Greenland ice sheet.
Photo: Konrad Steffen
How to tell if an ice sheet is growing or shrinking
The mass balance of an ice sheet, meaning the rate of change The component approach involves comparing the mass
of its mass, is vitally important because changes in mass bal- added by snowfall on the ice sheet to that lost into the sur-
ance are transformed directly into global sea-level change. rounding ocean. Mass input is based on estimates of snow-
Measurement is difficult and a range of techniques has to accumulation rates from counting annual layers in snow pits
be used to get an overall picture of change in the ice sheets. and ice cores, measuring depths to well-dated radioactive
There are two basic approaches – the integrated approach fallout horizons, or weather-model simulations. Newer re-
and the component approach. mote sensing methods are promising but not yet fully devel-
oped. Mass output by meltwater runoff is generally estimated
The integrated approach involves measuring changes in the from models; to that must be added the solid-ice flux, given
surface height (hence volume) or gravitational attraction (hence by the product of ice flow velocity and thickness at coastal
mass) of the ice sheet using instruments mounted in satel- margins. Ice thickness is generally measured by radar sound-
lites. These instruments include radar and laser altimeters and ing from airplanes, and ice velocity is measured by repeated
high-precision gravity-measuring systems. Laser altimeters can GPS surveys of ice markers, tracking of crevasses and other
detect small surface elevation changes, but are hampered by ice features in high-resolution satellite imagery, and analysis
persistent cloud cover. Satellite surveys began only in 2003. Air- of repeated satellite radar images. This last technique, in
craft laser measurements over Greenland began ten years ear- particular, has made it possible to measure the speed of ice
lier and, although they provide only limited coverage, flight lines movement over vast regions at high spatial resolution.
can be along specified routes such as glacier flow lines. Radar
altimeters have less precision, suffer errors associated with ra- All these techniques for measuring mass balance have sig-
dar penetration into the ice sheet surface, and give poor results nificant errors but because they offer independent estimates,
in rough or steep terrain, but their longer history is still a boon they provide an increased level of confidence in their collec-
for measurements of change. The gravity-based techniques can tive conclusions. However, interpretation of mass-balance
measure changes in overall mass to an astonishing level of pre- estimates is further complicated by high natural variability
cision, but the accuracy of ice-sheet mass balance estimates is that occurs on a range of time scales. Separation of long-
hampered by limited knowledge of how much mass change is term trends in ice mass from the effects of this variability
caused by uplift of the rock beneath by geologic forces. requires observations over long time periods.
104 GLOBAL OUTLOOK FOR ICE AND SNOW