antarctic Ice Sheet was lower by some 300 to 350 m than
today (Robin, 1987~. This idea is supported by the investigation by Lorius et al. (1985) of
INTO from the ice core collected by Soviet engineers at Vostok in the East Antarctic Ice
Sheet. At 125,000 yrBP, the oxygen isotope values were about 2 Ho higher than present.
Robin points out that this apparently higher temperature could be caused by a reduction in
surface elevation of about 300 m. Surface lowering of this amount for the East Antarctic
Ice Sheet would correspond to a volume of ice of about 2 million km3 and a correspond-
ing rise of sea level of 5 to 7 m during the last interglacial.
Eustatic Elects of Changes in Liquid Water on Land
In the absence of large-scale glaciation and deglaciation, a possible mechanism for
relatively rapid eustatic sea-level change could be changes in the mass of liquid water
sequestered on the continents, both above and below the ground surface. Such a mecha-
nism is needed to explain the apparently eustatic sea-level changes in a virtually ice-free
Earth during Mesozoic and early Cenozoic time described in papers by Haq et al. (1987)
and by Christie-Thick et al. (Chapter 71. The topic presented first is the possible variations
in groundwater.
A global climate change toward less precipitation will lower the water table in ground-
water aquifers, transfer water from the land to the sea, and raise sea level. Less precipita-
tion on land should result from atmospheric cooling, lower wind velocity, or changes in
atmospheric circulation which would alter the balance between precipitation over the
ocean and over the land. Increased precipitation resulting from atmospheric warming or
other causes will raise the water table and lower sea level.
Removal of fresh-water-bearing porous sediments chiefly sands and calcareous de-
posits by erosion should have the same effect as a decline of precipitation, while accre-
tion of such sediments will create a greater potential aquifer volume and hence be roughly
equivalent to a rise in the water table.
Land subsidence, which may now be occurring in many coastal areas, will reduce the
volume of sedimentary aquifers above sea level, and thus have much the same effect as
erosion of sediments or a decline in the level of the water table due to decreased precipi
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12
OVERVIEW AND RECOMMENDATIONS
ration. Emergence of previously submerged aquifers should have an opposite effect,
especially in carbonate terrains where karst formation can occur.
It is also possible that infiltration or discharge rates could change with time, leading to
larger or smaller accumulations of groundwater as the balance between infiltration and
discharge approaches a new equilibrium determined by changes in hydrostatic pressure in
the aquifer. For example, canyon cutting during times of low sea level should increase
discharge rates while deposition of unconsolidated coarse sediments should increase infil-
tration rates.
According to Hay and Leslie (Chapter 9) the late Cenozoic fluctuations in sea level
caused by glaciation and deglaciation resulted in an offloading of sediments from coastal
plain regions and continental shelves into the continental slopes and abyssal plains of the
oceans. Thus the potential groundwater reservoir that exists today may well represent a
minimum for much of geologic history. In past times, the potential change in sea level
resulting from fluctuations in groundwater storage could have been double that which
exists today.
These statements may be roughly quantified by assuming an equivalent rise or fall of the
water table by 100 m, 13.3 percent of the average height of continental surfaces above sea
level. Hay and Leslie (Chapter 9) estimate that porosities are close to 40 percent in the
upper layers of the sands and calcareous sediments resting on the continents in geosyn-
clines (intracratonic basins), coastal plains, and cratonic platforms. They calculate that
these two types of sediments make up 38 to 47 percent of the total deposits in these three
sedimentary environments, and that they are the only sediments that take part in significant
water exchange with the environment outside the aquifers.
Table 2 shows the volumes and pore space of the top 100 m of sands and calcareous
sediments in cratonic platforms, geosynclines, and coastal plains, computed from these
estimates by Hay and Leslie and supplemented by estimates made by Southam and Hay
( 198 1 ) and by Ronov ( 19821.
The total volume of pore space in the top 100 m of the continental sediments 2.5 x 106
km3 is equivalent to a rise or fall of sea level by 7 m. Hay and Leslie assume that the rates
of filling or discharge in these coarse sediments would be less than 13.5 x 103 km3/yr. Thus
more than 185 yr would be required to fill or empty an aquifer 100 m thick, corresponding
to a rate of sea-level change of less than 4 mm/yr. Where only a slight imbalance exists
between infiltration and discharge the times required for filling or emptying an aquifer 100
m thick could be tens to hundreds of thousands of years. For example, Meier (1984)
TABLE 2 Aquifers and Pore Space in Top 100 Meters of Sediments on Land
Volume of
sandy and Volume of
Area covered calcareous pore space
by sandy or sediments in in top
Area covered calcareous top 100 m 100 m of Sea-level
by sediments sediments of aquifers aquifersa equivalent
Location (lo6 km2) (percent) (106 km3) (106 km3) (meters)
Cratonic platforms 55 47 2.6 1.0 2.9
Geosyclines 59 38 2.3 0.9 2.6
Coastal plains and
shelves 31 47 1.4 0.6 1.7
Total 145 6.3 2.5 7.2
aAssuming 40 percent porosity
OCR for page 13
OVERVIEW AND RECOMMENDATIONS
13
estimates that global depletion of groundwater during this century has been between 1600
and 2400 km3/yr or 20 to 30 km3/yr. At this rate, filling or emptying a 100-m-thick global
aquifer would take 85,000 to 130,000 yr and the corresponding rate of rise or fall of sea
level would be less than 0.1 mm/yr.
Baumgartner and Reichel (1975) and Woods (1984) estimate the global volume of
groundwater at 8 x 106 km3, about 22 percent of the Earth's fresh water, equivalent to 22
m of sea level. This may be compared to 70 m of sea-level equivalent for the Greenland
and Antarctic ice sheets.
From data given by Hay and Leslie, assuming an average porosity of 20 percent for
sandy and calcareous sediments, a pore volume in sediments has been computed at 64.8 x
106 km3. But SS.S x 106 km3 of this pore space is below sea level and hence presumably
saturated with water. The pore space above sea level, which could be drained or filled by
the various mechanisms discussed herein, is 9.4 x 106 km3, equivalent to 27 m of sea level,
very close to the estimate of groundwater volume given by Baumgartner and Reichel.
However, as Hay and Leslie suggest, the porosity of the 750 m of sediments above sea
level may be 30 to 40 percent, corresponding to a pore volume of IS x 106 to 19 x 106 km3.
Geological evidence shows that large quantities of shallow water carbonates and non-
marine sands were laid down in mid-Paleozoic, late Paleozoic to early Mesozoic, and mid-
Cretaceous times. During these periods, their abundance was probably more than twice
that of the present time and the potential for groundwater storage was higher. Insofar as
these porous sediments occupied a greater percentage of the land area than their present
counterparts, infiltration rates must also have been higher than today, compared with rates
of discharge, which can occur only at the edges of sedimentary columns.
The mass of liquid water above ground in lakes and rivers is only a small fraction of the
mass of groundwater. Residence times in rivers vary from less than a week to about a year
depending on size and length and on the slope of the river bed.
The volume of water stored in lakes is about 0.22 x 106 km3 (Robin, 1987), probably
about 50 times the volume in rivers but only about 1 percent of the volume of groundwa-
ter above sea level. Changes in lake volume result from climate variation and change and
from human activities, primarily diversion of inflows for irrigation or other purposes. The
Aral Sea in the Uzbek Republic of the Soviet Union is a striking example (Micklin, 1971~.
This lake without an outlet, fed by the Amur Darya and Syr Darya rivers, was formerly the
world's fourth largest lake, behind the Caspian Sea, Lake Superior, and Lake Victoria. In
1960 its area was 68,000 km2, its average depth was 16 m, and its volume was 1090 km3.
Beginning in 1960, there was a large increase in diversions of the river flows for irrigation
caused by expansion and intensification of the irrigated areas. These increased diversions
were not compensated for by conservation measures as previously, and the lake began to
shrink rapidly. By the beginning of 1970, the area had decreased by 40 percent, the volume
had decreased by 66 percent, and the water depth had dropped to 9 m. This change in lake
volume must have been accompanied by a eustatic rise in sea level of slightly less than 2
mm. Destruction of the lake is still occurring; without drastic changes in irrigation
practices, it will have largely disappeared by the early part of the twenty-first century, and
there will be a further eustatic rise in sea level of about 1 mm.
On a worldwide basis, Robin (1987) estimates lake volumes are diminishing by 72 km3/
yr. He bases his estimates on the observed annual decline of the Caspian Sea by 10.9 km3,
assuming that this decline in the Caspian, the world's largest lake, is IS percent of the
global diminution of lake volumes. The corresponding eustatic rise in sea level should be
0.2 mm/yr or 20 mm/century. To this should probably be added the sea-level rise of about
1 mm due to the future decline of the Aral Sea.
Robin (1987) also points out that a long-term warming of the atmosphere by 3°C at low
latitudes and 6°C at higher latitudes should result in an increase of the atmospheric content
of water vapor, and a corresponding fall in sea level of about 7 mm.
OCR for page 24
24
O VER VIEW AND RECOMMENDA TI ONS
positional base level, can be identified in outcrops and boreholes as well as in seismic
profiles. Unlike transgressions and regressions of the shoreline or changes in
paleobathymetry, which are used in classical stratigraphy to gauge sea-level fluctuations,
the formation of regional unconformities is relatively insensitive to the rate of sediment
supply, and this constitutes the main advantage of the sequence stratigraphic approach.
According to Christie-Thick et al. (Chapter 7) and Christie-Thick (1989) most sequence
boundaries record times at which the rate of sea-level fall increased (or reached a maxi-
mum) or the rate of tectonic subsidence decreased. Those of eustatic origin should be
formed in all basins connected to the open ocean and should be nearly correlative, what-
ever the local tectonic history. Unconformities of tectonic origin may also be present, but
these are not expected to extend beyond a region more than a few hundred or a few
thousand kilometers across. The identification of a global sea-level signal therefore
depends on demonstrating that particular unconformities are present in widely separated
basins, and are synchronous (ideally to within 0.5 m.y.~.
More difficult than establishing the timing of sea-level change is the problem of
estimating amplitudes and rates of change. Amplitudes of sea-level oscillations may be
estimated through a combination of sequence stratigraphy and geophysical modeling of
subsidence history, but practical difficulties and sensitivity of results to model assumptions
may limit estimates to no better than a factor of 2 or 3 larger or smaller than true
amplitudes (Christie-Brick et al., Chapter 71. The principal uncertainties are in age
control, paleobathymetry, compaction history, the effects of sediment loading on the
lithosphere, and the tectonic subsidence that must be subtracted from corrected strati~ra-
phic data to obtain the sea-level signal.
A,
Harrison (Chapter 8) has suggested that 10 transgressive-regressive cycles of sedimen-
tation with average periods of 7 to 10 m.y. during the Cretaceous resulted from alternating
increases and decreases in the rates of seafloor spreading. These would have caused
eustatic rises and falls of sea level of about 15 m. The eustatic sea-level changes were
greatly amplified in the Western Interior Seaway by subsidence of the basin resulting from
subduction of the Pacific plate under the western part of the North American continent. In
Chapter 9, Hay and Leslie discuss possible changes in the volume of liquid water stored as
groundwater as a cause of some of these Cretaceous sea-level changes. These and many
other problems reviewed in Chapters 7 and 8 need to be solved before these curves can be
read as eustatic changes.
If stratigraphic sequence boundaries do turn out to be related to eustatic sea-level
change, the record from the Triassic (about 250 Ma) through to the present becomes rich
with events that need to be considered in the light of possible causal processes.
From 250 to 2500 Ma
Although there is some direct stratigraphic record of sea-level change that extends back
into the Paleozoic and Precambrian (Christie-Brick et al., 1988; Bond et al., 1988), most
of the data are derived from continental freeboard consideration. Geologic evidence on the
area of the continents covered with marine sediments indicates that continental freeboard,
the average elevation of the continents above sea level, has been approximately constant
since the Archean, 2500 Ma. Continental freeboard today, taking the mean of the conti-
nents, is about 750 m, and changes of more than a few hundred meters are apparently ruled
out by the geologic evidence.
Provided that the mass of seawater (plus water on land) has not varied significantly, an
approximately constant continental freeboard since the Archean, combined with a 50
percent decrease of heat flow from the mantle, has been shown by Schubert and Reymer
(1985) to require a net growth of the continents of about 25 percent (1 km3/yr) over the past
2500 m.y. They believe that post-Archean changes in the mean thickness of the continen
OCR for page 25
OVERVIEW AND RECOMMENDATIONS
25
tat crust are highly unlikely, hence the continents must have grown in area. If freeboard
has been within +200 m of its present value since the end of the Archean, the continents
have grown in area by anywhere from 10 to 40 percent. On the other hand, if the mass of
seawater has increased significantly since the Archean, isostasy requires that the thickness
of the continental crust should have likewise increased. For example, if the flux of water
to the ocean from the Earth's interior has been constant during the Earth's lifetime, the
entire growth of continental volume estimated by Schubert and Reymer can be accounted
for by an increase in continental thickness rather than continental area. Continental growth
is required as long as freeboard at the end of the Archean was not more than 400 m smaller
than it is today.
Continental ice ages, which must have caused a fall in sea level, occurred at intervals
during the Proterozoic and the Paleozoic. According to Crowell (1982), evidence from
what is now southern Ontario shows that there were large-scale glaciations between 2500
and 2100 Ma. One ice sheet may have reached southern Wyoming. A similar glaciation
probably occurred in South Africa sometime between 2700 and 2200 Ma, and possible
tillites are reported from Western Australia. During the span of 600 m.y. between 2700
and 2100 Ma, a period as long as the entire time in which multicelled animals and plants
have existed, several widespread glaciations, and presumably concomitant falls in sea
level, apparently occurred, separated by ice-free eras.
There is no record of glaciation anywhere on Earth during the ensuing 1100 m.y. until
the Late Precambrian, when continental glaciation flourished intermittently on all the
continents, except possibly Antarctica, from about 950 to 560 Ma. Apparently there were
three glaciation peaks, one about 940 Ma, another around 770 Ma, and a third about 615
Ma, in regions around the present North Atlantic, in central and southwestern Africa, and
in Brazil, western North America, and Australia. Each of these extreme glacial times must
have been a period of markedly lower sea level in comparison with earlier or later periods.
These Late Precambrian glaciations apparently occurred at low latitudes, distant from
the Earth's poles, for which no satisfactory explanation has come forth. Perhaps, there was
a reduced greenhouse effect or the patterns of oceanic and atmospheric circulation were
markedly different during Precambrian times from those of today, or of the past half billion
years.
The next widespread glacial epoch began in the supercontinent Gondwana near the end
of the Ordovician lasting into the Silurian, from about 450 to 400 Ma. The record extends
in scattered localities from northern Europe to South Africa and from the Sahara region to
Bolivia and Peru.
For 90 m.y., from 330 Ma to 240 Ma, a Late Paleozoic ice age existed in large areas of
the Gondwana supercontinent, beginning in what is now South America, reaching a climax
in South America, Africa, and Antarctica, and ending in Australia. There were apparently
several moderately sized ice caps instead of one huge one, because it does not appear that
sea level was drastically lowered. The center of the glaciated regions was near the South
Pole, as the supercontinent of Gondwana drifted across it.
There was no continental glaciation between Late Permian and Paleogene time, even
though Antarctica lay near the South Pole. This may have been the result of a greenhouse
effect caused by a high atmospheric CO2 content, related to high rates of seafloor spreading
and undersea volcanism (Berner et al., 19831.
DO CHANGES IN SEA LEVEL CAUSE CHANGES IN CLIMATE?
Climate variability and change cause local or regional variations in sea level over
months, years, or decades. Quantitatively the seasonal oscillation is the most important of
these, followed by interannual variations of 5 to 10 yr, related to the Southern Oscillation
in atmospheric pressure between the two sides of the Pacific Ocean, and manifested in E1
Nino in the eastern Pacific and in many other phenomena elsewhere.
OCR for page 26
26
OVERVIEW AND RECOMMENDATIONS
Longer-term global climatic changes can be reflected in eustatic sea-level change
through three processes: the waxing and waning of continental ice caps and alpine glaciers;
variations in the quantity of liquid water stored on land in lakes, rivers, and underground
aquifers; and steric changes in the volume of seawater resulting from warming or cooling.
It is also often suggested that a change in sea level can produce a change in climate. As
Barron and Thompson (Chapter 11) point out, this suggestion is consistent with the
apparent correlation of climate and sea level over geologic time. Several lines of evidence
show that during the past 70 m.y. globally averaged surface temperatures have declined by
6° to 12°C, global sea level has fallen by perhaps 200 m, and the total land area above sea
level has increased by about one-third.
There is a variety of physical mechanisms by which sea-level changes could directly
affect global climate: changes in albedo, regional changes in atmosphere-surface coupling,
changes in ocean circulation, changes at ice-sh~.~.t ocean marring ~nr1 rho in Not_
mosphere chemical composition.
~ =~ A ~ A ~ ~ ~ _ _ A ^ ~ _ ~^ ~ ^ ~ ~4/ ~ ~
Virtually every aspect of the Earth's climate is affected by the exchange of heat,
moisture, and momentum between the atmosphere and the underlying surface. Changes in
the surface energy balance are determined by changes in surface albedo and surface
wetness. A rise of sea level will increase the area covered by ocean water, which has a
much lower albedo and a much higher wetness than the land. A change in global albedo
by 0.01 will produce about a 1°C change in surface temperature. Such an albedo change
would require a major change in the land/sea ratio, caused by a rise or fall of 100 m or more
in sea level.
The change in wetness resulting from this assumed change in relative areas of sea and
land would lead to marked changes in evaporation, and therefore in precipitation, although
not necessarily in the same region, and also to a significant change in average summer
surface temperatures because of the much larger thermal inertia of the ocean compared to
the land. However, model calculations indicate that when both surface albedo and mois-
ture availability are altered simultaneously, they produce nearly complete compensating
effects. For example, modeled deforestation of the Amazon Basin, which would produce
the same effects on albedo and wetness as a fall in sea level, produced only a small net
surface temperature change in the deforested area and no rletect~hl~. aloha ~.~1~. climate
effects (Henderson-Sellers and Gornitz, 1985~.
In general, surface roughness is an order of magnitude higher over the land than over the
ocean. ConsequentlY a chance in ocean/land proportions caused bv a rise or fall of ~n
_ =~ ~_ ~ A ~ ~ ~ ~ ~ _
~1 1 ~
~ ~ ~ . _ ~ _ . . _ . ~ _ _ _
level may result In a marked redistribution of the surface areas of momentum exchange
between the land and ocean and the atmosphere. This could have a marked effect on
atmospheric circulation systems.
In some regions, for example, the Blake Plateau in the Western North Atlantic, it can be
shown from studies of seafloor erosion that the position of the Gulf Stream has varied
repeatedly by hundreds of kilometers with changes of sea level during the past 20 m.y. But
the extent of possible climatic change caused by such movements in current position is
unknown. The same lack of knowledge about climatic effects afflicts examples of seafloor
subsidence such as that of the Greenland-Iceland-Faroe Ridge between the Arctic and the
Atlantic oceans, and the Walvis Ridge off South Africa. Sea-level change can also isolate
or reconnect small basins along the ocean margins. Enhanced evaporation in partially
isolated basins can produce water masses of markedly different density than those of the
main ocean, and thereby affect deep water and mid-water formation.
A marked increase in atmospheric CO2 accompanied the deglaciation of the Northern
Hemisphere ice sheets about 10,000 yrBP; it probably had a significant warming effect on
the lower atmosphere. One hypothesis to explain at least part of this increase in atmos-
pheric CO2 involves the submergence of the continental shelves by the rise in sea level. In
tropical waters this resulted in a large increase in the growth of coral reefs and precipitation
of other calcareous sediments with a corresponding release of CO2 to the subsurface ocean
layers and the atmosphere.
OCR for page 27
OVERVIEW AND RECOMMENDATIONS
27
If bottom water temperatures were unchanged, the fall of the sea level by 20 to 130 m
during the last glaciation should also have resulted in the release of methane (a potent
greenhouse gas) from methane ices (clathrates) in the upper layers of continental slope
sediments. These sediments are believed to contain 2.2 mg/cm3 of methane, or a total of
3400 gigatons of methane (Revelle, 1983a). A fall of 100 m in sea level without a change
in ocean-bottom temperature should release 425 gigatons of methane. If the sea-level
change took 5000 yr, 0.085 gigatons would have been released each year. Methane in the
atmosphere has a half life of about 8 yr, implying that the release of methane from
continental slope sediments should have increased the preindustrial methane level of less
than 2 gigatons by about 50 percent. In fact, evidence from ice cores indicates that the
methane content of the atmosphere decreased by nearly 50 percent during the latter half of
the last glacial epoch and was at no time higher than the postglacial, preindustrial level of
650 parts per billion (Stauffer, 1988~. This information suggests that during the glacial
period the temperature of the ocean waters bathing the continental slope sediments de-
creased by more than 1°C than it is at present. This would prevent destabilization of the
sedimentary clathrates by the release of pressure that resulted from the drop in sea level.
As far as the apparent correlation between the 70-m.y. fall in eustatic sea level and the
drop in temperature over the same period are concerned, it now seems most reasonable to
ascribe both phenomena to the same set of processes within the Earth and not to attempt to
forge a causal relation between them. The high sea levels of late Cretaceous time were
most probably caused by intense oceanic lithosphere formation and seafloor spreading
from the mid-ocean ridges (see Harrison, Chapter 8~. This same process resulted in
perhaps a tenfold higher level of atmospheric CO2 and a correspondingly warmer climate,
perhaps 10°C warmer (Berner et al., 1983~.
FORECASTING CHANGES IN SEA LEVEL RELATED TO
GREENHOUSE GASES
On a time scale of 102 to 105 yr, global changes in sea level (eustatic changes) can result
mainly from the buildup or decay of alpine or continental glaciers, and from long-term
ocean volume or steric changes caused by temperature or salinity changes in waters below
the thermocline. The concern here is with forecasting eustatic changes related to the rise
of CO2 and other greenhouse gas concentrations in the atmosphere.
The past few thousand years have been a time of a high and relatively stable stand of sea
level after 100 millennia of rapidly varying levels during the last ice age. Regional steric
and other irregular variations of 10 to 20 cm from year to year, or from decade to decade,
have been common, but there has been at most only a small unequivocally detected long-
term trend in eustatic sea level. This situation can be expected to change with the advent
of greenhouse-gas-induced climate change.
Sundquist (Chapter 12) examined the probable future course of atmospheric CO2 con-
centrations over the next 1000 yr. It might be expected that oceanic biogeochemical
processes related to the dissolution of calcium carbonate sediments on the deep-sea floor
would, within a few centuries, reduce the content of free CO2 in seawater, and hence the
atmospheric content. This turns out not to be so, provided sufficient CO2 has been
generated by fossil fuel combustion.
The total remaining reserves of coal, oil, natural gas, oil shale, and oil sands that are
ultimately recoverable for human use are believed to correspond to about 7500 billion tons
of carbon. Of this amount approximately 3500 billion tons have already been identified.
Sundquist assumes that 2500 billion tons will ultimately be consumed in human activities,
with a peak rate of production in the middle of the next century of 16.8 billion tons/yr com-
pared to the annual carbon emissions in 1985 of 6 billion tons. He assumes that the
production rate will decline to 6 billion tons by about A.D. 2150 and go nearly to zero by
A.D. 2350. With this assumed history of hydrocarbon combustion, Sundquist finds that the
OCR for page 28
28
OVERVIEW AND RECOMMENDATIONS
CO2 content of the air rises from 350 parts per million by volume (ppmv)(700 billion tons)
in 1985 to 800 ppmv (1600 billion tons) around A.D. 2160, and slowly declines thereafter
to 550 ppmv (1100 billion tons) by A.D. 2700. The peak concentration is approximately
2.9 times the base concentration of 280 ppmv in 1880, from which increases of atmo-
spheric CO2 content are usually calculated. Also to be taken into account are the increasing
concentrations of methane, nitrous oxide, tropospheric ozone, and other minor greenhouse
gases that can be expected 150 yr from now.
General circulation models of the atmosphere constructed by the National Center for
Atmospheric Research, the NASA Goddard Institute of Space Sciences, the Geophysical
Fluid Dynamics Laboratory of NOAA, and others indicate that the estimated increased
concentrations of greenhouse gases should result in an average global temperature rise of
3° to 6°C in the atmosphere near the Earth's surface in the next 100 yr. [Recent modeling
studies show that more sophisticated parameterization of clouds, taking into account both
ice and liquid droplets, greatly reduces the sensitivity of the climate models to increased
CO2 (Cess et al., 1989; Mitchell et al., 19894. These newer models indicate a smaller
temperature increase than the 3° to 6°C range given above.]
The next question to ask is: How will this atmospheric temperature change affect the
, ~
world's oceans. This question has recently been studied by Frei et al. (1988~. They use
two kinds of models: a "pure diffusion" (PD) model in which heat is carried downward by
eddy diffusion, assuming vertical diffusion coefficients of 1.3 and 2 cm2/s and a modified
diffusion model in which cold, polar water sinks to the bottom of the ocean and mass is
conserved by assuming a slow, global upwelling. Frei et al. (1988) call this model an
upwelling-diffusion (UD) model; they assume that the coefficient of vertical eddy diffu-
sion is about 0.65 cm2/s and that the global average upwelling rate to balance deep and
bottom water formation is about 4 m/yr. A considerably smaller rate of upwelling and, cor-
respondingly, a higher rate of downward vertical diffusion would correspond to estimates
by Whitehead (1989) that cold deep and bottom water is formed at a rate of only 5 million
to 10 million m3/s instead of the rate of 40 million m3/s assumed by Frei et al. (19883.
Measurements of tritium distribution in the North Atlantic made by the "GEOSECS"
Expedition in 1972 (Ostlund et al., 1974) and 10 yr later by the "Transient Tracers in the
Ocean" Expedition (PCODF, 1981; Ostlund, 1983) indicate that the tritium "front" in the
Atlantic deep water between depths of 2500 and 5000 m moved about 800 km south during
this 10-yr period, indicating that the mass of deep water sinking in the Norwegian Sea and
cascading downward through the Denmark Strait, is 10 million to 20 million m3/s. Hence,
the rate of upwelling is probably smaller and the rate of downward diffusion greater than
assumed by Frei et al. (1988~.
Basically, a PD model transports heat relatively rapidly into the oceans, which slows the
atmospheric temperature response to the rising CO2 concentration but increases the rate of
sea-level rise. The UD model reduces heat penetration into the ocean, allowing the climate
to warm relatively rapidly but reducing the sea-level response.
Frei et al. estimate that the rise in sea level during the past 100 yr caused by thermal
(steric) expansion was between 3 and 8 cm, with the lower value corresponding to the UD
model. They project a rise of 10 to 50 cm during the next century resulting from thermal
expansion, the range arising from uncertainties in CO2 and trace gas concentrations and in
estimates of climate sensitivity to greenhouse warming.
To this estimate of steric sea-level rise in the next century must be added the NRC
(1985) Committee on Glaciology's estimate of the contribution to sea-level rise by ice
wastage in a CO2-enhanced environment. This would come from three sources: glaciers
and small ice caps, the Greenland Ice Sheet, and the Antarctic Ice Sheet. This NRC
committee estimates a sea-level rise by the year 2100 (the assumed time for a doubling of
atmospheric CO2) from ice wastage of 0.1 to 1.6 m with "most likely" values of 0.2 to 0.9
m; the "most likely" scenario can be expressed as 0.55 + 0.21 m if one assumes that this
OCR for page 29
OVERVIEW AND RECOMMENDATIONS
29
range expresses a standard deviation from the mean and if the "errors" are considered to be
independent.
Revelle (1983b), using a two-dimensional vertical diffusion model for ocean thermal
expansion, estimated a total sea-level rise of about 0.7 m. Also, in 1983, Hoffman et al.
(1983) forecast a larger global rise, between 1.44 m and 2.17 m by A.D. 2100. This was
estimated to result from thermal expansion of ocean waters and from ablation and partial
melting of alpine glaciers and the ice caps of Antarctica and Greenland. Of the total rise,
an average 0.72 m was estimated to result from ocean thermal expansion, and 0.72 to 1.45
m were added from ice discharge to maintain the relative contributions thought to have
contributed to sea-level rise over the past 100 yr. Because the range of rise related solely
to ocean thermal expansion was calculated to be 0.28 to 1.15 m, the ratio approach led to
an extreme upper limit of more than 3 m (of this amount, mountain glacier melting could
contribute, at moss, 0.3 to 0.5 m). Robin (1987) forecast a rise of 0.80 m by A.D.2100 with
a range of 0.20 to 1.65 m. Gornitz et al. (1982) calculated the component of sea-level rise
resulting from ocean thermal expansion between 1980 and 2050 as 0.20 to 0.30 m.
MacCracken et al. (1989), incorporating the Frei et al. (1988) model results and, with some
adjustments, the findings of Revelle (1983b), estimate a total sea-level change by the year
2100 of less than 0.5 to 1 m.
In all these estimates, the possible change, largely a result of human activities, in the
quantity of water stored in lakes, man-made reservoirs, and underground aquifers has been
neglected. Robin (1987) estimated the net effect of these three sources together at the
present time as causing an annual rise in sea level of 0.08 mm, or 8 mm/century.
Most calculations indicate that sea level will continue to rise for at least several hundred
years at average rates of centimeters per year or less. Of course, a considerably more rapid
rate would ensue if the West Antarctic ice cap should disintegrate during the next 1000 yr.
As we have seen, such a dissolution is probably impossible during the next several hundred
years because, as Bryan et al. (1988) have shown, deep ocean convection on the southern
side of the great circumpolar Antarctic Current may markedly delay much Antarctic
temperature change in the upper ocean layers.
The assumed rate of carbon combustion is highly uncertain. In order to reach an
atmospheric level of 800 ppmv by A.D. 2180, an average rate of CO2 production corre-
sponding to 9 billion tons of carbon per year during the next 200 yr is assumed. This is at
least 50 percent higher than the present rate of 6 billion tons/yr, which is estimated to
include tropical deforestation of greater than about 1 billion tons/yr (Machta, 1983~. On
the one hand, economic and social development of the now developing countries, which
make up the vast majority of mankind, may well require a considerable increase above
present levels of carbon combustion, and consequently increase the worldwide rate, per-
haps by a factor of 3 or more. On the other hand, as Goldemberg et al. (1987), Mintzer
(1987), and Bach (1988) have persuasively argued, it may be possible, through increases
. . ,
. . . . . . . . , , - . . ~ .~ , , ,
In energy use ettlclency and substitution of other energy sources tor lOSSll fuels, lo reduce
considerably the influx of CO2 to the atmosphere and ultimately to the oceans. With these
alternative energy sources in use, the atmospheric burden of CO2 might remain at all times
much below our estimated figure of 800 ppmv, and the consequent rise in air and sea tem-
perature and steric sea level would be considerably reduced.
In these calculations it has been tacitly assumed that the circulation of the deep water of
the world ocean will continue relatively unchanged, except that the deep water will be
somewhat warmer because of vertical and lateral mixing. In other words, the future ocean
circulation will be "surprise free." However, the warming of the atmosphere in high
latitudes, and the corresponding warming of the subsurface ocean waters, may greatly
reduce the volume of water sinking to the depths. The projected rise in sea level from
steric expansion would then be intermediate between the range estimated from the UD
model (10 to 50 cm), and the range computed from the PD model (20 to 110 cm).
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30
OVERVIEW AND RECOMMENDATIONS
HOW CAN WE IMPROVE THE MEASUREMENT OF SEA-LEVEL CHANGE?
The apparent trend of sea level at a particular place as measured by a tide gauge is the
sum of trends in motion of the gauge itself as the land on which it is mounted moves
vertically, the trend of change in steric sea level, and the trend of change in water mass
under the tide gauge. To understand what is happening one needs to be able to make
measurements that will separate these three components of the observed sea level. This
problem is addressed by Munk et al. (Chapter 141. Each component of the observed sea
level is considered separately.
The vertical motion of the tide gauge can be measured with fair accuracy in four
different ways: by measuring changes in the acceleration of gravity at the sea surface under
the gauge, by very-long-baseline interferometry (VLBI), by satellite laser ranging, and by
use of the satellite signals of the global positioning system (GPS'. Without repeated
measurements over several years, none of these methods is sufficiently accurate to deter-
mine the vertical position of the tide gauge.
The acceleration of the Earth's gravity, g, can be measured with an uncertainty of about
part in 108 by the methods described in Chapter 14. This accuracy corresponds to a
sensitivity to height changes of the tide gauge of about 30 mm. VLBI observing stations
yield estimates of intercontinental baselines with an rms scattering of 20 to 30 mm. Up to
the present, the scatter of the vertical components has been 3 or 4 times larger than this.
One of the major sources of error is atmospheric refraction caused by water vapor.
Improved water vapor radiometers are being developed and placed in operation. The rms
scatter of all components of the baseline should then be reduced to the 10- to 20-mm level.
However, reducing the scatter below the 10-mm level may be very difficult (Carter et al.,
19861.
Global Positioning System surveys of benchmarks separated by 8 to 50 km agree in the
height components by +10 to 30 mm. Carter et al. (1986) believe that these results are
about as good as can be expected reliably for the foreseeable future. However, there are
plans for increasing the accuracy of space-based geodetic techniques. It is possible that
satellite laser ranging (SLR) can improve position accuracy from the present 1-cm value to
1 mm in the next decade. If vertical positions can achieve this accuracy, we should be in
a position to improve our knowledge of eustatic change considerably. The Geodynamic
Laser Ranging System (GLRS) of the Earth Observing System (EOS) will allow a large
number of tide gauges to be included in the network.
At present, the single measurement errors in all four methods of measuring the changes
in the elevation of the tide gauge are comparable at +10 to 30 mm, and it seems likely that
they will remain for some time to come. In contrast, it is desirable that the motion of the
tide gauge be determined to a fraction of 1 mm/yr. Hence observational strategies will
have to rely on repetitive measurements spanning intervals of several years and even then
the desired accuracy can only be achieved by GLRS.
The change in the steric height of sea level can perhaps best be monitored with bottom-
mounted upward-looking fathometers plus tide gauges. (The alternative of measuring
bottom pressure with sufficient accuracy presents many difficulties, because of seemingly
inevitable unpredictable drift of the pressure gauges at pressures of a few tens of atmos-
pheres.) Because of the marked variation in the velocity of sound in water with changes
in temperature (some 23 times larger than the changes of specific volume or density with
temperatures, it should be quite practical to estimate the integrated changes in temperature
over the water column above the fathometer. The main problems in measuring steric
changes, as Roemmich (Chapter 13) points out, are that they tend to extend over great
depths and are not confined to or concentrated in the upper ocean. In the subtropical North
Atlantic, steric changes extend to at least 3000 m with maxima in the thermocline at depths
of 300 to 700 m and below the thermocline at about 1800 m. Hence the inverted
fathometer is likely to be most useful in the depths of the open ocean far from land. The
OCR for page 31
OVERVIEW AND RECOMMENDATIONS
31
logistical problems of maintaining operating fathometers in such depths and locations and
combining their measurements with surface mounted tide gauges may be difficult to solve.
In principle, however, the combination of inverted echo sounders plus one or more of
the four methods for measuring the vertical motions of the tide gauge, plus tide-gauge
measurements at the sea surface should allow us to separate the 3 major components of
changes in RSL (volume of water, mass of water, and changes in the elevation of the tide
gauge).
A further difficulty arises, as Roemmich shows in Chapter 13: steric height variations
occur over such a range of space and time scales that possible trends cannot be identified,
even in a 30-yr time series at a single station, no matter how dense the sampling. However,
the major steric changes seem to be relatively coherent over distances of several thousands
of kilometers. Munk et al. (Chapter 14) therefore suggest that 10 spatially independent
stations, each forming 5 independent samples over a 25-yr period, could be combined to
give a +7-mm standard deviation in a long-term trend of 10 to 25 mm in steric level. It
should be possible within the next few years to measure change in sea level with the
precision of 1 cm or less from satellite altimeter measurements such as those planned for
the TOPEX/POSEIDON experiment (B. Tapley, University of Texas, personal communi-
cation, 19891. Born et al. (1986) listed the magnitude of different sources of error in such
measurements. The accuracy of the measurements will be limited by the calibration of the
altimeter and of the estimated heights of fixed points on land. The effects of ionospheric
and tropospheric refraction can be eliminated through use of two-frequency altimeters and
a water vapor radiometer in the satellite. The drift of the altimeter, and not the absolute
calibration, is important for measurement in changes of sea level during the life of the
instrument.
RECOMMENDATIONS
1. Long-term sea-level measurements of sufficient accuracy over the world's oceans
could provide one of the most significant data sets for understanding global change,
particularly climatic change resulting from the greenhouse effect. It is for this reason that
the planning committees for the World Climate Research Program and the Intergovern-
mental Oceanographic Commission of UNESCO have given a very high priority to extend-
ing the global sea-level network in the Indian, South Atlantic, and South Pacific oceans.
This effort is being supported, insofar as available funds will allow, by the U.S. National
Oceanic and Atmospheric Administration (NOAA). We strongly recommend that the
national oceanographic and meteorological communities lend moral and intellectual
support to this sea-level program and to develop satellite altimeter methods for
changes in sea level.
2. A polar orbiting satellite equipped with a radar, preferably a laser, altimeter
should be operated on a continuing basis to measure changes in volume of the
Antarctic and Greenland ice sheets. These ice sheets may be the principal sources of
variations in sea level during the next century.
As reviewed by the Topographic Science Working Group (1988), detailed and repeated
height measurements by near polar-orbiting satellites are required to study the mass
balance and dynamics of ice sheets. Repeat surveys of the ice sheets at 1- to 5-yr intervals
with a vertical resolution of 10 cm are required for determination of elevation changes
indicative of changes in ice volume, thus providing a measurement of net mass balance.
Only refined radar altimeters or laser altimeter systems are capable of global coverage with
the requisite accuracy.
The output of ice in the mass balance equation occurs through iceberg discharge,
surface melting near the margin, melting at the bottom of ice shelves, evaporation, and
ablation. A 1-m difference in surface elevation of the ice shelves reflects a nearly 10-m
OCR for page 32
32
OVERVIEW AND RECOMMENDATIONS
difference in ice shelf thickness (Topographic Science Working Group, 19881. The
position of the grounding line can also be observed in elevation data because of a marked
change in slope. The output of the ice sheets is not known better than 30 to 100 percent
of the total snow accumulation, thus its measurement is critical in an assessment of the
mass balance of the ice sheets.
3. A geological record of sea-level change is well preserved in numerous basins for at
least the past 200 m.y., a span that includes the nearly ice-free Cretaceous Period and the
present ice age. Comparison of the record between glacial and nonglacial times will
provide an improved understanding of how depositional systems respond to sea-level
change, as well as insights about nonglacial mechanisms of sea-level change.
Support of programs, both national and international, that address the questions
of the sea-level record during the past 250 m.y. should be vigorously pursued. One
such program, the Global Sedimentary Geology Program (of the International Union of
Geological Sciences) on Cretaceous Resources, Events and Rhythms, addresses a variety
of questions that we have raised, viz., (a) Is there a global correlation of sequences? (b)
Are sequences caused by eustatic fluctuations and/or global tectonic variations, or are
sequences developed as a result of regional and local tectonic adjustments? (c) What are
the relationships between subsidence, sea level, sediment supply, erosion, and other factors
in mid-Cretaceous sedimentary basins?
4. To improve estimates of future steric changes in ocean volume caused by greenhouse
warming of the ocean water, coupled ocean-atmospheric general circulation models
should be improved and used to trace probable changes in ocean and atmospheric
temperature as the greenhouse gas concentrations in the atmosphere gradually in
crease.
5. To measure absolute elevation of tide gauges, measurements of position using
satellite laser ranging, the global positioning system, and very-long-baseline interfer-
ometry techniques and absolute gravity should be started.
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Representative terms from entire chapter:
ice sheet
