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Measuring sea level and the Earth -

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The Earth isn't a sphere. It is flattened at the poles. These days, the Earth is measured from
space. Sea level is measured by radar altimetry. The level of the sea changes by different amounts
in different places. Some areas of Sumatra have seas which are rising at a rate of 30mm/year. Other
areas in the Pacific are dropping by 20mm/year. A new mission, to be launched in October 2008 aims
at measuring the Earth's gravity field from space in very fine detail. This will provide a
reference point from which future changes can be measured.

Robyn Williams: Arizona is of course lumpy. Plenty of red sierras, mountains and hills, with
two-metre high cactuses all over. But can the sea be lumpy, and even be higher in some places than
others? Sounds daft, doesn't it! A bit like having hillocks in your bathwater or swimming pool.
Marek Ziebart from University College London is here to tell us what's going on and why. A new
mission is being sent off shortly to measure the surface variations.

Marek Ziebart: Actually go back to the time of Newton when a number of expeditions were made to
Greenland and South America to try to measure the shape of the surface of the Earth, and it was
that time when we realised that the Earth, rather than being spherical is in fact more flattened at
the poles somewhat. These days we're concerned with much finer resolution to understand the shape
of the Earth to use that as a reference surface against which to measure changes in the planet.

Robyn Williams: How do you measure that?

Marek Ziebart: There are many ways of doing it but the big revolution in the last, say, 20 years is
to measure the Earth from space by using probes in orbit that send down radio waves which bounce
off the surface and then we look at the pattern of those radio waves and from that we can
understand the shape of the fluid surface and the shape of the land masses.

Robyn Williams: Some people find it difficult to understand that the sea level might in fact be
different in different places. We assume that sea level is just that, universal. But how much does
it change?

Marek Ziebart: Over the last 12 years, using a technique called radar altimetry, we've been able to
measure changes in the oceans at very, very fine resolutions and over most of the ocean bodies.
Most people may have heard that we think sea level rises on average by about 1mm to 2mm per year,
but that's not true. In different places it changes very rapidly and by different amounts. I'll
give you an example. If you go to a place like Sumatra then our current estimates of sea level rise
there is about 30mm per year. In other places, in the Pacific for example, it could be dropping by
as much as 20mm per year. So the 1mm to 2mm per year is the average of all those effects, but the
signals in certain regions are very different.

Robyn Williams: Why doesn't it just reach the universal low level? Is it gravity?

Marek Ziebart: The shape of the surface is driven by the things that are pulling it. We have a mass
distribution within the Earth and that drives the shape of the fluid surface, but on top of that of
course is a dynamic system. The ice caps are melting, water is locked up in dams and in reservoirs,
so the whole thing is very dynamic.

Robyn Williams: So what is this new expedition that you're going to mount fairly shortly?

Marek Ziebart: The mission that we're talking about here is a mission called GOCE. The aim of this
mission is to measure the Earth's gravity field from space in unprecedented detail, and it has some
revolutionary technologies on board the spacecraft. It's the first time that we've attempted a
mission of this kind.

Robyn Williams: Tell me about it. When is it going to happen, and how?

Marek Ziebart: It's going to be launched from the Plesetsk Cosmodrome which is about 800 kilometres
north of Moscow. It will go into what we call a polar orbit. So it will go continually over the
poles of the Earth, and as the Earth turns beneath the spacecraft it will gradually sample the
gravity field from all the way around the Earth.

Robyn Williams: How long will it operate?

Marek Ziebart: It's scheduled for about an 18-month lifetime, but it could go on for longer. Some
of these missions can go on for years if the instruments keep working.

Robyn Williams: You paint the picture of this experiment having extraordinary implications for the
future of virtually everybody. Is it that big?

Marek Ziebart: It is that big, and the reason for it is because what this mission in particular
enables us to do is to understand what the energy transport mechanisms are in the ocean. The ocean
carries huge parts of the energy budget of the planet around the ocean, and so as that changes, as
climate changes, then it's going to affect everybody. Here in the UK everyone is aware of climate
change these days and our climate is very affected by changes in the Atlantic. Most of our weather
is driven by the moisture and the winds and the energy transport mechanisms from the Atlantic.

The difficulty we have at the moment is that although we know the shape of the surface of the ocean
from space, what we don't know is the physics of it. We can't model the physics of it because we
haven't got a reference surface against which to measure these changes in height in the ocean. What
this mission will enable us to do is to define that reference surface very clearly. So then we can
say because that reference surface is at such a height, then the water is flowing in such a way and
it's driven by such forcings and such energies.

Robyn Williams: Okay, so once you know the differences and you've got this immense accuracy, what
can you tell the policy people about what to do next?

Marek Ziebart: It gives us two things really. First thing is that there are many operational
missions working now which monitor the ocean continually. Once they've got the reference surface
defined very clearly, then we can say what the changes actually are. At the moment it's really hard
to say this is exactly what the changes are, and to a certain extent we owe it to the politicians
and the policy makers to be clear about what our predictions are. At the moment, because of the
uncertainties in the inputs to our climate change models, people can play a game on that. They can
say, 'Oh it's not really happening,' or it's this uncertain or that uncertain. We owe it to the
world's people and certainly our economies to try and iron out those uncertainties.

Robyn Williams: What's Australia's role in this, big or small?

Marek Ziebart: Australia has a very big role in this area of science, and actually invests
responsibly and heavily in the area. Australia has several observing systems that they support on
the Australian landmass, they have big networks of GPS receivers, satellite laser ranging
facilities and very long baseline interferometry, and it plays a key role in contributing to the
world observing system.

Robyn Williams: The question about Tuvalu, some people have suggested that the sea levels are
rising and Tuvalu is in danger. Others say that like any volcanic system, once it's coming up it
then sinks again. So it's sinking of its own weight rather than anything else. Will your experiment
show one way or the other what's going on?

Marek Ziebart: We'll provide a reference system and against that we can measure these changes very
accurately. So if anything, like Tuvalu, for example, let's say it's changing in height...we ask
the question 'how do we know?' And missions like GOCE will enable us to have a very clearly defined
reference system so we're actually looking at the geophysical signals and what they infer for
everybody.

Robyn Williams: And that launch is planned for October 5th, European time. Marek Ziebart is
Professor of Space Geodesy at University College London.