Environmental Engineering Reference
In-Depth Information
Mean, Local, Eustatic, and Steric Sea Level
Mean sea level (MSL) is a measure of the average height of the ocean's
surface (such as the halfway point between the mean high tide and the mean
low tide). Local mean sea level (LMSL) is defined as the height of the sea
with respect to a land benchmark, averaged over a period of time (such as
a month or a year) long enough that fluctuations caused by waves and tides
are smoothed out. One must adjust perceived changes in LMSL to account
for vertical movements of the land, which can be of the same order (mm
y
-1
) as sea level changes. Some land movements occur because of isostatic
adjustment of the mantle to the melting of ice sheets at the end of the last
ice age. The weight of the ice sheet depresses the underlying land, and
when the ice melts away the land slowly rebounds, whereas some coasts
are sinking as a result of isostatic adjustment due to collapsing forebulges
in the near-field ocean of previously glaciated regions (Mitrovica and Milne,
2002). Atmospheric pressure, ocean currents and local ocean temperature
changes also can affect LMSL. The term eustatic refers to global changes
in the sea level brought about by the alteration to the volume of the world
ocean (e.g., melting of ice sheets). The term steric refers to global changes
in sea level due to thermal expansion and salinity variations. The term
isostatic refers to changes in the level of the land masses due to thermal
buoyancy or tectonic effects and implies no real change in the volume of
water in the oceans.
Global Sea Level
Since 1870, global sea level has risen by about 0.2 m (Bindoff et al.,
2007). Since 1993, sea level has been accurately measured globally from
satellites. Before that time, the data come from tide gauges at coastal stations
around the world. Satellite and tide-gauge measurements show that the rate
of sea level rise has accelerated (Figure 4.20). Satellite measurements show
sea level is rising at 3.1±0.4 mm y
-1
since these records began in 1993
through 2003 (Figure 4.21). This rate has decreased for the most recent time
period (2003-2008) to 2.5±0.4 mm y
-1
due to a reduction of ocean thermal
expansion from 1.6±0.3 mm y
-1
to 0.37±0.1 mm y
-1
, whereas contributions
from glaciers, ice caps, and ice sheets increased from 1.2±0.41 mm y
-1
to
2.05±0.35 mm y
-1
, respectively (Cazenave et al., 2008). Statistical analysis
reveals that the rate of rise is closely correlated with temperature. Sea level
rise is an inevitable consequence of global warming for two main reasons:
ocean water expands as it heats up, and additional water flows into the
oceans from the ice that melts on land.
