national report of the federal republic of germany on the geodetic ...

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1. Monitoring <strong>of</strong> Long-Term Sea-Level Changes
and GPS Tide Gauge Benchmark Monitoring
The global sea level is a key quantity in <strong>the</strong> Earth's climate
system. Tide gauge data provide long-term information on
relative sea-level changes, whereas a GPS monitoring <strong>of</strong>
tide gauge benchmarks allows <strong>the</strong> separation <strong>of</strong> vertical
crustal movements and sea-level change in <strong>the</strong> global geocentric
coordinate frame.
In <strong>the</strong> sou<strong>the</strong>rn Baltic Sea, long-term sea-level data were
reduced in order to provide homogeneous time series, and
<strong>the</strong> regional pattern <strong>of</strong> sea-level rise was determined (LIEBSCH
et al. 2000; DIETRICH and LIEBSCH 2000). The analysis <strong>of</strong>
sea-level changes in <strong>the</strong> Mediterranean showed regional
differences in <strong>the</strong> sea-level rate, but since <strong>the</strong> 1990's an
increase <strong>of</strong> this rate can be observed at all <strong>of</strong> <strong>the</strong> analyzed
PSMSL stations (FENOGLIO-MARC and MARTINEZ-GARCIA
2001; FENOGLIO-MARC 2002a; FENOGLIO-MARC 2002b).
The GPS monitoring <strong>of</strong> tide gauge benchmarks is a key issue
<strong>of</strong> <strong>the</strong> IAG Special Commission 8 and <strong>of</strong> <strong>the</strong> IGS TIGA pilot
project. The SEAL project intends an integrated approach
<strong>of</strong> different research centres and contributes substantially
to this important task (REIGBER et al. 2001). Within <strong>the</strong> scope
<strong>of</strong> <strong>the</strong> EVAMARIA project, several Atlantic tide gauge
benchmarks are monitored by GPS (BOSCH et al. 2002;
HÄFELE et al. 2002). Antarctic tide gauges were also linked
to <strong>the</strong> ITRF by GPS observations (SCHÖNE et al. 1999).
2. Sea-Surface Changes from Altimetry, and
Its Consequences for Oceanography, Gravity
Field Determination, and Earth Rotation
For <strong>the</strong> study <strong>of</strong> <strong>the</strong> sea surface and its variability, satellite
altimetry is a powerful tool which provides data with almost
global coverage in high temporal resolution. The global sea
surface from altimetry has been used to estimate ocean mass
redistribution (GRUBER et al. 2000), and to discuss its impact
on <strong>the</strong> global height system definition (BOSCH 2002b). In
order to investigate <strong>the</strong> sea-level variability and its causes,
regional studies were carried out in <strong>the</strong> North Atlantic Ocean
(ESSELBORN et al. 2001; Esselborn and Eden 2001), in <strong>the</strong>
Mediterranean (FENOGLIO-MARC et al. 2000; FENOGLIO-
MARC and GROTEN 2003), and in <strong>the</strong> Carribean Sea (BOSCH
et al. 2002).
The role <strong>of</strong> sea-surface changes in gravity field determination
is discussed (REIGBER et al. 1999), and effects <strong>of</strong> <strong>the</strong> sea
level on earth rotation and length-<strong>of</strong>-day are investigated
Sea Level and Ice Sheets
R. DIETRICH 1
by several authors (HÖPFNER 2001b; JOCHMANN 2002;
WÜNSCH 2002b).
3. Altimeter Calibration/Validation and Intercomparison
<strong>of</strong> Techniques
For consistent sea-level observations, unbiased altimeter
data are required (see e.g. BOSCH 2002a). This involves <strong>the</strong>
determination <strong>of</strong> relative mission biases as well as absolute
<strong>of</strong>fsets and drift rates for single altimeter missions.
Within <strong>the</strong> calibration/validation activities for recent satellite
altimeter missions, GPS buoys in <strong>the</strong> North Sea (SCHÖNE
et al. 2000) were operated. The regional unification <strong>of</strong>
different altimeter missions was performed using crossover
differences in <strong>the</strong> Mediterranean (FENOGLIO-MARC 2001b;
FENOGLIO-MARC and GROTEN 2002b). For <strong>the</strong> Baltic Sea,
absolute altimeter bias and drift rates for several missions
were determined by comparison with tide gauges (LIEBSCH
et al. 2002).
Ano<strong>the</strong>r source for information about sea-surface heights
are oceanographic models. The intercomparison <strong>of</strong> tide gauge,
altimetry and oceanographic modelling in <strong>the</strong> Baltic Sea
showed a consistency <strong>of</strong> <strong>the</strong> different height observations
and information <strong>of</strong> a few centimetres (NOVOTNY et al. 2002).
4. Ice Sheet Topography, Dynamics, and Mass
Balance
Precise surface elevations <strong>of</strong> <strong>the</strong> Antarctic ice sheet were
determined from ERS radar altimetry on continental-wide
scale (IHDE et al. 2002). In Antarctica, regional investigations
to study <strong>the</strong> ice flow <strong>of</strong> outlet glaciers by SAR interferometry
were focussed on Mertz Glacier (PÖTZSCH et al. 2000) and
<strong>the</strong> region <strong>of</strong> Schirmacher Oasis (DIETRICH et al. 1999).
Fur<strong>the</strong>r, <strong>the</strong> mass balance <strong>of</strong> polar ice sheets was investigated
based on model computations (HUYBRECHTS 2001).
5. Ice – Ocean – Solid Earth Interactions
The dynamics <strong>of</strong> <strong>the</strong> ice sheets due to climatic and o<strong>the</strong>r
influences causes complex interactions with <strong>the</strong> ocean and
with <strong>the</strong> solid earth. The impact <strong>of</strong> ice mass changes on sea
level for glacial cycles was investigated (HUYBRECHTS 2002).
Fur<strong>the</strong>r, <strong>the</strong> impact <strong>of</strong> ice mass changes on gravity and geoid
was discussed (LE MEUR and HUYBRECHTS 2001).
The dynamics <strong>of</strong> ice shelves, especially <strong>of</strong> <strong>the</strong> grounding
zone due to tidal motions was studied in detail (RIEDEL et
al. 1999; RIEDEL 2002a; RIEDEL 2002b). The migration <strong>of</strong>
1 Reinhard Dietrich: Institut für Planetare Geodäsie, Technische Universität Dresden, Mommsenstraße 13, D-01062 Dresden, Germany; Fax.:
+49 - 351 - 4633 370 63, Tel.: +49 - 351 - 4633 346 52, E-mail: dietrich@ipg.geo.tu-dresden.de