SEA LEVEL
I.
A. Most important variable that controls position
of
shoreline. Datum from which we evaluate topographic
and bathymetric variations.
1. Never level over meaningful spatial or temporal
scale, irregularities due to:
a. oceanographic conditions (currents).
b. terrestrial (river discharge).
c. meteorological (wind/barometric pressure).
d. geophysical.
2. Local episodic changes confuse interpretation of
rates of change.
3. Of interest to us are changes in Quaternary period,
most interested in past 11,000 years.
4. Major changes in Quaternary due to growth/
disintegration of ice sheets, fluctuations caused
oscillations of 130m/20,000 years.
5. Rates of rise due to melting have been uneven due
to tectonics and glacio-isostatic processes.
6. Scenario in high latitudes. Weight of ice
depressed crust beneath and forced
sublithospheric material out from ice center.
Forms peripheral bulge outside and low region
beneath ice center.
7. Crustal bulge moves after ice melts.
8. Bulge movement determines resulting sea level
record.
B. Contemporary Sea Level
1. Measured by tide gage data, not distributed evenly.
Near population centers U.S., northwest Europe,
and Japan. Data from areas that are being uplifted
or subsiding.
2. Estimates using these data suggest an ongoing
eustatic sea level rise.
3. Taking into account vertical changes due to
tectonic/glacio-isostatic conditions, new data
suggests sea level has risen at a rate of 1.3 mm/yr.
during the past century.
4. Most of rise due to thermal expansion (increase
volume) related to temperature increase.
Remainder due to melting.
C. Methods of measuring sea level. It is easy to
demonstrate that sea level has changed. Difficult to
quantify. To
determine rates, need series of dates, if
rising, features
submerged.
1. Most commonly used indicators are C14 dates on
salt
marsh peats.
2. As sea level rises, marsh surfaces grow upwards
and landward over upland soils. Base of marsh
(high) marks the leading edge of the transgression.
Mean high water at this point.
3. Problems arise when material beneath peat is
compacted and thus lower than when the peat was
originally deposited.
4. Most peat is now buried (lagoons or shore face).
5. Generally, core across a valley and collect peat
samples.
6. Sometimes use wood fragments from tree stumps.
Trees die as a result of salt water inundation. Can
use shell material, but it has to be in place (know
relationship to HW).
D. Future Sea Level Change.
1. Understanding of past and present changes is
uncertain and therefore makes it difficult to
confidently project future changes.
2. Currently rising at 1.3 mm/yr., early in deglaciation
rise was much faster.
3. Problem using past trends as predictor for future is
the warming of the atmosphere due to introduction
of
CO2 into the atmosphere.
4. NAS revised estimates indicate and increase in
greenhouse gases and temperature and sea level
rise. Models used predict a temperature rise of
1.5 to 4.5C for CO2 alone and as much as 3-9C for
all gases.
5. By 2100, sea level is likely to increase 56-345 cm.
Best estimates are around 175 cm.