nd the elevation change is
N ICE CAPS
dian and other Arctic ice caps
e these ice masses are likely to
nte changes. Although their
ar less than that of Greenland
) respond to climate changes
heets; thus in some ways these
ice sheet may be more of an
their behavior in response to
es about the future of the
Greenland missions, airborne
nade of ice caps on Baffin,
, and Meighen islands, as well
The Canadian survey lines are
irveys have provided valuable
of the ice caps, their greatest
repeat surveys for elevation
> planned for May of 2000.
95 survey lines over several
vill be re-surveyed during the
oses of measuring elevation
icate the locations of coastal
dots indicate the location of
Such elevation surveys will provide the most direct large-scale
means of examining the state of balance of these caps. Unlike
mountain glaciers, whose variations are often manifested in the
advance and retreat of their termini, significant changes in ice
caps can occur with little indication along the perimeter. This
research is intended to detect and quantify any such changes,
and to do so in the context of the local climate. Initial results
from the Greenland measurements (Krabill et al., 1999) show
the most dramatic changes near the ice sheet edges, while the
more central regions show far more subtle changes. These
Greenland results underscore the importance of understanding
the current behavior of the surrounding temperate ice caps.
Meaningful interpretation of the observations, however requires
an understanding of the climate conditions in the region. Much
of this information is available from coastal weather station
data from the last 20 years from approximately 40 coastal
weather stations in the region. However, a more complete
interpretation requires some detailed knowledge of the in situ
temperature, accumulation, and mass balance history. An
understanding of the variability of these parameters over time,
and the degree to which they are (or are not) anomalous during
the time period that separates the repeat surveys is essential to
assessing the significance of observed elevation changes, and
understanding their causes.
Toward that end, collaborations have been established with
various Canadian glaciologists, who have a fairly
comprehensive set of relevant in situ observations spanning the
last 40 years in some cases. These data sets and their intended
uses are as follows:
Ice core data from 6 ice caps will provide long records of the
accumulation history (mass input) and its variability over time
for six major ice caps. Van der Veen (1993) has shown that the
statistical significance of observed elevation changes made
from repeat measurements separated by a time of n years is
given as:
1 Ay n
P(A - —[1- —,|—
(A1) 31 eg e en (1)
where P(A,) is the probability that the observed changes in
surface elevation are attributable to fluctuations in
accumulation is greater than A, and c. is the standard deviation
of accumulation. Using this relationship, data from these ice
cores will be used to assess the statistical significance of the
observed elevation changes. Moreover, the data will be used to
assess the degree to which the accumulation during the 1995-
2000 survey period has been anomalous.
Up to 40 years of mass balance measurements will provide
information on the recent mass balance history and its
variability over time. As with the ice cores, but on larger
spatial scales, this information will be used to assess the
statistical significance of the observed elevation changes and
the anomalous nature of the 1995-2000 survey period, by
adapting Equation (1) to account for mass balance rather than
accumulation.
Automatic weather station observations from 12 stations will
provide a recent temperature history in different ice cap zones,
which can be used to approximate ablation through degree-day
analyses. These data will also be used to assess the 1995-2000
climate characteristics for interpretation of the elevation
changes and the role of ablation in these changes. In addition,
the accumulation sensors mounted on nearly all of the stations
will complement the ice core accumulation data.
Detailed elevation maps of White and Thompson Glaciers,
developed from survey data in 1960 (Cogley et aL, 1996). Will
be compared to laser altimetry measurements This will allow
assessment of a long-term elevation changes of two major
drainage glaciers of the Müller Ice Cap on Axel Heiberg.
Although this comparison will be somewhat limited by the
accuracy of the earlier maps, the 40-year time separation
between the different data sets should prove to be quite
valuable.
Through a combination of the repeat laser altimetry surveys,
and all of the ancillary data sets, elevation changes (if
elevations have in fact changed) will be measured and
interpreted in the context of the present and recent climate.
Results from the Canadian ice cap missions will be an
important complement to those from Greenland, and they will
help provide a more complete picture of the current state of
balance of the Arctic ice masses.
In addition, two sets of baseline data (with 5 years of separation
between them) will be available for comparison to future GLAS
observations, which, in its 183-day repeat orbit will have
ground track separations in the vicinity of the ice caps of
approximately 6 km for the more southerly ones, and 1.5 km for
the more northern ones. Furthermore, the detailed topographic
information will be useful for pre-launch simulation of laser-
pulse return waveforms that GLAS will acquire over such ice
caps. Because these ice caps are generally rougher and more
steeply sloping than the large ice sheets, interpreting their
waveforms may be more complex. These detailed topography
measurements will improve these interpretations considerably.
4 FAST MOVING GLACIERS
4.1 Approach
For ice elevation change measurements, a series of surveys is
made over an ice mass with the intent of resurveying the same
flight lines at a later date. In the case of the Greenland ice
sheet, these surveys were made during the spring of 1993 and
1994 for the southern and northem portions of the ice sheet
respectively, and they were repeated in 1998 and 1999. Results
for the southern half of Greenland have been published by
Krabill et al, (1999), and the results from the north are
pending. In 1997, a detailed grid of the Jakobshavn Isbrae was
surveyed, during which time multiple measurements over or
near the ice stream were made. Some tracks were exact repeats
of others with a time separation of two to six days. These
surveys provided the initial proof of concept of velocity
