1. 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)

2. 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 we’re not there yet, of course.

3. 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.

4. For the moment, we’ll ignore the reference level terms and the bottom
For the moment, we’ll 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.

5. The important thing to note is that
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

6. 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.

7. 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.

8. 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 …

10. 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, it’s not clear that people from Colorado are very good at the sign of vertical changes either ….

12. 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 let’s hold substantive discussion and debate until the end.
Thanks … Gary