International Polar Year 2007–2008 - WMO

of the Earth. The patterns of uplift documented by
GPS measurements can document the positions of
major ice loads (Sella et al., 2007). Mantle viscosity and
the thickness and elastic rigidity of the lithosphere,
control the magnitudes and time scales for isostatic
response to glacial loading and unloading. These earth
properties, and their variation, can be inferred from
studies of seismic velocity and attenuation. Hence,
by simultaneously measuring crustal motions using
GPS and by gaining higher-resolution data on the
thickness and rheology of the crust and mantle from
seismological studies, the POLENET observational
programme will make an unprecedented leap in our
ability to model GIA in Antarctica and Greenland.
The first tests of glacial isostatic adjustment models
for Antarctica and Greenland based on GPS-derived
vertical crustal motions mainly show that GIA model
predictions do not match measured rates (Ohzono et
al., 2006; Mancini et al., 2007; Khan et al., 2008; Willis
et al., 2008c; Bevis et al., 2009a). This emphasizes the
need for improved ice and earth models, and for ‘GPStuned’
GIA modeling.
Satellite-based monitoring provides one means of
obtaining data on modern ice mass balance for entire
ice sheets, and ongoing analyses of both altimetry
and time-variable gravity data indicate mass loss
from the Greenland and Antarctic ice sheets that
appears to be accelerating (Rignot et al., 2008; Chen
et al., 2009; Velicogna, 2009). Uncertainties in both
types of measurements, however, are mainly due to
‘contamination’ by vertical displacement of the bedrock
beneath the ice sheets due to ‘rebound’, and a poorly
constrained ‘correction’ must be applied to remove
this component in order to derive ice mass change
(Alley et al., 2007). POLENET observing networks were
designed to directly measure solid earth phenomena,
including ‘rebound’, and provide the first synoptic
ground-based observations across the Greenland
and Antarctic ice sheets. The new observations will
thus greatly reduce the sources of uncertainties in
satellite-derived measurements. For example, GPS
data from Enderby Land, Antarctica, has shown that
an apparent region of positive ice mass change, which
could be interpreted as increasing ice mass, cannot be
ascribed to incorrectly modeled vertical crustal motion
due to GIA (Tregoning et al., 2009). POLENET GPS
measurements will thus complement the orbital data
sets to measure ice mass change to an unprecedented
level of detail and accuracy.
Earth’s response to any very recent changes in
ice mass, including rapidly accelerating ice loss
over decades, will be largely elastic. Outlet glaciers
in southeast Greenland showing accelerated flow
speeds and rapid ice discharge in the current decade
produced a detectable increase in uplift in the time
series of a nearby continuous GPS station, recording
a rapid elastic response of the crust (Khan et al., 2007).
The new array of continuous GPS stations in both
Greenland and Antarctica ensure that any such elastic
signals will be recorded and, using seismological
constraints on regional elastic structure and any
independent data on ice mass change, we will be able
to calibrate the relationship between ice mass change
and crustal deflection. This geodetic measurement
of earth’s elastic response will provide a new way to
recognize and measure periodic or accelerating ice
mass loss.
Ice dynamics
An understanding of the ‘solid Earth’ processes that
influence ice sheet dynamics is essential for predicting
the future behavior and stability of the polar ice
sheets. Coupled ice-sheet climate models require
estimates of heat flow and sediment thickness at the
base of the ice sheet, which can ‘lubricate’ the ice-rock
interface. Since these parameters cannot be measured
directly in Antarctica, seismic images provide a
‘remote sensing’ method to obtain information that
is vital to understanding ice sheet stability. Scientists
will use seismological investigations, integrated with
results from the geodetic studies, to provide firstorder
constraints on geological/tectonic parameters
important for understanding ice sheet dynamics.
New seismic data will be used to develop highresolution
seismic images that will constrain
lithospheric viscosity and thermal structure as well
as basal heat flow. High heat flow could produce subice
water, lowering bed friction, and may lead to the
formation of subglacial lakes. Tomographic images of
West Antarctica show the entire region is characterized
by slow upper mantle velocities suggestive of high
heat flow and thin lithosphere, but resolution is too
poor for detailed correlation of low velocity regions
with tectonic and glacial features.
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