Environmental Engineering Reference
In-Depth Information
2
Intermediate
-
eustatic
changes in global water volume and isostatic changes in basin
geometry driven by localized vertical displacements of crust operate over 10
1-5
years.
3
Long
-
tectonic cycles
alter ocean geometry at the longest time scales (10
5-8
).
The coastline is essentially in equilibrium with wave and tidal variations over short
periods. Our concern is primarily with the susceptibility of sea level and coastlines to
intermediate fluctuations, which are of greater significance despite far slower rates and
smaller magnitude than tides. Water volume depends on the global hydrological cycle,
but with over 97 per cent of global water mass held in the oceans, it possesses short-term
stability. Ocean basin and water mass dimensions are shown in Table 11.1.
Table 11.1 Principal ocean basin statistics
Ocean
Area
(10
6
km
2
)
% global
ocean
% Earth's
surface
Volume
(10
6
km
3
)
Mass
(10
23
kg)
Density (g
cm
-3
)
Mean
depth
(km)
Atlantic
94·3
24·9
17·7
340·3
3·5
>1·03
3·57
Arctic
12·2
0·9
0·6
13·7
0·13
~1·03
1·17
Indian
74·1
21·1
14·9
286·7
3·0
~1·03
3·84
Pacific 181·3 53·1 37·6 717·8 7·4 <1·03 3·94
Global 361·9 100·0 70·8 1358·5 14·03 1·03 3·73
Note
: The southern ocean is counted into the southern areas of the three main oceans; the figures
include all epicontinental seas, included with the most appropriate ocean.
EUSTATIC CONTROL OF SEA LEVEL
Eustasy is the control of sea level by water volume. Eustatic change is generally world-
wide and immediate because water effectively finds a common level. Change occurs
either by
steric
effects - adjustments to sea-water density via temperature or salinity - or
through net mass transfers between coupled stores. Thermal expansion would raise sea
level by approximately 0·8 m for a 1° C rise in global temperature before any ice melts.
Changes in atmospheric pressure also create measurable changes in sea height, falling in
anticyclonic (high-pressure) and rising in cyclonic (low-pressure) conditions at the rate of
about 1 cm hPa
−1
. The ocean-lithosphere couple, which cycles water through oceanic
crust via subduction and hydrothermal circulation, is assumed to be in equilibrium -
partly because it is difficult to assess!
We have a better grasp of ocean-atmosphere-cryosphere coupling and its component
terrestrial hydrological cycle. Intermediate-term instability is associated with the growth
and decay of Quaternary ice sheets which form the bulk of the planetary
cryosphere
(ice-
bound systems). Evaporated ocean water stored in terrestrial glaciers during a cold stage
causes a eustatic
fall
in sea level (Plate 11.2). Deglaciation causes a
rise
, corresponding
to the water-equivalent ice mass melted and returned to the oceans. The most recent
glacial/interglacial cycle of the past 125 ka experienced eustatic changes of about ± 130-
165 m. Modern sea level will rise by a further 60-80 m if the remaining ice sheets melt,
with major coastline implications. The rate of change can be rapid, with a rise from −130
