APPLICATION OF AIRCRAFT LASER ALTIMETRY TO GLACIER AND ICE CAP MASS BALANCE STUDIES 
   
W. Abdalati and W.B. Krabill 
Laboratory for Hydrospheric Processes 
NASA Goddard Space Flight Center 
waleed.abdalati @gsfc.nasa.gov 
Commission II, Working Group 3 
KEY WORDS: Glaciers, ice caps, ice sheets, aircraft, altimetry, laser. 
ABSTRACT 
The Arctic ice caps and glaciers serve as important indicators of climate Change and also make a small, but significant, contribution to 
changes in global sea level. Consequently, understanding their mass balance is of considerable interest. 
We are examining the current 
state of mass balance of the major Canadian ice caps in the greater Baffin Bay area in the context of recent climate conditions through 
analysis of precise elevation changes and recent climate history. Using the series of airborne laser/GPS elevation measurements made 
with the Airborne Topographic Mapper (ATM) in 1995, in conjunction with planned repeat surveys in the year 2000, we will 
quantitatively measure and interpret ice thickness changes in the context of recent climate conditions. 
Such elevation surveys provide 
the most direct large-scale means of examining the state of balance of these caps. In addition the application of airborne laser altimetry to 
the study of flow characteristics of glaciers is presented and discussed for three outlet glaciers on the Greenland ice sheet. 
This work is a complement to the current ATM-based ice sheet analyses that are being conducted for Greenland, but it is focused on 
smaller ice masses with different temporal responses to climate variations. Recent results of the Greenland missions have shown 
interesting thickening and thinning characteristics, with the most 
significant changes found in the more temperate parts of the ice sheet 
near the margins and in outlet glaciers. Consequently, the assessment of changes in the smaller Arctic ice masses will be an important 
complement to the results from Greenland, and they will help provide a more complete picture of the current state of balance of the 
Arctic ice masses. 
1 INTRODUCTION 
Understanding the behavior of Arctic ice masses in the 
changing present-day climate is important for a number of 
reasons. First, the Arctic is believed to be highly sensitive to 
changes in Global climate, primarily because of the positive 
albedo feedback effects associated with surface melt and snow 
metamorphism processes.  Recently-fallen snow is highly 
reflective with an albedo of as much as 90%, but as 
temperatures rise, and the surface begins to melt, the albedo 
reduces to 60 to 70%. This translates to a 3- to 4-fold increase 
in absorbed energy, which further enhances the melt process. 
Even in the absence of melt, warming temperatures increase 
grain growth, which in turn reduces the albedo, and creates a 
similar self-compounding effect, but to a lesser degree. 
Because of this unstable positive feedback, perturbations to the 
System are greatly amplified-in the Arctic, and other snow- and 
ice-covered areas. 
Secondly, the large ice masses influence sea level, which has 
been rising at a rate of as much as 2 mm/yr over the last 
century. In a warming climate, ice caps, glaciers and ice sheets 
can increase sea level if they are in a negative balance, by mass 
loss through ablation and discharge. However, it is possible 
that they can reduce the rise in sea level if they are in a positive 
balance, i.e. if they are gaining mass through increased 
accumulation. For the large ice sheets, Greenland and 
Antarctica, the current mass balance is not known. In the case 
the smaller ice masses, such as glaciers and ice caps, many 
appear to be shrinking, while others seem to be growing. 
Quantitative assessment of the state of balance of the larger ice 
masses, such as those in the Arctic and some of the smaller 
glaciers, is essential to understanding their behavior and 
associated implications in a changing climate. Currently, laser 
altimetry provides the best means for assessing ice mass 
balance through the measurement of large-scale elevation 
changes. 
In recent years, the utility of aircraft laser altimetry for 
measuring surface elevation changes on the Greenland ice sheet 
to better than 10 cm rms, has been clearly demonstrated 
(Krabill et al., 1995). This technology has been employed to 
investigate the mass balance of the Greenland ice sheet through 
a series of repeat elevation surveys separated by 5 years. Initial 
results for the southern half of the ice sheet (south of 
approximately 70°N) show interesting spatial variability, with 
very large thinning of some outlet glaciers in the east, and more 
subtle changes, both thickening and thinning, at the higher 
elevations (Krabill et al, 1999), These results suggest that 
some of the more dynamic areas of interest are the outlet 
glaciers and smaller ice masses, where the responses to climate 
forcing are more extreme. 
Building on that technology, we are now applying those same 
survey techniques to perform similar studies of Canadian ice 
caps and other smaller ice masses. This paper will summarize 
some of the applications of airborne laser altimetry to studying 
the dynamics of fast-moving glaciers, as well as describe plans