1. How representative are benchmark glaciers?
Andrew G. Fountain, Mathew Hoffman, Frank Granshaw, Portland State Univ
Jon Riedel, National Park Service
South Cascade Glacier
South Cascade Glacier, WA USGS John Scurlock
South Cascade has been used as the ‘benchmark’ glacier for the glacial reponse to climate change for all of the glaciers in the NWern US (Washington, Oregon)

2. Benchmark Glaciers of the US
Alaska
Gulkana -- Alaska Range
Wolverine -- Kenai Range
Continental US
South Cascade -- Cascade and
Olympic ranges of Washington
and Oregon
Our report here on the region around South
Cascade Glacier
Meier, 1985

3. Chickamin Glacier
Dome Peak
John Scurlock

4. Methods Aerial Photos Historic Maps GIS Database Repeat Photography
Gridded
Climate Data
Glacier
Inventory
(24K)
Lyell Glacier
Located in National parks, previous existing data)

5. South Cascade Glacier Original IHD glacier Monitored since 1959
US Geological Survey
Mass & Water Balance
4 Index Glaciers
Monitored since 1992
Nat. Park Service
Mass balance only
316 Remotely-sensed
1958, 1998
Area only
Park Complex

6. What is a representative glacier?
Topographic characteristics?
area slope
elevation aspect
Location? (any glacier will do)
Similar climatic response?
Pugh Glacier John Scurlock

7. Topographic Characteristics
Glacier Area
Benchmark Glacier
Index Glaciers 0%
10%
20%
30%
40%
50%
60%
70%
80%
90%
100% 1 2 3 4 5 6 7
1998 Area (km2)
Fraction of glaciers smaller

8. Topographic Characteristics
Elevation, Area, Slope
Benchmark Glacier
Index Glaciers
Remotely Sensed Glaciers
500
1000
1500
2000
2500
3000 2 4 6
Area (km2)
Mean Elevation (m) 20 40 60 80
Mean Slope (deg)
The benchmark glacier is larger (already said), slightly lower in elevation (difference may not be important); and of lower slope by far. The lower slope was a quality in the selection process to beable to reach all parts of the glacier.

9. Area Changes Number of Glaciers and Area Change 10% 0% -10% -20% -30%
-40%
-50%
-60%
-70%
Fractional Area Change
Number of Glaciers and Area Change
Depending on your choice of glacier it may or may not represent area change of the population

10. Area Changes Uncertainty
Is the choice of glacier representative of change within its size class?
Uncertainity is based on the uncertainty in the outlines of the 1958, 1998 glaciers, and is relatively larger for smaller glaciers.

11. From this cursory analysis, both topographic characteristics
area change
suggest that the benchmark glacier is not representative of the region. And the other index glaciers may be better.
Furthermore, the use of the benchmark glacier would over estimate the glacier change in this region.
John Scurlock

12. Mass Balance Variations)
The benchmark glacier, responding to the same climatic forcing as the index glaciers, exhibits year to year variations in net balance very similar to the index glaciers and most likely to all the glaciers in the region.

13. Mass Balance Variations
Cummulative Mass Change
( m weq)
As the cummulative mass change shows, however, small but consistent differences result in large cummulative differences over time.

14. Conclusions If the example from the North Cascades is typical,
probably most benchmark glaciers are not rep-
resentative of the regional glaciers, due to the
necessary selection criteria (IHD).
- different topographic characteristics
- different area/volume changes
Year to year variations in net mass balance are well
represented (probably by any glacier). But small
differences between glaciers yield large cummulative
differences.
The issue is how to transfer mass balance variations
from one glacier to another to more accurately depict
variations across the region.
First conclusion has important ramifications for using benchmark glaciers for estimating sea level.

15. Area Changes