20th Century Sea Level Change Estimates From Tide Gauges and Altimeters

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20th Century Sea Level Change Estimates From
Tide Gauges and Altimeters Speakers for this
session are Gary T. Mitchum (University of
South Florida) Mark A. Merrifield (University of
Hawaii) R. Steven Nerem (University of Colorado)
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Introduction to this session Our focus is on the
total sea level change. Other sessions will treat
the components. The hope is that this parameter
might usefully constrain climate change models,
even if we cannot precisely attribute the
changes.But were not there yet, of course.
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What is the session all about, and how does it
fit with the others? I will try to answer this
by reviewing (in a crude way) what ocean volume
variations depend on Start by considering a
small area element. The volume is
The reference terms will be discussed in later
geodetic sessions chaired by Neilan and Mitrovica.
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For the moment, well ignore the reference level
terms and the bottom. This is not too different
from what many of the historical studies have
done. That is, we will just focus on the sea
level (?) term. We can also write
to show explicitly how the total sea level
depends on the mass and density fields. These
separate components will be treated in the ice,
terrestial storage, and ocean thermal sessions.
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We will not focus on these attributions, but on
determining the total sea level. In particular we
want to know how well we can determine global
average of the total sea level. To look at this,
we will define
The important thing to note is that
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The main reason for this is low frequency
redistributions of mass and density. The session
by chaired by Lambeck is important to note on
this point, and see the poster by Roland Gehrels
in our session. The main point is that the noise
for the ocean volume calculation is very red. In
principle, altimeters defeat this problem by
enabling a true global average, which means that
the redistribution signals cancel out. What about
the tide gauges? This is a little more difficult.
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The tide gauge estimates of global sea level
change approximate the global integral of our
first equation with an an average over a discrete
set of points. Given the magnitude of the
redistribution signals, the question of how well
this discrete average estimates the global
integral is central. There is another LARGE
issue for the tide gauge estimates. In this case
the reference level for the sea level is
problematic. This is being addressed by GPS and
other methods now (see Neilan and Mitrovica
sessions), but important questions still remain.
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Outline for the session First we will have Mark
Merrifield of the University of Hawaii and
Director of the UH Sea Level Center discuss the
tide gauge estimates of sea level change. We do
this despite the fact that there is some evidence
that our Hawaiian colleague (Mark) is not clear
on what the SIGN of sea level change actually is
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We will then hear from Steve Nerem of the
University of Colorado about the estimates of sea
level change from satellite altimetry. But to be
fair to Mark, its not clear that people from
Colorado are very good at the sign of vertical
changes either .
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We hope you enjoy the session and we look forward
to a lively discussion. We will ask, however,
that you hold the discussion until after the
presentations. Questions aimed at clarifying the
material are welcome, but lets hold substantive
discussion and debate until the end. Thanks
Gary