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34C3 preroll music
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Herald: Hello everybody to the next talk,
here at stage Clarke. The next talk will
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be held in English. And here is a quick
announcement in German for the
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translation. Der nächste Vortrag wird in
Englisch sein. Und wir haben eine deutsche
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Übersetzung unter streaming.c3lingo.org.
Und wir haben das auch auf einer Folie.
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Und es wird auch eine französische
Übersetzung geben für diesen Vortrag.
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There will also be a french translation,
as well as an German translation for the
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next talk. And you can find everything
under streaming.c3lingo.org. And, I hope,
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displayed behind me. The next talk is
called "Watching the changing Earth".
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Satellite data and change in the
gravitational field of the earth can tell
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us a lot, especially when there's so much
public domain satelite data coming in from
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different projects or maybe CC-BY
satellite data. And how this is done, this
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new knowledge finding out of this big
heap of data, this will be explained by
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Manuel in the talk. He dropped stuff to
see if gravity still works, or, in fancy
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words, he does gravimetric methods and
sensory in geodesy. Is that pronounced
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right? I'm not sure, but give a big hand
and a round of applause for our speaker
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Manuel.
Applause
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No Audio
Manuel: Geiler Scheiß. Oh, das war Sound.
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So again, so hello and welcome to my
presentation on watching the changing
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earth. This year's call for papers for the
Congress offered me the opportunity to
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talk about my work in the related fields,
which is gravity. As far as Congress is
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concerned, a misunderstood force of
nature. So in the following couple of
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minutes, I want to talk about gravity,
gravitation, about the GRACE satellite
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mission, which maps the earth gravity
field every month, about the gravity
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fields, and I will show good results and
then we will go forward into the future.
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That's nice. So it's actually called,
actually called geodesy. Let me give you a
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short introduction on geodesy. Friedrich
Robert Helmert defined it in 1880 at as
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the science of mapping and measuring the
earth on its surface, and this still holds
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up today. It depends on your methods and
applications, but he was correct. The most
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known profession is probably land
surveying, people with colorful
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instruments and traffic cones. You find
them on construction sites, on the side of
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the road, but we actually have a lot of
applications not only in geodesy but in
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related fields like geophysics,
fundamental physics, if you want to build
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an autonomous car you need geodesists,
metrology. This talk is specifically about
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physical geodesy, which is the mapping of
the gravitational field of the earth, and
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in this case specifically with satellites.
So I drop stuff on the earth, which is
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terrestrial gravimetry, this talk is about
satellite gravimetry. Now gravity and
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gravitation, we usually talk about
gravitational potential. This is a scalar
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field. Gravitational acceleration is the
gradient of the gravitational potential
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and when we talk about gravity in geodesy,
it's usually the combination of attraction
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of the masses, gravitation, and the
centrifugal acceleration, but here we talk
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mostly about gravitation. And the
potential can easily be calculated, at
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least according to this very short
equation. We have G, which is the
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gravitational constant of the earth, or
other planets if you want to do. We have
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an ugly triple integral about the whole
earth, and this is basically what breaks
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the neck. We have to integrate about the
whole mass of the earth, we divide up into
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small parts and we need to know the
density of these parts. So, density times
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small volume, you have the mass of the
earth if you integrate over it. So what,
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the density of the whole earth is not
known. So if you want to calculate the
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potential sufficiently, you would need the
density of a penguin on the other side of
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the world. We don't know that. So, what
do you do if you cannot calculate the
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quantity? You write a proposal and get all
the funding. This is what happened about,
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let's say, twenty years ago, and the
result was the gravity recovery and
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climate experiment, or GRACE for short. In
this talk, we will only cover gravity
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recovery, so gravity field of the Earth.
As we can see, these are two satellites.
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They are flying in the same orbit, and the
main instrument is distance measurement
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between these two satellites, Here we see
the two satellites prior to its launch in
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2002, and this K-Band Microwave ranging,
which is the instrument, gives us a high
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resolution gravity field of the Earth.
This is spatial resolution of around 200
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kilometers (km). You might think 200 km is
not really high resolution, but we have it
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for the whole planet and not, let's say,
for Germany. And also we got the temporal
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variations. So for 15 years now, we have
each month, with only a few exceptions, a
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picture of the gravitational field of the
earth. The satellites fly in height of
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about 450 km, 220 km apart, and we see
here the orbits of a single day. So 15
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orbits per day, and we take one month of
data to generate one gravity field. The
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working principle is quite simple: The
distance between the two satellites is
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affected by gravity, so we measure the
distance and then we calculate gravity. In
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a homogenous gravity field, this is quite
simple: Let's say we take a spherical
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earth, it has only a single density, the
satellites fly along, and the distance
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between the two satellites does not
change. There is nothing to pull one or
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another, they just move along, not
changing the distance. Now we introduce a
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mass, let's say a mountain, this can be
any mass change or density change
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somewhere inside the earth, and the
leading satellite experiences a
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gravitational pull by this mass. And as
gravitation falls off with distance, it is
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a stronger than the pull experienced by
the trailing satellite. So the distance
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between the two satellites increases. Now,
the satellite, the trailing the leading
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satellite has passed the mass, and it is
still feeling its gravitational pull, but
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now it is being decelerated because the
mass is behind. And the trailing satellite
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is still being accelerated towards the
mass. This means the distance between the
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satellites decreases. And finally, the
second satellite passes the mass and it
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now also feels the gravitational pull
decelerating the satellite. The leading
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satellite is feeling less and less
gravitational pull and once both
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satellites left the gravitational
influence of this mass, we will have the
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same distance as prior to encountering the
mass. So the gravitational acceleration is
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a zero sum at this point. So of course,
the Earth is a little more complex than a
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single mountain or a single density
anomaly in the ground, but this is the
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basic concept. Now, how do we come from
these measurements to the actual
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potential? The formula is basically the
same as a couple of slides earlier. We are
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still calculating the potential. It looks
more complicated, but we don't have triple
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integrals anymore, and all these
quantities in here are basically easily
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calculated. We start with the
gravitational constant and the mass of the
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earth, which we can get from a physics
book, if we like. And then we have a
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couple of geometric quantities, a and r
are basically the size of my earth
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ellipsoid, the major axes and r is the
distance from a calculating point, let's
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say this podium, for which I want to know
the potential value to the center of the
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ellipsoid. And then we have lambda and
theta at the end, these are the
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geographical coordinates of this podium. P
is short for the associated Legendre
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functions, also depending solely on
geometry, not on the mass of the earth,
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depending on the software where you want
to implement this formula, it probably has
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already a function to calculate this, and
if not, it is easily done by yourself as
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the formulas look very long, but they are
quite simple. The interesting part are the
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two parameters C and S, these are
spherical harmonic coefficients. They
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include all the information about the
mass of the earth, as measured by the
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satellites. So we have the satellites in
space, and the user gets just the C and S
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coefficients, which are a couple of
thousand for the gravity field. Implements
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this formula and has a potential value.
So, these spherical harmonic coefficients
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are calculated from the GRACE Level 1B
products. These are the actual
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measurements done by the satellites. This
is the ranging information, the distance
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between satellites, satellite orbits, star
camera data, and so on. You add a couple
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of additional models for earth's gravity,
which you do not want to include in your
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satellite gravity field, and then you do
your processing. This is done by a couple
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of different groups JPL and GFZ, which is
a German research center for the
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geosciences. CSR is the center for space
research at university Austin. These three
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institutes also provides these GRACE Level
1B data. So they take the raw satellite
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data, process it to theGRACE Level 1B
products, which are accessible for all
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users, and then calculate further these
coefficients, C and S. But there are also
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additional groups who provide gravity
fields who calculate these coefficients,
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for example, Institute for Geodesy of the
University of Graz, or the Astronomical
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Institute of the University of Bern. They
all have slightly different approaches to
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topic and come to more or less the same
conclusions. There are countless papers,
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comparing these different gravity fields
with each other, but the user usually
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starts with the coefficients C and S, and
then it takes a formula like the one on
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top of this slide and calculates your
gravity value or whatever you want. Now,
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I'm talking about potential, I'm talking
about accelaration. These are not really
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useful quantities in day to day life. If
someone told to you in Greenland gravity
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decrease by 50 microGal, you have two
choices, you can say "wow, awesome" or you
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can say "oh no, we're all gonna die" It's
a 50:50 chance you'd say the correct
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thing. So we are looking for a more useful
representation of the changes in
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gravity field. Now gravity field reflects
mass redistributions and the most dynamic
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redistribution we have is water storage,
summer/winter, more snow, more rain, less
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water in summer, so we express our gravity
change in a unit called equivalent water
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height. This is the layer of water on the
surface with a thickness, equivalent to
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the mass change measured with the
satellites. This is also easily
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calculated. This is my last equation, I
promise, but this looks familiar. The
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second half of this equation, is basically
the same we saw one slide prior and the
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parameters in front of the sum is the
average density of earth, which is around
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5500 kg/m^3. We need the density or water,
let's say it 1000 kg/m^3. And in this
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fraction in the middle, we need to
parameter K, which are the so-called Love
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numbers. Now, this is not a numerical
representation of mutual attraction, but
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was put forward by, I think, Albert Love
in 1911, and they are parameters
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concerning the elastic response of the
earth to forces. So, if you put a lot of
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weight on a part of the earth, the earth
deforms and these parameters, describe the
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elastic response of the earth to such
loading. Now we have calculated our
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equivalent water height, let's say for two
months, let's say, in May 2002 and 15
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years later in May 2017 and we just
subtract these two gravity fields, these
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two equivalent waterheights, from these
two epoches. What we have left is the
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change in gravity between these 2 epochs,
15 years apart, expressed in water layer
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equivalent to the change in gravity
measured. And we can see a couple of
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features here. There should not be any
seasonal variations because it's the same
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month, just 15 years apart. So we see long
term gravity change between these two
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epochs. And what we see is, for example,
mass loss in the northern and southern ice
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shields, and we see two red blobs, one in
northern canada and one in northern
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europe, which are geophysical processes.
So this is glacial isostastic adjustment
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and during the last ice age the ice
shields deformed the earth downward.
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The material in the "Mantel" had to flow
aside, and now that the ice is gone, the
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lead is uplifting and the material in the
"Mantel" is flowing back. So it's flowing
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back and the earth is uplifting. This
process has been going on for 10000 years
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and will probably a couple of years
longer. Now how do you get your data?
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Everyone can get the GRACE Level 1B data,
which are the observations by the
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satellite, like again, ranging information
between the satelite, orbits,
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accelerometer data, star camera data and
so on. You can get them without hurdles at
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the ISDC, which is the information system,
a data center at the Geoforschungszentrum
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Potsdam, or at the Physical Oceanography
Distributed Active Archive Center run by
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JPL. And if you'd like, you can calculate
your own spherical harmonic coefficients
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for gravity fields. Or you can compare for
example, satelite orbits they give you
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with one you integrated yourself using
your own gravity field, to see if they fit
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together or not. You can get gravity field
models, if you'd like. A large collection
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is at the International Centre for Global
Earth Models. They have recent and
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historic gravity models all in the same
data format. So you only need to implement
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your software once from the 1970s to
today. They also have the proper
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references, the papers you want to read to
work with them. These are so-called Level
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2 Products. So, you can take a gravity
field from there, use the equation, I
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showed you earlier and calculate your
equivalent water height, if you'd like.
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If you don't want to do this, there is
someone to help you, a service called
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"TELLUS", which is a play on words I
don't want to go into detail about. They
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offer equivalent water heights calculated
for each monthly solution from the GRACE
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satellites. This tells us a lot about the
earth, if you look closer into it. In the
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following, I will use the monthly
solutions from the ITSG-GRACE 2016,
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provided by Institute for Geodesy at
University of Graz. The previous graph I
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showed you was also created with that
gravity model. I will not go into detail
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about further processing like filtering
and gravity reductions done to this, not
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enough time. So here are some results,
let's start with the most obvious one, the
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greenland ice shield, which has, as we saw
earlier, the greatest loss of mass
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according to the gravity field and we see
here, a water layer on the whole landmass,
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describing the loss of mass expressed as a
water layer of a certain thickness.
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So let's say in the southern tip, you have
one meter water layer. This would be
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equvalent in gravity to the actual mass
lost in Greenland. But we also see, that
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the signal is not very localized. So it's
not bound to the land mass. It's also in
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the ocean. This effect is called leakage.
If you do signal processing you will know
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this. There are methods to reduce leakage.
My next slide will show such a result, but
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I have done no reduction to this. So if
you use my formula I showed you, you will
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pretty much get a result like this. This
gives you a trend of around 280 gigatons
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per year in mass loss over the whole land
mass of greenland. And now gigatons is
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also not very useful an expression. One
cubic meter of water has a weight of a
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1000 kilos; one tonne, 1 gigatonne is
10^9 tonne, if you are familiar with ball
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sports, 1 soccer field with the 140 km
high water column has the weight of
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1 gigatonne, or if you are not fan of sports
ball, if you're more of a plane guy or girl
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the A380-800 has a maximum takeoff weight
of 575 tonne, so we need 1.7 mio of these
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airplanes for one gigatonne. So this is a more
beautiful representation of the process in
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greenland, done by NASA JPL. If you go to
the website of the GRACE project, they
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have a couple of these illustrations, they
obviously worked hard on the leakage.
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You can see localized where most of the
gravity, most of the mass is lost on the
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left and on the right you see accumulated
over time, the mass which is lost, and
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which trend it gives you. Also, if look
closely in the center of greenland, you
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see black lines, these are the ice flow,
as determined by radar interferometry.
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So now pretty much know where ice is lost,
where mass is lost. This goes into the
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ocean, and this would be a good idea to
see, to check our GRACE results, the mass
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we find missing on earth, so the melted
ice, and the additional mass in the ocean,
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does this agree with other methods who
determine the sea level rise. One of these
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methods is satellite radar altimetry, that
started in the 70's, but since 1991, we
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have lots of dedicated satellite missions,
which only job is basically mapping the
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global sea surface. So, they send down a
radar pulse, which is reflected at the sea
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surface. They measure the run time and
then they have a geometric representation
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of the global sea surface. Now, if we
compare this with the mass we calculated
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or we got from the GRACE result, calculate
a sea level rise rise from this additional
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mass in the ocean than these two systems
would not add up. The geometric sea level
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rise is higher than just the additional
mass. So there is the second process which
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is thermal expansion of the water. If
water gets warm it needs more space.
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In 2000 the deployment of so-called ARGO
floats started. These are free-floating
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devices in the ocean. Currently, there are
over 3000 and they measure temperature and
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salinity between sea surface and a depth
of 2000 meters. These are globally
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distributed. So, we have at least for the
upper layer of the ocean, how much thermal
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expansion there is. And what we want to
see is, do these components of additional
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mass in the ocean as determined by GRACE
and thermal expansion of the upper ocean
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layer come to the same result as
geometrical measurements done by satellite
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altimetry? On the left we see an image
taken from the last IPCC report on climate
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change from 2013. In green we see the
sealevel rise as measured with satellite
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altimetry in the time span 2005 to 2012
and in orange we see the combination of
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additional mass, as measured by GRACE, and
thermal extension as determined with ARGO
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inside the ocean. And these 2 graphs follow
each other quite well. On the right. We
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see a recent publication by Chen, Wilson
and Tapley, the latter one being one of
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the PIs of the GRACE mission, who
accumulated the data from 2005 to 2011. We
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basically come to the same conclusion. So
now if you really don't want to do the
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math, there are online services who make
the graphs for you. One of those is
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EGSIEM European Gravity Service for
Improved Emergency Control. If we can
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measure how much water is stored in a certain
area, we know that this amount of water
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has sooner or later to be removed from
this area. This can be a flood, for
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example, and with a mission like GRACE, we
can determine how much mass, how much
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water is there and are the rivers large
enough to allow for this water to be
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flowing away. That was the intention
behind this service. Oops, no, this is not
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the future. So, I wanted to do the life
demo but. So, yeah, the live demo did not
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work as expected. So, you will be greeted
with this graphic. You can plot for all
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areas in the world. The first thing you
have to do is you change your gravity
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functional, we want water heights. This is
what I talked about in this talk. Then you
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want to look at the data set and at the
bottom you see a large list of GRACE
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gravity fields. These are different
groups, I mentioned, providing these
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monthly solutions. And so we choose one of
these groups. Then we choose an area which
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we are interested in. You can freely
choose one area like here Finno-Scandia,
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or you can use pre-determined areas, for
example, the Amazon river basin or Elbe
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river or something like that. These areas
all over the world and you can see the
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gravity change in this area. So let's look
here at Finno-Scandia, and then you are
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greeted with a plot like this. This is
equivalent water height, even though this
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is a geophysical process. So we see here
the layer of water, which would have been
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added to the region as selected, and we
see a clear trend upward. Again, this is a
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geophysical process. This is not
additional ice or water or anything. Can I
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return to my...? No, I cannot. So, yeah,
live demo did not work. If you want to do
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this yourself. I have uploaded to the
Fahrplan all my resources, all my links.
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And the EGSIEM page also includes the
description of what is done in the backend
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and were the data comes from and what you
can see in the various fields. Now I want
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to give a last impression on the future,
because unfortunately while I was
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preparing my abstract for this conference,
one of the GRACE satellites was turned off
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due to age. It was launched in 2002,
planned for a five mission year; it
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survived 15 years, which is quite good,
but now we have no more ranging
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information between these satellites. We
had ranging information in micrometer
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accuracy, a couple of micrometer, and now
we cannot rely on these information
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anymore. And this means mo more gravity
fields with high spatial resolution, and
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I'm not sure about the temporal
resolution. So, the current work which is
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done is taking all satellites which are in
the low-enough orbits and calculate the
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gravity field from their positions,
because everything which is in low-earth
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orbit is affected by the Earth's gravity
field. So, if I take the satellite orbits,
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look "how does this orbit change" and the
reason is gravity, then I can calculate
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the gravity field. Unfortunately, not in
this higher resolution we are used to.
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And... But fortunately, there already is a
next-generation gravity field mission on
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its way. It arrived last week in the US,
where it will be launched in late March,
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early April by SpaceX. You might look at
this image and think, "I just saw this
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earlier" and you are quite correct: The
mission called "Grace Follow On" is a copy
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of Grace, which improved components, of
course, and now with lasers. We see not
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only the microwave ranging between the two
satellites, but additionally a laser
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interferometer. So, from micrometer
accuracy in the distance measurements we
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go to nanometer accuracy, hopefully. But
the main instrument will be the
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microwave ranging. So, in conclusion,
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I hope I showed you that the gravity field
can show mass transport on the surface and
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inside the Earth; that this offers, in
combination with other methods, new
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insights and also some kind of mutual
verification. If several different types
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of observations coming to the same
conclusion, none of them can be awfully
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wrong; and that the access to these
methods are relatively easy: the data is
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available, all the methods are described
in geodesy textbooks and the technical
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documentation; and there are other
applications, other than, let's say,
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climate change; you can look into drought
and flood prediction; the El Niño–Southern
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Oscillation you can predict from Grace's
gravity field data. So, lot's of work to do.
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So, this would be the end for my talk.
I thank you for your interest in the topic.
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applause
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Herald: Thank you, Manuel, for the talk.
And I think we have time for one or two,
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maybe two very short questions. Please be
seated during the Q&A session. Is there
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some questions? Okay, microphone 3,
please.
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Mic 3: Yeah, hi. In a quiet voice
Hi, hello? Can you hear me? Now loud
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Herald: Yeah.
Mic 3: Okay. Hey. So, my question is
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regarding acceleration. What's the
influence of Earth atmosphere and all the
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planetary bodies, like the moon, and does
it need to be accounted for?
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Manuel: The external gravity needs to be
accounted for, so the tidal effects of sun
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and moon would be one of those additional
models you put into the processing of the
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satellite data. The Earth's atmosphere has
an effect on the satellites themselves,
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which is measured onboard by
accelerometers and then reduced. And the
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gravitational effect of the atmosphere:
Part of this is averaged out, because we
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take a month of time series, and the rest
are also inclu... provide as extra
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products; at least by the Institute for
Geodesy in Graz. So atmosphere... the mass
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of the atmosphere is... has to be
accounted for, yes.
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Herald: Okay. Microphone 2 has vanished
all of a sudden. Then, microphone 1,
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please.
Mic 1: Hi. Is it possible to measure
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changes in the temperature of the oceans
or of the ocean streams, like... Can you
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see if El Niño is active by just measuring
the gravity... change in gravity fields?
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Manuel: As a precursor tool, El Niño, as I
understand it... certain regions of the
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ocean get warmer; it's a density change;
and, of course, this would be measured as
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part of ARGO and it's also in the GRACE
gravity field. There are probably papers
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on it. So, the last... the extend of the
last El Niño was predicted by GRACE. I
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don't know to what extend this was
correct, but...
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Mic 1: Okay, then.
Herald: Good. Then, that's all the time we
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have. A big round of applause for Manuel
and his talk, please.
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Applause
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34C3 Music
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