At 10:43 15.1.2001 -0500, you wrote:
>    Rising sea levels may or may not also be affected by rising land
>masses? Frinstance, does the rising of the Mid-Atlantic ridge tend to
>"push" more sea water toward the opposite shores? Does the rising in the
>"Ring of Fire" (volcanic activity along the Pacific) tend to "push"
>water toward other more dormant areas?
>        Of course, it all happens fairly slowly---but don't these rises
>tend to complicate estimates of the total rise in sea level?
>            Q-Man
>
>
As a person living on the Mid-Atlantic Ridge and working on crustal
movements I feel I have to respond to this. First of all, global sea level
is influenced by a lot of things. Basically, these factors can be divided
into two groups: 1. The ones that have to do with the volume of the water,
s.a. melting of the glaciers, change in climate, temperature of the ocean
(thermal expansion and such),
And: 2. The ones that have to do with the volume of the container, i.e. the
ocean basins. These factors include sedimentation, uplift or subsidence of
the crust in an area, and volcanism. So if the Hawaii Hotspot, for example,
increases its activity it will raise global sea level, but only slightly.
The activity of the Mid-Atlantic Ridge is much higher, making new crust at
the rate of about 2 cm per year, but it is steady-state, i.e. its rate does
not change much on the time scale of a million years. So it does not affect
global sea level. If, however, the rate changes, it will affect the average
temperature of the crust. If the rate increases the crust will expand, the
ocean bottom rise, and the oceanic container get smaller. This would result
in rising sea level (it is just like squeezing a plastc coke bottle). So as
long as the Mid-Atlantic Ridge (and all other active ridges) keeps on doing
what it does at a constant rate, sea-level will not be affected.


Long-term trends in relative sea level, as measured by tide gauge records,
generally show a rise of 2-4 mm/yr over the past 50 years or so for
locations along the Atlantic coast of the U.S. Estimates for the global sea
level change range between 1.6 and 2 mm per year. Most of this change is
thought to result from global warming, i.e. melting of glaciers and thermal
expansion of the ocean volume. Further global warming is expected to
increase the long-term, background rates of sea level rise. It has been
estimated that by 2050  global warming will be responsible for a 100 mm
rise in global sea level, in addition to the increase expected by
extrapolating current trends. Even higher numbers have been mentioned and
these estimates will definitely change as more knowledge accumulates.

Páll Einarsson
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Páll Einarsson
Science Institute, University of Iceland
Hofsvallagata 53
107 Reykjavik

Phone +354 525 4816,  fax +354 552 1347, email [log in to unmask]
http://www.raunvis.hi.is/~palli/
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