Geoscience Reference
In-Depth Information
10 Permafrost
In the Northern Hemisphere, permafrost regions extend over about 23 million km
2
(Zhang
et al.
2001
).
Permafrost is currently monitored mainly by means of ground-based point measure-
ments. Remote sensing systems are used as complementary tools, to map surface features
of permafrost terrain and monitor their changes driven by climate warming. Surface
indicators of permafrost terrains that can be identified by remote sensing images include
pingos, thaw lakes and basins, retrogressive thaw slumps, thermo-erosional valleys, ther-
mokarst mounds, ice wedge polygons, beaded drainage, palsa fields, slope failures, and
rock glaciers (IGOS
2007
). Precise topographical data are required for accurate geocoding
of the remote sensing imagery (optical and SAR) so that changes in permafrost features can
be tracked accurately. High resolution Digital Elevation Models (DEMs) are also required
for modelling hydrology, permafrost distribution, erosion and matter fluxes resulting from
permafrost degradation (McNamara et al.
1999
). DEMs derived from current satellite
systems (e.g., ASTER DEM) are lacking the accuracy needed for these tasks.
11 Glaciers and Ice Caps
Precise data on surface topography of mountain glaciers, ice caps and outlet glaciers of ice
sheets are needed as basic information for ice dynamic models, mass balance models, and
regional hydrological and climate models. Vertical accuracy on the order of 5 m is
acceptable for most of these applications, except for some special ice dynamic models. The
requirements in vertical accuracy are more stringent for measuring changes in surface
topography to infer glacier mass balance through annual (goal) or multi-annual volume
changes. The typical requirement in vertical accuracy for this application is B1 m (elevation
change). There is still high uncertainty in the mass balance of the world's glaciers and ice
caps (Lemke et al.
2007
). This is due to the fact that accurate mass balance measurements
are made only on few glaciers worldwide (Dyurgerov et al.
2005
). The representativeness of
this small sample is rather questionable, as there is a strong bias towards small glaciers that
are easily accessible. Extrapolating from these glaciers to global numbers causes large
uncertainty. To overcome this deficit requires spatially detailed, precise repeat measure-
ments of temporal changes in glacier surface topography for a large sample of the glaciers
worldwide. For calving glaciers, these measurements need to be complemented by estimates
of the calving flux to obtain the mass balance. In this context, radar altimetry and SAR
interferometry are providing highly important observations. The sustainability of these
observations is also promising in view of the Sentinel-1 and Sentinel-3 missions. The
limiting factor for use of SAR interferometry is the temporal variability of the radar signals
due to snow fall, drift and melting (Rott and Siegel
1997
) as well as due to signal de-
correlation in zones of strong sea ice deformation such as along glacier margins.
12 Ice Sheet
Radar altimetry (ERS-RA, Envisat-RA), IceSat-1 and CryoSat 2 have been the main
sensors for precise measurements of surface topography on the ice sheets for estimating
volume changes. Because the accumulation rate on the main accumulation zones of the ice
sheets is rather small, the requirements in vertical accuracy of the repeat measurements are
