Geoscience Reference
In-Depth Information
rather high (B10 cm/year minimum, B5 cm/year goal). Uncertainty of radar signal pen-
etration in firn and temporal trends in firn properties are reasons for differences in surface
elevation measured by radar and lidar (Brenner et al.
2007
). Further uncertainties in
computing mass changes of the ice sheets result from variability of firn layer thickness,
caused by regional variability of accumulation and temperature (Helsen et al.
2008
). These
are reasons for the rather large error bars in the mass balance estimates for Antarctica and
Greenland in the IPCC report (Lemke et al.
2007
). Significant reduction in the uncertainty
can be expected by applying different altimetric systems in synergy.
A combination of SAR interferometry and satellite altimetry strengthens the retrieval
method and reduces the uncertainties that in turn allows for more detailed studies of
topographical and mass changes as well as surface velocity and associated deformation as
reported by Rignot et al. (
1998
) and Shepherd and Wingham (
2002
) showing that
dynamically related thinning is penetrating deep into the interior of the West Antarctic, Pine
Island and Thwaites drainage basins. The temporal variability and corresponding signal
decorrelation time are again a limiting factor regarding appropriate use of interferometry.
CryoSat (Wingman et al.
2006
) is a separate mission developed mainly for measuring
ice sheet elevation and sea ice thickness and their changes. By accurately measuring
thickness change in both types of ice, CryoSat-2 will provide information to complete the
picture and lead to a better understanding of the role ice plays in the Earth system.
The planned Sentinel-1 mission is expected to offer unique operational and scientific
capacity due to its increase revisit period. Also, by a synergistic use of Sentinel-1 and the
other space-borne SAR missions, often operating at different wavelengths and modes,
certain ice types can be easily identified.
13 Sea Ice
Microwave satellite observations are routinely providing essential data on large-scale ice
concentration, area, type and large-scale motion. Moreover, measurement of the vertical
dimension of sea ice (ridges, freeboard, thickness, snow thickness) and thermodynamic
properties (temperature, heat flux) is possible by use of altimeters (e.g., IceSAT, CryoSat 2
for sea ice thickness [0.5 m) and infrared/microwave radiometers (e.g., SMOS for sea ice
thickness \0.5 m), although at varying degree of maturity with respect to retrieval accura-
cies. Many small-scale processes and phenomena related to sea ice deformation and marginal
ice zone thermodynamics can also be observed by high-resolution SAR (coupled with
optical/infrared images under cloud-free conditions), but there are no systematic and long-
term observations because the data coverage is insufficient. Snow depth and snow water
equivalent are also important variables in the presence of sea ice that need to be retrieved
more reliably from satellites. Data on snow depth can be obtained from satellite sensors such
as by combined use of IceSAT and CryoSat, and from optical (snow cover) and passive
microwave data (snow depth), or higher frequency (Ka band) SAR data, but the methods are
not adequately validated and need to be further carefully examined and improved.
14 Sea Level
Sea levels are rising in several places around the world potentially impacting human
populations (e.g., those living in coastal regions and on islands) and the natural environ-
ment (e.g., marine ecosystems). Global average sea level rose at an average rate of around
