Geoscience Reference
In-Depth Information
limitation in these assessments. Another source of uncertainty is poor knowledge of glacier
inventory data at that time, i.e., data on glacier location and surface area. Nevertheless,
these assessments provide continuous annual or pentadal time series of mass balance
reaching back into the mid-twentieth century.
2.2 Assessments by geodetic method
With the geodetic method, the glacier mass balance is estimated by repeated mapping,
either by ground-based surveys or remote sensing (laser, radar altimetry, stereoscopic
imagery). The change in glacier volume (obtained from the difference in glacier surface
elevations over the glacier area) multiplied by the average density of the removed or added
material gives the change in glacier mass. In contrast to in situ measurements, geodetic
observations generally have good regional but poor temporal coverage, since surveys are
often separated by multi-annual to multi-decadal gaps. On the other hand, the geodetic
method can observe the mass changes of tidewater glaciers, which are not included in
traditional glaciological measurements.
Initially, geodetic surveys have been mostly used to assess the mass changes of indi-
vidual glaciers (Cogley
2009a
,
b
); however, increasing availability of remote sensing data
(in particular satellite laser altimetry after the launch of ICESat in 2002) triggered a
number of studies covering entire glacier regions. Geodetic estimates of mass changes on
regional scale exist for Alaska (Arendt et al.
2002
; airborne laser altimetry, Berthier et al.
2010
; satellite remote sensing), Arctic Canada (Abdalati et al.
2004
; airborne laser
altimetry; Gardner et al.
2011
; satellite remote sensing), British Columbia (Schiefer et al.
2007
; satellite radar altimetry), Svalbard (Nuth et al.
2010
; Moholdt et al.
2010
; satellite
laser altimetry), Iceland (Bjornsson et al.
2013
; airborne and satellite remote sensing),
Russian High Arctic (Moholdt et al.
2012
; satellite laser altimetry), Austrian Alps
(Lambrecht and Kuhn
2007
; DEM from aerial photographs), Swiss Alps (Paul and
Haeberli
2008
; satellite radar altimetry), parts of central Asia (Gardner et al.
2013
; satellite
laser altimetry), Patagonia (Rignot et al.
2003
; Willis et al.
2012
; satellite remote sensing),
the peripheral glaciers in Greenland (Bolch et al.
2013
; Gardner et al.
2013
; satellite laser
altimetry), and the glaciers on the islands surrounding the Antarctic mainland (Gardner
et al.
2013
; satellite laser altimetry). Most of these estimates are derived for relatively short
recent (after 2000) time periods.
2.3 Assessments using satellite gravimetry
Gravimetric measurements have become a popular tool to estimate glacier mass changes
since the launch of the satellites of the Gravity Recovery and Climate Experiment
(GRACE) in March 2002. GRACE consists of a pair of satellites orbiting together and
measuring variations in the terrestrial gravity field, therefore detecting mass movements.
The twin satellites orbit the Earth 15 times a day, recording minute variations in the Earth's
gravitational pull. When passing over a region of larger gravity, the first satellite is pulled
ahead of the trailing satellite, thus increasing the distance between the satellites. Mass
changes are derived from the constantly changing distance between the twin satellites
combined with precise positioning measurements (Tapley et al.
2004
).
GRACE observes mass changes with high temporal resolution (e.g., sub-monthly), but
the spatial resolution is relatively poor (roughly 100 9 100 km). In contrast to the methods
above, no density assumptions are needed because mass change is measured directly.
However, since the satellites detect the total mass changes over a large area, and are unable
