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Sea Level Change
When discussing sea level changes, most people are referring
to relative sea level change, that is, the observed change in water level
at a particular point (relative to the level of the nearby land) from
all causes. It is necessary to note that sea level has never been constant.
It is subject to both long-term and short term rise and fall from a variety
of effects. Over very long periods of time (millions of years) the continental
land masses change, with coastlines rising and falling. Added to that
are very long-term climate changes alternating between interglacial highs,
like the present, and glacial lows when large amounts of ice are locked
into continental glaciers.
Most people are concerned with changes in their own lifetime, and the
various effects causing short-term changes. Those changes are complex,
including various natural effects and some man-made effects. Recent public
discussion has been centered on global warming, with the possibility of
melting glaciers adding water to the oceans, a topic addressed by the
SCAR Task Group on Ice Sheet Mass Balance and Sea Level (ISMASS, see their
Web page at http://www.antcrc.utas.edu.au/scar/ismass.html
for information), and warming of ocean water causing expansion of the
water mass. Water is generally viewed as incompressible, but that is not
actually correct. When dealing with oceanic depths, a very small expansion
due to a temperature change can become significant. While these changes
due to warming or cooling are estimated at a present rate of 1.8 mm/year
of sea level rise, they may be significant over long periods of time on
a low-lying coastline (it has been estimated that the melting of the entire
Greenland icecap would add 7 meters to global sea level). In the short
term, they are often overshadowed by other effects such as major subsidence
(examples are the compression of sediments in a river delta, or problems
from withdrawal of ground water), or realitively rapid uplifting of a
land mass.
Changes in land levels of the continental land masses vary depending on
the location. Some areas are rising (resulting in a drop in relative sea
level), and changes in water levels due to climate change may help offset
the land level change (examples will be found in areas such as Sweden
and Alaska). Some areas are falling (resulting in a rise in relative sea
level), and changes in water level due to climate change will add to the
problem. Usually these changes occur slowly, but in particular coastal
areas the change can be sudden and quite large. In the case of the 1964
Alaska earthquake, uplifting of the land was as much as 10 meters in one
location, with the change occurring in not more than about a minute. One
coastal town was left with a dry harbor due to the uplifting of the land.
Another effect on observations of short-term sea level changes is the
variation in ocean currents. An ocean current, by its nature, has an associated
gradient in the water surface. Ocean currents tend to be unsteady flow,
varying with time. Water masses may move in "slug motion" causing
variations in the currents; the shearing stress between a current and
the adjoining water mass may produce large eddies that affect surface
water levels; interaction of currents with bathymetry may produce eddies;
wind-driven surface currents will vary due to variations in the wind;
and temperature changes may cause shifts in currents in addition to thermal
expansion or contraction of the water mass.
Added to the natural effects are various man-made effects. Other than
possible contributions to the global warming mentioned above, the most
significant may be the withdrawal of groundwater, or sometimes petroleum,
in a coastal area. That will usually cause compression of the ground resulting
in a drop in the ground level, i.e., the relative sea level (this has
been observed in coastal areas in China and elsewhere). The other major
problem, particularly in river deltas, is the compression of sediments
resulting in a drop in the ground level. Natural compression can be large
but is offset by uncontrolled rivers which will generally spread sediment
across a delta, periodically renewing the sediment that is compressing.
Controlling rivers for navigation and flood control generally prevents
the uncontrolled spread of new sediment and results in a more severe drop
in the ground level.
Any significant change in relative sea level, either a rise or fall, can
have adverse economic and social effects such as operating difficulties
in ports and harbors and adverse effects on access to the coast and ocean;
coastal flooding or drying; loss of wetlands important to fisheries; loss
of fresh water supplies from possible salt water intrusion into the coastal
acquifer in the case of sea level rise; and a possible increase in coastal
erosion.
A large part of the problem has been caused by the expansion of the world
population, and the movement of people into coastal areas. Many heavily
populated areas are near or below sea level, examples ranging from Bangladesh
to the Netherlands. Many coral atolls in the Pacific have little or no
high ground, a problem of particular concern to some members of the Association
of Small Island States.
It should also be noted that climate change can cause an increase in major
cyclones or anti-cyclones (referred to as hurricanes, typhoons, tropical
storms, etc.) with will produce an increase in the frequency of large
storm tides.
For more information see the reports of Working Group 1 (Science) and
Working Group 2 (Impacts) of the Intergovernment Panel on Climate Change
(IPCC). The IPCC Web page is found at http://www.ipcc.ch
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