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OCR for page 185
11
INTRODUCTION
Sea Level and Climate Change
ERIC J. BARRON
Pennsylvania State University
STARLEY L. THOMPSON
National Center for Atmospheric Research
It has long been suggested that a change in global sea
level produces a change in climate. This hypothesis re-
ceives substantial support from the geologic record of sea-
level change. Over a broad range of time scales (104 to 107
yr), eustatic sea level and continental flooding are well
correlated with paleoclimatic data, particularly during the
past 100 million yr (m.y.~.
The relationship between sea level and climate has three
components. First, climate directly influences sea-level
variations largely through the processes that control the
growth and decay of continental ice. At peak glaciation
18,000 yr ago, approximately one-sixth of the planet was
covered by ice and continental glaciation, and sea level
was lower by 85 to 130 m than it is at present (CLIMAP,
1976).
Second, a variety of physical mechanisms exist whereby
sea-level changes can directly affect global climate. By
changing the nature of the atmosphere-surface interface,
sea-level changes can alter the transfer of heat, moisture
and momentum between the surface and the atmosphere.
In addition, sea level can alter ocean currents by introduc-
ing or removing geographic barriers. Ocean currents are a
principal means of transferring heat from the tropics to
polar regions and play a crucial role in controlling some
regional climates of the present. Other potential direct
influences of sea level on climate include effects on ice
~5
sheet size, ocean chemistry, and atmospheric content of
carbon dioxide and other trace gases.
Third, the correlation between sea level and climate
during Earth history may be the product of an indirect
association. One plausible indirect link is a relationship
between increased volcanism, higher atmospheric carbon
dioxide levels, and high global sea level caused by rapid
sea-floor spreading (Berner et al., 19831. In this case,
climate change would be directly related to changes in
carbon dioxide levels, and the correlation of climate change
with sea level would not be indicative of a direct cause-
and-effect relationship.
Only the direct influence of sea level on climate and the
indirect association between sea level and climate are
considered here. The direct influences of climate on sea
level, through land ice volume or ocean temperature change,
are considered in Chapters 10 and 13 (this volume), re-
spectively. Simulations with global dynamical climate
models and examples from the paleoclimatic record pro-
vide insight into the importance of various physical mecha-
nisms and the explanations of correlation between climate
and sea-level variations during earth history.
DIRECT EFFECTS OF SEA LEVEL ON CLIMATE
The potential direct effects of sea level on climate can
arise from physical mechanisms that fall into at least five
categories: (1) regional changes in atmosphere-surface
OCR for page 186
186
coupling, (2) changes in ocean circulation, (3) changes in
surface heat capacity, (4) changes at ice sheet-ocean mar-
gins, and (5) changes in ocean-atmosphere chemical
composition. The mechanisms are listed roughly in order
of increasing uncertainty, although insufficient work has
been completed in the investigation of the climatic effect
of sea-level changes in every category. Hence the discus-
sion that follows will necessarily rely heavily on work not
originally conducted with an eye to sea-level change. In
some cases, educated speculation will have to suffice until
more research specific to the problem of sea level and
climatic change is conducted.
More is known about atmosphere-surface coupling than
the other four categories, perhaps because atmospheric
and oceanographic scientists have studied and modeled
the general problem for many years. Ocean circulation
modeling has also been conducted for many years, but the
level of detail required for comprehensive study of the
global effects of sea-level change has exceeded the capa-
bility of even the largest computers. ECurrent global ocean
models use about a 500-km grid. Typical sea-level changes
(over geologic time) produce coast line changes on the
order of 100 to 1000 km. Thus sea-level changes are
either subgrid scale or, at best, barely resolvable (hence
poorly treated) by current global ocean models.] Changes
in ice sheet volume are usually thought of as influencing
sea level; interest in the influence of sea level on ice sheets
is relatively recent. Likewise, it is uncertain how sea-level
changes can have a direct impact on ocean-atmosphere
chemical composition, but such composition changes, if
they exist, could cause substantial changes in global cli-
mate by altering the "greenhouse" effect.
Atmosphere-Surface Coupling
Virtually every facet of Earth's climate is controlled or
affected by the transfer of heat, moisture, and momentum
between the atmosphere and the underlying surface. For
example, most of the heat that drives the atmospheric
circulation is first absorbed as solar energy at the surface
and then made available as sensible heat (direct tempera-
ture change) and latent heat of condensation (heat released
when evaporated moisture condenses to form rain or snow).
Furthermore, the long-term circulation of the atmosphere
is constrained to conserve absolute angular momentum.
Momentum transferred to the Earth when the atmosphere
"rubs" against the surface must be balanced by a reverse
momentum transfer elsewhere arising from changes in
wind speed and direction. Surface easterly winds repre-
sent absolute momentum transfer from the Earth to the
atmosphere, and surface westerlies vice versa. Changes in
the regional distribution of surface "roughness" can di
ERIC J. BARRON AND STARLEY L. THOMPSON
rectly affect local surface winds and, owing to the momen-
tum conservation constraint, global wind patterns as well.
Surface Heat and Moisture Balance Heat balances
typical of three surface types are shown in Figure 1 1.1. In
general, the surface of the Earth receives a net positive
flux of radiant energy (a gain of solar energy minus a loss
of infrared energy) that is balanced by sensible heat and
evaporative losses. Land in the subtropics is relatively dry
and has a relatively high albedo, that is, reflectivity for
solar energy. Most of the net radiative gain is balanced by
sensible heat loss owing to the lack of moisture to be
evaporated. In contrast, the adjacent subtropical oceans
have low albedo and lose heat almost exclusively by evapo-
ration. Changing subtropical land to ocean, e.g., by a sea-
level increase, would produce a major change in the amount
of heat and moisture received by the overlying atmosphere,
perhaps producing a change in regional climate. As Fig-
ure 1 1.1 shows, near equatorial land is qualitatively simi-
lar to ocean because abundant vegetation makes it rela-
tively dark and moist. Thus changing equatorial-type land
to ocean would produce a smaller climatic effect than
changing subtropical land to ocean.
A local change in albedo of the surface can produce a
large effect on the local surface energy balance, but what
effect can it have on global-scale climate? A simple analy
R Net Solar and I R Radiation ( Wm~2 )
E - Evaporation / Transpiration ( Wm~2)
S Sensible Heat ( Wm~2 )
a _ Surface Al bedo
50 -15 -35
Subtropical Land R: Eji S
85 - 80 -5
Subtropical Ocean
a=007 it L
i__ ~ ~ _
Near
60 -45 - 1 5
Equatoria I Land Rll ED So|
FIGURE 11.1 Heat balances for some representative surface
types. On average, net radiative heating of the surface is bal-
anced by evaporative and sensible heat loss. Evaporation is the
preferred mode of heat loss at warm temperature; thus subtropi-
cal oceans lose heat mainly through evaporation.
OCR for page 187
SEA LEVEL AND CLIMATE CHANGE
sis based on our current understanding of global climate
theory suggests that a global change in surface albedo of
0.01 will produce about a 1°C change in surface tempera-
ture. The effect of less than global-scale changes in sur-
face albedo will be smaller in general, but requires de-
tailed modeling calculations to determine the actual sensi-
tivity.
We can now make a rough estimate of the effect of sea-
level change on surface albedo and temperature. Suppose
that in a latitude zone with an initial land fraction of 0.4, a
sea-level rise reduces the land area fraction to 0.3 (a rather
large change, probably requiring a sea-level increase well
in excess of 100 m). Assume also that the difference in
surface albedos between the covered land and the ocean is
0.1. The average change in surface albedo in the zone is
thus (0.4 - 0.3) x 0.1 = 0.01. The predicted rise in surface
temperature of the zone (assuming no interaction with
adjacent zones or other effects) is about 1°C. Thus a
major change in sea level is not likely to cause a large
change in global-scale surface temperature from an albedo
effect only. This approximate result is confirmed by more
detailed calculations by Thompson and Barron (1981) that
examined the effect on albedo of shifting the continents to
the positions they had 100 m.y. ago (Ma). Factor of 2
changes in land area in the subtropics produced a change
of only a few watts per square meter (W/m2) in the surface
energy budget of the zone.
Referring again to Figure 11.1, we see that plausible
changes in surface wetness can produce much larger
changes in the surface energy balance than can plausible
changes in albedo. Sea-level change provides an ideal
way to produce an extreme change in surface wetness. No
calculations have been done with global climate models in
which sea level has been changed and only surface wet-
ness has been allowed to vary (i.e., no albedo changes or
other effects). Thus it is difficult to say just how important
changes in wetness are compared to albedo changes, but
we can get some indication from simulation studies that
have changed land surface properties to investigate other
problems. Shukla and Mintz (1982) performed two simu-
lations with a global climate model: one in which all land
was assumed to be completely dry (no evaporation al-
lowed) and the other assuming land to be completely wet
(like the ocean). Thus the simulations can give an indica-
tion of what might happen if very dry desert were changed
to ocean without any albedo change. The calculations
showed the dry land case producing summer land surface
temperatures 10° to 20°C higher than the wet case. Clearly,
evaporation from the surface can have a powerful cooling
effect.
Moisture flux from the surface not only transports heat
but causes changes in atmospheric water vapor content
that can affect cloudiness and precipitation. Yeh et al.
~7
(19841 have done simulations with a global climate model
in which soil moisture was perturbed to see the effect on
surface temperature and rainfall. They found, in general,
that increased surface evaporation led to increased precipi-
tation, but not necessarily in the same region as the in-
creased evaporation. Increasing evaporation in a surface
divergence region (a region of downward air motion) led
to increased precipitation in adjacent convergence regions,
but not locally. The explanation of the effect is that it is
difficult to form precipitation in regions of downward
motion regardless of the amount of water vapor in the air.
Excess humidity in subsidence regions is moved by winds
to regions of upward motion and then rained out. More
recently, Gordon and Hunt (1987) have further empha-
sized the role of simulated surface hydrology and precipi-
tation patterns.
From the above discussion, it can be concluded, for
example, that replacing land with ocean in the normally
dry subtropics might not lead to any local rainfall increase,
but might instead produce increased equatorial rainfall.
Aside from noting these potential local and nonlocal
mechanisms, more specific statements cannot be made
about the effects of sea-level change on rainfall without
more-detailed global climate model calculations.
What happens if surface albedo and moisture availabil-
ity are changed simultaneously? Henderson-Sellers and
Gornitz (1984) performed climate model experiments in
which the effects of Amazon deforestation were exam-
ined. The deforestation was simulated by changing the
surface albedo from 0.1 1 to 0.19 and by greatly decreasing
the potential for surface evaporation and transpiration.
These changes are similar to what one would expect from
a sea-level drop, i.e., replacing sea with land. The re-
searchers found that there was no net surface temperature
change in the deforested area. The surface albedo and
wetness changes for this particular case produced nearly
complete compensating effects. There was, however, a
local decrease in rainfall of about 10 percent over the
deforested area, as expected from the discussion above.
There were no global-scale climate effects that could be
reliably detected above the normal model variability. Even
though these results are only suggestive, they do indicate
that the direct climate effect of large sea-level changes
may not be large even on regional scales.
\ .~ ~ ~ ~ J .~ .^ ~ ~ ~ ~ _
Surface Roughness The transfer of heat, moisture, and
momentum between the atmosphere and surface is propor-
tional to the surface drag coefficient (CD), a nondimen-
sional quantity that expresses the proportionality between
the friction force per unit area at the surface and the square
of the surface wind speed. The value of CD is strongly
dependent on the "roughness" of the surface as measured
by the characteristic height of surface protrusions such as
OCR for page 188
188
TABLE 1 1.1 Typical Values
of Surface Drag Coefficient,
CD (X10-3) (from Garratt 1977)
D
Ocean
Wind speed 5 m/s 1
Wind speed 20 m/s 2
Land
Desert 3
Tropical forest 25
Typical 1 0
rocks or plants. Typical values of CD for land and ocean
are given in Table 11.1. In general, the drag coefficient
over ocean is an order of magnitude smaller than it is over
land. This has the effect of (a) tending to produce de-
creased surface fluxes over ocean and (b) producing a
higher surface wind speed over ocean by way of compen-
sation. [Substantial momentum transfer between the Earth
and the atmosphere is also created by large-scale orogra-
phy, the so-called "mountain pressure torque" (see Holton,
1979, pp. 264-265~. It is unlikely that reasonable sea-
level changes alone would produce climatologically im-
portant changes in topographic heights relative to sea level.]
It is not clear what effect a change in sea level could
have on large-scale climate through a change in CD. Some
sensitivity studies with climate models (e.g., Hansen et al.,
1983) have shown that the simulated climates are not very
sensitive to CD. On the other hand, it is well known that
changes in momentum exchange at the surface can have a
substantial effect on atmospheric circulation systems
(Horton, 1979, p. 260~. It is not possible at this time to say
how important sea-level-induced changes in surface rough-
ness are in relation to changes in albedo or wetness.
Combined Surface Effects Barron and Washington
(1984' performed two climate model simulations for con-
tinental positions of 100 Ma. One case was for present day
sea levels and the other included a sea level estimated to
be 330 m higher than the present that is thought to have
occurred during the Cretaceous period. This is a massive
sea-level increase that would have caused a 20 percent
decrease in global land area. Barron and Washington
found that the sea-level change produced systematic local
climate changes only in the subtropics and tropics. In
these regions, changing land to ocean produced local
cooling (the evaporation increase outweighed the albedo
decrease) and slight warming elsewhere. Zonally aver-
aged temperatures changed by less than 2°C at any latitude
(see Figure 11.2~. The global net effect on surface tem
ERIC J. BARRON AND STARLEY L. THOMPSON
perature was small. The postulated change in Cretaceous
sea level produced a smaller global climate effect than did
correspondingly large changes in topography and conti-
nental positions.
Ocean Circulation
In a number of instances, sea-level changes have been
invoked as a mechanism of modifying the ocean surface
and thermohaline circulations. The sea-level ocean circu-
lation mechanism involves bathymetric control of current
directions, importance of barriers in basin-basin commu-
nication, and the role of shallow epicontinental seas and
marginal basins in the formation of deep water. Each
example cited below illustrates the potential for sea-level
changes to influence climate, but none of the studies to
date provides quantitative links between sea-level-induced
circulation changes and climate change.
The high-velocity core of the Gulf Stream scours the
Blake Plateau, producing a 50- to 75-km-wide band of
erosional topography. Pinet and Popenoe (1982) used
seismic stratigraphy and a series of drill holes to show that
the axis of the Gulf Stream has shifted hundreds of kilo-
meters repeatedly during the past 20 m.y. The position of
the Gulf Stream axis is well correlated with sea-level
310
305
A 300
o
295
290
llJ
rL 285
L3J
280
275
270 _
26s
315 I ~ 1 l- I 1 1 1 1 1 1 1 1 1 1 l l 1
: _
_ ~
- ,/ Low Sea Level
~/ --- High Sea Level
~<
it'
. ,, 1,, I,, 1 1 1 1, 1 1 1 1 1
an 60 30 0 ~90
LAT ITUDE
FIGURE 11.2 A comparison of the zonally averaged surface
temperature for a general circulation model experiment with
Cretaceous continental positions with low sea level (present-day
total area) and no topography and an experiment with Cretaceous
continental positions with high sea level (about 20 percent of
continental area flooded) and no topography.
OCR for page 189
SEA LEVEL AND CLIMATE CHANGE
1.
it=
~ 3so~ ~ ~
~ ~ 1
If/ ~ CON N ECTE D
BASINS
GROWI N G
/ICE SHEET
2 ~WATER TRANSFER
/ ~: ~
~/ :'~i./'7/<
LOWERED / ¢///////////////
SEA LEVEL
3. ~
SEA ICE EVAPORI TES
4;~
LESS SALINE OCEAN
FIGURE 11.3 A hypothetical example of the potential for sea-
level change to affect climate through a mechanism that relies on
isolation of ocean basins. (1) The world ocean and a subtropical
sea communicate across a shallow strait. (2) A global climate
change induces ice sheets to grow. Lowered sea level isolates
the subtropical sea, which begins to evaporate. Fresh water is
transferred to the world ocean by precipitation. (3) Lower salin-
ity of the world ocean promotes a sea ice increase, which acts as
a positive feedback to the global cooling.
variations. During lowstands of sea level, a broad bathymet-
ric bulge, called the Charleston bump, deflects the Gulf
Stream offshore to a position across the Blake Plateau and
along the steep continental slope just north of the Blake
Spur. During sea-level highstands, the Gulf Stream paral-
lels the Florida-Hatteras Slope. The Gulf Stream has a
notable impact on regional climates, thus the potential for
climate change. However, the extent of possible climatic
change due to such movements in current direction in
response to sea-level variation is unknown.
The elevation of a barrier above sea level or subsidence
of a ridge below sea level has been cited as a mechanism
of climatic change in numerous instances. The subsidence
of the Greenland-Iceland-Faeroe Ridge (about 38 Ma)
probably resulted in a significant interaction between the
Arctic and Atlantic oceans; subsidence of the Walvis Ridge
off South Africa (about 20 Ma) greatly influenced the
Bengula current system; and formation of the Isthmus of
Panama (about 2.3 Ma) eliminated equatorial
Atlantic-Pacific flow (Berggren, 19821. Berggren stressed
the role of ocean gateways in modifying heat transport by
the oceans. Sea-level variations have the potential to have
breached or created surface circulation barriers during Earth
history, but few instances related specifically to sea level
are documented, and in every case the climatic implica-
tions are qualitative.
Sea-level change can isolate or reconnect small basins.
Particularly in subtropical high-evaporation zones or re-
gions where precipitation greatly exceeds evaporation,
isolation of a basin from the main ocean can result in large
density contrasts (i.e., hypersalinity or freshening of the
basin). Thierstein and Berger (1978) hypothesized that
reconnection of a temporarily isolated basin can result in
an injection event that favors either abyssal stratification
or surface stratification depending on the salinity charac-
teristics. Brass et al. (1982) showed that deep water can
form in marginal seas in the subtropics due to high evapo-
ration (warm, salty bottom water). These authors suggest
that changes in the size and configuration of marginal seas
in the subtropics owing to sea-level change may have
controlled deep-water formation during different periods
of earth history. Deep water tends to form in semirestricted
basins or marginal seas because the isolation allows the
water mass to obtain different density characteristics,
through atmospheric interaction, in comparison with the
main ocean. These marginal seas are certainly susceptible
to sea-level variations that may modify their size and
configuration. Basin isolation and induced salinity vari-
ations may also result in climate change because fresh
water freezes more readily (see hypothetical example in
Figure 11.3~. The potential of this mechanism to modify
climate is largely unexplored.
Annual Temperature Cycle
The heat capacity or thermal inertia of the surface is a
strong determinant of the annual cycle of temperature at a
given latitude. The thermal inertia of the ocean is depend-
ent on the depth of the mixed layer, ranging from tens of
meters to in excess of 200 m. In contrast, the thermal
inertia of land is roughly equivalent to a 1.0-m depth of
water. Extensive continental flooding is likely to modify
the local thermal inertia, and hence influence the annual
cycle of temperature.
In experiments using the seasonal zonal energy balance
climate model of Thompson and Schneider (1979) to in-
vestigate the contrast between the climate of the Creta-
ceous and that of the present (Barron et al., 1981), the
changes in land fraction associated with Cretaceous geog-
raphy resulted in a 3° to 5°C reduction in the amplitude of
the annual cycle of surface temperature in the Northern
Hemisphere mid-latitudes (i.e., the summers were cooler
and winters warmer than present). The mid-latitude ocean
fraction increased by approximately 20 percent in the
Cretaceous case. In this model the zonal thermal inertia
OCR for page 190
190
was computed from an area-weighted harmonic mean
assuming no land-sea heat transfer within a zone.
More recently, the general circulation model experi-
ments with Cretaceous geography and mean annual isola-
tion of Barron and Washington (1984) have been extended
to a seasonal mixed-layer experiment. The zonally aver-
aged amplitude of the annual cycle of surface temperature
in the Northern Hemisphere mid-latitudes was reduced 4°
to 8°C in comparison with a present-day control experi-
ment. The largest component of this zonally averaged
difference is regions that were land initially but become
oceanic when the sea level was increased. If this compo-
nent of the zonal average is removed, the change in ampli-
tude is 1° to 3°C.
The above two simulations illustrate the potential for
sea-level fluctuations to produce changes in seasonality.
However, each experiment described above includes a
number of variables that could influence seasonality (e.g.,
ice-albedo feedback), and consequently the importance of
thermal inertia has not been isolated.
Ice Sheets
The influence of sea level on ice sheets became a topic
of interest when the potential instability of the present
West Antarctic Ice Sheet became widely known. This ice
sheet is the smaller of two major ice masses covering
Antarctica. Unlike the East Antarctic Ice Sheet, the west-
ern ice sheet is mostly grounded below where sea level
would be if the ice were removed. Apparently, the ice
sheet exists only because floating ice shelves at its edge
act as buttresses to prevent the ice sheet from quickly
flowing out into the ocean. These ice shelves are them-
selves stabilized by friction against protruding islands,
underwater rises, and the sides of the bays in which the
shelves sit. Such marine ice sheet-shelf systems have been
shown to be potentially unstable to perturbations such as
ocean warming or sea-level rise (Thomas et al., 19791.
Evidence and modeling studies (Thomas and Bentley,
1978) indicated that the West Antarctic Ice Sheet was
much larger 18,000 yr ago during the height of the last
glacial episode. This increased size was made possible by
the almost 100-m lower sea level at that time. The hy-
pothesis is that the rise in sea level associated with the
melting of the great Northern Hemisphere ice sheets caused
a substantial collapse of the large West Antarctic Ice Sheet
until it stabilized at its present size. A further sea-level
rise could presumably act to unpin the buttressing ice
shelves that allow the remnant ice sheet to exist.
Although the massive East Antarctic Ice Sheet is in no
danger of total collapse, it too has undergone fluctuations
in size associated with sea-level changes. During the last
glacial maximum this ice sheet probably extended 75 to 90
ERIC J. BARRON AND STARLEY L. THOMPSON
km farther onto the continental shelf than at present (Alley
and Whillans, 1984~. The rise in sea level at the end of the
Ice Age produced a rapid retreat response at the ice sheet
edge that propagated as a wavelike reduction of ice sheet
thickness to the interior of the ice sheet.
Although the basic link between large sea-level changes
and ice sheet growth and decay is fairly clear, it is not
obvious that small sea-level changes in isolation have a
substantial effect on global climate. The principal effect
on climate would probably arise from the change in area of
the ice sheet and associated sea ice. A further regional
climatic effect would come from changes in ice sheet
topography. If large glacio-climatic changes in sea level
are needed to have significant effects on ice sheets, then
any sea-level-induced ice sheet changes would act only as
positive climatic feedbacks rather than actual driving forces
for large climatic changes. The level of positive feedback
could be quite small, e.g., a 50-cm rise in sea level due to
ocean thermal expansion could cause such a small reduc-
tion in ice sheet size that any positive feedback on ocean
temperature would be negligible.
Ocean Chemistry
The most uncertain potential direct effect of sea level
on climate involves the conjectured relation between sea
level, ocean-atmosphere chemistry, and the greenhouse
effect. Studies of cores drilled in ice sheets have shown
that atmospheric CO2 concentration was about two thirds
of its present value during the last glacial maximum. Since
CO2 is a greenhouse gas, the change in atmospheric com-
position apparently acted to substantially augment the
postglacial warming (Thompson and Schneider, 1981).
It is not clear what caused the atmospheric CO2 in-
crease at the time of deglaciation, but it is known that the
changes must have originated in the oceans (Broecker,
1984~. An initial hypothesis (Broecker, 1982) was that
increased sea level due to melting Northern Hemisphere
ice sheets covered previously exposed continental shelf.
Increased sedimentation of organic matter onto the shelf
would have removed phosphorous from the ocean. Since
phosphorous is a limiting nutrient, such a loss would de-
crease ocean phytoplankton productivity, reduce the up-
take of CO2 in surface water, and hence produce an atmo-
spheric CO2 increase. A second hypothesis (Berger, 1982)
was that the postglacial sea-level rise encouraged deposi-
tion of carbonate sediments on the shelf, an action that
would have generated an increased CO2 content in the
surface water and the atmosphere. The source of carbon-
ate to be deposited, and hence CO2, would have been
dissolution of deep-sea sediments. However, more recent
hypotheses of the glacial to interglacial CO2 change have
not directly involved sea-level changes (Broecker, 1984~.
OCR for page 191
SEA LEVEL AND CLIMATE CHANGE
Whether sea-level changes can directly affect atmospheric
CO2 concentration is an unresolved question.
INDIRECT SEA-LEVEL AND CLIMATE
ASSOCIATIONS
The global cooling trend over the past 70 m.y. is well
correlated with a gradual decline in global sea level or
increase in global land area. The global cooling from the
Cretaceous to the present day has been estimated to be in
the range of 6° to 12°C in globally averaged surface tem-
perature (Barron, 1983~. The decrease in global sea level
is estimated to be as much as 300 to 400 m (Hardenbol et
al., 1982) and an increase in total land area of 20 percent
(Barron et al., 1980~. Early comparisons between sea-
level variations associated with this long-term trend and
paleotemperature (e.g., Damon, 1968) have been used to
infer a strong causal relationship between sea level and
climate over the past 100 m.y.
Barron and Washington (1984) performed an extreme
sea-level sensitivity experiment with a general circulation
model of the atmosphere. This experiment compared mean
annual simulations for Cretaceous geography with flooded
continents and with present land area (nonflooded conti-
nents), which were described earlier. The globally aver-
aged surface temperature response to increased sea level
and decreased land area in the model was -0.2°C (Figure
11.2~. In the subtropics, where the majority of the flood-
ing occurred, increased evaporative cooling (about 60
W/m2) and a small increase in cloud cover (2.5 percent)
compensated for the surface albedo change.
The results of the sea-level climate model sensitivity
experiment bring into question the direct explanation of
the global cooling trend as a function of sea-level and
surface-albedo variations. The alternative possibilities are
(1) inadequate model sensitivity, most likely because of a
lack of a seasonal cycle or a fully resolved coupled ocean
model, or (2) an indirect association of sea level and paleo-
temperatures. Point (1) is discussed in detail by Barron
and Washington (1984), and here we will introduce only
one plausible indirect association between sea level and
paleotemperatures.
Berner et al. (1983) and Lasaga et al. (1985) performed
calculations with a geochemical model based on the
carbonate-silicate geochemical cycle to test the possibility
of atmospheric CO2 concentration changes on time scales
of 1 m.y. over the past 90 m.y. On this time scale, CO2
removal from the atmosphere is largely dependent on
weathering of exposed silicate rocks (with higher sea level
this area decreases). One explanation of higher global sea
levels is increased rates of seafloor spreading (see Harri-
son, Chapter 8, this volume). If seafloor spreading rates
are high, then a logical implication is greater volcanism
191
and hence higher CO2 input into the atmosphere (see Arthur
et al., 1985, for a discussion on substantially higher vol-
canic input estimates for the Cretaceous). Berner et al.
(1983) suggested that this substantially higher volcanic
input of CO2 cannot be compensated by increased rate of
weathering of exposed silicate rocks. In this scenario,
high paleotemperatures would result from a CO2-induced
warming that would be associated with high sea level, but
the warming would not be a direct response to sea-level
. .
variations.
The problems with the atmospheric CO2 model of Berner
et al. (1983) include estimating actual rates of CO2 de-
gassing through time, estimating actual seafloor spreading
rates (Berner et al. used four different estimates of seafloor
spreading variations), and determination of how directly
weathering of silicate and carbonate rocks responds to the
level of atmospheric CO2 concentrations (Berner and
Barron, 1984~. In addition, Shackleton (1985) suggested
that the carbon isotope record may limit the potential
variability of atmospheric CO2. Despite some unresolved
problems, the CO2 model of Berner et al. (1983) and
Lasaga et al. (1985) would solve the problem of explain-
ing the long-term correlation between sea level and paleo-
temperatures in light of small climate model sensitivity to
large-scale continental flooding, and is a good example of
a potential indirect association between sea level and cli-
mate.
SUMMARY
The direct effects of sea level on climate include changes
in atmosphere-surface coupling, ocean circulation, ther-
mal inertia, ice sheet-ocean interactions, and changes in
ocean-atmosphere chemical composition. In only a few
cases has the direct effect of a specific variable been iso-
lated. In the majority of the climate model simulations,
the model experiments provide insight into the importance
of various physical mechanisms, but a series of sensitivity
experiments should be conducted to isolate the importance
of variables such as surface albedo, surface wetness, sur-
face roughness, and thermal inertia. Many of the argu-
ments presented for the importance of various mecha-
nisms (e.g., ocean gateways that influence oceanic heat
transport) are purely qualitative. The first steps toward
placing these concepts on a more firm physical foundation
must be taken if we are to demonstrate the potential role of
sea level in climate change.
Further, correlations between sea level and climate in
the geologic record may not be a product of the direct
mechanisms described here. The possibility exists, for
example, that the causes of sea-level change may influ-
ence climate (e.g., tectonic control on sea level and vol
OCR for page 192
192
canic CO2 emissions), and the paleoclimatic associations
with sea level may be indirect.
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Representative terms from entire chapter:
ice sheet
